Complete machine configuration for waste plastic flake twin screw compound pellet plant

Complete machine configuration for waste plastic flake twin screw compound pellet plant

What a flake-to-pellet compounding line has to do

A waste plastic flake twin screw compound pellet plant converts washed, dried, irregular flake into a uniform, dosable, contaminant-controlled pellet that a downstream molder or extruder can run without changing its process window. Kerke, a Wanplas factory with 12+ years of dedicated experience in compounding extrusion, a 19,997+ square meter plant, more than 2,000 machines running in over 70 countries, and a team of 100+ specialists, builds these lines around one design rule: every machine in the chain exists to remove one specific source of variability from the flake.

That framing matters because flake is not a raw material in the conventional sense. Virgin resin arrives as a pellet with a known melt flow rate, a known additive package, a known bulk density, and a moisture level set by the supplier. Flake arrives with a bulk density that swings with flake geometry, a moisture content that depends on how the washing line was run that shift, a residual contamination load that varies bale by bale, and a thermal history that has already consumed part of the stabilizer package. A compounding line that treats flake as if it were pellet will produce pellet with unstable melt flow rate, visible black specks, inconsistent color, and a moisture level that causes splay in the customer’s molding shop.

This article walks the complete equipment chain machine by machine. For each unit it gives what the machine actually does, the key specifications that must appear on the technical schedule, the selection logic that decides between configurations, and the mistakes that repeatedly show up on lines that were configured on price rather than on material behavior. It then adds the adaptation tables that link flake type to line configuration, two Kerke machine families with specification tables, a requirement-to-configuration recommendation table, workshop layout and utility figures, and the acceptance criteria that should govern sign-off.

The scope here is deliberately narrow and deep. It is about hardware configuration and parameters, not about site selection, staffing, or permitting. Everything below assumes the flake has already been washed and dried to a usable state, and that the question on the table is which machines, in which sizes, with which options, should be written into the line specification.

Whole-line overview and material flow path

The material path through a flake compounding plant is linear, but the control loops around it are not. Understanding the flow first makes every later machine decision easier to justify. The sequence below describes a mainstream configuration for washed post-consumer or post-industrial flake going to a single-stage co-rotating twin-screw extruder with water-bath strand pelletizing.

Flake is discharged from a bulk bag or a buffer bunker into a screw loader or a pneumatic conveying line. It passes a permanent magnetic drum and a metal detector before it can reach any hardened surface. It falls into a hopper above the main loss-in-weight feeder, which meters it into the first barrel section of the twin-screw extruder at a controlled mass rate. Additives, masterbatch, fillers, or stabilizers are dosed by separate loss-in-weight feeders, either into the main throat or through a side feeder further along the barrel. Inside the barrel the flake is conveyed, melted, dispersed, homogenized, and devolatilized under vacuum. The melt leaves the extruder, passes a melt pump that stabilizes pressure, then a screen changer or continuous filter that removes solid contamination, then the die. Strands are drawn across a cooling water bath, dewatered by an air knife, and cut by a strand pelletizer. Pellets pass a centrifugal dewatering unit, a vibrating fluidized bed dryer, a vibrating classifier that removes fines and long cuts, and a dedusting stage. Finished pellet is conveyed to a blending silo for homogenization across production hours, then to an automatic weighing and bagging station.

Two return paths run against the material flow. Fines and rejected pellet from the classifier can be routed back to the feeder inlet at a controlled percentage. Filter backflush or screen change purge is collected separately and treated as reprocessing material, never fed back blind. The table below summarizes the chain as a process schedule.

Step	Equipment	Function in the chain	Key control variable
1	Bulk bag unloader / buffer bunker	Holds a shift buffer of flake, breaks bridging	Level, bridging alarm
2	Screw loader or pneumatic conveying	Transfers flake to the feeder hopper	Conveying rate, filter differential pressure
3	Permanent magnetic drum	Removes ferrous fragments before the barrel	Magnetic field strength in gauss
4	Metal detector	Detects non-ferrous and stainless fragments, triggers reject	Detection sensitivity in mm
5	Loss-in-weight main feeder	Meters flake at a controlled mass rate	Feed accuracy, refill cycle
6	Additive and micro feeders	Dose masterbatch, stabilizer, compatibilizer	Dosing accuracy at low rate
7	Twin-screw extruder	Melts, disperses, homogenizes, devolatilizes	Melt temperature, torque, screw speed
8	Side feeder	Introduces filler, fiber, or light regrind downstream of melting	Side feeder speed, fill level
9	Vacuum devolatilization	Extracts moisture, monomer, and volatile residues	Vacuum level in mbar
10	Melt pump	Stabilizes discharge pressure to the die	Inlet and outlet pressure
11	Screen changer or continuous filter	Removes solid contamination from the melt	Differential pressure across the screen
12	Die and pelletizing system	Forms and cuts the pellet	Melt pressure, cutter speed
13	Dewatering and drying	Brings pellet moisture to target	Residual moisture percentage
14	Classifier and dedusting	Removes fines, long cuts, and dust	Screen aperture, air volume
15	Blending silo	Averages out hour-to-hour variation	Silo turnover, blending ratio
16	Weighing and bagging	Packs into 25 kg bags or bulk bags	Weighing tolerance

A flake compounding line should be specified backwards from the pellet quality specification the customer will accept, not forwards from the flake that happens to be available this month.

Flake infeed, conveying and metal removal

The infeed section decides how much unplanned downtime the rest of the line will suffer. Flake bridges, flake segregates by size, and flake carries metal. Every one of those three problems produces a stoppage somewhere downstream if it is not handled at the front of the line.

Screw loader versus pneumatic conveying

A screw loader is the safer default for light, filmy, or fibrous flake. It moves material positively, it does not classify by particle weight, and it does not need a large filter surface. Its limitations are layout inflexibility and a mechanical wear surface. Pneumatic conveying is preferred when the flake must travel a longer distance, cross an aisle, or feed several hoppers from one source. Its penalties are fines generation from particle-to-wall impact, segregation of light and heavy fractions at the receiver, and a filter that needs regular reverse-pulse cleaning. For flake with a bulk density below roughly 0.15 t/m³, a vacuum conveying receiver sized generously is the usual answer, because a screw loader will simply churn without conveying.

Bulk density is the parameter that gets forgotten

Bulk density controls whether the extruder can actually swallow the specified throughput. Washed PET bottle flake typically sits at 0.30 to 0.40 t/m³, rigid HDPE flake at 0.30 to 0.45 t/m³, and washed film flake at 0.10 to 0.25 t/m³ depending on flake size and curl. A twin-screw extruder rated at 800 kg/h on pellet feed can be volumetrically limited to a much lower figure on low-density film flake unless the feed throat, the feed screw geometry, and where relevant a crammer feeder are all matched to the actual material. The correct practice is to measure bulk density on a real sample before sizing the machine, not to accept a generic figure.

Metal protection layers

Metal protection should always be two layers, never one. A permanent magnetic drum or magnetic grid removes ferrous fragments passively and continuously. A metal detector then catches austenitic stainless steel, aluminum, copper, and brass, which a magnet cannot hold. Placing the detector after the magnet is deliberate: the magnet removes the high-frequency ferrous hits so the detector does not trip constantly and desensitize the operator to alarms.

Protection unit	Typical specification	What it catches	Selection note
Permanent magnetic grid	8,000 to 12,000 gauss surface field	Ferrous fragments above roughly 1 mm	Lowest cost layer, needs manual cleaning each shift
Permanent magnetic drum	9,000 to 12,000 gauss, self-cleaning rotation	Ferrous fragments, continuous duty	Preferred for continuous lines, no shift interruption
Metal detector, gravity chute type	Fe 1.5 to 2.5 mm, non-Fe 2.0 to 3.5 mm, stainless 3.0 to 5.0 mm	Aluminum, copper, brass, stainless	Sensitivity depends on chute aperture; smaller aperture detects smaller particles
Metal detector, conveyor type	Fe 2.0 to 3.0 mm, non-Fe 2.5 to 4.0 mm	Same, with belt reject gate	Used where flake is delivered on a belt rather than by gravity
Reject flap valve	Response under 100 ms	Diverts the contaminated slug	Reject volume must be large enough for detector latency

Loss-in-weight gravimetric feeding package

On a starve-fed co-rotating twin-screw extruder, the feeders do not assist the process, they define it. Screw speed sets shear and residence distribution, but throughput and formula ratio are set entirely by the feeders. A gravimetric feeding package with poor accuracy will make every downstream quality metric unstable regardless of how good the extruder is.

Why loss-in-weight rather than volumetric

A volumetric feeder meters by screw revolutions and assumes constant bulk density. Flake violates that assumption continuously: bulk density changes with flake size distribution, with hopper level, with static charge, and with humidity. A loss-in-weight feeder weighs the hopper continuously and closes a control loop on actual mass discharged, correcting for bulk density drift in real time. On a flake line, the difference between the two is not a refinement, it is the difference between a controlled compound and an approximate one.

Feeder set for a typical flake compounding line

A practical configuration uses one main loss-in-weight feeder for the flake, one or two feeders for filler or a second polymer stream, and one micro feeder for the additive package. Kerke supplies volumetric metering systems, twin-screw side feeders, crammer feeders, loss-in-weight feeders, and liquid feeders, so the package can be matched to material behavior rather than forced into one standard.

Feeder position	Type	Typical range	Accuracy	Material handled
Main feed	Twin-screw loss-in-weight, wide throat	50 to 3,000 kg/h	±0.5% of setpoint	Washed flake, regrind
Main feed for low bulk density	Loss-in-weight with crammer	30 to 1,200 kg/h	±0.5% to ±1.0%	Film flake below 0.15 t/m³
Second polymer	Twin-screw loss-in-weight	20 to 800 kg/h	±0.5%	Virgin resin for dilution, compatibilizer carrier
Filler side stream	Loss-in-weight into side feeder	50 to 1,500 kg/h	±0.5%	Calcium carbonate, talc, glass fiber
Additive micro feed	Single-screw micro loss-in-weight	0.5 to 50 kg/h	±1.0% at low rate	Antioxidant, chain extender, color masterbatch
Liquid additive	Metering pump with mass flow feedback	0.2 to 40 kg/h	±1.0%	Liquid lubricant, coupling agent, oil

Three configuration details separate a feeder package that holds accuracy from one that only holds it on the acceptance day. First, the refill cycle must be fast and repeatable, because a loss-in-weight feeder runs volumetrically during refill and any long refill window degrades average accuracy. Second, the feeders must be mounted on a vibration-isolated platform; a pelletizer or a blower transmitting vibration into a load cell will produce a noisy weight signal that the controller interprets as flow variation. Third, the micro feeder needs an agitation design suited to the specific additive, since many antioxidant and processing aid powders bridge readily at low discharge rates.

The twin-screw main machine: core specification set

The extruder is the machine that determines what the line can and cannot do. Ten specifications describe it completely enough for a technical comparison, and a purchase decision made without all ten is a decision made on an incomplete picture.

Screw diameter and throughput

Screw diameter sets the volumetric envelope. For a co-rotating parallel twin-screw extruder the practical throughput scales roughly with the cube of diameter for the same material and screw speed, though in real machines torque and heat transfer limits flatten that curve. On recycled flake, the achievable throughput at a given diameter is usually 60 to 85 percent of the pellet-fed figure, because bulk density is lower and devolatilization consumes barrel length.

L/D ratio

L/D is the length available to do process work. Each function consumes barrel length: feeding, melting, mixing, side feeding, venting, pressure build-up. The table in the next section shows how the length budget is spent. As a general configuration rule for flake work, L/D 36 handles clean single-polymer flake, L/D 40 is the mainstream choice, L/D 44 is used when two vacuum zones are needed, and L/D 48 is reserved for heavily filled formulations, reactive extrusion, or lines that combine side feeding with twin-stage devolatilization.

Specific torque

Specific torque, expressed in Nm/cm³, is the single most useful number for comparing two extruders of the same diameter. It describes how much torque the gearbox and shafts can deliver per unit of free volume. A machine with higher specific torque can run at higher screw speed with the same melt temperature, or at the same speed with a higher filler loading, because it is not torque-limited first. Standard machines sit around 7 to 9 Nm/cm³. High-torque configurations reach 11 to 13 Nm/cm³ and are the correct choice for filled recycled compounds where torque, not volume, is the limit.

Barrel construction, heating and cooling

Barrels are built in modular sections so the configuration can be changed. A typical machine has 8 to 12 barrel sections, each individually heated by cast or ceramic band heaters and individually cooled by a soft water circuit with solenoid control. Barrel liners are either nitrided for standard duty or bimetallic for abrasive duty. On recycled flake carrying mineral filler, glass fiber, or residual sand from washing, bimetallic liners and correspondingly wear-resistant screw elements are not an upgrade, they are a requirement; nitrided surfaces on abrasive recycled feed can lose their service life by a large multiple.

Specification	Typical value or range	Why it matters on flake
Screw diameter	20 mm laboratory to 133 mm production	Sets volumetric capacity and the throughput ceiling
L/D ratio	36 / 40 / 44 / 48	Determines how many process functions fit in one barrel
Specific torque	7 to 9 standard, 11 to 13 high torque, Nm/cm³	Decides whether filled recycled compound is torque limited
Maximum screw speed	500 to 800 rpm depending on size	High speed raises output but also shear heat on degraded polymer
Main drive power	Sized to torque and speed, 11 kW to above 1,000 kW	Installed power base for the whole plant
Gearbox	Hardened and ground helical gears, forced lubrication, oil cooler	Continuous duty at full torque, oil temperature monitored
Barrel sections	8 to 12 modular sections	Allows venting and side feeding to be relocated
Barrel heating	Cast aluminum or ceramic band heaters, zone controlled	Zone control prevents local overheating of degraded polymer
Barrel cooling	Soft water circuit, solenoid valve per zone	Fast response to shear heat spikes from filler loading
Barrel liner	Nitrided standard, bimetallic for abrasive feed	Determines wear life on filled or sand-bearing recycled flake
Screw shaft	Splined high torque shaft, hardened tool steel elements	Element interchangeability and torque transmission margin
Melt temperature sensing	Immersion thermocouple at the discharge, plus pressure transducer	Actual melt temperature, not barrel setpoint, controls degradation

Screw element configuration and shear management

The screw configuration is where a compounding line is actually engineered. Two extruders with identical nameplate specifications will produce different pellet if their screw configurations differ, and on recycled flake the configuration matters more than on virgin resin because the polymer has already lost part of its thermal stability margin.

The three element families

Conveying elements move material forward. They are described by pitch: a long pitch conveys fast with low fill and low shear, a short pitch conveys slower with higher fill and builds pressure. Kneading blocks do the dispersive and distributive work. They are described by the stagger angle between discs: 45 degrees forward is a moderate, forward-conveying mixing element; 60 degrees is more aggressive; 90 degrees is neutral-conveying and gives the highest local shear intensity per unit length. Reverse elements, whether reverse-pitch conveying screws or reverse kneading blocks, create a melt seal by forcing material to back up, which is how a vacuum zone is isolated and how residence time is extended locally.

How the length budget is spent

On a 40 L/D machine handling washed film flake with a filler side stream, a workable length allocation is: 0 to 6 L/D feeding and conveying with long pitch elements; 6 to 12 L/D first melting section with a 45 and 60 degree kneading block group ending in a short reverse element; 12 to 16 L/D conveying and side feeding entry; 16 to 22 L/D dispersive mixing with 60 and 90 degree blocks for filler wetting; 22 to 26 L/D distributive mixing with narrow discs; 26 to 28 L/D melt seal ahead of the vacuum zone; 28 to 34 L/D vacuum devolatilization under low fill with long pitch elements; 34 to 40 L/D pressure build-up with progressively shorter pitch to the discharge.

Element type	Designation	Primary function	Shear intensity	Where used on a flake line
Long pitch conveying	1.5D to 2.0D pitch	Fast transport, low fill	Very low	Feed zone, vacuum zone
Standard conveying	1.0D pitch	Transport with moderate fill	Low	Transition and transfer sections
Short pitch conveying	0.5D to 0.75D pitch	Pressure build-up	Low to medium	Discharge zone ahead of the melt pump
Forward kneading block	KB45	Melting and gentle distributive mixing	Medium	Primary melting group
Forward kneading block	KB60	Stronger dispersion, moderate forward conveying	Medium to high	Filler dispersion, pigment wetting
Neutral kneading block	KB90	Maximum local shear, no net conveying	High	Agglomerate breakdown, carbon black dispersion
Reverse kneading block	KB45 reverse	Melt seal, extended residence	High	Ahead of vacuum zones and side feeders
Reverse conveying element	Reverse pitch screw	Full melt seal with lower shear than reverse blocks	Medium	Ahead of vacuum on shear-sensitive polymer
Toothed mixing element	Gear or turbine element	Distributive mixing at low shear	Low to medium	Color homogenization on degraded polymer

The practical rule for recycled flake is to buy dispersion with the minimum shear that achieves it. Recycled polyolefin arriving from a washing line has already been through at least one melt history, and often two. Every additional degree of melt temperature above what the formula needs accelerates chain scission for polypropylene and cross-linking for polyethylene, both of which show up as melt flow rate drift in the finished pellet. A configuration with two moderate KB45 and KB60 groups will frequently give better final pellet properties than a single aggressive KB90 group, even though the aggressive configuration looks stronger on paper.

Kerke designs screw assemblies with computer-aided element layout, uses a kneading co-type geometry with strong self-cleaning behavior, and maintains element interchangeability across the series so that a configuration can be revised in the field when the incoming flake stream changes. In practice a recycling plant will change flake sources several times in the life of the machine, and the ability to re-stagger the screw for a new material without buying a new shaft set is worth more than any single configuration chosen at purchase.

Vacuum devolatilization system

Devolatilization is the process step that most directly separates a recycled pellet that molds cleanly from one that produces splay, voids, and odor complaints. Waste plastic flake carries surface moisture from washing, absorbed moisture in hygroscopic polymers, residual detergent and surfactant, printing ink solvent, adhesive residue from labels, and low molecular weight degradation products created by earlier heat histories. All of these must leave the melt inside the barrel, because anything still dissolved in the melt at the die becomes a defect in the pellet.

Single-stage versus twin-stage vacuum

A single vacuum zone with a water ring pump reaching roughly 30 to 100 mbar absolute is adequate for washed, pre-dried polyolefin flake whose main volatile load is residual surface moisture. It is simple, tolerant of water vapor, and cheap to maintain. A twin-stage arrangement places two separate vacuum zones along the barrel, usually one after the melting section and one further downstream, and uses a deeper vacuum on the second stage. The first stage takes the bulk load of water vapor and light volatiles; the second stage, at 5 to 20 mbar absolute, removes the residual fraction that the first stage cannot reach because the driving partial pressure difference has already been consumed.

Pump technology selection

A water ring pump is the standard choice where the extracted vapor is dominated by water. It handles wet gas without damage, tolerates carry-over, and is inexpensive to service, but its attainable vacuum is limited by the vapor pressure of the sealing water, which is why it plateaus around 30 mbar. A rotary vane pump reaches a much deeper vacuum but is intolerant of liquid water and polymer carry-over, so it must be protected by a condenser and a knock-out separator. A roots blower installed in front of a backing pump raises pumping speed at low absolute pressure, which is what actually determines devolatilization rate in a large machine. On heavily printed film flake, a condenser ahead of the pump is not optional; without it, the pump will accumulate solvent and lose performance within weeks.

Configuration	Attainable vacuum	Suitable feed	Residual volatile outcome	Complexity level
Atmospheric vent only	Ambient	Dry, clean post-industrial regrind	Removes entrained air only	Low
Single stage, water ring pump	30 to 100 mbar absolute	Washed and dried polyolefin flake	Moisture typically below 0.10%	Low
Single stage with condenser	25 to 60 mbar absolute	Lightly printed film flake	Moisture below 0.08%, reduced odor	Medium
Twin stage, water ring plus rotary vane	5 to 20 mbar absolute	Printed film, mixed post-consumer flake	Moisture below 0.05%, low VOC carry-over	Medium to high
Twin stage with roots booster	2 to 10 mbar absolute	PET flake, engineering polymers, odor-critical grades	Moisture below 0.02%, lowest residual volatiles	High

Two hardware details govern whether a vacuum zone performs as designed. The vent port must have a stuffing screw or a vent insert that stops melt from creeping up into the port; a vented barrel that vents polymer instead of vapor will be shut down for cleaning several times per shift. And the vacuum line must include a cyclone separator with an accessible collection vessel, because fine powder and oligomer will always be entrained. Sizing the separator too small is one of the most frequent causes of falling vacuum performance three months after commissioning.

Side venting and side feeding positions

Side devices solve two problems that cannot be solved at the main feed throat: getting low bulk density material into a barrel that is already partly full of melt, and letting entrained air escape without dragging powder out with it.

Side feeder

A twin-screw side feeder introduces material into a barrel section downstream of the melting zone. On a flake compounding line it is used for three purposes. First, mineral filler such as calcium carbonate or talc is added after the polymer has melted, so the filler is wetted by a melt rather than mixed with cold flake; this improves dispersion and reduces wear on the melting section. Second, glass fiber is added late in the barrel to preserve fiber length, since fiber added at the main throat is chopped down by the melting elements. Third, very low bulk density regrind or film flake can be introduced through a side feeder with a dedicated venting port, because the main throat cannot swallow it volumetrically.

Side feeder positioning is a length trade-off. Feeding at 14 to 18 L/D leaves plenty of barrel length for dispersion but exposes the added material to more shear. Feeding at 22 to 26 L/D preserves fiber length or heat-sensitive additive but leaves less mixing length afterwards. On a 40 L/D machine handling filled recycled polyolefin, 16 to 20 L/D is the common compromise.

Side venting

A side vent, sometimes described as a side degassing unit, is a twin-screw device mounted at a vent port whose screws rotate to push escaping powder back into the barrel while allowing gas to leave. It is essential when large volumes of air enter with low bulk density feed, because a plain open vent in that situation will blow powder into the workshop and into the vacuum line. Side venting is normally placed in the first third of the barrel, upstream of melting, where the entrained air load is highest.

A compact summary of where each device belongs: atmospheric or side venting in the first third of the barrel to release entrained air; side feeding in the middle third for filler and fiber; vacuum devolatilization in the final third where the melt is fully formed and the volatiles can actually diffuse out. Placing a vacuum port too early, before the polymer has fully melted, wastes the port because unmelted flake simply plugs it.

Melt pump and discharge pressure stabilization

A co-rotating twin-screw extruder is starve fed, which means its throughput is set by the feeders and its discharge pressure is a consequence of whatever the downstream resistance happens to be at that instant. That is an excellent arrangement for mixing and a poor one for pelletizing, because pellet geometry depends on a stable volumetric flow at the die.

A melt gear pump installed between the extruder and the filtration unit, or between the filter and the die, converts a fluctuating flow into a near-constant one. It draws melt at a low, controlled inlet pressure and delivers it at whatever outlet pressure the die requires. The control loop holds inlet pressure constant by adjusting pump speed, which isolates the extruder from downstream disturbance.

The measurable benefits on a recycled flake line are specific. Outlet pressure fluctuation typically drops from a range of plus or minus 8 to 15 bar on a direct-coupled machine to plus or minus 1 to 3 bar with a melt pump under closed-loop control. Pellet weight distribution tightens correspondingly, which matters for customers who dose by volume. Because the extruder no longer needs to generate the full die pressure, the pressure-build zone can be shortened and the screw can run at lower fill in the discharge section, which lowers melt temperature by several degrees. On many configurations that releases 10 to 20 percent additional usable throughput at the same melt temperature ceiling.

Parameter	Without melt pump	With melt pump	Practical consequence
Die pressure fluctuation	±8 to ±15 bar	±1 to ±3 bar	Stable strand diameter, fewer strand breaks
Pellet weight scatter	Higher	Reduced	Tighter dosing accuracy for the end user
Melt temperature at die	Baseline	3 to 8 degrees Celsius lower	Less degradation of already-recycled polymer
Usable throughput	Baseline of 100 index points	110 to 120 index points	More output at the same quality window
Filter loading tolerance	Throughput falls as screens load	Throughput held until change point	Consistent output between screen changes
Complexity and maintenance	Low	Medium	Additional heated unit, seals, and drive to service

The melt pump is most valuable on lines running underwater or water-ring die-face pelletizing, where die pressure stability directly controls pellet shape, and on lines with heavy filtration where screen loading would otherwise cause output to drift downward between screen changes. On a simple strand line with clean post-industrial flake and light filtration, a melt pump is a reasonable option rather than a necessity.

Melt filtration and screen changing

Filtration is the step that most directly determines the commercial grade of the finished pellet. Two lines with identical extruders can sell into completely different markets purely because one filters to 120 mesh continuously and the other filters to 40 mesh with manual screen changes. The correct filtration choice comes from the contamination level of the flake and the required run continuity, not from a default preference.

Screen changer types

A manual slide-plate screen changer is the simplest device and is acceptable only on very clean post-industrial regrind, because every change interrupts flow. A hydraulic single-piston screen changer with two positions allows a change without stopping the extruder but produces a brief pressure and flow disturbance. A dual-piston, four-position hydraulic screen changer is the workhorse configuration for recycled flake: it has four screen cavities across two pistons, so half the filtration area can be changed while the other half stays in service, giving a continuous flow path with only a minor pressure ripple. Backflush versions redirect a small melt stream through a loaded screen to clear it, extending the interval between physical screen changes.

Continuous laser filters

For heavily contaminated post-consumer flake, particularly material carrying paper fiber, wood, rubber, or aluminum foil residue, a continuous self-cleaning laser filter is the appropriate technology. Melt passes through a perforated drum whose holes are laser-drilled to a defined diameter, typically 90 to 250 micrometers. A scraper continuously removes retained contamination and discharges it through a discharge screw. The advantages are uninterrupted operation, a constant differential pressure, and a filtration fineness that would clog a conventional screen pack within minutes on the same feed. The trade-offs are higher complexity, a continuous small polymer loss with the discharged contamination, and a higher maintenance skill requirement.

Filtration solution	Typical fineness	Contamination tolerance	Flow continuity	Best matched flake	Relative cost level
Manual slide plate changer	40 to 100 mesh	Very low	Interrupted at each change	Clean post-industrial regrind	Low
Hydraulic single piston, two position	40 to 120 mesh	Low	Brief disturbance at change	Sorted post-industrial flake	Low to Medium
Dual piston, four position hydraulic	40 to 150 mesh	Medium	Continuous, minor ripple	Washed post-consumer flake	Medium
Dual piston with backflush	60 to 150 mesh	Medium to high	Continuous	Washed flake with variable load	Medium to High
Continuous laser filter	90 to 250 micrometers	High	Fully continuous	Heavily contaminated post-consumer flake	High
Laser filter plus fine polishing screen	Laser plus 120 to 200 mesh	High	Fully continuous	Flake destined for demanding applications	Very High

Screen pack construction and differential pressure

A screen pack is not a single mesh. A working pack is layered: a coarse support screen of 20 to 30 mesh next to the breaker plate to carry the load, a main filtration layer at the target mesh, and often an intermediate layer to distribute flow. A common polyolefin recycling pack is 20 mesh support, 40 mesh intermediate, and 80 mesh main filtration, with the option to move the main layer to 120 mesh when the flake quality allows.

Differential pressure across the filter is the operating signal that governs screen life. Pressure transducers before and after the filter feed a differential calculation into the PLC. A typical control scheme sets a warning threshold at roughly 60 to 80 bar differential and an automatic or prompted change point at 100 to 120 bar, adjusted for the specific polymer and the design pressure of the unit. Running past the change point does not just cost throughput; it raises melt temperature through pressure-driven shear and it can force contamination through a distorted screen.

Pelletizing method comparison

Pelletizing is the last shaping step and the one most often mismatched to the material. Four methods dominate flake compounding lines, and each is genuinely the right answer for a defined set of materials. Choosing by throughput alone, without considering melt viscosity, tackiness, and moisture sensitivity, is the most common configuration error on recycling lines.

Water-bath strand pelletizing

Melt is extruded as strands through a strand die, drawn across a cooling water bath, dried by an air knife, and cut by a rotating knife against a fixed blade. It is the most tolerant configuration: it accepts wide viscosity variation, it shows the operator immediately when the melt is unstable because strands break, and it is the easiest to restart after an interruption. The output is a cylindrical pellet with a diameter set by the die and a length set by the haul-off to cutter speed ratio. Its limits are throughput per die area and the need for an operator to re-thread strands after a break, though automatic strand conveying reduces that.

Water-ring die-face cutting

Melt is cut at the die face by rotating blades inside a housing through which a ring of water flows. The pellet is thrown into the water ring, cooled, and carried to a centrifugal dryer. It removes the strand break problem entirely and handles low-viscosity melt that would not form stable strands. Pellet shape is lenticular rather than cylindrical. It suits polyolefin regrind well and reaches higher throughput than strand cutting for the same footprint.

Underwater pelletizing

The die plate is submerged in a temperature-controlled water chamber and the cutter runs against the die face fully underwater. Pellets are immediately quenched and carried away in the process water. It gives the most spherical, most uniform pellet, the highest capacity ceiling, and the best dust control. It is the standard for engineering plastics, PET, thermoplastic elastomers, and any product with a demanding geometry specification. It is also the least tolerant configuration: it demands a stable melt pressure, precise cutter gap control, and careful startup, and it carries the highest complexity level of the four.

Air-cooled hot-face cutting

Pellets are cut at the die face and conveyed away in an air stream that provides the cooling. There is no process water at all, which makes it attractive for moisture-sensitive or water-reactive formulations and for filled compounds. It works only where the compound solidifies fast enough not to agglomerate in the air stream, so it is largely restricted to highly filled, high-melting, non-tacky materials.

Method	Suitable materials	Pellet shape	Typical pellet moisture after drying	Practical capacity ceiling	Relative cost level	Complexity
Water-bath strand pelletizing	PP, PE, PS, ABS, most filled compounds, mixed recycled flake	Cylindrical, 2 to 4 mm	0.05 to 0.20%	Up to roughly 1,500 to 2,500 kg/h per line	Low	Low
Water-ring die-face cutting	PP, PE, low-viscosity polyolefin regrind, wax-modified compounds	Lenticular	0.05 to 0.15%	Up to roughly 2,000 to 3,000 kg/h	Medium	Medium
Underwater pelletizing	PET, PA, PC, TPE, engineering compounds, high-output polyolefin	Near spherical, very uniform	0.02 to 0.10%	Above 5,000 kg/h with suitable die plate	High	High
Air-cooled hot-face cutting	Highly filled compounds, non-tacky high-melting materials	Irregular to lenticular	Dry, no water contact	Up to roughly 800 to 1,500 kg/h	Medium	Medium
Air-cooled strand pelletizing	Filled masterbatch, moisture-sensitive compounds	Cylindrical	Dry, no water contact	Up to roughly 500 to 900 kg/h	Low to Medium	Low
Eccentric water mist hot cutting	Compounds needing partial water cooling without immersion	Lenticular	0.10 to 0.30%	Up to roughly 1,000 kg/h	Medium	Medium

Kerke builds all six cutting configurations listed above as part of its cutting and pelletizing system range, which means the pelletizer can be chosen for the material rather than the material being forced onto whichever cutter the line was built around. On mixed flake operations that expect to change feedstock, the strand line remains the most forgiving default, with the option to add a water-ring or underwater system later on a second line dedicated to a specific grade.

Dewatering and drying of the pellet

Pellet moisture is a specification that end users test on arrival, and it is the reason a technically good compound gets rejected. The dewatering and drying train has to bring pellet from a water-wet state to a defined residual moisture without adding thermal history.

A centrifugal dewatering unit is the first stage. Pellet and water enter tangentially, a rotor throws the water through a perforated screen, and the pellet leaves at the top. It removes the bulk surface water mechanically, which is far cheaper in energy terms than evaporating it. A well-sized centrifugal dryer will take pellet from fully wet to roughly 0.3 to 0.8 percent surface moisture in a single pass.

A vibrating fluidized bed dryer is the second stage. Pellet travels along a vibrating perforated deck while warm air passes upward through the bed. Because the pellet is fluidized, every surface contacts the air stream and residence time is uniform. Typical air temperature is 60 to 80 degrees Celsius for polyolefin, with residence times of 2 to 6 minutes. This stage takes the pellet to the 0.05 to 0.15 percent range that most polyolefin customers accept.

For hygroscopic polymers a third stage is required, because the moisture is not on the surface but dissolved in the polymer. A dehumidifying dryer with a desiccant circuit and a dew point of minus 40 degrees Celsius is used, with hopper residence times of 4 to 6 hours for PET and 3 to 4 hours for PA. PET additionally requires crystallization before drying, since amorphous PET will agglomerate in a hot hopper.

Stage	Equipment	Typical operating condition	Moisture achieved	Applies to
Bulk water removal	Centrifugal dewatering unit	Rotor speed set by pellet size, ambient	0.3 to 0.8% surface	All water-cut pellet
Surface drying	Vibrating fluidized bed dryer	60 to 80 degrees Celsius, 2 to 6 minutes	0.05 to 0.15%	PP, PE, PS, filled compounds
Extended surface drying	Fluidized bed with extended deck	70 to 90 degrees Celsius, 6 to 10 minutes	0.03 to 0.08%	ABS, PS for demanding applications
Crystallization	Crystallizing dryer with agitation	160 to 180 degrees Celsius, 20 to 40 minutes	Prevents agglomeration	PET pellet before hopper drying
Deep drying	Desiccant dehumidifying dryer	Dew point minus 40 degrees Celsius, 4 to 6 hours	Below 0.005% for PET, below 0.02% for PA	PET, PA, PC, PBT

A configuration detail that repeatedly causes trouble: the fluidized bed dryer needs enough air volume for the actual pellet throughput, and air volume requirement scales with the water load, not with the pellet mass. A line that changes from strand pelletizing to water-ring cutting carries substantially more water into the dryer for the same pellet output, and the existing dryer will underperform unless the fan and heater are resized.

Classification, dedusting, blending and bagging

The back end of the line converts a stream of correctly made pellet into a saleable, homogeneous, correctly packed product. It is routinely under-specified because it does not look like process equipment, and it is routinely the reason a customer complains.

Vibrating classifier

A two-deck vibrating classifier removes both ends of the size distribution. The upper deck retains oversize, which on a strand line means fused doubles and long cuts, and on a die-face line means agglomerates formed at startup. The lower deck passes fines and dust to a separate collection point. Typical deck apertures for a 3 mm pellet are 5 to 6 mm on the upper deck and 1.5 to 2 mm on the lower deck. Removing long cuts matters because a molding machine feed throat will bridge on them, and removing fines matters because fines melt faster than pellet and cause localized degradation in the customer’s process.

Dedusting

A gravity or cyclone dedusting unit passes pellet through a counter-current air stream that lifts dust and angel hair away from the pellet stream. It is placed after classification and before the blending silo. Dust content is a specification that demanding pellet buyers test, and on recycled material the dust load is inherently higher than on virgin resin because the pellet surface is less regular and because fines are generated at every conveying transfer. Reducing the number of pneumatic transfer steps between the pelletizer and the bagging station is a design decision that reduces dust more effectively than any dedusting machine.

Blending silo

Recycled compound produced from waste flake varies hour by hour, because the input varies hour by hour. A blending silo averages that variation. A gravity blending silo with multiple internal draw-down tubes takes material from many levels simultaneously, so material produced at different times is combined on discharge. A silo sized to hold 4 to 8 hours of production gives a meaningful reduction in the variation of melt flow rate and color that reaches the customer. This single machine often does more for perceived quality consistency than an upgrade anywhere else in the line.

Weighing and bagging

Two packing formats cover almost all recycled pellet trade. Bulk bags of 500 to 1,000 kg are used for industrial customers with bulk handling. Bags of 25 kg are used where the material is resold or handled manually. An automatic net-weigh bagging station with a gross weight check, a sealing unit, and a palletizing conveyor is standard. Weighing tolerance of plus or minus 0.1 to 0.2 percent of the nominal bag weight is a reasonable specification; tighter tolerance mainly adds cost without commercial benefit on recycled grades.

Back-end unit	Key specification	Quality metric it protects	Sizing guidance
Two-deck vibrating classifier	Upper 5 to 6 mm, lower 1.5 to 2 mm	Long cuts, doubles, fines content	Deck area sized for 1.5 times line output
Cyclone dedusting unit	Counter-current air, adjustable air volume	Dust and angel hair content	Air volume matched to pellet fall rate
Gravity blending silo	Multi-tube internal draw-down	Melt flow rate and color variation	4 to 8 hours of production volume
Bulk bag filling station	500 to 1,000 kg, load cell weighing	Weight accuracy, dust escape	One station per 1,500 kg/h of output
25 kg bagging station	Net weigh, ±0.1 to ±0.2% tolerance	Declared weight compliance	Throughput matched to line rate plus 20%
Metal check at packing	Fe 1.5 mm, non-Fe 2.5 mm	Final metal assurance to the customer	Optional but valuable for demanding buyers

Electrical control system and installed power

The control system is what turns a collection of machines into a line. On a recycled flake plant it carries an additional burden that a virgin compounding line does not have: it must record what was made from which input, because recycled pellet is increasingly sold with documentation about its source and its process conditions.

Architecture

A mainstream configuration uses a PLC with a color touch screen human-machine interface as the line master. The main drive runs on a vector inverter or a servo drive with torque feedback. Barrel temperature zones run on independent controllers reporting to the PLC. Feeders have their own controllers on a fieldbus, so recipe changes propagate to all feeders at once. The vacuum system, pelletizer, dryer, and classifier are interlocked so that a fault in any of them triggers a controlled sequence rather than an uncoordinated stop.

Functions worth specifying explicitly

Recipe management should store the complete parameter set including all feeder setpoints, all temperature zones, screw speed, melt pump speed, vacuum setpoint, and cutter speed, and should apply a recipe as an atomic change rather than field by field. Data logging should record throughput, torque percentage, melt pressure, melt temperature, filter differential pressure, vacuum level, and energy consumption at a sampling interval of one to five seconds, with retention of at least 90 days. Alarm management should distinguish warnings from trips and should log the first-out cause. A remote service interface over a secured connection allows a factory engineer to view live PLC data and assist with diagnosis without traveling; this is the single feature that most shortens downtime on an exported line.

Installed power versus actual load

Installed power on the electrical schedule is not the power the plant will draw. Heaters are sized for startup, not for steady state, and they cycle at a low duty once the barrel is hot because most of the process heat comes from shear. The table below gives a typical distribution for a mid-size line built around a 250 kW main drive.

Consumer	Installed rating share	Typical steady-state load factor	Notes
Main extruder drive	55 to 62%	60 to 80%	Load rises with filler content and screw speed
Barrel heating	10 to 14%	10 to 25%	High at startup, low duty in steady production
Vacuum system	3 to 6%	85 to 100%	Runs continuously at near full load
Melt pump and filtration	3 to 5%	50 to 70%	Hydraulic unit is intermittent
Pelletizing and water circuit	6 to 10%	70 to 85%	Includes cutter drive and process water pumps
Dewatering and drying	6 to 9%	60 to 80%	Fan and heater on the fluidized bed
Conveying, classification, dedusting	4 to 7%	50 to 70%	Blowers dominate this group
Control, lighting, auxiliaries	2 to 3%	80 to 100%	Continuous small load

A reasonable planning figure is that the measured plant load in steady production is 55 to 70 percent of installed power. Specific energy for washed polyolefin flake pelletizing on a well-configured twin-screw line generally falls between 0.20 and 0.35 kWh per kilogram of pellet, including all auxiliaries. Filled compounds move toward the upper end, and lines with deep twin-stage vacuum add roughly 0.02 to 0.04 kWh per kilogram. These figures should be written into the acceptance protocol, because they are measurable and they are the fairest way to compare two competing line configurations.

Line adaptation by flake type

The same block diagram supports very different machines depending on what the flake is. This section maps the five flake families that dominate commercial recycling onto concrete configuration decisions.

PP and PE film flake

Low bulk density is the defining problem. The feed system needs a crammer feeder or a side-fed arrangement, and the extruder is volumetrically limited rather than torque limited. Melting is easy, dispersion is easy, and the real work is devolatilization of printing ink residue and residual moisture. Screw configuration stays gentle, with KB45 groups and long-pitch vacuum sections. Filtration must be generous because film flake carries the highest contamination load of the common streams.

PET bottle flake

PET is hydrolysis sensitive, so moisture is the controlling variable. Crystallization and deep drying before extrusion are mandatory unless the line is designed for high-vacuum devolatilization in the barrel. Intrinsic viscosity drops during processing, so a chain extender dosed by micro feeder is common. Melt temperature control is tight, filtration is fine, and underwater pelletizing is preferred for pellet uniformity and for the crystallization step that follows.

HDPE and PP rigid flake

This is the most straightforward family. Bulk density is workable, contamination after washing is moderate, and the polymer tolerates the process well. A standard 40 L/D machine with single-stage vacuum, a dual-piston screen changer at 60 to 80 mesh, and strand or water-ring pelletizing covers most of it. The main configuration question is whether the incoming flake carries mineral filler or sand, which decides bimetallic barrels and hardened screw elements.

ABS and PS flake

Styrenic polymers are shear sensitive and prone to color shift and yellowing when overheated. The configuration priority is low melt temperature: moderate screw speed, gentle kneading blocks, and a melt pump to allow a shorter pressure-build zone. Residual monomer and volatiles require effective vacuum. Filtration in the 80 to 120 mesh band is normal because these materials are usually destined for visible parts where a black speck is a rejection.

PA and PC flake

Engineering polymers demand the highest specification across the board: deep drying before extrusion, twin-stage vacuum, tight melt temperature control, fine filtration, and underwater pelletizing. PA is hygroscopic and hydrolysis sensitive at melt temperature; PC is very sensitive to both moisture and residence time. Both benefit from high specific torque so the machine can run at higher output without raising melt temperature.

Flake type	Recommended L/D	Screw configuration emphasis	Vacuum stages	Filtration	Pelletizing method	Barrel and element material
PP / PE film flake	44	Gentle KB45, long-pitch vacuum zones, crammer or side feed	Two, with condenser	Laser filter or 40 to 60 mesh with backflush	Water-bath strand or water ring	Nitrided standard, bimetallic if sand present
PET bottle flake	40 to 44	Short melting group, low shear, chain extender micro dosing	Two, roots assisted	80 to 120 mesh or laser filter	Underwater with downstream crystallizer	Bimetallic
HDPE / PP rigid flake	36 to 40	Standard KB45 and KB60 melting group	One	60 to 80 mesh, dual piston four position	Water-bath strand or water ring	Nitrided, bimetallic for filled feed
ABS / PS flake	40	Low shear, toothed distributive elements, melt pump	One to two	80 to 120 mesh	Water-bath strand	Nitrided
PA / PC flake	44 to 48	High specific torque, controlled residence, tight temperature	Two	100 to 150 mesh or laser filter	Underwater	Bimetallic with corrosion-resistant option
Filled or glass-fiber recycled compound	44 to 48	Side feeding at 16 to 20 L/D, KB60 dispersion group	One to two	40 to 80 mesh	Water-bath strand or underwater	Bimetallic mandatory

Kerke KTE Series twin screw extruder

The KTE Series is the parallel co-rotating twin-screw platform that Kerke builds for compounding and recycled flake pelletizing, spanning KTE-16B for laboratory formula trials up to KTE-135D for high-output production. Every unit in the series shares the same modular barrel architecture, the same splined high-torque shaft system, and the same computer-aided screw element library, so a configuration proven on a laboratory machine transfers to a production machine without redesign. Aspect ratio, barrel structure, screw arrangement, exhaust configuration, feeding package, and electrical control are all specified per project rather than fixed.

For waste flake compounding the D-suffix high-torque configurations are the relevant part of the range, because filled and contaminated recycled feed is torque limited more often than volume limited. The table below gives the indicative configuration set. Final figures are confirmed against the actual material, its bulk density, and the required pellet specification.

Model	Screw diameter	L/D options	Max screw speed	Main drive power	Specific torque	Indicative flake output	Typical duty
KTE-20B	21.7 mm	36 / 40 / 44	600 rpm	4 to 11 kW	7 to 9 Nm/cm³	3 to 20 kg/h	Laboratory formula trials
KTE-35B	35.6 mm	36 / 40 / 44	600 rpm	15 to 37 kW	8 to 10 Nm/cm³	30 to 120 kg/h	Pilot line, small batch grades
KTE-52D	51.4 mm	40 / 44 / 48	600 rpm	55 to 90 kW	11 to 13 Nm/cm³	150 to 450 kg/h	Entry production, specialty flake
KTE-65D	62.4 mm	40 / 44 / 48	600 rpm	90 to 160 kW	11 to 13 Nm/cm³	300 to 800 kg/h	Mainstream rigid flake pelletizing
KTE-75D	71 mm	40 / 44 / 48	600 rpm	160 to 250 kW	11 to 13 Nm/cm³	500 to 1,300 kg/h	Film flake and filled recycled compound
KTE-95D	93 mm	40 / 44 / 48	550 rpm	315 to 500 kW	11 to 13 Nm/cm³	1,200 to 2,600 kg/h	High-output single-line plant
KTE-135D	133 mm	40 / 44 / 48	500 rpm	800 to 1,200 kW	11 to 13 Nm/cm³	2,800 to 6,000 kg/h	Large-scale regional recycling plant

Screw assemblies for the KTE Series are laid out with computer-aided design and use a kneading co-type geometry with strong self-cleaning behavior, which matters on recycled feed because material that stagnates in a dead zone degrades and eventually appears as a black speck. The element set realizes the full function chain of transport, plasticizing, shearing, dispersion, homogenization, exhaust, and pressure building, and elements are interchangeable so a configuration can be revised when the flake stream changes.

Kerke SE Series, double-stage system and cutting systems

Not every flake pelletizing duty is best solved by a single twin-screw machine. Kerke builds two additional platforms that cover the cases where a single-stage twin-screw configuration is either more machine than the job needs or not enough machine for the material.

SE Series single screw extruder

The SE Series is a single-screw platform aimed mainly at plastic recycling, with outputs from 30 kg/h to 800 kg/h. Where the duty is straightforward re-pelletizing of a clean, single-polymer flake with no compounding requirement, a single-screw machine offers a simpler mechanical package, lower specific energy for pure conveying and melting duty, and lower wear-part complexity. It is also the standard second stage on a double-stage system.

Model	Screw diameter	L/D	Max screw speed	Main drive power	Indicative output	Typical duty
SE-75	75 mm	30 to 33	120 rpm	22 to 37 kW	80 to 180 kg/h	Small re-pelletizing duty
SE-90	90 mm	30 to 33	110 rpm	37 to 55 kW	150 to 300 kg/h	Clean single-polymer flake
SE-105	105 mm	30 to 33	100 rpm	55 to 90 kW	250 to 450 kg/h	Second stage on double-stage system
SE-120	120 mm	30 to 33	90 rpm	90 to 132 kW	400 to 650 kg/h	Mid-output re-pelletizing
SE-150	150 mm	30 to 33	80 rpm	132 to 200 kW	600 to 800 kg/h	High-output single-polymer flake

KTE-SE double-stage extrusion system

A double-stage system, sometimes described as a mother-baby arrangement, places a twin-screw first stage in series with a single-screw second stage. The twin-screw stage does the mixing, dispersion, and devolatilization at low pressure and low melt temperature. The single-screw stage does nothing but build pressure for the filter and the die. The advantage is that the two functions are decoupled: the twin-screw never has to run at high discharge pressure, which lowers melt temperature and shear history, and the single-screw provides very stable pressure to the pelletizer. It is the right answer for heat-sensitive recycled material, for compounds needing very deep devolatilization, and for lines where high filtration resistance would otherwise force the twin-screw into an unfavorable operating point.

Cutting and pelletizing systems

Kerke supplies water-cooled strand pelletizing, air-cooled strand pelletizing, air-cooled die-face hot cutting, water-ring die-face hot cutting, eccentric water mist hot cutting, and underwater granulation as a matched range. The pelletizer is selected from the melt viscosity, tackiness, moisture sensitivity, and target pellet geometry as set out in the comparison table earlier in this guide, rather than being fixed by the extruder choice.

Auxiliary equipment

The line is completed with high-speed mixers for premixing additive packages, plastic granulators and pulverizers for handling reject material and startup purge, water chillers for the process water circuit, and extruder core parts including screw elements, barrel liners, and shafts held as spares.

Application industries for the finished pellet

The configuration decisions above only make sense in relation to what the pellet is sold into. Different downstream processes tolerate very different levels of variation, and the specification of the line should be set by the most demanding market the plant intends to serve.

Injection molded products

Injection molding is the largest single outlet for recycled polyolefin and styrenic pellet. It is sensitive to melt flow rate consistency because a shift in melt flow rate changes fill behavior and can move a part out of dimensional tolerance. It is also sensitive to moisture, which produces splay on the part surface, and to black specks on visible parts. Crates, pallets, household items, buckets, garden furniture, tool housings, and automotive under-hood components are typical outputs.

Pipe extrusion

Recycled pellet used in non-pressure pipe, cable protection pipe, and drainage systems demands consistent melt strength and low contamination, because a contaminant in a pipe wall is a stress concentration point. Ash content and filler consistency matter more here than color.

Sheet and board extrusion

Sheet and thick board lines are comparatively tolerant of color variation when the product is unseen or laminated, but they are intolerant of gels and unmelted particles, which appear as surface defects. Filtration fineness is the controlling specification for this outlet.

Packaging

Non-food packaging, industrial packaging, transport packaging, and secondary packaging absorb large volumes of recycled polyolefin. The controlling parameters are odor, which comes back to devolatilization, and color consistency, which comes back to blending and to the color masterbatch dosing accuracy.

Automotive interior components

Automotive interior applications impose odor and emission limits alongside mechanical specifications. This is the market where a twin-stage vacuum system pays for itself, because residual volatile content is directly measured. Wheel arch liners, trunk trim, air ducts, and under-carpet components are common recycled-content parts.

Building and construction materials

Wood-plastic composite decking, geotextile components, insulation carriers, and construction site protection products accept a wider quality window and higher filler loading. This is the natural first market for a plant that is still tuning its process, and a wood-plastic composite formulation is one of the standard duties for a filled twin-screw configuration with side feeding.

Beyond these outlets, the same KTE platform is used across the broader compounding field that Kerke serves: color masterbatch, filler masterbatch, black masterbatch, additive masterbatch, textile masterbatch, engineering plastic compounds, biodegradable plastic, cable compounding, PVC compounding, thermoplastic elastomers, wood-plastic composites, and R-PET flake recycling. A plant that starts with a single recycled flake grade frequently expands into masterbatch or compounding on the same machine platform, which is a good reason to specify feeding and screw flexibility at purchase rather than later.

Requirement to line configuration recommendation

The table below translates the three variables that a buyer actually knows at the enquiry stage — target output, flake type, and contamination grade — into a concrete starting configuration. It is a starting point for discussion, not a substitute for a trial run on the real material.

Target output	Flake type	Contamination grade	Recommended main machine	Filtration solution	Pelletizing method	Vacuum configuration
100 to 250 kg/h	Clean post-industrial PP or PE regrind	Low	KTE-52D, L/D 40	Hydraulic single piston, 80 mesh	Water-bath strand	Single stage water ring
200 to 450 kg/h	Washed HDPE rigid flake	Low to medium	KTE-52D or KTE-65D, L/D 40	Dual piston four position, 60 to 80 mesh	Water-bath strand	Single stage water ring
300 to 800 kg/h	Washed PP or PE film flake, printed	Medium to high	KTE-65D, L/D 44, crammer feed	Continuous laser filter, 130 micrometers	Water ring die-face	Twin stage with condenser
500 to 1,300 kg/h	Mixed post-consumer polyolefin flake	High	KTE-75D, L/D 44, bimetallic barrels	Continuous laser filter plus polishing screen	Water ring or water-bath strand	Twin stage, roots assisted
500 to 1,200 kg/h	Filled or glass-fiber recycled compound	Medium	KTE-75D, L/D 48, side feeder at 18 L/D	Dual piston four position, 40 to 60 mesh	Water-bath strand	Single to twin stage
600 to 1,500 kg/h	PET bottle flake	Low to medium	KTE-75D, L/D 44, bimetallic, chain extender dosing	100 to 120 mesh or laser filter	Underwater with crystallizer	Twin stage, roots assisted
400 to 900 kg/h	ABS or PS flake for visible parts	Low	KTE-65D, L/D 40, melt pump	Dual piston four position, 100 to 120 mesh	Water-bath strand	Single to twin stage
1,200 to 2,600 kg/h	Washed polyolefin flake, single stream	Medium	KTE-95D, L/D 44	Continuous laser filter	Underwater or water ring	Twin stage, roots assisted
Above 2,800 kg/h	Regional-scale mixed polyolefin flake	Medium to high	KTE-135D, L/D 44 to 48	Continuous laser filter, twin unit	Underwater	Twin stage, roots assisted
Heat-sensitive grades, any output	Degraded or multi-history polymer	Any	KTE-SE double-stage system	Matched to contamination grade	Water-bath strand or underwater	Twin stage on the first stage
30 to 800 kg/h	Clean single-polymer re-pelletizing, no compounding	Low	SE Series single screw	Hydraulic screen changer, 60 to 100 mesh	Water-bath strand	Atmospheric or single stage

Workshop layout and utility demand

A flake compounding line is a long machine. The layout constraints are usually discovered late, when the building is already chosen, and they are expensive to fix at that point. The figures below give planning envelopes for three representative line sizes.

Length is driven by the extruder plus the strand bath plus the drying and classification train. Height is driven by the feeder platform above the extruder and by the blending silo. A gravimetric feeding platform sits typically 2.5 to 3.5 meters above the extruder axis, and clearance above it is needed for bulk bag handling, which is what pushes the clear height requirement to 6 meters and above on larger lines.

Planning item	Line at 300 to 500 kg/h	Line at 800 to 1,300 kg/h	Line at 2,000 to 3,000 kg/h
Line footprint including access	140 to 200 m²	260 to 380 m²	450 to 700 m²
Total workshop area with storage	500 to 800 m²	1,000 to 1,600 m²	2,000 to 3,200 m²
Clear height required	5.5 to 6.5 m	6.5 to 8 m	8 to 10 m
Overhead crane	3 to 5 t optional	5 to 10 t recommended	10 to 16 t required
Installed electrical capacity	180 to 300 kW	400 to 650 kW	900 to 1,600 kW
Cooling water flow, soft water circuit	10 to 18 m³/h	25 to 45 m³/h	55 to 100 m³/h
Process water for pelletizing	8 to 15 m³/h	20 to 35 m³/h	45 to 80 m³/h
Compressed air at 0.6 to 0.8 MPa	0.6 to 1.2 m³/min	1.5 to 2.5 m³/min	3 to 5 m³/min
Dedusting extraction air volume	3,000 to 6,000 m³/h	8,000 to 14,000 m³/h	18,000 to 30,000 m³/h
Floor loading at the extruder base	Reinforced foundation, leveled	Reinforced foundation with anchor bolts	Engineered foundation, vibration isolated
Operators per shift	2 to 3	3 to 4	4 to 6

Two layout details save money later. First, leave a clear withdrawal length equal to the full barrel length on the operator side of the extruder, so screws can be pulled without dismantling adjacent equipment. Second, place the process water tank and chiller so the pipe run to the pelletizer is short; long process water runs increase pump load and make water temperature control sluggish, which shows up as inconsistent pellet cooling.

Acceptance testing and commissioning

A line specification is only enforceable if the acceptance criteria are written into it. The sequence below is the practice that protects both sides, and it should be agreed in writing before manufacturing starts.

Before shipment

The machine is assembled completely at the factory and run without material to verify drive function, gearbox oil temperature and pressure, barrel heating ramp and zone stability, thermocouple and pressure transducer response, vacuum pump ultimate pressure, hydraulic function of the screen changer, cutter drive, and every safety interlock. A material trial run follows on the customer’s own flake sample where a sample has been supplied, which is by far the most useful test available because it produces real pellet from real feedstock before any money moves.

After installation

Commissioning covers alignment, utility connection verification, a repeat of the no-load functional test, a heating soak, a startup with a purge material, and then a graduated loaded run: 30 percent of rated throughput, then 60 percent, then 100 percent, with samples taken at each level. Throughput is verified by weighing bagged output over a timed period, not by reading the feeder setpoint. Specific energy is measured at the incoming panel across the same period.

Acceptance item	Method	Typical criterion
Throughput	Timed weighing of bagged output at steady state	Meets or exceeds the contract figure on the agreed material
Specific energy	Panel energy meter over the same timed period	0.20 to 0.35 kWh/kg for polyolefin flake, agreed per material
Melt flow rate consistency	Sample every 30 minutes over an 8-hour run	Variation within the agreed band for the grade
Ash content	Laboratory ashing of pellet samples	Matches the declared filler content within tolerance
Pellet moisture	Moisture analyzer on the finished pellet	Below 0.10% for polyolefin, tighter for hygroscopic grades
Black specks	Visual count on a defined pellet mass or on a pressed plaque	Below the agreed count per unit weight
Color deviation	Spectrophotometer against an agreed reference	Delta E within the agreed limit for the grade
Pellet size distribution	Sieve analysis	Fines and oversize below the agreed percentage
Continuous running	Extended run at rated output	Stable operation without unplanned stoppage over the agreed period
Noise level	Measurement at the operator position	Within the agreed workplace limit
Safety interlocks	Function test of every guard, trip, and emergency stop	All functions verified and recorded

One caution about throughput acceptance: the contract figure must state the material, its bulk density, its moisture, and the pellet specification it is measured against. A rated output measured on clean pellet feed and a rated output measured on 0.12 t/m³ printed film flake are not the same number, and disputes at acceptance almost always trace back to a throughput figure that was never tied to a material.

Service and support from Kerke

A pelletizing line is a long-lived asset that changes duty during its life, so the support package matters as much as the machine specification. Kerke, as a Wanplas factory, applies the group service commitments alongside its own technical support.

Material trial before purchase

Customers are invited to send a representative flake sample for a trial run on a laboratory or pilot machine. The trial produces real pellet, a measured throughput, a measured specific energy, and a screw configuration recommendation. On recycled feedstock this is the only reliable way to size the machine, because published throughput figures for pellet feed do not transfer to flake.

Testing before shipment and installation support

Every machine is assembled and function-tested at the factory before dispatch, and where the customer supplies material the loaded trial run is documented with the results. On site, engineers handle installation and commissioning, work through the graduated loaded run described above, and hand over with the operating parameters recorded in the control system as a stored recipe.

Spare parts policy and wear-part planning

The Wanplas group policy provides USD 500 free parts every year and free replacement of parts damaged within the warranty period. Beyond that entitlement, the practical approach is to plan wear parts by running hours and processed tonnage rather than by calendar. The table below gives indicative service lives that should be adjusted for the actual abrasiveness of the feedstock.

Wear part	Indicative life on clean flake	Indicative life on abrasive or filled flake	Planning note
Kneading block elements	10,000 to 18,000 running hours	4,000 to 9,000 running hours	Hold a full melting-group set as spare
Conveying elements	14,000 to 22,000 running hours	7,000 to 12,000 running hours	Wear concentrates near the melting group
Barrel liners, nitrided	12,000 to 20,000 running hours	4,000 to 8,000 running hours	Specify bimetallic if filler is present
Barrel liners, bimetallic	25,000 to 40,000 running hours	12,000 to 20,000 running hours	The main reason to pay for bimetallic upfront
Screen changer seals	6,000 to 12,000 running hours	4,000 to 8,000 running hours	Replace during a planned barrel service
Pelletizer knives	800 to 2,000 running hours	300 to 900 running hours	Regrind schedule tied to pellet cut quality
Die plate	15,000 to 25,000 running hours	6,000 to 12,000 running hours	Inspect hole geometry at every knife change
Vacuum pump service kit	Annual	Every 6 to 9 months	Condenser condition drives this interval
Gearbox oil and filter	Every 4,000 to 6,000 hours	Every 3,000 to 5,000 hours	Oil analysis is cheaper than a gearbox rebuild

Training, remote support and factory visits

Operator and maintenance training covers startup and shutdown sequence, recipe handling, screw configuration principles, screen change procedure, wear inspection, and troubleshooting of the common defects. A remote service interface allows Kerke engineers to review live PLC data and assist with diagnosis without traveling, which materially shortens response time for exported lines. The Wanplas open factory policy applies: customers are welcome to visit the plant, inspect machines under manufacture, and watch a trial run in person.

Frequently Asked Questions

Conclusion

The complete machine configuration for a waste plastic flake twin screw compound pellet plant is not a catalog choice, it is a chain of linked decisions that all trace back to two facts about the incoming material: its bulk density and its contamination load. Bulk density decides the feeding package and the volumetric sizing of the extruder. Contamination load decides the filtration technology, the wear material of the barrels and screw elements, and how much vacuum capacity the line needs. Every other decision, from L/D ratio to pelletizing method to dryer sizing, follows from those two starting points and from the pellet specification the plant intends to sell against.

The configuration principles worth carrying away are straightforward. Protect the barrel with two independent layers of metal removal. Feed gravimetrically, never volumetrically, on a material whose bulk density moves. Buy specific torque rather than raw motor power when the compound is filled. Configure the screw for the minimum shear that achieves the required dispersion, because recycled polymer has no spare thermal stability to give away. Match filtration technology to the actual contamination load rather than to an aspiration. Choose the pelletizer from melt behavior, not from throughput alone. Size the vacuum system by pumping speed at working pressure, not by the ultimate figure on the nameplate. And write measurable acceptance criteria, tied to a named material, into the contract before manufacturing begins.

Kerke, a Wanplas factory with 12+ years of dedicated compounding extrusion experience, a 19,997+ square meter plant, more than 2,000 machines running in over 70 countries, and a full range spanning the KTE Series twin-screw platform from KTE-16B to KTE-135D, the SE Series single-screw platform, the KTE-SE double-stage system, six pelletizing configurations, and the complete feeding and auxiliary package, builds these lines as matched systems rather than as assembled parts. The group commitments apply throughout: testing before shipment, on-site installation and commissioning, operator and maintenance training, remote technical support, USD 500 free parts every year, free replacement of parts damaged within warranty, and an open factory policy.

If a flake pelletizing line is on the plan, the most productive next step is to send a representative sample of the actual flake for a trial run, together with the target output and the pellet specification the material has to meet. A trial produces a measured throughput, a measured specific energy, a screw configuration recommendation, and real pellet that can be tested downstream, which together replace guesswork with evidence. Customers are equally welcome to share a target capacity and material description for a tailored whole-line configuration proposal, or to visit the plant in person, inspect machines under manufacture, and watch a trial run before any commitment is made.