Low cost recycled plastic modification formula for twin screw devolatilization extruder

Building a low cost recycled plastic modification formula for a twin screw devolatilization extruder is not an exercise in buying cheaper additives. It is an exercise in removing the invisible contaminants that make recycled resin behave unpredictably, and then spending the smallest possible additive budget to pull mechanical properties back into a usable window. Compounders who reverse that order — dosing impact modifier, compatibilizer and odor scavenger into a wet, volatile-laden regrind stream — consistently pay more per accepted kilogram and still lose customers to smell complaints and black specks.

Recycled polyolefins and polyesters arrive at the hopper with four handicaps that virgin resin does not have: a broadened and shifted molecular weight distribution from prior heat histories, absorbed moisture, low molecular weight degradation products that carry odor, and residual print ink, adhesive and fragrance from the article’s first life. Every one of those handicaps is addressed more cheaply by devolatilization hardware and screw geometry than by chemistry. That is the central engineering argument of this guide, and it is why the devolatilization section of a twin-screw compounding line deserves as much design attention as the kneading section.

Kerke Extrusion Equipment, a Wanplas factory, has spent more than twelve years designing parallel co-rotating twin-screw compounding extruders for exactly this class of difficult, variable feedstock. From a 19,997 square meter manufacturing base, Kerke has placed more than 2,000 machines in over 70 countries, supported by a technical team of more than 100 people whose slogan — “We Know Compounding Extruder” — reflects a narrow, deliberate specialization in the KTE series of parallel co-rotating twin-screw extruders, from the KTE-16B laboratory unit up to the KTE-135D production machine. This guide sets out the formula families, screw configurations, vent settings, process windows and quality control routines that Kerke’s application engineers use when a customer’s brief is simply: make this recycled material cheap, stable and sellable.

Everything that follows is written in relative cost language. Where a cost comparison is needed, this article uses Low, Medium, High, Very High and Premium ratings, or an index in which a baseline recipe equals 100 index points. Absolute prices move weekly with resin, energy and freight markets and would be misleading in a technical reference; the relationships between formula choices, however, are stable and are what a process engineer actually needs.

1. What “Low Cost” Really Means in Recycled Compounding

Low cost in recycled compounding is a three-part equation: additive loading, yield, and specific energy consumption. Most compounders track only the first, which is why so many “cheap” formulas are expensive in practice. The dominant hidden cost in this business is not the additive package at all — it is batch-to-batch instability that triggers downstream rejection, and the resulting loss of a qualified customer who then re-qualifies with virgin resin.

Consider the arithmetic. An additive package typically represents a modest share of a filled recycled polyolefin compound’s total material cost. Trimming that package by a fifth produces a small saving on the material index. A single returned lot, by contrast, absorbs the freight both ways, the reprocessing labor, the machine hours to rerun, and — most damaging — the customer’s confidence. When a converter has to stop a running injection molding machine because impact strength dropped between two pallets of the same compound grade, the compounder rarely gets a second chance to explain that the regrind bale changed.

The three real cost drivers

Cost driver	What it covers	Typical share of total cost	How much a formula engineer can move it	Cost sensitivity
Base feedstock	Washed regrind, flakes, bale-sorted material, in-house scrap	Largest single block	Moderate — controlled mostly by sourcing and sorting discipline	High
Additive package	Antioxidants, compatibilizer, impact modifier, filler, lubricant, odor adsorbent, chain extender	Small to moderate	Large in percentage terms, small in absolute terms	Medium
Yield and rejection	Start-up scrap, off-spec lots, screen change losses, customer returns	Highly variable, often underestimated	Very large — the single biggest lever	Very High
Specific energy consumption	Motor load, barrel heating, vacuum pump, chiller, pelletizer drives	Moderate and rising	Moderate — screw design and melt temperature control	Medium
Labor and downtime	Screen changes, screw cleaning, die changes, purging	Moderate	Moderate — self-wiping geometry and continuous filtration	Medium

Read that table as a priority list. The additive package is the most visible cost and the easiest to cut, which is exactly why it is cut too aggressively. Yield is the least visible and the most valuable. A formula that adds 0.1 phr of secondary antioxidant and thereby holds melt flow rate within a narrower band across twenty consecutive lots is a cheaper formula, even though its bill of materials index is higher.

A relative cost index for comparing recipes

To keep recipe comparisons honest without quoting money, this article uses a simple index. An economy recycled polypropylene compound with a minimal antioxidant package and no impact modifier is defined as 100 index points on the material axis. Other recipes are expressed relative to that baseline. A separate quality axis tracks the probability that a lot meets specification on first pass.

Recipe tier	Material cost index (baseline = 100)	First-pass acceptance	Effective cost per accepted kilogram	Verdict
Bare economy, no devolatilization	96	Low	Highest of the four	False economy
Economy with two-stage devolatilization	100	Medium to High	Low	Best value for commodity parts
Standard with compatibilizer and impact modifier	108 to 114	High	Low to Medium	Best value for technical parts
High-performance with full stabilization	118 to 128	Very High	Medium	Justified only by specification

The first row is the trap. Removing the vacuum vent, running a shorter barrel and skipping the secondary antioxidant lowers the visible index by four points and destroys first-pass acceptance. This is the most common mistake among new entrants to recycled compounding, and it is a hardware mistake disguised as a formula mistake.

2. Why Devolatilization Is the Foundation of Any Recycled Formula

Devolatilization is the foundation of any recycled formula because volatiles consume additives, disrupt melt continuity and carry the odor that ends most commercial conversations. Additives dosed into an undevolatilized melt are partly spent neutralizing species that a vacuum port would have removed for the cost of electricity alone.

Recycled polymer streams carry four distinct classes of volatile matter, and each behaves differently in the barrel.

Class one: moisture

Water arrives with washed and mechanically dewatered flake, and is also absorbed from ambient humidity during storage. Polyolefins are not hygroscopic in the chemical sense, but flake geometry holds surface water stubbornly — 0.3 to 1.5 percent by weight is common after centrifugal dewatering alone. Polyester and polyamide streams absorb water into the polymer itself and hydrolyze at melt temperature, cleaving chains and dropping intrinsic viscosity irreversibly. Moisture that reaches the melt zone flashes to steam, causes surging at the die, produces voids in the strand and prints as silver streaks in downstream molded parts.

Class two: residual monomer and oligomers

Every polymer contains a small residual fraction of monomer, dimer and low molecular weight oligomer. In recycled streams this fraction grows because previous processing cycles generated chain scission products. Polyester regrind carries acetaldehyde and cyclic trimer; styrenics carry residual styrene; polyolefins carry short-chain paraffins and olefinic fragments. These species are volatile at melt temperature and are exactly what a deep vacuum port is designed to strip.

Class three: thermo-oxidative degradation products

This is the odor class that customers complain about. Repeated heat histories in the presence of oxygen generate aldehydes, ketones and carboxylic acids on the polymer backbone and as free small molecules. Short-chain aldehydes in particular have very low odor thresholds — a few parts per million in the headspace is enough for a human nose to register the smell in a finished part. No practical additive loading removes them once they are formed in bulk; the correct response is to strip the existing ones under vacuum and to prevent the formation of new ones with an efficient antioxidant system and a conservative melt temperature.

Class four: residual ink, adhesive, fragrance and product residue

Post-consumer packaging carries printing ink solvents, hot-melt adhesive fractions, label varnish, and fragrance or surfactant residue from the original contents. Detergent and personal-care bottles are notorious for this. These compounds are chemically diverse, often relatively high boiling, and are the reason that a single atmospheric vent is rarely enough. Deep vacuum, sufficient residence time in the vented zone, and in stubborn cases steam stripping are the tools that work.

Volatile class	Typical species	Typical level in post-consumer regrind	Removal method	Consequence if left in
Moisture	Free and absorbed water	0.3 to 1.5 percent (polyolefin flake); 0.2 to 0.6 percent (polyester flake)	Pre-drying plus atmospheric vent	Surging, voids, silver streaks, hydrolysis of polyester
Monomer and oligomer	Acetaldehyde, cyclic trimer, short paraffins	Tens to hundreds of parts per million	Vacuum vent at minus 0.085 to minus 0.095 MPa	Odor, taste transfer, blooming
Degradation products	Aldehydes, ketones, carboxylic acids	Variable, rises with each heat history	Vacuum vent plus antioxidant plus lower melt temperature	Strong odor, yellowing, further chain scission
Ink, adhesive, fragrance	Solvents, resin fractions, terpenes, surfactants	Highly variable by source	Deep vacuum, longer vented residence, steam stripping	Odor, black specks, gel formation, color drift

3. Devolatilization Engineering on a Twin-Screw Extruder

Effective devolatilization on a twin-screw extruder is a two-stage job: an atmospheric vent that releases the bulk of steam and easy volatiles early, followed by a vacuum vent operating at minus 0.080 to minus 0.095 MPa that strips the low molecular weight residue after the melt is fully homogeneous. Attempting both duties at a single port is the most frequent design shortcut in recycled compounding, and it is the reason many lines cannot hit an odor specification regardless of formula.

Stage one: the atmospheric vent

The atmospheric vent sits shortly after the first melting section, typically between 14 and 22 L:D depending on total barrel length. At this point the material is molten but the melt is still relatively open and gas can escape without a pressure driving force. The vent’s job is quantity, not refinement: it releases the steam load from surface moisture and the most volatile fraction. Running an atmospheric vent hot enough to keep the port free of condensate — but not so hot that condensed oligomer bakes into a crust — is a small detail with large maintenance consequences. A port jacket held near the barrel setpoint, with an easily removable insert, is the practical answer.

Stage two: the vacuum vent

The vacuum vent belongs in the last third of the barrel, after the final mixing section and before the pressure build-up zone. By this point the compound is homogeneous, the filler is dispersed, and any remaining volatiles are the difficult ones. Vacuum level is the primary lever:

Vacuum level (gauge)	Achievable residual volatile content	Typical odor improvement	Equipment requirement	Energy cost level
Atmospheric only	Baseline	None	Open vent port	Low
Minus 0.05 to 0.07 MPa	30 to 45 percent reduction	Around half a VDA 270 grade	Single-stage water ring pump	Low
Minus 0.080 to 0.090 MPa	55 to 70 percent reduction	Around one VDA 270 grade	Water ring pump with good sealing water cooling	Medium
Minus 0.090 to 0.095 MPa	70 to 85 percent reduction	One to one and a half VDA 270 grades	Two-stage pump or pump with booster, tight barrel sealing	Medium to High
Deep vacuum with steam stripping	Above 85 percent reduction	Up to two VDA 270 grades	Water injection port, stripping section, condenser	High

Screw elements in the vent zone

The vent zone is a screw design problem before it is a vacuum pump problem. Three rules govern it:

Shallow-channel, long-pitch conveying elements under the port. Long pitch reduces the degree of fill and increases free surface area, which is what drives mass transfer of volatiles out of the melt. A degree of fill above roughly 30 percent under the port sharply reduces stripping efficiency and raises the risk of material creeping up the vent throat.

A sealing element before the port. A short reverse-pitch or narrow kneading block upstream of the vent creates a melt seal that prevents vacuum from propagating backwards toward the feed opening. Without it, deep vacuum pulls unmelted feed and powder additive directly into the vent line.

A pressure-building element after the port. Downstream of the vent, conveying pitch is reduced progressively to rebuild pressure for the melt filter and die. That transition should be gradual; an abrupt pressure jump immediately after a vacuum port raises local shear and can undo the temperature discipline the formula depends on.

Vent zone element	Function	Typical geometry	Design note
Upstream melt seal	Isolates vacuum from feed zone	Narrow kneading disc block or short reverse element	Keep short — a long seal wastes barrel length and adds shear
Vent conveying section	Maximizes free surface for mass transfer	Long-pitch, shallow-channel conveying, 3 to 5 L:D	Target degree of fill below 30 percent
Vent stuffer (optional)	Prevents melt creeping into the vacuum throat	Twin-screw side stuffer driven at low speed	Essential above 800 kg/h or with low viscosity melt
Downstream transition	Progressive pressure rebuild	Stepped-down pitch conveying, 4 to 6 L:D	Avoid a single abrupt pitch change
Water injection port	Steam stripping for stubborn odor	Metered water injection into a filled zone before the vent	Typically 0.5 to 2 percent water on throughput

Preventing vent flooding

Vacuum port flooding — melt climbing into the vent throat and eventually into the vacuum line — is the most common devolatilization fault on recycled lines, and it has four causes worth checking in order: degree of fill too high under the port, melt viscosity too low because of an overheated or heavily degraded feed, throughput surging from inconsistent bulk density regrind, and a blocked or partially closed vent insert. A vent stuffer solves the symptom mechanically, but the durable fixes are a longer-pitch vent section, steadier gravimetric feeding, and a melt temperature 10 to 15 degrees Celsius lower than a virgin resin recipe would use.

Steam stripping as a low-cost odor tool

Injecting a controlled quantity of water into a filled melt zone upstream of the vacuum vent is one of the highest-value, lowest-cost odor interventions available. The water flashes to steam and acts as a stripping agent, lowering the partial pressure of the odor species and carrying them out through the vent. Typical injection is 0.5 to 2 percent on throughput; the equipment is a metering pump, an injection nozzle rated for melt pressure, and a condenser on the vacuum line. Compared with buying a specialty odor-absorbing additive at a High or Premium cost level, steam stripping is a one-time capital item with a Low running cost, and it is more effective on the low molecular weight aldehyde fraction that dominates polyolefin odor.

4. Feedstock Characterization Before Formulating

No recycled formula can be written responsibly without a characterization sheet for the incoming batch. Recycled feedstock is not a grade; it is a distribution. The purpose of incoming testing is to locate where the current bale sits inside that distribution so that the additive package can be adjusted by the smallest amount necessary rather than over-specified as insurance.

The test list below is deliberately short. Every item on it changes a formula decision. Tests that are interesting but do not change a decision belong in development work, not in routine incoming control.

Test	Method reference	What it tells the formulator	Typical range for post-consumer polyolefin	Formula decision it drives
Melt flow rate	ISO 1133 / ASTM D1238	Average molecular weight and degree of prior degradation	rPP 4 to 35 g/10min; rHDPE 0.3 to 8 g/10min	Whether a chain extender or a lower-MFR virgin blend is needed
Density	ISO 1183	Polymer identity and filler content already present	0.90 to 1.05 g/cm3 depending on mix	Filler dosing headroom and stiffness expectation
Ash content	ISO 3451	Existing mineral filler, pigment and inorganic contamination	0.5 to 12 percent	How much fresh filler can be added before impact collapses
Moisture	Karl Fischer or loss on drying	Water load entering the barrel	0.1 to 1.5 percent	Pre-drying requirement and atmospheric vent sizing
Odor grade	VDA 270 six-grade scale	Severity of the volatile problem	Grade 3.5 to 6 untreated	Vacuum level, steam stripping, adsorbent dosage
Yellowness index	ASTM E313	Extent of thermo-oxidative history and pigment carryover	YI 8 to 40	Antioxidant level, whether natural color is achievable
Tensile and notched impact	ISO 527 / ISO 179	Baseline mechanical capability of the regrind	Highly variable	Impact modifier level, or whether the target is achievable at all
Differential scanning calorimetry	ISO 11357	Multiple melting peaks reveal cross-contamination	One to three peaks	Compatibilizer selection and dosage
Infrared spectroscopy	ATR-FTIR	Polymer identification and rough blend ratio	Qualitative to semi-quantitative	Whether the batch is a single polymer or a mixed polyolefin

Reading the odor grade correctly

The VDA 270 six-grade odor scale is the reference most automotive and appliance customers use, and it is worth knowing how the grades translate into commercial outcomes. Grade 1 means not perceptible; grade 2 perceptible but not disturbing; grade 3 clearly perceptible but not disturbing; grade 4 disturbing; grade 5 strongly disturbing; grade 6 unacceptable. Most untreated post-consumer polyolefin regrind sits between 4 and 5.5. Interior automotive parts commonly require 3.0 or better; consumer appliance housings often accept 3.5; non-visible industrial parts may accept 4. Knowing the target grade before formulating decides whether a Low-cost adsorbent is sufficient or whether steam stripping hardware is required.

Interpreting differential scanning calorimetry on mixed streams

A single sharp melting peak near 163 to 168 degrees Celsius indicates a reasonably clean polypropylene stream. A second peak near 125 to 133 degrees Celsius indicates high-density polyethylene contamination; near 108 to 115 degrees Celsius indicates low-density or linear low-density polyethylene. The relative enthalpy under each peak gives a workable estimate of the blend ratio, which is precisely the number that sets compatibilizer dosage. Running this one test on every incoming bale of mixed material is the difference between a compatibilizer level that is correct and one that is guessed.

5. Recycled PP Formula Family

Recycled polypropylene is the workhorse of low-cost compounding, and it responds well to a tiered formula strategy: an economy tier for non-critical parts, a standard tier for general technical parts, and a high-performance tier where notched impact and surface quality both matter. The tiers differ mainly in impact modifier and compatibilizer content, because those are the two components with the strongest effect on both properties and cost.

All three recipes below assume the regrind has passed through a two-stage devolatilization barrel. Without that, every antioxidant figure would need to rise and the odor grade column would be unachievable.

Component	Economy tier (parts)	Standard tier (parts)	High-performance tier (parts)	Function
rPP base regrind, washed and dried	100	100	100	Matrix
Talc, 2000 to 5000 mesh	10 to 20	15 to 20	18 to 22	Stiffness, dimensional stability, nucleation
CaCO3, 1250 mesh, surface activated	0 to 12	0 to 8	0	Cost dilution, lower reinforcement than talc
POE elastomer, MFR 0.5 to 5 g/10min	0 to 3	6 to 10	12 to 18	Impact modification, low-temperature toughness
PP-g-MAH compatibilizer, grafting rate 0.8 to 1.2 percent	0 to 1	1.5 to 2.5	3 to 4	Filler and elastomer interfacial coupling
Hindered phenol primary antioxidant	0.10 to 0.15	0.15 to 0.20	0.20 to 0.25	Radical scavenging during processing and service
Phosphite secondary antioxidant	0.10 to 0.15	0.15 to 0.20	0.20 to 0.30	Hydroperoxide decomposition, color protection
HALS light stabilizer	0	0 to 0.2	0.3 to 0.5	Only where outdoor exposure is specified
Odor adsorbent, zeolite or porous mineral carrier	0 to 0.3	0.3 to 0.6	0.5 to 1.0	Residual odor capture after devolatilization
Lubricant, metal stearate or amide wax	0.15 to 0.25	0.20 to 0.30	0.25 to 0.40	Filler dispersion, torque reduction, surface finish
Nucleating agent	0	0 to 0.1	0.1 to 0.2	Cycle time and stiffness improvement

Expected property windows for the three rPP tiers

Property	Economy tier	Standard tier	High-performance tier	Test reference
Melt flow rate, 230 degrees Celsius / 2.16 kg	12 to 30 g/10min	8 to 20 g/10min	6 to 14 g/10min	ISO 1133
Notched Charpy impact, 23 degrees Celsius	2.5 to 5 kJ/m2	7 to 15 kJ/m2	20 to 45 kJ/m2	ISO 179-1eA
Notched Charpy impact, minus 20 degrees Celsius	1.5 to 2.5 kJ/m2	3 to 6 kJ/m2	7 to 14 kJ/m2	ISO 179-1eA
Flexural modulus	1700 to 2300 MPa	1600 to 2100 MPa	1300 to 1800 MPa	ISO 178
Tensile strength at yield	20 to 26 MPa	19 to 24 MPa	17 to 22 MPa	ISO 527
Odor grade after two-stage devolatilization	3.5 to 4.0	3.0 to 3.5	2.5 to 3.0	VDA 270
Material cost index (baseline = 100)	100	110	126	Relative
Cost level	Low	Medium	High	Relative

Why compatibilizer dosage is not proportional to filler

A common error is to scale PP-g-MAH linearly with filler content. Maleic anhydride grafted polypropylene works at the interface, so its required dosage scales with interfacial area, which scales with filler surface area, not filler mass. Moving from 1250 mesh calcium carbonate to 5000 mesh talc at the same loading multiplies the interfacial area substantially and demands more compatibilizer. Conversely, using pre-activated calcium carbonate whose surface has already been treated reduces the coupling agent requirement. Getting this right is worth several index points on a large-volume grade.

6. Recycled PE Formula Family

Recycled high-density polyethylene formulation is dominated by one question that does not apply to polypropylene: has the molecular weight been reduced by prior processing, and can it be rebuilt? Polyethylene tends to cross-link as well as chain-scission under thermo-oxidative stress, so recycled polyethylene often shows both a lower melt flow rate in some fractions and gels in others. Chain extenders and careful shear management are the two levers that matter most.

The three application families below — blow molding, pipe and injection molding — have genuinely different requirements, and a single “rHDPE compound” grade serves none of them well.

Component	Blow molding grade (parts)	Pipe and conduit grade (parts)	Injection grade (parts)	Function
rHDPE base regrind	100	100	100	Matrix
Virgin HDPE, high molecular weight	0 to 15	15 to 30	0 to 10	Melt strength and long-term property recovery
Chain extender, epoxy-functional styrene-acrylic oligomer	0.2 to 0.5	0.3 to 0.6	0 to 0.2	Molecular weight and melt strength rebuild
CaCO3, 1250 mesh, activated	0 to 10	0	10 to 25	Cost dilution and stiffness in non-pressure parts
POE or metallocene plastomer	0 to 4	3 to 6	0 to 5	Impact and environmental stress crack resistance
PE-g-MAH compatibilizer	0 to 1	0.5 to 1.5	1 to 2	Filler coupling and blend compatibility
Hindered phenol primary antioxidant	0.15 to 0.20	0.20 to 0.30	0.12 to 0.18	Melt and service stabilization
Phosphite secondary antioxidant	0.10 to 0.20	0.15 to 0.25	0.10 to 0.15	Color and hydroperoxide control
Carbon black masterbatch, 40 percent concentration	0 to 5	5 to 6	0 to 4	Ultraviolet protection for buried or exposed parts
Processing aid, fluoropolymer-free polymer type	0.05 to 0.15	0.05 to 0.15	0 to 0.1	Melt fracture suppression and die build-up control
Odor adsorbent, porous mineral carrier	0.3 to 0.6	0 to 0.3	0.3 to 0.8	Residual odor capture

Expected property windows for the three rHDPE grades

Property	Blow molding grade	Pipe and conduit grade	Injection grade	Test reference
Melt flow rate, 190 degrees Celsius / 2.16 kg	0.25 to 0.7 g/10min	0.15 to 0.5 g/10min	2 to 8 g/10min	ISO 1133
Melt flow rate, 190 degrees Celsius / 5 kg	0.9 to 2.2 g/10min	0.5 to 1.6 g/10min	7 to 25 g/10min	ISO 1133
Density	0.950 to 0.958 g/cm3	0.948 to 0.956 g/cm3	0.955 to 1.05 g/cm3 (filled)	ISO 1183
Notched Izod impact, 23 degrees Celsius	10 to 20 kJ/m2	18 to 35 kJ/m2	4 to 9 kJ/m2	ISO 180
Environmental stress crack resistance, F50	60 to 250 hours	250 to 900 hours	30 to 120 hours	ASTM D1693 condition B
Flexural modulus	950 to 1200 MPa	900 to 1150 MPa	1200 to 1900 MPa	ISO 178
Odor grade after two-stage devolatilization	3.0 to 3.5	3.5 to 4.0	3.0 to 3.5	VDA 270
Material cost index (baseline = 100)	112	124	98	Relative
Cost level	Medium	High	Low	Relative

How chain extenders rebuild melt strength

An epoxy-functional styrene-acrylic oligomer chain extender carries multiple reactive epoxy groups per molecule. Those groups react with the carboxyl and hydroxyl end groups generated by prior chain scission, linking shorter chains back into longer ones and introducing a controlled degree of long-chain branching. The practical result is a rise in melt strength and a drop in melt flow rate, both of which are exactly what a blow molding or pipe grade needs. Dosage discipline is essential: too little produces no measurable change, while too much produces gels, surface defects and a compound that will not flow into thin sections.

Chain extender dosage (parts)	Effect on melt flow rate	Effect on melt strength	Gel risk	Recommended for
0.1	Barely measurable	Slight increase	None	Fine trimming of an almost-correct batch
0.2 to 0.3	Drops 20 to 35 percent	Clear improvement	Low	Injection and general purpose grades
0.4 to 0.6	Drops 40 to 60 percent	Strong improvement, parison sag controlled	Medium	Blow molding and pipe grades
0.8 and above	Drops beyond 65 percent	Diminishing return	High	Not recommended for general compounding

Chain extender reaction requires melt residence time and good distributive mixing, but not high shear. On a twin-screw extruder the correct placement is a downstream side or liquid feed port into a well-mixed but moderately sheared zone, followed by 8 to 12 L:D of distributive mixing before the vacuum vent. Feeding chain extender at the main hopper wastes a substantial part of it on premature reaction in the melting zone.

7. Recycled PET Formula and Intrinsic Viscosity Recovery

Recycled polyester compounding is governed by a single unforgiving rule: water plus heat equals irreversible hydrolysis. Every part of the rPET process — crystallizing, drying, high vacuum devolatilization, chain extension — exists to keep intrinsic viscosity from falling and then to rebuild what has already been lost. Get the drying wrong and no additive package will save the batch.

Crystallizing and drying, the non-negotiable first step

Amorphous recycled polyester flake softens and agglomerates if heated directly to drying temperature, so it must be crystallized first at 130 to 150 degrees Celsius under agitation, then dried at 160 to 170 degrees Celsius for 4 to 6 hours with a dehumidified air dew point of minus 40 degrees Celsius or better. The target residual moisture entering the barrel is below 50 parts per million; above 100 parts per million, intrinsic viscosity loss during a single extrusion pass can exceed 0.05 dL/g regardless of vacuum settings.

Component	Fiber grade (parts)	Sheet and strapping grade (parts)	Engineering compound grade (parts)	Function
rPET flake, crystallized and dried	100	100	100	Matrix
Chain extender, epoxy-functional styrene-acrylic oligomer	0.3 to 0.5	0.5 to 0.8	0.4 to 0.7	Intrinsic viscosity and melt strength recovery
Hydrolysis stabilizer, carbodiimide type	0.2 to 0.4	0.3 to 0.5	0.3 to 0.6	End-group capping, long-term hydrolytic resistance
Nucleating agent, ionomer or mineral type	0 to 0.3	0.2 to 0.5	0.5 to 1.0	Crystallization rate and cycle time
Hindered phenol primary antioxidant	0.10 to 0.15	0.10 to 0.20	0.15 to 0.25	Thermal stabilization at high melt temperature
Phosphite secondary antioxidant	0.05 to 0.10	0.10 to 0.15	0.10 to 0.20	Color protection
Impact modifier, reactive acrylic core-shell or elastomeric ethylene copolymer	0	0 to 3	5 to 10	Notched impact for technical parts
Glass fiber, chopped, silane sized	0	0	15 to 30	Stiffness and heat deflection temperature
Lubricant, high-temperature stable type	0.1 to 0.2	0.1 to 0.3	0.2 to 0.4	Torque and surface finish

Intrinsic viscosity and its application map

Intrinsic viscosity is the single number that decides where a recycled polyester compound can be sold. The relationship is direct and worth memorizing.

Intrinsic viscosity (dL/g)	Typical origin	Suitable applications	Processing note	Cost level
0.50 to 0.58	Heavily degraded post-consumer flake, multiple heat histories	Staple fiber, non-woven, low-grade filler applications	Runs easily but has no melt strength	Low
0.60 to 0.68	Standard washed bottle flake after one extrusion pass	Filament fiber, thin sheet, thermoformed trays	Requires nucleation for fast crystallization	Low to Medium
0.70 to 0.78	Chain-extended and high-vacuum devolatilized flake	Thick sheet, strapping, engineering compounds	Chain extender dosage control is critical	Medium
0.80 and above	Solid-state post-condensed material	Bottle grade, high-strength strapping	Requires solid-state polycondensation, not just extrusion	High to Very High

A twin screw devolatilization extruder can reliably lift intrinsic viscosity from around 0.58 dL/g to 0.70 to 0.78 dL/g using chain extension combined with deep vacuum. It cannot reach bottle grade on its own — that step needs solid-state polycondensation. Being clear about this boundary with customers prevents the most damaging kind of specification failure, which is agreeing to a target the process cannot physically reach.

Why high vacuum matters even more for polyester

Polyester devolatilization does double duty. It strips acetaldehyde and cyclic trimer, which are the odor and taste offenders, and it shifts the hydrolysis equilibrium. Removing water vapor from the melt drives the ester hydrolysis reaction backwards, slowing further chain cleavage during the residence time in the barrel. On a recycled polyester line, a vacuum level of minus 0.090 to minus 0.098 MPa with a two-stage pump is not a luxury — it directly determines how much intrinsic viscosity survives the pass, and therefore which market the pellets can serve.

8. Mixed Polyolefin Low-Cost Formula

Mixed polypropylene and polyethylene streams are the cheapest feedstock available and the hardest to formulate, because the two polymers are thermodynamically immiscible. Without a compatibilizer the blend forms a coarse two-phase morphology with weak interfaces, and notched impact collapses to values that no technical application will accept. With the right compatibilizer at the right dosage, the same blend becomes a serviceable general-purpose compound at a Low cost level.

How blend ratio changes properties

The property curve across the polypropylene to polyethylene composition range is not linear, and it is not symmetric. Small amounts of the minority phase act as a dispersed impact modifier; intermediate ratios produce co-continuous morphology with the worst mechanical performance; the minimum sits broadly around a 50/50 to 60/40 composition.

PP / PE ratio	Morphology	Notched Charpy impact, uncompatibilized	Notched Charpy impact, with 3 parts compatibilizer	Flexural modulus	Practical verdict
100 / 0	Single phase	3 to 5 kJ/m2	Not applicable	1400 to 1600 MPa	Reference point
90 / 10	PE droplets in PP matrix	4 to 7 kJ/m2	8 to 13 kJ/m2	1300 to 1500 MPa	Best value ratio, mild toughening effect
70 / 30	Coarse dispersed phase	3 to 5 kJ/m2	9 to 16 kJ/m2	1150 to 1350 MPa	Workable with compatibilizer, poor without
50 / 50	Co-continuous, weak interfaces	2.5 to 4 kJ/m2	7 to 12 kJ/m2	1000 to 1200 MPa	Worst case, needs the highest compatibilizer dosage
30 / 70	PP droplets in PE matrix	4 to 8 kJ/m2	12 to 22 kJ/m2	900 to 1100 MPa	Surprisingly good, PE matrix is forgiving
0 / 100	Single phase	12 to 25 kJ/m2	Not applicable	850 to 1100 MPa	Reference point

Compatibilizer selection logic

Compatibilizer type	Mechanism	Typical dosage (parts)	Best blend ratio range	Cost level
Ethylene-propylene rubber, amorphous	Interfacial emulsification by segmental affinity to both phases	4 to 10	Any ratio, especially 50/50	Medium
POE elastomer, MFR 0.5 to 5 g/10min	Interfacial modification plus independent toughening	4 to 8	PP-rich blends	Medium to High
Styrenic block copolymer, hydrogenated	Strong interfacial anchoring of both phases	3 to 6	Any ratio, best surface quality	High
PP-g-MAH, grafting rate 0.8 to 1.2 percent	Reactive coupling, mainly effective with fillers present	1 to 3	Filled mixed polyolefin	Medium
Combined POE plus PP-g-MAH	Toughening plus interfacial coupling in one package	5 plus 1.5	Filled 70/30 to 50/50 blends	Medium

A working low-cost mixed polyolefin recipe

Component	Parts	Notes
Mixed polyolefin regrind, PP/PE around 70/30	100	Verify ratio by differential scanning calorimetry on every bale
CaCO3, 1250 mesh, surface activated	15 to 25	Side fed downstream of first melting section
POE elastomer, MFR 0.5 to 5 g/10min	5 to 7	Primary compatibilizer and toughener
PP-g-MAH compatibilizer, grafting rate 0.8 to 1.2 percent	1.5 to 2.5	Couples filler to the polyolefin matrix
Hindered phenol primary antioxidant	0.15	Mandatory on any mixed stream
Phosphite secondary antioxidant	0.15	Controls yellowing from mixed heat histories
Odor adsorbent, porous mineral carrier	0.4 to 0.8	Mixed streams carry the widest odor spectrum
Lubricant, amide wax	0.25	Improves filler dispersion and surface gloss
Carbon black masterbatch, 40 percent concentration	2 to 4	Black color hides the color variability of mixed feed

Expected performance for this recipe: melt flow rate 3 to 12 g/10min at 230 degrees Celsius and 2.16 kg, notched Charpy impact 8 to 15 kJ/m2 at 23 degrees Celsius, flexural modulus 1200 to 1500 MPa, odor grade 3.5 to 4.0 after two-stage devolatilization, and a material cost index of approximately 92 against the economy rPP baseline. That combination is well suited to crates, pallets, drainage components, cable drums, garden furniture frames and industrial packaging.

9. Additive Cost-Effectiveness Ranking

The most useful tool a recycled compounder can own is a ranked list of additives by performance gained per unit of cost spent. The list below is ordered by cost-effectiveness, not by importance, and it produces a clear dosing sequence: add the top items first, and only descend the list when a specification demands it.

Rank	Additive	Function	Typical dosage (phr)	Performance gain	Cost level	Cost-effectiveness
1	Phosphite secondary antioxidant	Decomposes hydroperoxides, protects color and melt flow rate stability	0.10 to 0.25	Melt flow rate drift cut by half or more across passes; yellowness index improved several points	Low	Excellent
2	Hindered phenol primary antioxidant	Radical scavenging during processing and in service	0.10 to 0.25	Prevents progressive chain scission and long-term embrittlement	Low	Excellent
3	Lubricant, metal stearate or amide wax	Filler wetting, torque reduction, surface finish	0.15 to 0.40	Torque reduced 5 to 12 percent, better dispersion, higher output	Low	Excellent
4	Talc, 2000 to 5000 mesh	Stiffness, dimensional stability, nucleation	10 to 25	Flexural modulus up 30 to 60 percent, heat deflection temperature up	Low	Very good
5	CaCO3, 1250 mesh, activated	Cost dilution with modest stiffness gain	10 to 25	Material index reduced; impact reduced less than with untreated grades	Low	Very good
6	PP-g-MAH compatibilizer, grafting rate 0.8 to 1.2 percent	Interfacial coupling between filler, elastomer and matrix	1 to 4	Impact and weld line strength up 30 to 80 percent in filled systems	Medium	Very good
7	Nucleating agent	Faster crystallization, higher stiffness, shorter cycle	0.1 to 0.3	Cycle time down 5 to 15 percent in injection applications	Medium	Good
8	Chain extender, epoxy-functional styrene-acrylic oligomer	Molecular weight and melt strength rebuild	0.2 to 0.8	Intrinsic viscosity or melt strength restored to usable levels	High	Good where required, wasteful elsewhere
9	POE elastomer, MFR 0.5 to 5 g/10min	Impact modification	5 to 18	Notched impact multiplied several times	Medium to High	Fair — high loading makes it costly in absolute terms
10	Odor adsorbent, zeolite or porous carrier	Residual odor capture	0.3 to 1.0	Around half a VDA 270 grade when used after devolatilization	Medium	Fair — cheap only if the vacuum system has done its job first
11	HALS light stabilizer	Ultraviolet and long-term thermal protection	0.2 to 0.6	Outdoor service life extended substantially	High	Fair — specify only where exposure is real
12	Hydrolysis stabilizer, carbodiimide type	End-group capping in polyester systems	0.2 to 0.6	Hydrolytic aging resistance greatly improved	High	Necessary for polyester, irrelevant for polyolefin
13	Reactive acrylic core-shell impact modifier	High-efficiency toughening in polar polymers	4 to 10	Large impact gain in polyester compounds	Premium	Poor for low-cost work, reserve for technical grades
14	Specialty odor-masking fragrance	Covers rather than removes odor	0.1 to 0.5	Perceived odor improved, measured VOC unchanged	High	Poor — customers detect the substitution

The recommended dosing sequence

Build the formula in this order and stop as soon as the specification is met. First, the antioxidant pair and lubricant — these three are always present, always cheap, and always pay for themselves. Second, filler to the level that stiffness and cost require. Third, compatibilizer sized to filler surface area. Fourth, impact modifier, added only in the increments needed to clear the notched impact target. Fifth, functional specialties such as chain extender, nucleating agent, light stabilizer or hydrolysis stabilizer, each justified by a named requirement in the customer specification. Anything that cannot be traced to a specification line should not be in the recipe.

10. Odor and VOC Reduction Without Expensive Additives

Odor is the number one commercial obstacle for recycled compounds, and it is best attacked with process settings before chemistry. Four process levers — deeper vacuum, more vented barrel length, lower melt temperature, and steam stripping — together deliver more odor improvement than any affordable additive package, and their running cost is electricity rather than consumables.

Lever one: raise the vacuum level

Moving from minus 0.06 MPa to minus 0.09 MPa is often the single largest available improvement, and on an existing machine it may cost nothing more than servicing the vacuum pump, replacing worn barrel seals and cooling the sealing water properly. Warm sealing water is the most commonly overlooked cause of poor vacuum on water ring pumps; every few degrees of sealing water temperature reduction translates directly into achievable vacuum depth.

Lever two: add vented barrel length

Mass transfer of volatiles out of a polymer melt is time-dependent. Extending the vented section from 3 L:D to 5 L:D increases exposure time under vacuum by roughly two-thirds at the same screw speed, and a second vacuum port in series is more effective than one port at a slightly deeper vacuum. This is a design decision at machine specification time, which is why odor targets should be discussed before an extruder is configured rather than after commissioning.

Lever three: lower the melt temperature

Every 10 degrees Celsius of melt temperature reduction meaningfully slows thermo-oxidative degradation, which means fewer new aldehydes and ketones are created inside the barrel. On recycled polyolefin, running 10 to 15 degrees Celsius below the equivalent virgin recipe is usually possible if the screw is configured for gentle plastication — moderate kneading block angles, fewer reverse elements, and adequate barrel length rather than aggressive shear in a short machine. Lower melt temperature also reduces specific energy consumption, so it improves two cost drivers at once.

Lever four: steam stripping

Metered water injection into a filled melt zone, described earlier, is the strongest process tool available for stubborn odor from ink, adhesive and fragrance residue. It requires a condenser and a water separation arrangement on the vacuum line, and it slightly increases energy consumption, but the additive-free odor reduction it delivers is difficult to match by any other means at a comparable cost level.

Intervention	Typical VDA 270 improvement	Capital cost level	Running cost level	Applies to
Vacuum pump service and seal renewal	0.3 to 0.7 grade	Low	Low	Any existing line
Upgrade to two-stage vacuum system	0.5 to 1.0 grade	Medium	Low	Lines limited to minus 0.07 MPa or worse
Second vacuum port in series	0.4 to 0.8 grade	Medium	Low	Machines with spare barrel length
Extend vented section from 3 to 5 L:D	0.3 to 0.6 grade	Medium	None	New machine specification
Reduce melt temperature 10 to 15 degrees Celsius	0.3 to 0.6 grade	None	Negative — saves energy	Any line with screw design headroom
Steam stripping with water injection	0.5 to 1.0 grade	Medium	Low	Post-consumer packaging streams
Odor adsorbent at 0.5 parts	0.3 to 0.5 grade	None	Medium	Final gap closing only
Fragrance masking	Perceived only	None	High	Not recommended

The practical route to a grade 3.0 compound from a grade 5.0 feedstock is therefore cumulative: a two-stage vacuum system, a properly sized vented section, a disciplined melt temperature, steam stripping where the feedstock warrants it, and a modest adsorbent dosage at the end. Trying to close a two-grade gap with additives alone requires loadings at a High to Premium cost level and still tends to fail on the aldehyde fraction.

11. Screw Configuration for Recycled Compounding

Screw configuration for recycled material follows one governing principle: plasticize gently, mix thoroughly, vent generously. Recycled polymer has already absorbed one or more full heat histories, so the shear intensity that works perfectly for virgin resin actively destroys it. Kerke designs screw assemblies with computer-aided layout and a self-wiping kneading geometry precisely so that dispersion can be achieved with lower peak shear.

Zone	Position (L:D)	Element type	Function	Design note for recycled feed
Feed and conveying	0 to 6	Long-pitch conveying elements	Intake of low bulk density regrind	Use extra-long pitch; flake bulk density can be a third of pellet bulk density
First melting zone	6 to 12	Forward kneading blocks at 45 degrees plus one 90 degree block	Controlled melting without excessive shear peak	Avoid reverse elements here; use narrow discs rather than wide ones
First mixing zone	12 to 16	Combination of 45 degree kneading and toothed mixing elements	Distributive homogenization of additives	Distributive rather than dispersive at this stage
Atmospheric vent	16 to 20	Long-pitch shallow conveying with upstream seal	Steam and light volatile release	Keep degree of fill low; heat the port jacket to avoid condensate
Side feeding zone	20 to 24	Conveying elements matched to side feeder speed	Filler, glass fiber or chain extender introduction	Never side feed into a filled zone; keep the receiving section starved
Second mixing zone	24 to 32	Kneading blocks at 45 degrees plus toothed and gear-type mixers	Filler dispersion and elastomer phase refinement	This is where dispersion happens; use narrow discs and moderate block length
Vacuum vent	32 to 38	Upstream melt seal, long-pitch vent conveying, optional stuffer	Deep devolatilization	3 to 5 L:D of open conveying; fill below 30 percent
Pressure build-up	38 to 44	Progressively shorter pitch conveying	Melt pressure for filter and die	Step pitch down gradually to limit local shear heating

Kneading block angle selection

Element	Shear intensity	Mixing character	Residence time effect	Use on recycled feed
30 degree forward kneading block	Low	Mainly distributive	Slight increase	Freely — safe for heat-sensitive recycled material
45 degree forward kneading block	Medium	Balanced distributive and dispersive	Moderate increase	The workhorse element; use as the backbone of both mixing zones
90 degree neutral kneading block	High	Strongly dispersive	Large increase	One or two only, in the first melting zone
Reverse kneading block	Very high	Dispersive plus melt sealing	Very large increase	Only as a short melt seal before a vent; never as a mixing strategy
Toothed mixing element	Low	Distributive with high surface renewal	Slight increase	Excellent for recycled work — mixing without degradation
Reverse conveying element	Medium to high	Sealing and residence time control	Large increase	Short sections only, before vents and side feeders

12. Process Window Tables by Material

Process windows for recycled material are narrower than for virgin resin and sit at lower temperatures. The tables below give starting points for a 44 to 48 L:D twin-screw extruder with one atmospheric and one vacuum vent. They are starting points, not final settings; every recycled feedstock requires confirmation on a trial run.

Recycled polypropylene compound

Parameter	Setting	Parameter	Setting
Barrel zone 1 (feed)	Water cooled, 40 to 60 degrees Celsius	Barrel zone 6	195 to 205 degrees Celsius
Barrel zone 2	150 to 165 degrees Celsius	Barrel zone 7 (vacuum vent)	190 to 200 degrees Celsius
Barrel zone 3	175 to 190 degrees Celsius	Barrel zone 8	190 to 200 degrees Celsius
Barrel zone 4	190 to 200 degrees Celsius	Die head	195 to 210 degrees Celsius
Barrel zone 5 (atmospheric vent)	195 to 205 degrees Celsius	Actual melt temperature	200 to 218 degrees Celsius
Screw speed	350 to 500 rpm	Torque utilization	55 to 75 percent
Vacuum level	Minus 0.080 to minus 0.092 MPa	Melt pressure at screen	3 to 7 MPa
Specific energy consumption	0.16 to 0.24 kWh/kg	Pelletizing	Water strand or water ring die face

Recycled high-density polyethylene compound

Parameter	Setting	Parameter	Setting
Barrel zone 1 (feed)	Water cooled, 40 to 60 degrees Celsius	Barrel zone 6	190 to 200 degrees Celsius
Barrel zone 2	145 to 160 degrees Celsius	Barrel zone 7 (vacuum vent)	185 to 195 degrees Celsius
Barrel zone 3	170 to 185 degrees Celsius	Barrel zone 8	185 to 195 degrees Celsius
Barrel zone 4	185 to 195 degrees Celsius	Die head	190 to 205 degrees Celsius
Barrel zone 5 (atmospheric vent)	190 to 200 degrees Celsius	Actual melt temperature	195 to 215 degrees Celsius
Screw speed	300 to 450 rpm	Torque utilization	60 to 80 percent
Vacuum level	Minus 0.080 to minus 0.090 MPa	Melt pressure at screen	4 to 9 MPa
Specific energy consumption	0.18 to 0.26 kWh/kg	Pelletizing	Water strand or underwater

Recycled polyester compound

Parameter	Setting	Parameter	Setting
Pre-crystallizing	130 to 150 degrees Celsius with agitation	Drying	160 to 170 degrees Celsius, 4 to 6 hours, dew point minus 40 degrees Celsius
Barrel zone 1 (feed)	Water cooled, 50 to 70 degrees Celsius	Barrel zone 6	265 to 275 degrees Celsius
Barrel zone 2	230 to 250 degrees Celsius	Barrel zone 7 (vacuum vent)	260 to 272 degrees Celsius
Barrel zone 3	260 to 275 degrees Celsius	Barrel zone 8	260 to 272 degrees Celsius
Barrel zone 4	265 to 278 degrees Celsius	Die head	265 to 280 degrees Celsius
Barrel zone 5 (atmospheric vent)	265 to 278 degrees Celsius	Actual melt temperature	272 to 292 degrees Celsius
Screw speed	200 to 350 rpm	Torque utilization	50 to 70 percent
Vacuum level	Minus 0.090 to minus 0.098 MPa	Melt pressure at screen	3 to 6 MPa
Specific energy consumption	0.20 to 0.30 kWh/kg	Pelletizing	Water strand with rapid quench, or underwater

Mixed polyolefin compound

Parameter	Setting	Parameter	Setting
Barrel zones 2 to 4	150 to 195 degrees Celsius, rising	Barrel zones 5 to 8	185 to 200 degrees Celsius
Die head	190 to 205 degrees Celsius	Actual melt temperature	198 to 216 degrees Celsius
Screw speed	320 to 450 rpm	Torque utilization	60 to 80 percent
Vacuum level	Minus 0.085 to minus 0.095 MPa	Melt pressure at screen	4 to 8 MPa
Specific energy consumption	0.18 to 0.27 kWh/kg	Filtration	Continuous or double-piston screen changer recommended

13. Kerke KTE Series Twin Screw Extruder

The KTE series is Kerke’s parallel co-rotating twin-screw compounding platform, spanning from the KTE-16B laboratory machine used for formula trials to the KTE-135D production machine. For recycled compounding the series is specified with a longer barrel than a standard masterbatch configuration, at least one atmospheric and one vacuum vent, and a screw assembly designed around gentle plastication rather than maximum shear.

Every KTE machine is built around a modular barrel and a computer-aided designed segmented screw assembly. The kneading elements use a self-wiping co-type geometry, which matters enormously in recycled work: self-wiping surfaces leave no stagnant melt to carbonize into black specks, and segment interchangeability means a screw configured for recycled polypropylene can be reconfigured for recycled polyester by changing elements rather than buying a new shaft. The functions the assembly must deliver — material transport, plastication, shearing, dispersion, homogenization, exhaust and pressure building — are allocated deliberately across the barrel rather than left to chance.

KTE series specifications for recycled compounding

Model	Screw diameter (mm)	Typical L:D ratio	Max screw speed (rpm)	Main motor power (kW)	Specific torque class	Output for recycled compounding (kg/h)	Vent ports
KTE-16B	16	40 to 48	600	3 to 4	Standard	1 to 10	1 to 2
KTE-20B	21.7	40 to 48	600	4 to 7.5	Standard	5 to 25	1 to 2
KTE-26B	26	40 to 52	600	11 to 15	Standard to high	20 to 60	2
KTE-36B	35.6	40 to 52	600	22 to 37	High	50 to 150	2
KTE-52B	51.4	44 to 56	600	55 to 90	High	150 to 400	2 to 3
KTE-65B	62.4	44 to 56	600	90 to 160	High	300 to 700	2 to 3
KTE-75D	71	44 to 56	600	160 to 250	High torque	500 to 1100	2 to 3
KTE-95D	93	44 to 56	500 to 600	315 to 500	High torque	1000 to 2200	3
KTE-135D	133	44 to 56	500	700 to 1200	High torque	2500 to 6000	3 to 4

Output figures above are for recycled compounding with filler and a two-stage devolatilization configuration. Clean virgin masterbatch work on the same machines runs considerably higher; recycled feed with low bulk density flake and a long vented section deliberately trades throughput for quality, and any supplier quoting a single output number without stating the material and configuration is quoting a marketing figure rather than an engineering one.

Configuration options that matter for recycled feedstock

Option	Standard configuration	Recycled compounding configuration	Why it matters
Barrel length	40 to 44 L:D	48 to 56 L:D	Room for two vents plus side feeding without shortening the mixing zones
Barrel liner	Nitrided	Bimetallic wear-resistant liner	Mineral filler and residual contamination are abrasive
Screw element material	Nitrided steel	High-alloy tool steel or powder metallurgy grade	Extends element life against filler abrasion
Feed opening	Standard	Enlarged with optional crammer feeder	Low bulk density flake needs help entering the barrel
Vent arrangement	One vent	Atmospheric plus vacuum, optional second vacuum	The foundation of odor and volatile control
Vacuum system	Single-stage water ring pump	Two-stage system with condenser and separator	Reaches minus 0.09 MPa and beyond reliably
Filtration	Manual screen changer	Double-piston or continuous screen changer	Recycled feed blinds screens far faster than virgin
Control system	PLC with touch screen	PLC with recipe management and data logging	Batch traceability is what wins repeat orders

Application industries served by the KTE platform

Kerke’s KTE machines are in daily production across masterbatch and compounding sectors: color masterbatch, filler masterbatch, additive masterbatch, black masterbatch and textile masterbatch; engineering plastic compounds, biodegradable plastic compounds, cable compounding, PVC compounding, thermoplastic elastomer compounding and wood-plastic composites; plus recycled polyester flake processing, pet food processing and textured vegetable protein food processing lines. For the recycled modification work described in this article, the most common end products are automotive interior trim substrates, appliance housings and internal brackets, logistics crates and pallets, garden and outdoor furniture components, drainage and cable protection parts, industrial packaging, and non-pressure building components.

14. Kerke Feeding and Auxiliary Systems

Feeding accuracy is the most underestimated determinant of formula consistency. A recipe specified to a hundredth of a part means nothing if the antioxidant feeder drifts by five percent between shifts. On recycled lines, where the base material itself already varies, feeding precision is the only variable fully under the operator’s control — which makes it the place where consistency is won or lost.

Kerke supplies a complete feeding and downstream package alongside the KTE extruders, covering volumetric metering, loss-in-weight gravimetric feeding, twin-screw side feeders, crammer feeders for low bulk density material, liquid feeders for chain extenders and liquid additives, and a full range of pelletizing systems.

Equipment	Capacity range	Accuracy	Applicable material	Recycled compounding role
Loss-in-weight feeder, single screw	0.4 to 400 kg/h	Plus or minus 0.5 to 1 percent	Pellets, powders, additive blends	Main feed and all critical additive streams
Loss-in-weight feeder, twin screw	0.2 to 250 kg/h	Plus or minus 0.5 percent	Poor-flowing powders, talc, adsorbents	Fine mineral and stabilizer dosing
Volumetric metering feeder	1 to 500 kg/h	Plus or minus 2 to 3 percent	Free-flowing pellets and regrind	Non-critical bulk streams only
Twin-screw side feeder	50 to 3000 kg/h	Matched to main feed rate	Mineral filler, glass fiber, high-loading additives	Downstream filler introduction after first melting
Crammer feeder	20 to 1500 kg/h	Volumetric with level control	Flake, fluff, film scrap, low bulk density regrind	Forces low-density recycled feed into the barrel
Liquid feeder with metering pump	0.1 to 60 kg/h	Plus or minus 1 percent	Liquid chain extenders, oils, liquid stabilizers	Precise injection of reactive liquid additives
Water strand pelletizing system	50 to 3000 kg/h	Pellet length plus or minus 0.3 mm	Polyolefin and polyester compounds	Standard choice for most recycled compounds
Water ring die face hot cutting	100 to 2500 kg/h	Uniform spherical pellets	Low viscosity and adhesive compounds	Good for tacky recycled polyolefin grades
Underwater pelletizing system	200 to 6000 kg/h	Very uniform pellet geometry	High output polyolefin and polyester	Best pellet consistency at high throughput
Air-cooled die face hot cutting	30 to 800 kg/h	Moderate uniformity	Moisture-sensitive compounds	Avoids water pickup on hygroscopic grades
High-speed mixer	100 to 1000 liters	Batch homogeneity	Powder premixes and additive blends	Pre-blending stabilizer packages before dosing
Plastic granulator and pulverizer	100 to 1500 kg/h	Screen-dependent particle size	Purgings, off-spec strand, rework	In-house scrap recovery back into the line

Why gravimetric feeding pays for itself on recycled lines

Volumetric feeders meter by volume, which means their mass output changes whenever bulk density changes. Recycled flake bulk density can vary noticeably between bales, between the top and bottom of a silo, and even with ambient humidity. A loss-in-weight gravimetric feeder measures actual mass loss over time and corrects continuously, holding the recipe regardless of bulk density drift. On a filled recycled compound where the customer specification includes an ash content window, gravimetric feeding on both the main and filler streams is not optional — it is the mechanism that keeps the ash result inside the window from lot to lot.

15. Melt Filtration and Black Spot Control

Black spots are the most visible quality defect in recycled compound, and they come from three distinct sources that require three distinct remedies: carbonized stagnant melt inside the machine, degraded material at worn clearances, and unfiltered solid contamination arriving with the feedstock. Attacking only the third — by adding finer screens — while ignoring the first two is why some lines never fully solve the problem.

Source one: carbonized stagnant melt

Any dead zone in the flow path will eventually hold melt long enough for it to carbonize, and then release it as black particles. The classic locations are behind worn screw element flights, at poorly matched barrel joints, at the transition into an oversized adapter, in an oversized die manifold, and around the vent port throat. The structural answer is self-wiping screw geometry with tight, well-maintained clearances and a streamlined melt path with no cavities. Kerke’s kneading co-type elements are designed for exactly this self-cleaning behavior, and the practical maintenance rule is to measure screw element and barrel liner wear at scheduled intervals rather than waiting for spots to appear in product.

Source two: degradation at worn clearances

As barrel liners and screw elements wear, the clearance between them grows. Material trapped in an enlarged clearance circulates rather than conveying, accumulates heat history and eventually degrades. This mechanism explains why black spot complaints often begin gradually on a machine that ran cleanly for years. Wear-resistant bimetallic liners and high-alloy screw elements extend the interval substantially on abrasive filled recycled compounds.

Source three: solid contamination in the feedstock

Wood fiber, paper label residue, aluminium foil fragments, rubber, cross-linked polymer gel and unmelted higher-melting polymer all arrive with post-consumer material. Melt filtration is the barrier, and mesh selection is a balance: finer screens catch more but blind faster and raise melt pressure and shear.

Screen mesh	Approximate opening	What it removes	Blinding rate on post-consumer feed	Typical application
20 to 40 mesh	Coarse	Large solids, label fragments, wood pieces	Low	First-stage protection on heavily contaminated feed
60 to 80 mesh	Medium	Medium contamination, most visible specks	Moderate	General recycled polyolefin compounding
100 to 150 mesh	Fine	Fine specks, small gels	High	Technical grades and light-colored compounds
200 mesh and finer	Very fine	Micro gels and fine carbonized particles	Very high	Fiber grade polyester and thin film applications only

Filtration technology	Continuity	Melt loss	Suitable contamination level	Capital cost level	Best fit
Manual plate screen changer	Requires line stop	Low	Very low	Low	Clean in-house scrap only
Hydraulic single-piston screen changer	Brief pressure interruption	Low	Low to medium	Low to Medium	Lightly contaminated regrind
Double-piston screen changer	Continuous, no line stop	Low	Medium	Medium	The standard choice for recycled compounding
Continuous belt or plate filter	Fully continuous	Medium	High	High	Heavily contaminated post-consumer streams
Backflush laser filter	Fully continuous with self-cleaning	Medium to high	Very high	Very High	Film and fiber recycling with paper and wood load

16. Quality Control Plan for Recycled Compounds

A recycled compound business is a consistency business. The technical content of the formula is replicable by any competent competitor; what is not easily replicated is a supply of pellets that behaves the same way in the customer’s molding machine every week. That outcome comes from three mechanisms: continuous in-line process monitoring, a disciplined batch testing schedule, and physical homogenization of output.

In-line monitoring parameters

Parameter	Sensor location	Normal behavior	What a deviation indicates	Alarm response
Melt pressure	Before screen changer and before die	Steady within plus or minus 0.3 MPa	Rising: screen blinding. Falling: feed interruption or viscosity drop	Change screen or check feeder
Melt temperature	Immersion probe before die	Within 6 degrees Celsius of setpoint	Rising: excessive shear or worn elements	Reduce screw speed, review screw configuration
Motor torque	Drive controller	Steady within plus or minus 4 percent	Fluctuating: inconsistent feed or bulk density drift	Check crammer and gravimetric feeder performance
Vacuum level	Vacuum line gauge	Steady at setpoint	Weakening: seal leak, warm sealing water, port fouling	Service pump, clean vent insert, cool sealing water
Feeder mass flow	Each loss-in-weight feeder	Within plus or minus 1 percent of setpoint	Drift: bridging, refill fault, load cell drift	Check hopper flow and recalibrate
Water bath temperature	Strand cooling bath	Stable within 3 degrees Celsius	Rising: cooling capacity fault, causes soft pellets	Check chiller and flow rate

Batch testing schedule

Test	Frequency	Sample point	Acceptance criterion	Action on failure
Melt flow rate	Every 2 hours and every lot change	Pellet stream after pelletizer	Within plus or minus 15 percent of grade nominal	Hold lot, adjust chain extender or feed blend
Ash content	Every 4 hours	Pellet stream	Within plus or minus 1.5 percentage points of target	Recalibrate filler feeder
Moisture	Every shift	Pellets after drying or storage	Below grade limit	Extend drying, check silo sealing
Pellet appearance and black spot count	Every hour, visual on a fixed sample mass	Pellet stream	Below agreed spots per unit mass	Change screen, inspect screw wear
Notched impact and tensile	Every lot	Injection molded test specimens	Within specification band	Hold lot, review elastomer and compatibilizer dosing
Odor grade	Every lot for odor-critical grades	Pellets, VDA 270 procedure	At or better than agreed grade	Raise vacuum, review melt temperature, adjust adsorbent
Color and yellowness index	Every lot for colored grades	Molded plaque	Within agreed color tolerance	Adjust pigment, review antioxidant package
Intrinsic viscosity, polyester only	Every lot	Pellets	Within plus or minus 0.02 dL/g of target	Adjust chain extender, verify drying performance

Homogenizing silos: the cheapest consistency tool available

Even with perfect process control, output varies slowly over a production run as the feedstock bale changes. Blending several hours of production in a homogenizing silo before packing averages that variation out. A gravity-blend silo with internal blending tubes, sized to hold four to eight hours of output, typically cuts lot-to-lot melt flow rate variation by a third to a half. In terms of customer-perceived consistency per unit of capital spent, a homogenizing silo outperforms almost any additive intervention, and it is one of the first upgrades Kerke’s engineers recommend to compounders whose test results are acceptable on average but too scattered.

17. Requirement to Machine and Formula Selection Table

The table below maps common recycled compounding briefs to a recommended Kerke KTE configuration, a formula tier from the earlier sections, and the critical options that must be specified at order time rather than retrofitted later.

Customer scenario	Recommended Kerke model	Barrel L:D	Formula tier	Critical configuration
500 kg/h recycled PP injection grade with talc, appliance and crate parts	KTE-65B or KTE-75D	48	Standard rPP tier	Twin-screw side feeder for talc, one atmospheric plus one vacuum vent, double-piston screen changer, water strand pelletizing
1000 kg/h recycled HDPE pipe and conduit grade	KTE-75D or KTE-95D	52	rHDPE pipe grade with chain extender	Liquid feeder for chain extender, deep vacuum two-stage system, carbon black masterbatch dosing, continuous filtration
Recycled PET sheet grade, intrinsic viscosity target 0.72 dL/g	KTE-75D with high-torque drive	48 to 52	rPET sheet and strapping formula	Crystallizing dryer with dew point minus 40 degrees Celsius, minus 0.095 MPa vacuum, high-temperature barrel package, fine filtration
Mixed polyolefin low-end products, crates and drainage parts, 800 kg/h	KTE-75D	52	Mixed polyolefin recipe	Crammer feeder for low bulk density flake, side feeder for calcium carbonate, two vacuum ports, wear-resistant bimetallic liner
Odor-critical automotive interior substrate from post-consumer PP	KTE-65B or KTE-75D	56	High-performance rPP tier	Two vacuum vents plus steam stripping water injection, condenser and separator, low-shear screw configuration, homogenizing silo
Formula development and small-batch trials before scale-up	KTE-16B or KTE-26B laboratory unit	40 to 48	All tiers, trial quantities	Segmented screw kit, both vent types, quick-change die, data logging control
2500 kg/h and above, large-scale recycled polyolefin compounding	KTE-95D or KTE-135D	52 to 56	Economy or standard tier depending on end use	High-torque drive, three vents, continuous filtration, underwater pelletizing, homogenizing silo
Difficult materials that cannot be processed in one pass	Double-stage extrusion system, mother-baby configuration	Combined	Case-specific	First stage for melting and coarse devolatilization, second stage for gentle finishing and deep vacuum

18. Common Problems and Fixes

Nearly every problem in recycled compounding traces back to one of four root causes: volatiles that were not removed, shear that was too aggressive, feeding that was not steady, or contamination that was not filtered. The troubleshooting table below is organized by symptom, with causes listed in the order they should be checked.

Symptom	Probable causes, in check order	Corrective actions
Black spots in pellets	Carbonized melt in dead zones; worn screw elements or barrel liner; insufficient filtration mesh; contamination in feedstock; degraded material in the die manifold	Inspect and replace worn elements; step filtration one mesh finer; shorten screen change interval; purge and clean the die and adapter; verify self-wiping element condition; improve upstream sorting
Strong odor in finished pellets	Vacuum level too shallow; vented section too short; melt temperature too high; degree of fill too high under the vent; no steam stripping on a heavily contaminated feed	Service the vacuum pump and seals; cool the sealing water; extend vent conveying pitch; reduce melt temperature 10 to 15 degrees Celsius; add a second vacuum port or water injection; add adsorbent only after these steps
Notched impact strength drops between lots	Feedstock composition changed; elastomer feeder drifting; filler over-dosed; excessive shear degrading the matrix; compatibilizer under-dosed for the filler surface area	Run incoming characterization on every bale; switch critical streams to gravimetric feeding; verify ash content; replace 90 degree kneading blocks with 45 degree blocks; recalculate compatibilizer against filler surface area; install a homogenizing silo
Melt fracture and rough strand surface	Melt temperature too low at the die; die land geometry too aggressive; viscosity too high after chain extension; no processing aid; output above the die’s capability	Raise die temperature moderately; open the die land; reduce chain extender dosage; add 0.05 to 0.15 parts of processing aid; increase die hole count
Vacuum port flooding	Degree of fill too high under the port; melt viscosity too low; throughput surging; blocked vent insert; missing melt seal upstream	Fit longer-pitch conveying under the port; install a vent stuffer; steady the feed with gravimetric control and a crammer; clean or redesign the vent insert; add a short sealing element upstream
Pellet size variation	Strand diameter fluctuating from output surging; pelletizer blade wear; feed roll pressure incorrect; water bath temperature drifting; die hole partially blocked	Stabilize feeding and torque; regrind or replace pelletizer blades; reset feed roll pressure; stabilize bath temperature; clean the die plate
Yellowing or color drift	Antioxidant package insufficient; melt temperature too high; residence time too long; oxygen ingress at the feed throat; pigment carryover from the feedstock	Raise phosphite secondary antioxidant to 0.2 parts; reduce melt temperature; shorten high-shear zones; consider nitrogen blanketing at the feed; move the grade to a darker color
Torque fluctuation and unstable output	Bulk density variation in the flake; bridging in the feed hopper; crammer feeder speed mismatched; screw wear; moisture flashing in the melting zone	Use gravimetric feeding and a level-controlled crammer; fit hopper agitation; match crammer speed to intake; measure element wear; improve pre-drying
Gels in the compound	Cross-linked polyethylene fraction in the feed; chain extender over-dosed; localized overheating; unmelted higher-melting contaminant	Improve filtration; reduce chain extender dosage; verify barrel zone temperatures; check differential scanning calorimetry for a foreign polymer peak
Intrinsic viscosity below target in polyester	Drying incomplete; dew point too high; vacuum insufficient; chain extender fed at the main hopper; melt temperature too high	Verify residual moisture below 50 parts per million; confirm dew point of minus 40 degrees Celsius; raise vacuum toward minus 0.095 MPa; move chain extender to a downstream feed port; reduce barrel temperature

19. Service and Support

A twin screw devolatilization extruder configured for recycled work is a long-service-life asset, and its economics depend as much on support as on the initial specification. Kerke, a Wanplas factory, structures its service around the Wanplas group commitments and around the specific realities of recycled compounding, where feedstock changes constantly and formulas need periodic revisiting.

Before shipment

Every machine is assembled and tested before it leaves the factory. For recycled compounding configurations, Kerke’s engineers verify screw assembly against the agreed configuration drawing, run the vacuum system to confirm it reaches the specified level, check feeder calibration across the intended dosing range, and perform a running test to confirm drive, heating, cooling and control functions. Where a customer supplies representative feedstock in advance, a trial run on the actual material is arranged so that the machine ships with a validated process window rather than a generic one.

Installation, commissioning and training

Kerke provides installation and commissioning support, either on site or through guided remote commissioning. Operator training covers screw assembly and disassembly, element identification and configuration logic, vacuum system operation and maintenance, screen changer operation, feeder calibration, recipe management in the control system, and the safety procedures for high-temperature melt and rotating equipment. For recycled work the training places particular emphasis on how to read torque, melt pressure and vacuum trends as early warnings of a feedstock change.

Spare parts and warranty

The Wanplas group spare parts policy provides USD 500 free parts per year, with free replacement of parts that fail within the warranty period. For recycled compounding lines the recommended stock list is short and specific: a set of wear-prone screw elements from the filler side feeding and second mixing zones, spare screen packs in the operating mesh range, vacuum pump seals and sealing water filters, pelletizer blades, and thermocouples. Holding these items locally converts the most common stoppages from multi-day events into one-shift events.

Remote support and factory visits

Machines specified with data-logging control systems can be supported remotely, with Kerke engineers reviewing torque, temperature, pressure and vacuum trend data to diagnose process drift without travel. The Wanplas open factory policy applies to Kerke’s 19,997 square meter manufacturing base: customers and prospective customers are welcome to visit, inspect machines under construction, and witness trial runs.

Formula development and trial runs

Formula work on recycled material is empirical. Kerke maintains laboratory twin-screw extruders — the KTE-16B and KTE-26B class machines — specifically so that a customer’s actual feedstock can be compounded at small scale before a production machine is configured. Wanplas can arrange small-batch formula verification and machine trials, so that the screw configuration, vent arrangement and additive package are proven on the real material rather than assumed from a datasheet. For a recycled compounding project, this trial step is the single most effective way to avoid specifying a machine that is one vent port or four L:D short of what the material actually needs.

20. Frequently Asked Questions

Can a twin screw devolatilization extruder remove odor from recycled plastic without additives?

To a large extent, yes. A correctly configured two-stage vent layout — an atmospheric vent after the first melting zone plus a vacuum vent at minus 0.085 to minus 0.095 MPa in the final third of the barrel — typically improves the VDA 270 odor grade by one to one and a half grades with no additive at all. Adding steam stripping can extend that to two grades on heavily contaminated post-consumer packaging streams. Adsorbent additives are then used only to close the last small gap, which is exactly the sequence that keeps a low cost recycled plastic modification formula genuinely low cost.

What is the minimum additive package for a low cost recycled PP formula?

An economy recycled polypropylene recipe needs only three things: a hindered phenol primary antioxidant at 0.10 to 0.15 parts, a phosphite secondary antioxidant at 0.10 to 0.15 parts, and a lubricant at 0.15 to 0.25 parts. Filler is added for stiffness and cost dilution, not for stabilization. Impact modifier and compatibilizer belong in the recipe only when the notched impact target exceeds what the base regrind delivers on its own, which the incoming characterization sheet will tell you before you spend anything.

How much intrinsic viscosity can a chain extender recover in recycled PET?

With correct crystallizing at 130 to 150 degrees Celsius, drying at 160 to 170 degrees Celsius for 4 to 6 hours to a dew point of minus 40 degrees Celsius, and high vacuum devolatilization at minus 0.090 to minus 0.098 MPa, an epoxy-functional styrene-acrylic oligomer chain extender at 0.3 to 0.8 parts commonly lifts intrinsic viscosity from around 0.58 dL/g to 0.70 to 0.78 dL/g. That range covers sheet, thermoformed tray, strapping and engineering compound applications. Bottle grade above 0.80 dL/g requires solid-state polycondensation and cannot be reached by extrusion chain extension alone.

Which Kerke KTE model suits 500 kg/h of recycled PP injection grade compound?

A KTE-65B covers 300 to 700 kg/h of filled recycled polypropylene and is the economical choice; a KTE-75D covers 500 to 1100 kg/h and leaves room for future growth. Either should be specified with a 48 L:D barrel, one atmospheric and one vacuum vent, a twin-screw side feeder for talc, gravimetric feeding on the main and additive streams, and a double-piston screen changer. The extra barrel length is what allows both vents and the side feeder to coexist without shortening the mixing zones.

Why do black spots appear in recycled compound pellets and how are they eliminated?

Black spots have three sources: carbonized melt sitting in dead zones inside the machine, degraded material circulating in enlarged clearances at worn screw elements or barrel liners, and solid contamination arriving with the feedstock. The remedies are correspondingly three: self-wiping screw geometry with a streamlined melt path and no cavities, scheduled wear measurement with timely element and liner replacement, and appropriately specified melt filtration with a disciplined screen change interval. Adding finer screens alone will not solve a problem whose origin is inside the barrel.

Does adding more filler always lower the cost of a recycled compound?

No, and this is the most expensive misconception in the business. Up to roughly 20 to 25 parts of talc or activated calcium carbonate, the material index falls while properties stay acceptable. Beyond about 30 parts, notched impact and weld line strength fall sharply, surface quality deteriorates, and the compound starts to fail at the customer’s molding machine. Rejection cost then rises faster than the material index falls, so the true cost per accepted kilogram increases. The optimum filler level is always the one that clears the mechanical specification with a small margin, not the maximum the melt will carry.

Should filler be added at the main hopper or through a side feeder?

Through a side feeder, downstream of the first melting zone, in essentially all cases above about 10 parts loading. Feeding mineral filler at the main hopper means the filler passes through the entire melting section, where it abrades the screw elements and barrel liner, absorbs shear energy that should be melting polymer, and disrupts the solids conveying that low bulk density recycled flake already struggles with. Side feeding into a starved zone after melting gives better dispersion at lower shear and dramatically extends component life.

How much can specific energy consumption be reduced on a recycled compounding line?

Realistically, a line running 0.24 kWh/kg on recycled polypropylene can usually be brought to 0.17 to 0.20 kWh/kg through three changes: replacing aggressive 90 degree and reverse kneading blocks with a greater number of 45 degree and toothed elements, lowering barrel setpoints by 10 to 15 degrees Celsius now that shear heating is reduced, and stabilizing feeding so the drive is not repeatedly accelerating against surges. Those same three changes reduce degradation, improve odor grade and extend screw life, which is why gentle plastication is the recurring theme of this guide.

21. Conclusion

A genuinely low cost recycled plastic modification formula for a twin screw devolatilization extruder is built in a specific order, and the order matters more than any individual ingredient. First, characterize the feedstock so that decisions are made on data rather than insurance. Second, strip the melt with a properly engineered two-stage vent system — atmospheric venting for the steam load, deep vacuum at minus 0.085 to minus 0.095 MPa for the low molecular weight residue, and steam stripping where the odor specification demands it. Third, dose the smallest additive package that clears the specification, starting with the antioxidant pair and lubricant that always pay for themselves and descending the cost-effectiveness ranking only as far as the requirement forces you. Fourth, protect consistency with gravimetric feeding, appropriate melt filtration, in-line monitoring and a homogenizing silo.

The recipes in this guide — three tiers for recycled polypropylene, three application grades for recycled high-density polyethylene, three intrinsic viscosity routes for recycled polyester, and a working mixed polyolefin formula — are starting points calibrated to real production windows. They assume a barrel long enough to hold two vents and a side feeder, a screw configured for gentle plastication rather than maximum shear, and feeding accurate enough that a formula written to a hundredth of a part actually arrives in the melt that way. Without that hardware foundation, the same recipes will underperform and the additive levels required to compensate will erase the cost advantage entirely.

Kerke, a Wanplas factory with more than twelve years of specialization in parallel co-rotating twin-screw compounding extruders, a 19,997 square meter manufacturing base, more than 2,000 machines in service across over 70 countries and a technical team of more than 100 people, configures KTE series machines from the KTE-16B laboratory unit to the KTE-135D production machine specifically for this class of work. Barrel length, vent arrangement, screw configuration, feeding accuracy and filtration technology are specified against the material and the target specification, not against a catalog default.

If you are planning a recycled compounding line or trying to lift an existing one out of a batch-consistency problem, send the details that matter: the feedstock type and its typical characterization data, the target properties and odor grade, the required output, and the end application. Kerke’s application engineers will propose a machine configuration, a screw layout and a formula tier matched to that brief, and Wanplas can arrange small-batch formula verification and a trial run on your actual material before anything is committed. Factory visits are welcome, and witnessing a trial on your own feedstock remains the most reliable way to confirm that a configuration will do what the specification says it will.