How to launch biodegradable PLA compound factory with special twin screw extruder

Why a PLA compound factory is not a conventional compounding startup

Launching a biodegradable PLA compound factory is a chemistry project disguised as a machinery project. The equipment list looks familiar to anyone who has built a masterbatch or engineering plastic compounding plant, but the material rules are different: polylactic acid degrades by hydrolysis during melt processing, tolerates a processing window barely 30 degrees Celsius wide, and delivers a melt so weak that conventional strand handling becomes a daily production problem. A plant that copies a polyolefin compounding layout and hopes the recipe will adapt will produce pellets, but it will not produce a certifiable, reproducible, sellable compound.

This guide sets out a complete project methodology for launching a PLA compound factory with a special twin screw extruder, organized into seven sequential phases with explicit deliverables and go criteria at each gate. It covers how to define your product portfolio before you buy anything, how to develop and validate formulations at pilot scale, how to plan the compostability certification route so it runs in parallel rather than after the fact, how to specify a twin-screw extruder configuration that protects molecular weight instead of destroying it, how to size the building and utilities, and how to run commissioning and ramp-up so that the plant reaches stable quality rather than merely stable output.

Kerke, a Wanplas factory, has focused exclusively on parallel co-rotating twin-screw compounding extruders for more than 12 years. From a 19,997 square meter plant, Kerke has delivered more than 2,000 machines now running in over 70 countries, supported by a team of more than 100 people covering research and development, screw design, manufacturing and commissioning. Biodegradable plastic compounding is one of the core application industries the KTE series was developed for, alongside masterbatch, engineering plastic, cable compounding and thermoplastic elastomer work. That experience is the basis for the specific configuration recommendations in this article: they come from machines that are already running PLA, PLA/PBAT and starch-based blends in production plants, not from a generic catalog.

One framing point before the roadmap. The biodegradable compound business sits at a particular place in the plastics value chain. You are buying a specialty resin whose cost base is driven by fermentation and polycondensation economics rather than by olefin cracking, you are selling into converter markets whose demand is created largely by regulation on single-use plastics, and your defensible margin comes from formulation know-how rather than from raw tonnage. Every phase below is designed around that reality. The plant that wins is the plant that can develop a new certified grade quickly and reproduce it batch after batch, not the plant with the largest nameplate capacity.

The seven-phase launch roadmap with deliverables and go criteria

A biodegradable compound plant should be launched in seven gated phases, each with a defined deliverable and a pass criterion that must be met before capital is committed to the next phase. The most common project failure is not technical, it is sequencing: buying equipment in phase four before the formulation of phase two is frozen, or starting certification in phase seven when it should have started in phase three. The table below is the project skeleton that the rest of this article fills in.

Phase	Core activity	Deliverable	Go criterion before next phase	Typical duration
1. Market and product positioning	Define target end products, converter processes and regional regulation drivers	Product portfolio document: 3 to 6 grade families with target property windows	Each grade family has a named converter process and a written property specification	1 to 2 months
2. Formulation development and pilot	Screen base polymer ratios, modifiers, fillers and stabilizers on a laboratory or pilot twin-screw extruder	Frozen formulation sheets with mixing sequence, temperature profile and screw configuration	Three consecutive pilot batches within the property window; molecular weight retention verified	3 to 6 months
3. Certification route planning	Select certification scheme per market, prepare sample geometries, book test slots	Certification plan with standard list, sample matrix and submission calendar	Samples submitted for biodegradation and disintegration testing on the frozen recipe	1 month to plan, 9 to 14 months to complete
4. Line configuration and equipment selection	Translate the frozen recipes into extruder size, L/D, screw design, venting, feeding and pelletizing	Signed technical specification and layout drawing for the compounding line	Trial run on the supplier machine with your own raw material confirms output and quality	1 to 2 months, plus manufacturing lead time
5. Building and utilities	Workshop area, clear height, electrical capacity, cooling water, compressed air, humidity control, warehousing	Utility load schedule and civil drawings with foundation and service routing	Power connection and chilled water capacity confirmed against installed load	2 to 4 months in parallel
6. Installation, commissioning and trial production	Mechanical and electrical installation, no-load and loaded run, recipe transfer from pilot to production scale	Commissioning report with achieved output, specific energy and first-article quality data	Production-scale batch matches the pilot property window within agreed tolerance	3 to 6 weeks
7. Ramp-up and quality system	Yield improvement, operator training, statistical control of key properties, certification file completion	Documented quality system with control charts, plus first commercial deliveries	Batch-to-batch melt flow rate variation within plus or minus 10 percent over 20 batches	3 to 6 months

Two structural rules govern this sequence. First, phase three runs in parallel with phases four and five rather than after them, because compostability testing is the longest single lead time in the whole project and it cannot start until a recipe is frozen. Second, phase two must be executed on a machine whose mixing mechanism is the same as the production machine. A pilot trial done on a batch mixer or a single-screw laboratory unit will not transfer, because the shear history, residence time distribution and devolatilization behavior are fundamentally different from a co-rotating twin-screw extruder.

Each gate should have a written pass or fail decision with a named owner. In practice the most valuable gate is the one at the end of phase two, because that is the last point at which a project can change direction cheaply. Once the recipe is frozen and samples are in certification testing, changing the base polymer ratio or the filler system means restarting a 9 to 14 month clock.

Phase 1: Market and product positioning

Product positioning determines equipment specification far more than annual tonnage does. Before any machine discussion, define which converter processes your compound must serve, because film blowing, injection molding, thermoforming, sheet extrusion and fiber spinning each demand a different melt strength, crystallization rate and molecular weight target from the same base polymer.

Start by listing the end products in your target market. A regional plant serving a single-use plastics restriction typically starts with three or four grade families: a blown film grade for shopping bags and bin liners, an injection molding grade for cutlery and rigid closures, an extrusion grade for drinking straws, and possibly a sheet grade for thermoformed trays. Each of these is a different compound even though all of them are described commercially as biodegradable PLA compound.

Mapping converter process to compound requirement

The film grade is the most demanding. Blown film requires melt strength that neat PLA simply does not have, so the film family is normally a PLA/PBAT blend with a chain extender to raise melt viscosity and elasticity, and it usually carries the lowest permissible moisture specification because any hydrolytic chain scission shows up immediately as bubble instability. The injection grade goes the opposite direction: it needs fast crystallization for short cycle time and adequate heat resistance, so it carries a nucleating agent and often mineral filler, and it tolerates a slightly higher melt flow rate. The straw and rigid extrusion grade sits between the two, needing dimensional stability and enough melt strength to hold a profile without excessive drawdown. The thermoforming sheet grade needs a wide sag window and controlled crystallization so that the sheet forms cleanly.

Write these as numerical property windows, not as adjectives. A usable specification for a film grade states melt flow rate at 190 degrees Celsius and 2.16 kg within a defined band, elongation at break above a stated percentage, tensile strength above a stated value, and a maximum permitted yellowness index. This document becomes the acceptance criterion for phase two and the basis for the quality control plan in phase seven.

Volume planning and grade concentration

New entrants consistently overestimate the number of grades they can run profitably and underestimate the changeover cost. Every recipe change on a compounding line costs purge material, cleaning time and a period of off-specification production. A plant running four grade families on one line will lose a meaningful share of its available hours to changeover unless campaigns are planned in blocks. The practical rule is to concentrate 70 to 80 percent of planned tonnage into one or two grade families, keep the remainder as development and specialty volume, and only add a second production line when the dominant family alone justifies it.

Positioning also determines whether you need a dedicated development machine. If your commercial proposition is formulation service rather than commodity supply, a small laboratory extruder is not optional. It lets you screen recipes without stopping production, and it is the only economical way to answer a converter who asks for a modified grade in two weeks.

PLA material behavior and the processing constraints it imposes

Every difficult aspect of a PLA compounding line traces back to four material facts: the polymer hydrolyzes in the melt, it has a narrow thermal processing window, it crystallizes slowly, and its melt strength is low. Understanding these four facts converts a long list of apparently unrelated equipment requirements into a coherent design logic.

Property	Typical value for commercial PLA grades	Consequence for the compounding line
Melting point	150 to 175 degrees Celsius depending on stereochemistry	Melt zone can run cooler than polyolefin practice; barrel profile must not overshoot
Glass transition temperature	55 to 65 degrees Celsius	Amorphous pellets soften and agglomerate in a hot dryer hopper; pre-crystallization required
Heat deflection temperature, unmodified	Approximately 55 to 60 degrees Celsius	Nucleating agent and filler needed for any application above ambient service temperature
Melt processing window	180 to 210 degrees Celsius	Barrel and melt temperature control must be tight; shear heating becomes a design constraint
Melt flow rate, common grades	Roughly 3 to 8 g/10min at 190 degrees Celsius and 2.16 kg	Defines the incoming material band and the batch acceptance test
Hydrolysis sensitivity	Chain scission accelerates sharply above 250 ppm moisture in the melt	Drying and venting are not auxiliary items, they are core process equipment
Crystallization rate	Slow without nucleation; half-time can be minutes	Pellets stay tacky after cutting; downstream crystallizer often required
Melt strength	Low, drops further with temperature	Strand pelletizing is fragile; die-face or underwater cutting preferred
Thermal and oxidative stability	Yellowing and molecular weight loss under prolonged heat	Short residence time, no dead zones, oxygen exclusion at the feed throat

Hydrolysis is the dominant failure mechanism

Water in molten PLA cleaves ester bonds. The reaction is fast at melt temperature and it is not reversible. A batch fed at 500 ppm moisture rather than 100 ppm can lose a large fraction of its number-average molecular weight in a single pass, and the loss shows up as a melt flow rate that has drifted far above specification, brittle strands, poor mechanical properties and a compound that fails its own certification sample geometry. Because the pellet still looks normal, this failure is often diagnosed as a formulation problem when it is actually a drying problem.

The drying specification is therefore non-negotiable. Use a desiccant dryer capable of holding minus 40 degrees Celsius dew point at the process air inlet, dry at 80 to 100 degrees Celsius for 4 to 6 hours, and verify residual moisture below 250 ppm as a working maximum, tightening to below 100 ppm for film and fiber grades. Two practical details are frequently missed. First, amorphous PLA pellets will soften and bridge in the hopper if heated directly to 90 degrees Celsius, because that is well above the glass transition temperature; run a pre-crystallization stage with agitation at a lower temperature, or buy pre-crystallized feedstock. Second, dried PLA reabsorbs moisture quickly on exposure to workshop air, so the transfer path from dryer to feed throat must be closed and the dryer must be sized for the line rate with margin, not for the average.

Drying parameter	Standard compound grade	Film and fiber grade	Verification method
Dew point at dryer outlet	Minus 40 degrees Celsius	Minus 40 degrees Celsius, monitored continuously	Inline dew point sensor with alarm
Drying temperature	80 to 90 degrees Celsius	90 to 100 degrees Celsius	Hopper thermocouples, top and bottom
Drying time	4 hours minimum	5 to 6 hours	Hopper volume divided by line rate
Pre-crystallization for amorphous pellets	Agitated, 60 to 75 degrees Celsius	Agitated, 60 to 75 degrees Celsius	Visual check for agglomeration
Target residual moisture	Below 250 ppm	Below 100 ppm	Coulometric or loss-on-heating moisture analyzer, every batch
Transfer to feed throat	Closed conveying, insulated	Closed conveying, dry air purged	Moisture check at the throat, not only at the dryer
Additive and filler drying	Mineral fillers dried separately	Mineral fillers dried separately	Filler moisture below 0.05 percent before side feeding

The narrow window and the shear problem

PLA is processed between roughly 180 and 210 degrees Celsius. Below that band the melt is too viscous and unmixed; above it, thermal degradation and yellowing accelerate quickly. What makes this difficult on a twin-screw extruder is that melt temperature is not set by the barrel heaters alone. At production screw speeds, most of the energy entering the polymer is mechanical, delivered as shear through the screw elements. A screw configuration designed for dispersing carbon black into polyethylene will push a PLA melt straight through the top of its window even with the barrel heaters set low.

This is the single most important reason a PLA line needs a special twin screw extruder configuration rather than a standard one. The design objective inverts: instead of maximizing dispersive energy, you are managing a shear budget. You need enough mixing to distribute a chain extender at under 1 phr uniformly through the melt, and enough dispersive capability to break down filler agglomerates, while keeping peak melt temperature below the degradation threshold and total residence time short. Section seven of this guide translates that objective into concrete element choices.

Slow crystallization and low melt strength

Unmodified PLA crystallizes slowly enough that a pellet leaving a die-face cutter is essentially amorphous. Amorphous pellets above their glass transition temperature are tacky, which is why PLA pellets block in warehouse bags during hot weather and why the pelletizing water temperature must be controlled. Nucleating agents such as talc at 1 to 3 percent, or organic nucleators at lower loadings, raise the crystallization rate substantially and are used both to improve the compound and to make the pellet handleable.

Low melt strength has a direct equipment consequence. Strand pelletizing depends on a melt strand surviving the air gap and water bath under tension. PLA strands break, and PLA/PBAT strands both break and stick. Die-face cutting methods that never form a free strand are therefore preferred, and the pelletizing section of this article compares the practical options.

Phase 2: Formulation development and pilot trials

Formulation development is where a biodegradable compound business creates its value, and it is the phase that most new plants compress dangerously. Budget three to six months and a dedicated laboratory or pilot extruder. The deliverable is not a recipe on paper, it is a frozen formulation sheet that specifies ingredient list, addition points, mixing sequence, barrel temperature profile, screw speed, vacuum level and screw configuration, validated by three consecutive batches that land inside the property window.

The modification toolbox

Neat PLA is too brittle, too slow-crystallizing and too heat-sensitive for most end products. Practically every commercial biodegradable compound is a modified system. The table below summarizes the modification families, typical loading ranges and the process implications each one creates on the extruder. Loadings are expressed either as weight percentage of the total compound or as parts per hundred resin, following normal compounding practice.

Modification system	Typical loading	Purpose	Process implication on the twin-screw line
PLA/PBAT blend	30/70 to 70/30 by weight	Toughening, elongation at break, film processability	Two-component loss-in-weight feeding; morphology depends on shear and viscosity ratio; melt temperature must suit both polymers
Multifunctional epoxy chain extender	0.3 to 1.0 phr	Rebuilds molecular weight, raises melt strength and melt elasticity	Needs reaction length after addition, ideally 12 to 16 L/D of distributive mixing; overdosing causes gels and torque spikes
Mineral nucleating agent, talc	1 to 3 percent	Faster crystallization, better heat resistance, less pellet tack	Side feeding preferred; dust control needed; abrasive over long campaigns
Organic nucleating agent	0.3 to 1.5 percent	Crystallization control with less optical impact	Sensitive to thermal history; add downstream of the main melt zone
Citrate ester plasticizer	5 to 15 percent	Flexibility, lower processing temperature, softer film hand	Liquid injection pump with heated line; risk of migration and screw slip if overdosed
Calcium carbonate filler	10 to 40 percent	Stiffness, cost dilution, controlled degradation behavior	Twin-screw side feeder after the melt zone; vent required to remove entrained air and moisture
Starch-based blend component	10 to 40 percent	Bio-content increase, compostability, cost structure	High volatile load; requires strong devolatilization; heat sensitive, tight residence time control
Carbodiimide hydrolysis stabilizer	0.5 to 1.5 phr	Protects the compound during processing and in service	Add with the main feed or via side feeder; verify dispersion, poor dispersion gives local weak zones
Maleic anhydride grafted compatibilizer	1 to 5 percent	Interface between PLA and filler, starch or second polymer phase	Reactive; benefits from distributive mixing rather than aggressive kneading
Slip and anti-blocking additives	0.2 to 1.5 percent	Film opening, reduced friction in converting	Usually added as a masterbatch through a small loss-in-weight feeder

Two combination effects deserve attention. First, chain extender and hydrolysis stabilizer are complementary but not interchangeable. The chain extender repairs chains that have already been cut, raising melt viscosity back toward target; the carbodiimide suppresses the hydrolysis reaction itself. A film grade normally uses both. Second, plasticizer and nucleating agent pull in opposite directions on heat resistance, so a compound that is both soft and heat resistant requires careful balancing and usually mineral filler as well.

Structuring the trial program

Run the trial program as a structured screening rather than a sequence of one-off experiments. A workable structure is three stages. Stage one screens base polymer ratio at fixed additive levels to locate the mechanical property region. Stage two optimizes the additive package at the chosen ratio, varying chain extender and nucleating agent while holding everything else constant. Stage three varies process parameters at the chosen formulation: screw speed, barrel profile, vacuum level and feed rate, to map the process window and find out how tolerant the recipe is.

Stage three is the one that new entrants skip, and it is the most valuable for production. A recipe that only works at exactly 250 rpm and exactly 190 degrees Celsius will fail in production, because a real plant experiences feed rate fluctuation, ambient temperature swings and raw material lot variation. What you want from phase two is not the single best data point, it is the widest robust window.

Verifying that molecular weight survives

Include molecular weight tracking in every trial series. Gel permeation chromatography before and after compounding tells you how much chain scission your process caused, and it separates a formulation problem from a process problem in a way that mechanical testing cannot. A well-run PLA compounding pass should lose only a modest fraction of weight-average molecular weight; a large drop points to moisture, excessive melt temperature, excessive residence time or all three. If your laboratory cannot run gel permeation chromatography in-house, use melt flow rate drift as a proxy and send periodic samples out for confirmation.

The pilot machine matters here. A laboratory twin-screw extruder with the same co-rotating geometry, the same element library and the same self-cleaning kneading profile as the production machine gives results that scale. Kerke builds laboratory twin-screw extruders specifically for formula trials, tests and research work, using the same computer-aided screw assembly design philosophy as the production KTE series, which is what makes recipe transfer from pilot to plant predictable rather than experimental.

Phase 3: Certification route planning

Compostability certification is the longest lead time in the entire project and it must be started as soon as a recipe is frozen, in parallel with equipment manufacturing rather than after commissioning. A plant that waits until it has production material before beginning certification adds close to a year to its time-to-revenue, and risks discovering that the frozen formulation needs adjustment after the line is already built.

Which schemes apply

The certification landscape for biodegradable compounds has three layers: compostability, food contact, and management systems. Compostability standards define what may be labeled compostable and are the commercial gatekeeper for bags, cutlery and packaging in most regulated markets. The European standard EN 13432 and the North American standard ASTM D6400 are the two dominant schemes, with ISO 17088 providing an internationally harmonized specification. Food contact compliance is a separate track, governed in Europe by EU 10/2011 and in the United States by the relevant FDA requirements for food contact substances, with GB series national standards applying in China. ISO 9001 covers the quality management system that certification bodies and industrial customers will audit.

Standard or scheme	Scope	What is tested	Sample requirement	Indicative timeline
EN 13432	Compostability of packaging in industrial composting	Chemical characterization, biodegradation, disintegration, ecotoxicity on plant growth	Finished article in final thickness plus compound pellets; several kilograms	9 to 14 months overall
ASTM D6400	Compostable plastics specification for North America	Biodegradation to carbon dioxide, disintegration, absence of regulated metals, plant toxicity	Final article geometry, multiple thicknesses if the grade covers a range	9 to 14 months overall
ISO 17088	International specification for compostable plastics	Harmonized criteria closely aligned with the two above	Same sample basis	Runs in parallel with the above
Biodegradation test	Component of the schemes above	Conversion to carbon dioxide under controlled composting conditions	Ground compound sample	Up to 180 days of test time alone
Disintegration test	Component of the schemes above	Fragmentation of the final article at pilot composting scale	Final article at maximum intended thickness	Approximately 12 weeks
EU 10/2011	Plastic materials in contact with food, European market	Overall and specific migration, declaration of compliance	Final article or representative plaque	2 to 4 months
FDA food contact requirements	Food contact status, United States market	Composition review, extractables where applicable	Formulation disclosure plus test articles	2 to 6 months depending on route
GB series national standards	Chinese market requirements for degradable products and food contact	Performance, labeling and food contact criteria per applicable part	Final article samples	2 to 5 months
ISO 9001	Quality management system	Documented processes, traceability, corrective action	Plant audit, not product samples	3 to 6 months after system is operating

Three practical points about certification. First, compostability certificates are granted against a specific formulation and, for disintegration, a specific article geometry and thickness. If you change the filler loading or move from 25 micron film to 40 micron film, you may need supplementary testing. Design the sample matrix at the start to cover the thickness range you intend to sell. Second, the certificate follows the material through the chain, so your converter customers will ask for the certificate number and the scope wording; write the scope to cover the grades you actually plan to sell. Third, heavy metal and regulated substance limits apply to the whole compound, which means every pigment, filler and additive in the recipe must have documentation. Collect those declarations during phase two, not during the audit.

Building the certification file alongside the plant

The documentation package that supports certification overlaps almost completely with the quality system you need anyway: raw material specifications and supplier declarations, batch records with process parameters, test records for each production lot, traceability from finished pellet back to raw material lot, and a change control procedure. Set this up in phase three on paper, so that phase seven only has to populate it with real data rather than invent it.

Phase 4: Configuring the special twin screw extruder

A twin screw extruder for biodegradable compounding differs from a standard compounding machine in five specific ways: it is longer, it is deliberately not over-torqued, its screw configuration favors distributive over dispersive mixing, it has more devolatilization capacity, and it has more precise feeding. Each of these follows directly from the material behavior described earlier.

Machine architecture: co-rotating parallel twin-screw

The base architecture is a co-rotating parallel twin-screw extruder with a segmented barrel and a modular screw built from splined shafts. This architecture is chosen because the intermeshing co-rotating geometry is self-wiping, which eliminates the stagnant zones where heat-sensitive polymer would sit and degrade. Self-cleaning also means a recipe change purges out quickly, which matters when a plant runs several biodegradable grade families on one line. Kerke’s KTE screw elements use a kneading co-type geometry with an outer to inner diameter ratio of 1.55, designed by computer-aided assembly so that transport, plasticizing, shearing, dispersion, homogenization, exhaust and pressure building are each assigned to defined zones rather than left to chance.

Length: why 40 to 52 L/D

General compounding runs comfortably at 36 to 40 L/D. Biodegradable systems benefit from 40 to 52 L/D, and the extra length is not for more mixing. It buys three things. It provides physical separation between the chain extender addition point and the die, giving the reactive additive 12 to 16 L/D of residence to react and distribute before pressure builds. It allows a second vacuum vent to be placed far enough downstream of the first to be useful. And it allows a longer low-shear melt conveying section before the die, which lets melt temperature settle downward by a few degrees before the melt meets the pelletizer, a small effect that shows up directly in yellowness index.

There is a limit. Beyond roughly 52 L/D, additional length adds residence time and thermal exposure without adding function, which is counterproductive for a hydrolysis-sensitive polymer. Length is a tool, not a virtue.

Torque grade: choosing moderate on purpose

High torque density machines are marketed as universally better, and for glass-fiber reinforced engineering compounds they are. For PLA systems they are a trap. A very high torque grade allows the operator to push screw speed and specific energy far beyond what the polymer tolerates, and the resulting damage is invisible until the laboratory reports the melt flow rate. Specify a moderate torque grade in the range typically used for standard compounding duty, size the machine so that the target output is achieved at a comfortable drive load, and treat spare torque as a safety margin rather than as an operating range. In practice a PLA compounding line runs at 200 to 400 rpm and a specific mechanical energy input in the region of 0.12 to 0.22 kWh/kg, well below what filled polyolefin compounding demands.

Screw configuration: the shear budget

The screw is where a PLA line is won or lost. The configuration principles are as follows.

• Feed and melting zone. Use deep-flighted conveying elements for stable intake, then a short melting section with 30 degree forward kneading blocks. Thirty degree blocks generate distributive mixing with modest energy input, which is exactly what is needed to melt PLA without overheating it. Avoid stacking long kneading sections here.
• Mixing zone. Combine 60 degree kneading blocks for dispersion of fillers and pigments with distributive elements such as gear mixers or slotted elements for additive distribution. Use 90 degree neutral blocks sparingly and only where a melt seal is genuinely required, because they are efficient at converting drive power into heat.
• Reverse elements. Reverse-flighted elements build a strong melt seal and high local shear. On a PLA line they are used with restraint, typically only one short reverse element ahead of a vacuum vent to ensure the seal, and never as a general mixing device. Overuse of reverse elements is the most common cause of yellowing on lines converted from polyolefin duty.
• Side feed entry. Place the filler side feeder after the polymer is fully melted, so that mineral filler is wetted into an existing melt rather than being dragged through the melting zone as an abrasive dry powder. Follow the side feeder with distributive mixing and then a vent.
• Reaction section. After chain extender addition, use distributive elements over a long stretch. The goal is uniform contact, not high shear; the reaction needs residence and mixing, not energy.
• Metering and pressure build. End with a conveying section long enough to generate stable die pressure at low shear. Do not build pressure with kneading blocks close to the die.

Temperature control works alongside the screw. Run a profile that rises from the feed zone into the melt zone and then flattens or declines slightly toward the die, with the melt temperature target inside 180 to 210 degrees Celsius measured at the die adapter, not inferred from the barrel setpoints. Barrel cooling capacity matters as much as heating capacity on this duty; a barrel that cannot remove shear heat will not hold the window regardless of how the heaters are set.

Devolatilization: dual vacuum venting

Biodegradable compounds carry a heavy volatile load: residual moisture from the polymer, moisture and air entrained with mineral filler, water released by starch components, plasticizer volatiles and low molecular weight reaction products. A single atmospheric vent is not enough. Specify two vacuum vent stages operating at minus 0.08 to minus 0.09 MPa, with the first placed after the melt zone and the second after the side feed and distributive mixing section. Each vent needs an adequate melt seal upstream and a vent insert designed to resist polymer flooding at high output. Fit a vacuum system with a liquid separator and a filter, because PLA vapor condensate will otherwise foul the pump.

Vacuum level should be monitored as a process parameter, logged with the batch record, and alarmed. A vent that has partially blocked will not stop the line, it will simply produce compound with bubbles, elevated moisture and reduced molecular weight for however long it takes someone to notice.

Feeding: loss-in-weight accuracy of plus or minus 0.5 percent

Formulation accuracy is only as good as the feeders. Use gravimetric loss-in-weight feeders on the base polymers and on every additive that materially affects properties, with a dosing accuracy of plus or minus 0.5 percent. This is not a luxury on a chain-extended recipe: a chain extender dosed at 0.5 phr with a plus or minus 10 percent feeder error swings the actual loading between 0.45 and 0.55 phr, which is enough to move melt viscosity out of the converter’s window. Use a twin-screw side feeder for mineral filler, a crammer feeder for low bulk density powders, and a heated liquid feeder for plasticizer. Mount feeders on a platform isolated from line vibration, and calibrate on a fixed schedule.

The complete feeding system for a typical PLA/PBAT line with filler comprises: two main loss-in-weight feeders for the polymers, one loss-in-weight feeder for the additive masterbatch or powder blend, one twin-screw side feeder with its own loss-in-weight scale for filler, and one liquid feeder if plasticizer is used. Kerke supplies volumetric metering systems, twin-screw side feeders, crammer feeders, loss-in-weight feeders and liquid feeders as part of the line, which avoids the integration gaps that appear when feeding is bought separately from the extruder.

Kerke KTE machines for biodegradable compounding

The KTE series covers parallel co-rotating twin-screw extruders from KTE-16B up to KTE-135D, with aspect ratio, barrel structure, screw arrangement, exhaust configuration, feeding system and electrical control all optimizable per application. For a biodegradable compound project, two tiers matter: a development and pilot tier that carries phase two, and a production tier that carries phases four through seven. The blocks below give the configuration data for each.

Pelletizing and post-treatment for low melt strength compounds

Pelletizing is where PLA compound plants lose the most yield, and the choice between water ring die-face cutting and underwater granulation should be made from the recipe, not from the price list. Both avoid the free-hanging strand that low melt strength material cannot support, but they behave differently on tacky, slow-crystallizing compounds.

In water ring die-face cutting, the melt is extruded through a heated die plate and cut immediately at the plate face by rotating blades, with a ring of water on the housing wall carrying pellets away and cooling them. In underwater granulation, the die plate is fully submerged in a temperature-controlled water chamber and pellets are cut directly into flowing water, then dewatered centrifugally. Underwater granulation gives more uniform, near-spherical pellets and handles very low viscosity melts, at the cost of a more complex water and temperature control loop.

Criterion	Water-cooled strand pelletizing	Water ring die-face cutting	Underwater granulation
Suitability for neat PLA	Poor, strands break	Good	Excellent
Suitability for PLA/PBAT blends	Poor, strands break and stick	Acceptable with tight water temperature control	Excellent
Suitability for filled PLA	Fair, stiffer strands survive better	Good	Good
Suitability for plasticized soft compounds	Poor	Fair, tackiness risk	Good with chilled water
Pellet shape uniformity	Cylindrical, variable length	Lens shaped, moderate spread	Near spherical, tight distribution
Startup and changeover simplicity	Simple	Moderate	Most complex
Water temperature requirement	25 to 40 degrees Celsius	20 to 35 degrees Celsius	20 to 35 degrees Celsius, closely controlled
Sensitivity to die plate temperature	Low	High, freeze-off risk	High, requires heated die plate
Startup scrap generation	Moderate	Moderate	Low once stable
Relative capital level	Low	Medium	High
Relative maintenance demand	Low	Medium	Medium to high

Water temperature is a quality parameter

Hold pelletizing water at 20 to 35 degrees Celsius. Above that band, freshly cut amorphous PLA pellets remain above or close to their glass transition temperature long enough to stick to each other and to the dewatering equipment, producing doubles, clusters and blocked screens. Below that band, thermal shock at the die face raises the risk of freeze-off and of stress cracking in filled compounds. A dedicated chiller loop with a temperature controller is the right solution; using untreated cooling tower water is a false economy that shows up as pellet defects every summer.

Die plate temperature control is the other half of the equation. The plate must stay hot enough for the polymer to flow freely through the holes while the surrounding water is cold. Electric or oil-heated die plates with insulated inserts are used for this duty. If the plate runs cold, holes freeze off progressively, throughput drops, pressure rises and the pellet size distribution widens.

Downstream: dewatering, screening, crystallization and packing

The post-pelletizing train for a biodegradable compound is longer than for a polyolefin compound, because pellets leave the cutter amorphous and wet.

• Centrifugal dewatering. Removes surface water immediately after cutting. Residence must be short, because warm wet pellets are the tackiest they will ever be.
• Vibrating screen classification. Removes fines, doubles and oversize before they reach the silo. On PLA lines this screen earns its place daily, because clusters that reach a customer’s dosing unit cause a complaint that costs far more than the screen.
• Pellet crystallization. An agitated crystallizer at approximately 90 to 110 degrees Celsius raises pellet crystallinity so that the pellets no longer block during warehouse storage or in a customer’s drying hopper. For grades that will be stored or shipped in hot climates, this step is effectively mandatory.
• Final drying and cooling. Brings residual surface and absorbed moisture down before packing, then cools the pellets so that they are not bagged warm.
• Packing. Use moisture barrier packaging: aluminum foil laminate bags with a desiccant sachet for 25 kg units, or lined octabins for bulk. Purge with dry air or nitrogen where the grade is moisture critical. A compound that leaves the plant at 200 ppm and arrives at the converter at 900 ppm has been damaged by packaging, not by processing.

Kerke supplies the cutting and pelletizing system as part of the line, covering 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, so the cutting method can be matched to the recipe rather than the recipe being forced to suit a fixed cutter.

Phase 5: Plant layout and utilities

Utility planning fails when it is based on the extruder nameplate alone. The extruder main drive is usually less than half of the connected load of a compounding line, and for a biodegradable plant the drying, chilling and humidity control loads are unusually large. Size the electrical supply, cooling and air systems from a complete load schedule built during phase four, before civil work starts.

Utility or facility item	Basis of sizing	Indicative figure for a KTE-75D line	Notes for biodegradable duty
Extruder main drive	Machine rating	160 to 200 kW installed	Average draw typically 40 to 60 percent of rating on PLA recipes
Barrel heating	Zone count and zone rating	50 to 80 kW installed	High at startup, low in steady state; heat-up allowance must be in the peak load
Feeders, side feeder, vacuum, control	Component sum	25 to 45 kW	Vacuum pumps run continuously
Pelletizing and dewatering	Cutter, pump, centrifuge	30 to 55 kW	Underwater systems sit at the upper end
Drying and crystallizing	Line rate and moisture target	60 to 120 kW	The single largest auxiliary load on a PLA plant
Chiller	Pelletizing water plus barrel cooling	90 to 150 kW cooling duty	Dedicated loop, temperature controlled to 20 to 35 degrees Celsius
Total connected load	Sum of the above plus lighting and handling	Approximately 400 to 550 kW	Apply a demand factor of 0.5 to 0.65 for the running average
Cooling water flow	Heat rejection requirement	10 to 18 m³/h at 3 to 5 bar	Separate the process loop from the machine cooling loop
Compressed air	Conveying, valves, screen changer	1.5 to 3 m³/min at 0.6 to 0.8 MPa	Dry air also used for material transfer and packing purge
Workshop floor area for one line	Machine, feeders, downstream, aisles	Approximately 500 to 800 m²	Add 200 to 400 m² for laboratory, packing and staging
Clear height	Feeder platform and silo access	7 m minimum, 8 to 9 m preferred	Loss-in-weight feeders sit on an elevated platform above the feed throat
Floor loading	Machine base and silo footings	1.5 to 2 t/m² in the machine bay	Provide an isolated foundation for the extruder base frame
Workshop humidity	Moisture reabsorption control	Relative humidity below 60 percent, 45 to 55 percent preferred	Dehumidification in humid climates pays for itself in reduced drying load
Raw material warehouse	Resin storage stability	Below 30 degrees Celsius, relative humidity below 60 percent	Keep bags sealed until use; PLA absorbs moisture from ambient air
Dust extraction	Filler handling points	Local extraction at side feeder and bag dump	Talc and calcium carbonate handling requires containment

Layout principles specific to biodegradable compounding

Three layout decisions have outsized effects. First, place the dryers as close to the feed throat as physically possible and use closed, insulated conveying between them; every meter of open transfer is moisture reabsorption. Second, give the pelletizing water loop its own room or bay with drainage, because water management determines how clean the plant stays and how quickly the cutter can be serviced. Third, put the laboratory inside the production building with a direct line of sight or short walk to the extruder. Biodegradable compounding is a test-and-adjust operation; a laboratory in a separate building slows the feedback loop from minutes to hours and quietly degrades quality control.

Plan the raw material warehouse for sealed storage and first-in-first-out rotation. Resin bags that have been open on a shop floor for a week are effectively a different material from the same lot stored sealed, and the difference will appear in the melt flow rate of the finished compound.

Phase 6: Installation, commissioning and trial production

Commissioning a biodegradable compounding line is not the same as starting it. Starting it means the motor turns and pellets emerge; commissioning means the line reproduces the pilot property window at production rate, with documented process parameters that an operator can repeat. Plan three to six weeks and structure the work in five steps.

Step 1: Mechanical and electrical installation

Set and level the base frame on its foundation, align the gearbox and drive, connect barrel cooling and heating circuits, install feeders on their platform, connect the vacuum system with its separator, and install the pelletizing water loop and chiller. Check that every barrel zone thermocouple reads correctly and is mapped to the right zone in the control system, a mundane check that catches a surprising number of wiring errors. Verify that the vacuum system holds minus 0.09 MPa with the vents blanked, before any polymer enters the machine.

Step 2: No-load and heat-up verification

Run the screws empty at low speed to confirm rotation direction, gearbox temperature and oil circulation. Heat the barrel through a full cycle and record heat-up time and zone stability. A zone that overshoots by more than a few degrees on the way up will overshoot in production too, and on a polymer with a 30 degree window that matters.

Step 3: Purge and first material pass

Start with a purge compound or an easy-flowing polyolefin to establish flow through the die and cutter, then transition to PLA. Do not start directly on the target recipe. Run neat dried PLA first at low rate to confirm melt temperature behavior, vacuum stability and pelletizing behavior before adding fillers and reactive additives.

Step 4: Recipe transfer from pilot to production scale

Scale-up on a co-rotating twin-screw extruder is not a simple ratio. Specific energy input, residence time distribution and surface-to-volume ratio all change with machine size, so a recipe transferred from a KTE-35D to a KTE-75D needs its screw speed, feed rate and barrel profile re-optimized even though the formulation is identical. Approach it systematically: match specific mechanical energy first, then adjust barrel profile to bring melt temperature into the window, then tune vacuum and feed rates. Record every step; this record becomes the standard operating procedure.

Step 5: First-article qualification

Produce a full batch at target rate and test it against the phase one property specification: melt flow rate, moisture, ash content, tensile strength, elongation at break, notched Izod impact, yellowness index and color difference against the reference. Compare against the pilot batch data. The commissioning report should state achieved output, specific energy consumption in kWh/kg, melt temperature at the die, vacuum levels, and the full property result set. Kerke runs machines under load before shipment and supports installation and commissioning on site, which shortens this phase considerably compared with a self-commissioned installation.

Phase 7: Ramp-up and the quality control system

A biodegradable compound plant reaches commercial credibility when its batch-to-batch variation is small enough that converters stop re-qualifying every delivery. The target to design for is melt flow rate variation within plus or minus 10 percent batch to batch, held over at least 20 consecutive batches, together with stable color and mechanical properties.

Test	Method basis	Frequency	Typical acceptance approach	What a drift indicates
Melt flow rate	190 degrees Celsius, 2.16 kg	Every batch, plus start and end of long runs	Within plus or minus 10 percent of the grade target	Upward drift means chain scission from moisture, heat or shear
Residual moisture	Coulometric or loss on heating	Incoming resin, at feed throat, finished pellet	Below 250 ppm, below 100 ppm for film and fiber grades	Drying system capacity, hopper bridging or transfer line leakage
Ash content	Burn-off residue	Every batch on filled grades	Filler loading within plus or minus 1 percentage point	Side feeder calibration or filler bridging
Tensile strength and elongation at break	Injection molded or pressed specimens	First batch of a campaign and periodically	Within the grade property window	Formulation error or degraded molecular weight
Notched Izod impact	Standard notched specimen	Campaign start and on complaint investigation	Within the grade property window	Poor dispersion of the impact modifier phase
Yellowness index	Colorimeter on pressed plaque or pellets	Every batch	Below the grade limit	Thermal degradation, oxygen ingress, excessive residence time
Color difference	Colorimeter against reference	Every batch on colored grades	Within agreed tolerance	Pigment dosing or dispersion problem
Molecular weight by gel permeation chromatography	Weight-average and number-average	Weekly or on process change	Retention above the agreed fraction of incoming resin	Cumulative hydrolytic or thermal degradation
Melt stability hold test	Repeat melt flow rate after extended melt residence	On formulation change	Limited change over the hold period	Insufficient stabilizer package
Pellet geometry and fines	Screen analysis and visual inspection	Every batch	Fines below the agreed limit, no clusters	Pelletizing water temperature, blade wear or crystallization step

From testing to control

Testing every batch is necessary but not sufficient. Put the two or three most sensitive properties on control charts and react to trends, not only to out-of-specification results. On a PLA line the earliest warning signals are melt flow rate creeping upward and yellowness index creeping upward together; that combination almost always means the melt is seeing too much heat history, and it is far easier to correct at the trend stage than after a customer rejects a delivery.

Traceability closes the loop. Every batch record should link the finished pellet lot to raw material lots, feeder setpoints, barrel profile, screw speed, vacuum readings, melt temperature and the operator on shift. When a converter reports a problem three weeks later, that record is the difference between a one-hour investigation and a week of guessing. It is also exactly what a certification body and an ISO 9001 audit will ask to see.

Operator capability

The ramp-up phase is as much about people as about the machine. Operators moving from polyolefin compounding bring habits that are actively harmful on PLA: raising screw speed to increase output, raising barrel temperature to clear a torque alarm, running with a vent partially blocked because the line is still producing. Training should be explicit that on a hydrolysis-sensitive polymer, the line either runs inside the window or it should be stopped. Kerke provides formulation and process training as part of the project handover, covering screw configuration logic, temperature and vacuum management, and startup and shutdown sequences specific to biodegradable recipes.

Application industries and end products

Biodegradable PLA compounds serve a defined and rapidly formalizing set of end products, most of them driven by single-use plastic regulation. Knowing which end products your compound targets is what turns a generic pellet into a sellable grade, because each product family has its own converting process, property window and certification requirement.

Film and bag applications

Compostable shopping bags, produce bags, bin liners and mailer films are the largest volume outlet for PLA/PBAT compounds. These are blown film applications, so melt strength and bubble stability dominate the specification. Typical compounds sit in the PLA-lean part of the blend range with a chain extender, slip and anti-blocking additives, and often a small mineral loading. Agricultural mulch film is a related but distinct family, where the compound must hold mechanical integrity through a growing season and then degrade in soil, which changes both the formulation and the certification scope.

Rigid and semi-rigid articles

Cutlery, knives, forks and spoons are injection molded from stiff, nucleated and often mineral-filled PLA compounds where heat resistance matters because the article contacts hot food. Drinking straws are extruded from compounds that need dimensional stability and resistance to collapse in cold liquid. Thermoformed trays, lids and cups use sheet compounds with a controlled crystallization profile so the sheet forms cleanly and the finished part holds shape. Rigid closures and containers use compounds closer to the injection cutlery family, with the addition of impact modification where drop performance is required.

Fiber, filament and technical applications

PLA fiber for nonwovens, wipes and textile applications requires the tightest molecular weight control of any application, because spinning amplifies every defect. Compounds for this duty carry the strictest moisture limit and usually a hydrolysis stabilizer. Three-dimensional printing filament is a specialty outlet with high value per ton and modest volume, where color consistency, diameter stability and freedom from gels and contamination determine acceptance. Both of these applications favor a plant with a laboratory extruder and strong analytical capability, because they are formulation-led businesses.

Matching application to compound family

In practice a plant with four grade families can cover most of this landscape: a film family based on a PLA/PBAT blend with chain extender, a rigid injection family based on nucleated and filled PLA, an extrusion family for straws and profiles, and a sheet family for thermoforming. Additional value comes from customer-specific variants of these four rather than from entirely new chemistries, which is why the laboratory extruder and the loss-in-weight feeding accuracy discussed earlier are strategic rather than incidental investments.

Requirement-to-model selection guide

The table below maps a target annual output, formulation system and filler loading to a recommended Kerke KTE model and the configuration options that matter for that duty. Annual capacity assumes 7,000 operating hours and normal availability; confirm against your own shift pattern and product mix.

Target annual output	Formulation system	Filler or additive loading	Recommended Kerke model	Key configuration points
Development only, no commercial volume	Any biodegradable system	Any	KTE-16B laboratory twin screw extruder	Modular barrel, micro loss-in-weight feeders, fast recipe change for screening work
200 to 600 t	PLA/PBAT film grade, pilot or specialty	0 to 10 percent	KTE-35D	44 to 48 L/D, two vacuum vents, liquid feeder for chain extender carrier
600 to 1,200 t	Nucleated rigid PLA for cutlery	10 to 20 percent talc or CaCO3	KTE-35D or KTE-52D	Side feeder after melt zone, moderate torque grade, water-ring die-face cutting
1,200 to 2,000 t	PLA/PBAT blend, general purpose	0 to 20 percent	KTE-52D	48 L/D, dual vacuum venting at minus 0.08 to minus 0.09 MPa, distributive reaction section
2,400 to 4,200 t	PLA/PBAT film grade with chain extender	0 to 15 percent	KTE-65D	48 to 52 L/D, three loss-in-weight feeders, underwater granulation, pellet crystallizer
2,500 to 4,000 t	Filled rigid PLA for thermoforming sheet	20 to 40 percent CaCO3	KTE-65D	Twin-screw side feeder with own scale, wear-resistant elements, vent after side feed
3,800 to 6,600 t	Multi-grade plant, film plus rigid campaigns	0 to 40 percent	KTE-75D	52 L/D, quick-change screw set, recipe management in the PLC, underwater granulation
3,000 to 5,500 t	Starch-based biodegradable blend	20 to 40 percent starch component	KTE-75D or KTE-SE double-stage system	Maximum devolatilization capacity, low shear profile, short residence time
7,000 to 12,000 t	Single dominant grade family, campaign running	0 to 30 percent	KTE-95D	Underwater granulation, automated packing, energy sub-metering, dual vent with backup vacuum
Above 15,000 t	National supply agreement, dedicated grades	Any	KTE-135D	Dedicated line per grade family, full automation, silo storage with dry air blanket
Any volume, plasticized soft compound	Citrate ester plasticized PLA	5 to 15 percent plasticizer	KTE-52D to KTE-75D depending on tonnage	Heated liquid injection, chilled pelletizing water, extended crystallization step
Any volume, materials needing gentler shear	Special heat-sensitive systems	Any	KTE-T three screw extruder	Alternative mixing geometry with a different shear history profile

Why PLA compound plant launches fail

Most failed biodegradable compound projects fail for one of six reasons, and all six are avoidable at the planning stage. They are listed here in rough order of how often they appear.

1. Inadequate drying

The most frequent and most damaging. A dryer sized for average rather than peak rate, a dew point sensor that has drifted, an open transfer line, or amorphous pellets bridging in a hopper that is too hot: any of these puts wet resin into the barrel, and molecular weight falls off a cliff. The compound looks fine and fails every mechanical test. The fix is to treat drying as process equipment with its own instrumentation, alarms and daily verification, and to measure moisture at the feed throat rather than trusting the dryer display.

2. A screw configuration copied from polyolefin practice

Plants that convert an existing PP or PE compounding line, or that accept a general-purpose screw build from a supplier who has not run biodegradable material, end up with too much kneading, too many reverse elements and too much shear energy. The symptom is yellow pellets, an upward-drifting melt flow rate and a machine that cannot hold the temperature window at commercial output. The fix is a purpose-built configuration: distributive bias, restrained reverse elements, a dedicated reaction section and a longer low-shear metering zone.

3. Insufficient devolatilization

One atmospheric vent, or two vents where the second has no melt seal, leaves moisture and volatiles in the melt. The result is bubbles in the strand or pellet, voids in the converted part, and elevated moisture in the finished compound. Because the line still produces pellets, this fault often runs for weeks. Specify two vacuum stages at minus 0.08 to minus 0.09 MPa, monitor vacuum as a logged process value, and alarm on loss of vacuum.

4. Pellet agglomeration and blocking

Warm pelletizing water, no crystallization step, and packing pellets while they are still warm produce bags of blocked material that a converter cannot dose. This complaint damages commercial credibility disproportionately, because it is visible before any test is run. Control pelletizing water to 20 to 35 degrees Celsius, install the crystallizer, cool before packing, and use moisture barrier packaging.

5. Formulation and equipment capacity mismatch

A plant sizes its extruder on the output of an unfilled recipe and then wins business for a 35 percent filled grade, or the reverse: it buys a large machine and then runs small specialty batches with constant changeover. Both destroy the economics. Size the line from the recipe mix you actually intend to run, weighted by expected tonnage, and check the output figures for each recipe family separately rather than using a single nameplate number.

6. Certification started too late

The project that runs phases one to six efficiently and then begins compostability testing has added the better part of a year before it can sell into regulated markets. Worse, if the test result requires a formulation change, the clock restarts. Freeze the recipe early enough to submit samples while equipment is being manufactured, and design the sample matrix to cover the article thicknesses you intend to supply.

An indexed framework for investment and return

Because equipment configurations, local construction practice and utility tariffs vary enormously between regions, the only portable way to compare biodegradable compounding investments is to index them. In the framework below, a single KTE-65D line with dual vacuum venting, gravimetric feeding, water-ring die-face pelletizing and a basic laboratory is defined as the baseline at 100 index points for total installed project cost, and 100 index points for conversion cost per ton. Everything else is expressed relative to that baseline, as a percentage, a multiple, or a qualitative level from Low to Premium.

Configuration	Installed project cost index	Conversion cost per ton index	Relative capacity multiple	Relative complexity
KTE-35D pilot line, minimal automation	52	148	0.15x	Low
KTE-52D line, dual vent, water-ring cutting	78	116	0.45x	Low to medium
KTE-65D baseline line	100	100	1.0x	Medium
KTE-65D with underwater granulation and crystallizer	118	96	1.0x	Medium to high
KTE-75D line, full gravimetric, underwater granulation	142	91	1.6x	High
KTE-95D line, campaign running, automated packing	205	84	2.9x	High
KTE-135D dedicated line	340	78	6.5x	Very high
Add laboratory extruder and analytical capability	Plus 8 to 14 points	Neutral to slightly higher	No change	Medium
Add workshop dehumidification	Plus 4 to 8 points	Minus 2 to 4 points	No change	Low
Add second vacuum stage and upgraded vacuum system	Plus 3 to 6 points	Minus 2 to 5 points through lower reject rate	No change	Low

Reading the framework

Three patterns are visible. First, conversion cost per ton falls with scale but with diminishing returns; the step from the KTE-52D tier to the KTE-65D baseline is worth 16 index points, while the step from the KTE-95D to the KTE-135D is worth only 6. Second, the quality-oriented additions at the bottom of the table cost very little in installed terms and reduce conversion cost, because on a hydrolysis-sensitive polymer the dominant hidden cost is off-specification production rather than energy. Third, the pilot line looks expensive per ton and is, but it is not a production asset; it is a development asset whose return appears in grade development speed rather than in tonnage.

Utilization is the dominant variable

For a first biodegradable compound plant, capacity utilization moves conversion cost per ton more than any equipment decision. A line running at 45 percent utilization carries roughly twice the fixed cost per ton of the same line at 85 percent. This is why phase one insists on concentrating tonnage into one or two grade families: changeover time is the mechanism by which a technically excellent plant ends up with weak economics. Track utilization as availability multiplied by performance multiplied by quality yield, and treat changeover reduction as the first improvement project after ramp-up.

Relative cost levels for planning

For early planning conversations, the useful relative levels are these. Extruder and pelletizing package: High as a share of installed project cost. Drying, chilling and utilities: Medium to High, and consistently underestimated on biodegradable projects. Building and civil work: Medium, driven mainly by clear height and floor loading. Laboratory and analytical equipment: Low in absolute terms, Premium in terms of return per point invested. Certification: Medium in cost terms but Very High in schedule impact. Working capital for resin inventory: High, because biodegradable feedstocks are specialty materials with longer procurement lead times than commodity resins.

Service and support

A biodegradable compound plant launch is a knowledge transfer project as much as an equipment purchase, and the support package should be evaluated on that basis. Kerke, as a Wanplas factory, applies the Wanplas brand service standards across every KTE line, and the elements below are the ones that matter most on a first biodegradable project.

Testing before shipment

Every machine is assembled, wired and run under load in the Kerke workshop before it ships. For biodegradable projects, customers are welcome to send their own raw materials so that the trial run is done on the actual recipe rather than on a generic test material. Running your own PLA or PLA/PBAT blend on the machine before it leaves the factory converts a specification discussion into measured output, measured melt temperature and a pellet sample you can test. It is the single most effective risk reduction step available before commissioning.

Installation and commissioning

Kerke supports on-site installation and commissioning, covering mechanical setting and alignment, control system startup, first material pass, recipe transfer from pilot to production scale and first-article qualification. Because scale-up on a co-rotating twin-screw extruder involves re-optimizing screw speed, feed rate and barrel profile rather than simply scaling a ratio, having the machine builder present during recipe transfer materially shortens the time to stable production.

Spare parts policy

Kerke follows the Wanplas group policy of USD 500 free parts per year, together with free replacement of parts damaged within the warranty period. For a compounding line the practical value of this policy lies in the consumable and wear items that a new plant tends not to stock: thermocouples, heater bands, seals, vent inserts, cutter blades and screw element spares. Agree the recommended spare parts list during the technical specification stage of phase four so that the first year of operation is not interrupted by a missing item with a long lead time.

Formulation and process training

Training is delivered around the specific recipes the plant intends to run, not as a generic machine course. Topics include the logic of the screw configuration and why elements are placed where they are, barrel profile and melt temperature management within the 180 to 210 degrees Celsius window, vacuum system operation and troubleshooting, feeder calibration procedure, startup and shutdown sequences that avoid material sitting hot in the barrel, and changeover and purge procedure between grade families. Operators trained on polyolefin compounding need this explicitly, because several habits that are harmless on polyolefin are damaging on PLA.

Remote support and lifecycle service

KTE lines are supplied with PLC control that includes recipe management and data acquisition, which makes remote diagnostic support practical. When a plant reports a quality drift, reviewing logged barrel temperatures, screw speed, torque, vacuum level and feeder rates usually identifies the cause faster than any description over a call. Longer term support covers screw element replacement and reconfiguration as the product mix evolves, which is common on biodegradable plants: a plant that starts with film grades and later adds rigid grades often needs a modified element set rather than a new machine.

Open factory, audits and trial runs

Kerke operates an open factory policy in line with the Wanplas brand. Prospective buyers are welcome to visit the plant, inspect machining and assembly, review quality documentation and meet the engineering team. The trial and sample center is available for running customer materials, which for a biodegradable project means you can validate a formulation concept on a production-geometry machine before committing to a line configuration. Factory audits and witnessed acceptance testing before shipment are supported on request.

Frequently asked questions

What residual moisture level does PLA compounding actually require?

Below 250 ppm at the feed throat as a working maximum, and below 100 ppm for film and fiber grades where molecular weight retention is critical. Achieve it with a desiccant dryer holding minus 40 degrees Celsius dew point, drying at 80 to 100 degrees Celsius for 4 to 6 hours, with a pre-crystallization stage for amorphous pellets. Measure at the feed throat rather than at the dryer outlet, because the transfer path is where dried material picks moisture back up.

Why can a PLA line not simply use a standard PP or PE screw configuration?

Polyolefin configurations are designed to deliver high dispersive energy through aggressive kneading blocks and reverse elements. PLA has a processing window of only about 30 degrees Celsius and degrades by chain scission under high shear, so that energy shows up as yellowing and molecular weight loss rather than as better mixing. A biodegradable configuration uses distributive mixing in place of much of the dispersive work, restrained reverse elements, a dedicated low-shear reaction section after chain extender addition, and a longer metering zone to let melt temperature settle before the die.

What L/D ratio should a biodegradable compounding extruder have?

Between 40 and 52. Below 40 there is not enough length to place two vacuum vent stages properly and still give a reactive additive the 12 to 16 L/D of distributive mixing it needs. Above 52, extra length adds residence time and thermal exposure without adding function, which works against a hydrolysis-sensitive polymer. Within that band, choose the longer end for chain-extended film grades and starch-based systems, and the shorter end for simple filled rigid grades.

Water ring or underwater pelletizing for PLA compounds?

Underwater granulation is the more robust choice for PLA and PLA/PBAT blends, because it eliminates the free strand entirely, handles low viscosity melts and produces uniform near-spherical pellets. Water ring die-face cutting is a lower complexity, lower capital option that performs well on stiffer mineral-filled PLA compounds. Whichever is chosen, hold pelletizing water at 20 to 35 degrees Celsius and control die plate temperature independently, because both tackiness and freeze-off are temperature problems.

How long does compostability certification take, and when should it start?

Plan 9 to 14 months from sample submission to certificate for EN 13432 or ASTM D6400, with the biodegradation test alone taking up to 180 days and disintegration testing running around 12 weeks on the final article geometry. It should start as soon as the formulation is frozen at the end of phase two, running in parallel with equipment manufacturing and building work. Starting after commissioning adds close to a year before the plant can sell into regulated markets.

What causes pellet yellowing, and how is it diagnosed?

Yellowing is thermal and thermo-oxidative degradation. The usual causes are melt temperature peaks above 210 degrees Celsius, an aggressive screw configuration converting drive power into heat, excessive residence time in dead zones or an oversized melt pipe, insufficient venting, or oxygen ingress at the feed throat. Diagnose by combining yellowness index with melt flow rate: both rising together points to heat and shear, melt flow rate rising alone points to moisture, and color deteriorating alone points to oxygen or a localized hot spot.

How much annual capacity does one biodegradable compounding line deliver?

A KTE-65D running a PLA/PBAT blend at 350 to 600 kg/h yields roughly 2,400 to 4,200 tons per year at 7,000 operating hours. A KTE-75D on the same basis yields roughly 3,800 to 6,600 tons, and a KTE-95D roughly 7,000 to 12,000 tons. Filled recipes run at higher mass throughput than unfilled ones because the mineral fraction carries less specific energy demand, while starch-based blends run lower because devolatilization limits the rate.

Can one line run film grades and rigid grades, or are two lines needed?

One line can run both, provided it is configured for the harder duty and the plant accepts changeover time. The practical approach is to build the line at 48 to 52 L/D with dual vacuum venting, a twin-screw side feeder and recipe management in the control system, then run film and rigid grades in campaigns rather than alternating batch by batch. A second line becomes justified when either family alone approaches full utilization, or when changeover losses exceed the cost of dedicated capacity.

What is the minimum viable plant configuration to start?

A defensible minimum is one production extruder sized to the dominant grade family, a desiccant drying system with margin, dual vacuum venting, gravimetric loss-in-weight feeding on all critical streams, a die-face pelletizing system with temperature-controlled water, a pellet crystallization step, moisture barrier packing, and a laboratory capable of melt flow rate, moisture, ash, color and basic mechanical testing. Cutting any of these produces a plant that can make pellets but cannot guarantee them, and in a certified biodegradable market a compound that cannot be guaranteed is difficult to sell.

Conclusion

Launching a biodegradable PLA compound factory succeeds or fails on sequencing and on respecting the material. The sequencing discipline is straightforward once stated: define the product portfolio before buying anything, freeze formulations on a pilot machine that behaves like the production machine, start certification the moment the recipe is frozen, configure the extruder around the recipes rather than around a catalog, size utilities from a real load schedule, commission until the property window is reproduced rather than until pellets appear, and ramp up with a quality system that catches trends rather than only failures.

Respecting the material means accepting that PLA is not a forgiving polymer. Moisture below 250 ppm and preferably below 100 ppm, a melt held inside 180 to 210 degrees Celsius, a shear budget managed through screw design rather than screw speed, two vacuum vent stages at minus 0.08 to minus 0.09 MPa, feeding accurate to plus or minus 0.5 percent, and pelletizing water held at 20 to 35 degrees Celsius: these are not optional refinements, they are the conditions under which the chemistry works. A special twin screw extruder configuration is what makes those conditions achievable at commercial output.

Kerke, a Wanplas factory with more than 12 years dedicated to compounding extrusion, a 19,997 square meter plant, more than 2,000 machines running in over 70 countries and biodegradable plastic as one of its core application industries, builds KTE series lines specifically for this duty, from the KTE-16B laboratory extruder used in formulation development through the KTE-65D and KTE-75D production tier to the KTE-95D and KTE-135D scale tier, with matched feeding, venting and pelletizing systems and, where the recipe demands it, the KTE-SE double-stage system or the KTE-T three-screw extruder.

If you are planning a biodegradable compound plant in 2026, the most useful next step is to share your target formulation systems, the end products you intend to serve and your target annual capacity. From that, a complete line configuration can be proposed, covering extruder size and L/D, screw configuration logic, venting and feeding layout, pelletizing method, utility load schedule and floor plan. Send your own raw material for a trial run at the sample and testing center so that the configuration is validated on your recipe before anything is manufactured, and visit the factory to see the machining, assembly and testing process for yourself. Tell us the recipe and the tonnage, and the line will be designed around them.