A twin screw compounding plastic factory is not one machine with accessories attached. It is a continuous material-handling system in which roughly thirty distinct machines must hand material to each other at matched rates, matched temperatures and matched cleanliness levels. The co-rotating twin-screw extruder is the heart of that system, but it typically represents only part of the installed equipment count and a smaller share of the floor area. Engineers who plan a compounding plant around the extruder alone almost always discover the gaps late: an undersized dryer that throttles the line on hygroscopic resin, a feeder turndown that cannot deliver a 0.3 percent additive stream, a cooling water circuit that cannot absorb the motor heat load in summer.
This article is an equipment manual rather than a business plan. It walks the material flow from the ton bag on the receiving dock to the palletized finished product, and for each station it sets out what the machine does, the key specifications that matter, the selection logic behind those specifications, the common configuration variants, and how the station couples to the equipment upstream and downstream of it. Kerke, a Wanplas factory that has specialized in parallel co-rotating twin-screw compounding extruders for more than twelve years, builds complete lines to this pattern, and the equipment logic below reflects how those lines are actually assembled and commissioned.
A compounding line is only as accurate as its worst feeder, only as fast as its slowest cooling stage, and only as clean as its least-controlled conveying step. Specify the whole chain, not just the extruder.
Line Architecture and the Master Equipment List
A twin screw compounding line divides into seven functional blocks: bulk handling and storage, gravimetric feeding, extrusion, cooling and pelletizing, downstream finishing and packing, utilities, and the laboratory. Material flows in one direction through the first five blocks while utilities and the laboratory support all of them. Understanding the blocks matters because responsibility for scope, interfaces and control usually splits along exactly these lines when a plant is procured from multiple suppliers.
The layout is normally arranged on two or three levels. Bulk bag discharge and drying occupy ground level near the raw material store. Feeders sit on a mezzanine platform above the extruder feed throat so that material drops by gravity into the barrel. The extruder, pelletizing section and packing line occupy the ground floor in a straight run, typically requiring a clear length of 15 to 40 metres depending on capacity. Utilities are grouped outside or in a separate plant room to keep noise, vibration and heat away from the production hall.
The table below is the master equipment list. It covers every station discussed in this article, indicates whether the station is mandatory for a functional line or optional, and rates the relative investment level on a Low to Premium scale rather than in currency, because pricing depends on capacity, material of construction, automation level and country of origin.
| No. | Equipment | Function | Key Specification | Status | Relative Investment |
|---|---|---|---|---|---|
| 1 | Ton bag unloading station | Discharge bulk bags of resin, filler or masterbatch | Hoist 1 to 2 t, vibrating discharge, dust hood | Recommended | Low |
| 2 | Vacuum conveyor | Pneumatic transfer of pellets and powder | 300 to 3000 kg/h, DN38 to DN76 pipe | Essential | Low |
| 3 | Storage silo | Buffer raw material between deliveries and the line | 1 to 20 m³, carbon or stainless steel, level sensor | Recommended | Low to Medium |
| 4 | Dehumidifying dryer | Remove bound moisture from hygroscopic resin | Dew point −40°C, 50 to 1000 kg hopper | Essential for PA, PC, PET | Medium |
| 5 | Crusher (granulator) | Reclaim start-up lumps, strands and off-spec pellets | 7.5 to 45 kW, screen 6 to 12 mm | Recommended | Low |
| 6 | Loss-in-weight feeder | Gravimetric metering of resin and additives | Accuracy ±0.5%, 0.5 to 2000 kg/h | Essential | Medium to High |
| 7 | Volumetric feeder | Low-cost metering for tolerant single-recipe duty | Screw or disc, accuracy typically ±2% to ±5% | Optional | Low |
| 8 | Liquid injection pump | Inject oil, silicone or coupling agent into the melt | Plunger or gear type, 0.5 to 50 L/h | Optional | Medium |
| 9 | Side feeder | Introduce fiber or powder downstream of melting | Twin-screw, single or double side, vented option | Essential for filled grades | Medium |
| 10 | Central weighing control | Recipe management and batch traceability | PLC plus HMI, recipe library, batch reports | Recommended | Medium |
| 11 | Co-rotating twin-screw extruder | Melt, disperse, devolatilize and pressurize | Ø20 to Ø135 mm, L/D 36 to 52:1, 300 to 1200 rpm | Essential | High to Premium |
| 12 | Main motor and gearbox | Deliver torque to the screw shafts | AC inverter drive or PMSM, hardened-tooth gearbox | Essential | Included with extruder |
| 13 | Barrel heating and cooling | Zone temperature control along the barrel | 6 to 12 zones, ±1 to 2°C, soft-water circuit | Essential | Included with extruder |
| 14 | Vacuum devolatilization system | Remove moisture, monomer and volatiles from the melt | −0.08 to −0.095 MPa, cyclone plus filter | Essential | Medium |
| 15 | Screen changer | Filter gels, char and contamination from the melt | 40 to 200 mesh, manual to continuous types | Essential | Low to High |
| 16 | Melt pump (gear pump) | Stabilize die pressure and lift output | Outlet pressure fluctuation below ±2% | Optional | High |
| 17 | Die head | Form strands or feed the pelletizing die face | Strand die 20 to 120 holes, Ø2.5 to 4 mm | Essential | Low to Medium |
| 18 | Water bath with air knife | Cool and dry strands before cutting | 2 to 6 m length, blower-fed air knife | Essential for strand lines | Low |
| 19 | Strand pelletizer | Cut cooled strands into pellets | 20 to 120 m/min, pellet length 2 to 4 mm | Essential for strand lines | Medium |
| 20 | Water ring or underwater pelletizer | Die-face cutting for low-viscosity or high-output grades | Rotating blade head, process water loop | Alternative to strand cutting | High to Premium |
| 21 | Centrifugal dryer | Spin-dry pellets after water contact | Residual surface moisture typically below 0.1% | Essential with wet cutting | Low to Medium |
| 22 | Vibrating screen (classifier) | Remove longs, fines and twins from the pellet stream | Two or three decks, 2 to 6 mm apertures | Essential | Low |
| 23 | Blending silo | Homogenize several production batches | 2 to 30 m³, gravity or agitated blending | Recommended | Medium |
| 24 | Metal detector and magnetic separator | Protect customers’ machines from metal contamination | Rare-earth grate magnet plus free-fall detector | Recommended | Low to Medium |
| 25 | Automatic weighing and bagging machine | Fill and seal 25 kg bags or bulk bags | 25 kg valve bag or 500 to 1000 kg ton bag | Optional | Medium to High |
| 26 | Palletizer and stretch wrapper | Stack and wrap finished bags for shipment | Robotic or gantry, 300 to 900 bags/h | Optional | High |
| 27 | Cooling water system | Reject heat from barrels, gearbox and pelletizing | Cooling tower, pumps, plate exchanger, softener | Essential | Medium |
| 28 | Air compressor and dryer | Supply clean dry air for valves and conveying | 0.6 to 0.8 MPa, refrigerated or desiccant dryer | Essential | Low to Medium |
| 29 | Dust and fume extraction | Capture powder dust and vent-stack fume | Pulse-jet bag filter, activated carbon or oxidizer | Essential | Medium |
| 30 | Electrical distribution and control | Power, protection, automation and data | Transformer, control cabinets, PLC, HMI, SCADA | Essential | Medium to High |
| 31 | Laboratory equipment set | Verify melt flow, mechanical and thermal properties | MFR, tensile, impact, HDT, ash, color, moisture | Recommended | Medium to High |
Bulk Unloading, Conveying, Storage and Drying
The upstream block decides how much manual handling a compounding plant will need forever after. Its machines are individually inexpensive relative to the extruder, but they set labor cost, dust exposure and the moisture baseline that the extruder must then correct. Specifying this block properly is the cheapest quality improvement available to a new compounding factory.
Ton bag unloading station
A bulk bag discharger lifts a 500 to 1000 kg bag on an electric hoist or forklift frame, clamps the outlet spout into a dust-tight glove box, and discharges into a hopper below. The essential options are a vibrating discharge cone or paddle activators to break powder bridges, a bag-massaging device for compacted calcium carbonate or talc, and an integral dust hood connected to the central extraction system. Frame capacity should be specified one size above the heaviest bag actually used, and headroom of at least 3.5 metres is needed for a hoist-type frame. The station feeds either the vacuum conveyor or directly into a silo, and for powder duty a rotary valve at the outlet prevents flooding.
Vacuum conveyor and pneumatic transfer
Vacuum loaders move resin pellets, filler powder and regrind from the discharge point to silos, dryers and feeder hoppers. Conveying capacity in a compounding plant normally spans 300 to 3000 kg/h with pipe diameters from DN38 to DN76; larger diameters are chosen when the material has poor flow, when the conveying distance exceeds roughly 30 metres, or when the fill rate has to keep pace with a large main feeder. Single-station loaders with a self-contained motor suit small lines, while central feeding systems with a shared vacuum pump and a sequencing valve manifold suit multi-feeder compounding lines. Pipe bends should be long-radius and, for glass-fiber-filled products, bends in the finished-pellet circuit should be avoided altogether because they break fibers and generate fines. Filter cartridges with reverse-jet cleaning are standard on powder duty.
Storage silos
Silos buffer bulk deliveries and decouple the line from the receiving dock. Compounding plants typically use 1 to 20 m³ vessels; small vertical hoppers of 1 to 5 m³ sit indoors above the feeders, while 10 to 20 m³ outdoor silos hold base resin delivered by bulk tanker. Carbon steel with an epoxy lining is normal for commodity polyolefin, while stainless steel is specified for food-contact, medical, optical and light-colored production where iron contamination is unacceptable. Every silo needs a level indicator, either a rotary paddle for simple high-low signalling or a radar or load-cell system when inventory data feeds a plant management system. A cone angle of at least 60 degrees and an aeration pad prevent bridging with powders.
Dehumidifying dryer
Hygroscopic resins must be dried before compounding or the extruder will spend its devolatilization capacity removing water instead of process volatiles, and hydrolytic degradation will cut molecular weight irreversibly. A desiccant dehumidifying dryer with a closed-loop molecular-sieve rotor delivers a dew point of about −40°C, which is the practical requirement for PA, PC, PET, PBT, TPU and PMMA. Hopper volume is sized as throughput multiplied by residence time, with 25 to 30 percent added for surge. The dryer discharges directly into the main loss-in-weight feeder, and the transfer must be short and enclosed because dried PA regains a significant fraction of its moisture within minutes of exposure to humid air.
| Resin | Drying Temperature | Drying Time | Target Moisture | Dryer Type |
|---|---|---|---|---|
| PA6 / PA66 | 80 to 100°C | 4 to 6 h | Below 0.10% | Desiccant, −40°C dew point |
| PC | 110 to 120°C | 3 to 4 h | Below 0.02% | Desiccant, −40°C dew point |
| PET | 150 to 170°C | 4 to 6 h | Below 0.005% | Crystallizing dryer plus desiccant |
| PBT | 120 to 130°C | 3 to 4 h | Below 0.02% | Desiccant, −40°C dew point |
| ABS / PS | 80 to 90°C | 2 to 3 h | Below 0.10% | Hot air or desiccant |
| PP / PE | Not required | Not required | Surface moisture only | Hot air if stored outdoors |
| Calcium carbonate / talc | 100 to 120°C | 1 to 2 h | Below 0.30% | Powder dryer or heated silo |
Crusher and reclaim
Every compounding line produces start-up purge, thick strand ends, transition material and off-spec pellets. A crusher with a 7.5 to 45 kW drive and a 6 to 12 mm screen turns that material into regrind that can be re-introduced through a dedicated feeder at a controlled percentage. Blade clearance must be adjustable, and a soundproof cabinet is worthwhile because purge crushing is one of the noisiest operations in the plant. For heavily filled or fiber-reinforced products, wear-resistant blades and a cyclone separator on the discharge are needed to control fines.
Gravimetric Feeding: The Accuracy Core
Feeding accuracy determines compound consistency more directly than any other variable on a compounding line. A co-rotating twin-screw extruder is a starve-fed machine, which means the feeders, not the screw, set the throughput and the recipe ratio. If a 2 percent additive stream drifts by 10 percent of its own value, the finished compound moves outside specification regardless of how well the screws mix.
Loss-in-weight feeders
A loss-in-weight feeder, also called a gravimetric feeder, sits on load cells and controls the metering screw speed so that the measured rate of weight loss matches the setpoint. Well-built units hold short-term accuracy within about ±0.5 percent of setpoint, with linearity across a turndown ratio of roughly 20:1 to 100:1 depending on the metering geometry. Feed rates on compounding lines span 0.5 kg/h for a high-activity additive up to 2000 kg/h for base resin on a large line. The refill valve, refill frequency and hopper geometry matter as much as the load cells, because the controller runs in volumetric mode during refill and any bridging shows up as a rate excursion.
Three metering geometries cover almost all duties. A single-screw metering unit handles free-flowing pellets and granules. A twin-screw metering unit, usually with intermeshing concave or auger screws, handles powders that would otherwise flood or bridge, including calcium carbonate, talc, carbon black and flame retardant. A vibratory tray unit handles friable, fragile or fiber-like materials such as chopped glass strand and some metallic pigments, because it moves material without shear. A typical compounding line specifies one main resin feeder plus two to six additive feeders; the practical rule is that the smallest stream should still run above roughly 10 percent of the chosen feeder’s maximum rate, otherwise a smaller feeder is required.
| Material Form | Recommended Feeder Type | Typical Accuracy | Practical Feed Range | Notes |
|---|---|---|---|---|
| Free-flowing pellets, base resin | Single-screw loss-in-weight | ±0.5% | 20 to 2000 kg/h | Refill from silo or dryer, agitator optional |
| Fine powder (CaCO₃, talc, TiO₂) | Twin-screw loss-in-weight | ±0.5% to ±1% | 5 to 800 kg/h | Flexible wall with paddle agitation prevents bridging |
| Carbon black, low bulk density | Twin-screw with vertical agitator | ±1% | 2 to 200 kg/h | Sealed hopper and dust extraction required |
| Chopped glass fiber | Vibratory tray | ±1% | 5 to 500 kg/h | Zero shear preserves fiber length before the barrel |
| Flake or regrind | Single-screw with crammer | ±1% to ±2% | 10 to 600 kg/h | Irregular bulk density needs a forced-fill device |
| Micro-dose additive below 0.5% | Micro loss-in-weight, twin-screw | ±0.5% | 0.5 to 20 kg/h | Consider pre-diluting into a carrier masterbatch |
| Liquid oil, silicone, coupling agent | Plunger or gear metering pump | ±1% | 0.5 to 50 L/h | Heated line and non-return injection nozzle |
| Tolerant single-recipe bulk stream | Volumetric screw feeder | ±2% to ±5% | 10 to 1000 kg/h | Lowest cost, no compensation for density drift |
Volumetric feeders
A volumetric feeder meters by screw revolutions rather than by weight. It cannot compensate for changes in bulk density, so accuracy sits in the ±2 to ±5 percent range. Volumetric units still make sense for a dedicated single-recipe line running one consistent material, for a non-critical filler stream where the specification has generous tolerance, and as an economical way to establish a starter line that can be upgraded to gravimetric control later. The mechanical interface and the mounting frame should be identical so that a future upgrade is a bolt-out, bolt-in exercise.
Liquid injection pumps
Liquid additives such as white oil, silicone oil, plasticizer, coupling agent and peroxide are injected directly into the barrel through a heated, non-return nozzle, usually positioned after the first melting zone. Plunger metering pumps in the 0.5 to 50 L/h range give repeatable displacement against back pressure; gear pumps are used where the liquid is viscous and pulsation must be minimized. The pump should be linked to the central weighing control so that the liquid stream is ratioed to total throughput, and the supply tank should be on a load cell so that consumption is verified by weight, not by pump stroke count alone.
Side feeders
A side feeder is a short twin-screw unit that pushes material into the barrel through a side port after the base resin has already melted. It is essential for glass fiber, where introducing roving or chopped strand into the melt rather than the feed throat preserves residual fiber length and therefore mechanical performance. It is equally important for high filler loadings, because a side feeder bypasses the feed-throat volume limit that otherwise caps the throughput of low-bulk-density powders. Configurations include single side feeder for standard filled grades, double side feeder for two-stage filling or fiber plus mineral, and a vented side feeder whose upper barrel section removes entrained air so that powder can be forced in at a higher rate. Placement is specified in barrel diameters from the feed throat, and the screw elements around the side port must be conveying elements, never restrictive kneading blocks.
Central weighing and recipe control
A central control system ties every feeder, the liquid pump and the extruder drive into one ratio-controlled group. Its practical value lies in recipe management, so that an operator selects a product code rather than typing eight setpoints, and in batch traceability, so that each production lot carries a record of actual feed rates, screw speed, melt pressure and temperature. Systems typically expose data over an industrial protocol so that production records reach the plant’s business system without manual transcription.
The Twin-Screw Extruder Main Machine
The co-rotating parallel twin-screw extruder performs melting, dispersive and distributive mixing, devolatilization and pressure generation in a single continuous pass. Its capability is defined by four numbers: screw diameter, L/D ratio, specific torque and maximum screw speed. Every other decision on the machine follows from these.
Kerke builds parallel co-rotating machines across the full compounding range, from laboratory units around 20 mm screw diameter through the mid-range 35, 50 and 65 mm classes to 75, 95 and 135 mm production machines. Screw diameter sets free volume and therefore volumetric capacity; L/D sets available residence time and the number of process operations that can be arranged along the barrel; specific torque, expressed as torque divided by the cube of the centre distance, sets how much viscous work the machine can apply before the shafts limit; and screw speed determines shear rate and the achievable output within that torque envelope.
| Screw Diameter Class | Typical L/D | Typical Screw Speed | Indicative Duty | Typical Application |
|---|---|---|---|---|
| 20 to 25 mm | 40 to 48:1 | 300 to 900 rpm | Laboratory and sampling | Formulation development, small trial batches |
| 35 mm | 40 to 48:1 | 400 to 900 rpm | Pilot and small production | Color masterbatch, specialty compounds |
| 50 mm | 40 to 52:1 | 400 to 1000 rpm | Small to mid production | Filled PP, cable compound, engineering plastics |
| 65 mm | 40 to 52:1 | 400 to 1200 rpm | Mid production | Glass-fiber PA, filler masterbatch, TPE |
| 75 mm | 36 to 48:1 | 400 to 1200 rpm | Mid to large production | High-output filled compounds |
| 95 mm | 36 to 48:1 | 300 to 900 rpm | Large production | Dedicated single-product lines |
| 135 mm | 36 to 44:1 | 300 to 800 rpm | Very large production | Commodity filler and polyolefin compounding |
Main motor and gearbox
The drive train is an AC motor with a variable frequency drive, or increasingly a permanent-magnet synchronous motor that holds high efficiency across a wider speed range and reduces cooling demand. The motor connects through a coupling to a high-torque distribution gearbox with hardened, ground gear faces. Because a twin-screw gearbox must transmit large torque through two closely spaced output shafts, its thrust bearing arrangement is the limiting component and the main reason for specific-torque differences between machine ranges. A forced lubrication station with an oil pump, filter and oil cooler is standard from mid-range sizes upward, and oil temperature should be interlocked so that the drive trips before the bearings are damaged. Gearbox oil analysis at defined intervals is the single most useful predictive maintenance measure on the machine.
Barrel heating and cooling
Barrels are built in segments, conventionally 4D long, so that the process layout can be reconfigured: closed segments, feed segments, side-feed segments, vent segments and combination segments are bolted together in the order the process requires. Each segment or pair of segments forms a control zone, giving 6 to 12 zones on a typical compounding machine. Heating is by ceramic or cast-aluminium electric heater bands; cooling is by soft water circulating through drilled channels or cast cooling tubes, switched by solenoid valves or modulated by proportional valves. Proportional control gives tighter regulation, typically holding ±1 to 2°C, which matters for heat-sensitive products such as PVC compound, flame-retardant systems and biodegradable polyesters. Soft water is mandatory: hard water scales the narrow cooling channels and the loss of cooling authority is usually discovered only when a zone starts to overshoot.
Screw and Barrel Configuration
Screw configuration is where a general-purpose extruder becomes a specific process machine. Modern compounding screws are segmented, with elements keyed onto splined shafts, so a single machine can be reconfigured for different products in a few hours. Knowing what each element family does is essential for both purchasing and troubleshooting.
- Conveying elements transport material forward. Pitch is selected to match the duty: long pitch at the feed throat and vent sections for maximum intake and free surface area, shorter pitch before restrictive sections to build a melt seal.
- Kneading blocks provide the mixing work. A 30-degree stagger gives forward conveying with mild mixing, 45 degrees is the general-purpose dispersive choice, 60 degrees increases shear intensity and residence time, and 90 degrees is neutral with strong dispersive action but no conveying capacity.
- Reverse elements create a pressure dam that fully fills the preceding section, forcing melting to complete and forming the seal ahead of a vacuum vent. They also raise local melt temperature sharply, so they must be used deliberately.
- Tooth mixing elements, sometimes called gear or slotted mixing elements, give strong distributive mixing with low shear, which suits color dispersion and heat-sensitive systems.
- Eccentric elements generate elongational flow with a low temperature rise, useful for polymer blends where droplet break-up matters more than raw shear.
Element metallurgy is a selection decision in its own right. Nitrided steel is adequate for unfilled and lightly filled duty. Tool steel with heat treatment suits moderately abrasive filler. Powder-metallurgy grades with high carbide content, and bimetallic barrel liners, are needed for glass fiber, mineral filler above roughly 40 percent, and flame-retardant systems containing halogen or phosphorus, where corrosive attack combines with abrasion. Specifying wear-resistant metallurgy from the start is far less costly than replacing a worn barrel section after twelve months.
Barrel arrangement follows the process sequence. A standard high-filler configuration runs feed, melting with 45-degree kneading blocks, a reverse element to seal, side-feed section, mixing, atmospheric vent, further mixing, vacuum vent, then the metering section to the die. A glass-fiber configuration keeps the mixing after the side feeder deliberately gentle, using long-pitch conveying and tooth elements rather than aggressive kneading, because every additional kneading block shortens residual fiber length and reduces the tensile and impact performance of the finished compound.
Devolatilization, Filtration and Melt Conveying
Between the last mixing section and the die, three machines determine whether a technically correct compound reaches the pelletizer in saleable condition: the vacuum system, the screen changer and, optionally, the melt pump. These are frequently under-specified because they sit in the shadow of the extruder itself.
Vacuum devolatilization system
Vacuum venting removes residual moisture, monomer, oligomer, solvent and reaction by-products from the melt. A water ring vacuum pump is the standard choice on compounding lines because it tolerates the condensable vapor load, and it achieves roughly −0.08 to −0.09 MPa gauge. Where deeper vacuum is required, for example on recycled material with a high volatile load or on polyamide where residual monomer affects odor and blooming, a Roots blower is added ahead of the water ring pump to reach about −0.095 MPa. The vacuum train needs a cyclone separator to catch entrained powder, a knock-out pot for condensate, and cartridge filtration to protect the pump. A vent stuffer, a small screw that pushes escaping material back into the barrel, prevents vent flooding on foamy or low-viscosity melts.
Two-stage devolatilization, with an atmospheric vent early in the barrel followed by a vacuum vent later, is the standard configuration for recycled and moisture-laden feedstock, because it removes the bulk of the water before the melt reaches the deep-vacuum section. Vacuum lines should be short, sloped back to the knock-out pot and fitted with an isolation valve at each vent so that a single vent can be cleaned during a product change without breaking vacuum on the rest of the system.
Screen changers and melt filtration
A screen changer holds the filter pack that removes gels, char, unmelted filler agglomerates and foreign contamination from the melt. The selection driver is contamination level and the cost of stopping. Manual slide-plate changers are the simplest and require a brief line stop to change screens. Hydraulic quick-change plate types cut the changeover to a few seconds but still cause a short pressure disturbance. Dual-piston, four-position designs allow one screen cavity to be changed while the other stays in production, giving continuous operation. Continuous belt-screen changers advance a roll of filter mesh automatically according to differential pressure and are the choice for heavily contaminated recycled feedstock.
| Screen Changer Type | Typical Mesh Range | Changeover Behaviour | Best Suited To | Relative Investment |
|---|---|---|---|---|
| Manual slide plate | 40 to 100 mesh | Line stop, 1 to 3 minutes | Clean virgin resin, short runs, lab and pilot lines | Low |
| Hydraulic quick-change plate | 60 to 150 mesh | Seconds, brief pressure dip | General compounding with clean feedstock | Low to Medium |
| Dual-piston four-position | 80 to 200 mesh | Continuous, no line stop | Masterbatch and engineering compounds needing fine filtration | Medium to High |
| Continuous belt screen | 40 to 120 mesh | Fully automatic, pressure controlled | Recycled and contaminated feedstock, long campaigns | High |
| Backflush laser-drilled filter | Equivalent to 100 to 200 mesh | Self-cleaning by backflush cycle | Very high contamination, continuous 24-hour running | Premium |
Filter fineness is a compromise. Coarse packs of 40 to 60 mesh protect the die from gross contamination at low pressure drop. Fine packs of 150 to 200 mesh are used for fiber-grade and film-grade compounds where a single gel can cause a customer line break, but they raise back pressure, increase melt temperature and shorten screen life. Every screen changer should carry a differential pressure transducer with an alarm setpoint so that screens are changed on data rather than on the operator’s judgement.
Melt pump
A melt pump, or gear pump, is fitted between the screen changer and the die when pressure stability matters. Because a twin-screw extruder is starve-fed, its discharge pressure fluctuates with feeding variation and screw pumping behaviour; a melt pump isolates the die from that fluctuation and typically holds outlet pressure variation within about ±2 percent. The benefits are more uniform strand diameter, better pellet size consistency, the ability to run finer filtration without losing output, and often a modest output increase because the extruder no longer has to generate all the die pressure itself. The trade-offs are extra residence time, an additional heated component to purge during color changes, and higher capital and maintenance cost, which is why melt pumps are common on masterbatch and fiber-grade lines and less common on general filler lines.
Die head
The die head converts melt flow into the geometry the pelletizer needs. A strand die carries 20 to 120 holes of Ø2.5 to 4 mm on a heated plate, with hole count and diameter matched to output so that strand velocity stays in the range the water bath can cool. Strand dies for filled compounds need a streamlined internal manifold with no dead spots, because stagnant melt degrades and produces black specks. A water ring die and an underwater die instead present a flat face against which rotating blades cut, and both require a cutter chamber, blade pressure control and a process water circuit. Die plates should be electrically or oil heated to prevent freeze-off at the outer holes, which is the most common cause of uneven strand break.
Cooling and Pelletizing Equipment
Pelletizing method is the single most consequential downstream decision in a compounding plant because it dictates the cooling equipment, the water treatment, the drying step, the pellet shape delivered to the customer and the difficulty of product changeover. The four practical routes are strand pelletizing, water ring die-face cutting, underwater pelletizing and air-cooled die-face cutting.
Water bath and air knife
On a strand line, extruded strands run through a stainless steel cooling trough 2 to 6 metres long. Length is chosen from output, strand diameter and the crystallization behaviour of the polymer; a semi-crystalline high-output product needs a longer bath than an amorphous low-output one. Water temperature is normally held between 25 and 60°C, since water that is too cold quenches the strand surface and leaves a hot core that deforms in the cutter. Adjustable strand guide rollers and a slotted overflow weir keep strands parallel and prevent them from touching and fusing. At the exit, an air knife fed by a high-pressure blower strips surface water; effective drying matters because water carried into the cutter accelerates blade wear and produces wet pellets that then need more centrifugal drying capacity.
Strand pelletizer
A strand pelletizer draws cooled strands between a feed roll pair and cuts them against a fixed bed knife using a helical rotor. Line speed capability of 20 to 120 m/min covers most compounding duties, and pellet length is set by the ratio of feed speed to rotor speed, typically producing 2 to 4 mm pellets. Key specification points are the rotor blade material, with tool steel adequate for unfilled duty and tungsten carbide required for glass fiber and mineral filler; the feed roll surface, where a rubber-coated top roll grips soft strands and a knurled steel roll grips rigid ones; and the ease of blade clearance adjustment, because a poorly set gap is the main cause of tails, fines and twins. A soundproof enclosure and an interlocked guard are standard safety requirements.
Water ring and underwater pelletizing
Water ring pelletizing cuts the melt at the die face with a rotating blade head while a ring of water sprayed around the cutting chamber cools and transports the pellets. It avoids strand handling entirely, which suits low-viscosity materials, hot-melt adhesives and products that cannot form a stable strand. Underwater pelletizing goes further, submerging the entire die face in a temperature-controlled process water circuit; pellets are cut, quenched and carried away in one operation, producing near-spherical pellets with very low dust. Both routes require a process water system with a pump, heat exchanger, filter and temperature control, and both add a centrifugal dryer downstream. Their weakness is changeover: purging and cleaning a submerged die and cutter chamber takes longer than swapping strands, so job-shop compounders running many colors usually stay with strand cutting.
| Pelletizing Method | Suitable Materials | Pellet Shape | Typical Output Range | Maintenance Difficulty | Relative Cost |
|---|---|---|---|---|---|
| Strand pelletizing, water bath | Most filled and unfilled thermoplastics, engineering compounds | Cylindrical, 2 to 4 mm | 50 to 3000 kg/h | Low, blade and roll adjustment | Low to Medium |
| Water ring die-face cutting | Low-viscosity grades, filler masterbatch, adhesives | Lens to oval | 100 to 2000 kg/h | Medium, blade pressure and die face | Medium to High |
| Underwater pelletizing | Elastomers, TPE, hot-melt, high-output single product | Near spherical, low dust | 300 to 6000 kg/h | High, water loop and seal management | Premium |
| Air-cooled die-face cutting | Moisture-sensitive grades, PVC compound, some masterbatch | Irregular to lens | 50 to 800 kg/h | Medium, blade wear and fines control | Medium |
Centrifugal dryer and vibrating screen
Any wet-cut pellet stream passes through a centrifugal dryer, a vertical rotor inside a perforated screen basket that throws water outward while lifting pellets upward. Correctly sized, it leaves surface moisture below roughly 0.1 percent, which is sufficient for immediate packing of non-hygroscopic products. Rotor speed and paddle angle should be adjustable, since soft or waxy pellets can smear at high speed. A vibrating screen classifier follows, usually with two or three decks: the top deck rejects long or fused pellets and strand tails, the working deck passes on-specification product, and the bottom deck collects fines and dust. Screens are simple machines that pay for themselves the first time an oversize twin pellet would otherwise have blocked a customer’s dryer or feed throat.
Downstream Handling, Blending and Packing
The finishing block converts a continuous pellet stream into a homogeneous, verified, traceable product in the packaging a customer specified. Its machines are conventional, but the interfaces between them are where contamination and mislabelling risks concentrate.
Blending silos
Compounding output varies slightly across a production run as feeders refill, ambient conditions shift and screens load up. A blending silo of 2 to 30 m³ homogenizes that variation. Gravity blenders use internal tubes with staggered inlets so that material from many horizontal layers discharges simultaneously; agitated blenders use a slow internal screw. Blending matters most for color-critical products and for compounds sold against a narrow melt flow rate window, where lot-to-lot consistency is the entire value proposition. Silos should have level measurement, a dust filter on top and a sampling point at the discharge.
Metal detection and magnetic separation
Two devices protect downstream customers. A rare-earth grate magnet or drawer magnet installed in a free-fall chute captures ferrous particles from wear, broken blades and stray hardware. A free-fall metal detector downstream of the magnet detects non-ferrous and stainless fragments and triggers a fast reject flap. Both are cheap insurance: a single metal fragment reaching a customer’s injection molding machine can damage a screw, a hot runner or a mold cavity, and the resulting claim will exceed the cost of the equipment many times over. Detection sensitivity should be verified at each shift start with certified test spheres and the result logged.
Weighing, bagging and palletizing
Packing configurations follow the customer profile. A 25 kg valve bag line uses a net weigh hopper, a bag placer or manual bag placement, an impeller or air-packer filling head, and a heat sealer or sewing head, at rates typically between 200 and 600 bags per hour. Bulk bag stations fill 500 to 1000 kg bags on a weighing platform with a bag inflation and hook system. Open-mouth bag lines with form-fill-seal heads are used where a printed film bag is preferred. Automatic checkweighing after filling should be standard, since bag weight disputes are a common commercial friction point. Downstream, a robotic or gantry palletizer stacks bags in a stable pattern and a stretch wrapper secures the load. Palletizing is the most common phase-two automation upgrade, because it removes the heaviest manual work in the plant and its benefit scales directly with volume.
Utility Systems Behind the Line
Utilities are routinely under-budgeted in compounding projects, and the consequences appear as summer capacity loss, temperature control drift and unplanned stops. A compounding line converts a large fraction of its drive power into heat that must be removed, and it moves powders that must be captured.
Cooling water system
Two water circuits are normally required. A soft-water closed loop serves the barrel cooling channels and, where fitted, the gearbox oil cooler; it must be soft or demineralized because the drilled barrel channels are narrow and scale is effectively irreversible. An open circuit from a cooling tower serves the pelletizing water bath, the process water for die-face cutting, the vacuum pump seal water and the plate heat exchanger that rejects heat from the closed loop. A chiller is added where the strand bath must run below ambient wet-bulb temperature or where a specific product needs rapid quenching, and a mold temperature controller style unit is sometimes used to hold the die or the underwater process water at a set temperature. Water treatment, filtration and a biocide regime protect the whole system.
Compressed air, dust extraction and electrical distribution
Compressed air at 0.6 to 0.8 MPa drives conveying valves, pneumatic screen changers, air knives, bag handling and cleaning stations. A refrigerated dryer is the minimum, with a desiccant dryer where instrument air quality is specified, plus an air receiver sized to absorb the demand peaks that pulse-jet filter cleaning creates. Dust collection uses pulse-jet bag filters at the bulk bag station, powder feeders, crusher and packing points, with capture velocity at each hood specified rather than assumed. Extruder vent stacks and die-face fume need a separate extraction path; activated carbon adsorption handles moderate loads, while a catalytic or regenerative thermal oxidizer is used where local emission limits or heavy stabilizer packages demand it.
Electrical design starts from the connected load: main motor, barrel heaters, feeders, vacuum pumps, water pumps, dryer, pelletizer, air compressor, dust collector and packing line. Because heaters and the main drive rarely draw peak power simultaneously, the transformer is sized on a diversified load with headroom for future expansion. Control architecture is a PLC with an HMI at the extruder, distributed control for the feeder group, and a supervisory layer that logs process data. Data interfaces to a plant management system commonly use OPC UA, with EUROMAP 63 still encountered on older plastics equipment; specifying the interface at purchase avoids expensive retrofitting later.
| Line Capacity | Cooling Water Demand | Indicative Connected Load | Compressed Air | Dust Extraction Airflow |
|---|---|---|---|---|
| 100 kg/h | 8 to 15 m³/h | 60 to 110 kW | 0.3 to 0.6 m³/min | 1500 to 3000 m³/h |
| 300 kg/h | 15 to 30 m³/h | 120 to 220 kW | 0.6 to 1.2 m³/min | 3000 to 6000 m³/h |
| 500 kg/h | 25 to 45 m³/h | 200 to 350 kW | 1.0 to 2.0 m³/min | 5000 to 9000 m³/h |
| 1000 kg/h | 45 to 80 m³/h | 350 to 600 kW | 1.5 to 3.0 m³/min | 8000 to 15000 m³/h |
Laboratory and Quality Control Equipment
A compounding factory sells measured properties, not pellets, which makes the laboratory a production asset rather than an overhead. The minimum set verifies flow, mechanical performance, thermal performance, filler content and appearance; the extended set adds flammability and a pilot compounding line for formulation work.
The instrument that gets used most is the melt flow rate tester, because melt flow rate is the fastest indicator that a batch has drifted, whether from feeder error, excessive shear or moisture-driven degradation. Tensile testing verifies the reinforcement level actually achieved, impact testing exposes poor dispersion of impact modifier, and heat deflection and Vicat testing confirm the thermal claims on the datasheet. A muffle furnace measures ash content, which is the direct check on filler and glass loading and therefore on feeder calibration. A colorimeter closes the loop on color matching, and a moisture analyzer verifies both incoming resin and finished pellet moisture.
| Laboratory Equipment | Property Measured | Reference Standard | Why It Matters on a Compounding Line |
|---|---|---|---|
| Melt flow rate tester | Melt mass-flow rate, melt volume-flow rate | ISO 1133, ASTM D1238 | Fastest daily check for degradation and feeder drift |
| Universal testing machine | Tensile strength, elongation, flexural properties | ISO 527, ISO 178 | Confirms reinforcement level and fiber length retention |
| Impact tester, Izod and Charpy | Notched impact strength | ISO 180, ISO 179 | Detects poor impact modifier dispersion |
| HDT and Vicat apparatus | Heat deflection temperature, softening point | ISO 75, ISO 306 | Validates thermal claims for engineering compounds |
| Muffle furnace | Ash and filler content | ISO 3451 | Direct verification of filler and glass loading |
| Colorimeter | Color difference, whiteness, yellowness | ISO 105 series, ASTM D2244 | Objective color approval and batch matching |
| Moisture analyzer | Moisture content | ISO 15512 | Controls drying and prevents hydrolytic degradation |
| Flammability test chamber | Burning behaviour rating | UL 94 | Required for flame-retardant compound development |
| Laboratory twin-screw extruder | Small-batch compounding trials | Not applicable | Develops recipes without occupying the production line |
The laboratory twin-screw extruder deserves particular attention. A 20 to 25 mm laboratory machine with the same screw geometry family as the production line allows a formulation to be developed on a few kilograms of material and then scaled with confidence, because specific energy input and screw configuration translate across the range. Kerke supplies laboratory twin screw extruders alongside its production machines specifically so that development work and production work stay on comparable geometry, which is what makes scale-up predictable rather than experimental.
Complete Line Configurations by Throughput Class
Equipment selection changes qualitatively, not just quantitatively, as target output rises. At 100 kg/h a compounding plant can run semi-manually with minimal automation; by 1000 kg/h the material handling, utilities and packing must all be mechanized or labor and downtime consume the throughput advantage.
| Station | 100 kg/h | 300 kg/h | 500 kg/h | 1000 kg/h |
|---|---|---|---|---|
| Extruder class | Ø35 mm, L/D 40 to 44 | Ø50 mm, L/D 44 to 48 | Ø65 mm, L/D 44 to 48 | Ø75 to 95 mm, L/D 40 to 48 |
| Loss-in-weight feeders | 1 main plus 1 to 2 additive | 1 main plus 2 to 3 additive | 1 main plus 3 to 4 additive | 1 main plus 4 to 6 additive |
| Side feeder | Optional, single | Single, twin-screw | Single or double | Double, one vented |
| Bulk handling | Manual bag tipping | Ton bag station plus vacuum conveyor | Ton bag station, silo, central conveying | Outdoor silos, tanker intake, central conveying |
| Drying | 50 to 100 kg hopper dryer | 150 to 300 kg desiccant dryer | 300 to 500 kg desiccant dryer | 600 to 1000 kg desiccant dryer |
| Screen changer | Manual slide plate | Hydraulic quick-change | Hydraulic or dual-piston | Dual-piston four-position or continuous |
| Melt pump | Not typical | Optional | Recommended for fine filtration | Recommended |
| Pelletizing | Strand, 2 m bath | Strand, 3 m bath | Strand 4 to 5 m bath, or water ring | Strand 5 to 6 m bath, or underwater |
| Classification | Single-deck screen | Two-deck screen | Two or three-deck screen | Three-deck screen plus fines cyclone |
| Blending | Batch mixer, optional | 2 to 5 m³ silo | 5 to 10 m³ silo | 15 to 30 m³ silo |
| Packing | Manual weigh and seal | Semi-automatic bagging | Automatic bagging | Automatic bagging plus palletizer |
| Control | Extruder PLC and HMI | Extruder plus feeder group control | Central recipe control | Central control with SCADA data logging |
Two patterns are worth noting. First, the number of feeders scales faster than throughput, because product complexity rather than volume drives feeder count; a 300 kg/h specialty compounding line may carry more feeders than a 1000 kg/h commodity filler line. Second, packing automation becomes mandatory around 500 kg/h, because manual bagging above that rate needs continuous multi-operator coverage that quickly exceeds the cost of the machine.
Must-Have Versus Optional: Phasing the Equipment
Not every machine in the master list has to arrive on day one. The reliable way to phase a compounding plant is to install everything that touches the melt path and everything that is difficult to retrofit, then add accuracy, automation and capacity later. Melt-path equipment defines what the plant can physically make; peripheral equipment defines how efficiently it makes it.
Install from the start: the extruder with its final L/D and barrel configuration, the vacuum system, the screen changer body, the die, the water bath at final length, the pelletizer, the classification screen, the cooling water system at final capacity, the electrical supply and cable trays, and the mezzanine platform with space for the maximum planned feeder count. Extending a barrel, lengthening a water bath, upsizing a transformer or enlarging a cooling tower after the plant is running always costs more and stops production.
Defer if necessary: additional loss-in-weight feeders beyond the initial recipe requirement, the liquid injection pump, the melt pump, the second side feeder, the metal detector, automatic bagging, palletizing, the blending silo and the extended laboratory. Each of these can be integrated later provided the physical space, the utility connections and the spare capacity in the control cabinet were reserved. The reservation is the critical discipline: leaving a mounting frame, a blanked barrel port, a spare drive slot and a labelled cable route costs very little during construction and turns a future upgrade into a weekend job.
Two commonly deferred items deserve a caution. Skipping the metal detector is a false economy given the damage a single fragment can cause in a customer’s molding machine. Skipping the ash furnace and melt flow rate tester means the plant cannot verify its own filler loading or detect degradation, which leaves quality control dependent on customer complaints. Both are modest investments relative to the risk they retire.
Selecting a supplier who can deliver the whole line as one integrated scope removes most interface risk. Kerke, a Wanplas factory, builds parallel co-rotating twin-screw extruders together with feeding systems, side feeders, cutting and pelletizing systems, high-speed mixers, granulators, pulverizers and water chillers, and supplies them as complete compounding lines. Established international platforms from Coperion, Leistritz, JSW and Berstorff serve the same market at the premium end, and the practical comparison for most buyers is between a fully integrated line with a single commissioning responsibility and a multi-vendor assembly that must be coordinated on site. Wanplas, the parent brand behind Kerke and its sister factories, applies group-wide commitments on parts support, capacity verification and quality standards across all of its equipment.
Frequently Asked Questions
What is the minimum viable equipment set for a twin screw compounding line?
A functional line needs one main loss-in-weight feeder, a co-rotating twin-screw extruder with vacuum devolatilization, a screen changer, a strand die, a water bath with air knife, a strand pelletizer, a vibrating screen, a semi-automatic bagging station, plus cooling water and compressed air. That set will make saleable pellets from a simple recipe. Everything else in the master list improves accuracy, expands the product range, reduces labor or protects quality, but is not required for the line to run.
Which machines can be added later without rebuilding the line?
Additional loss-in-weight feeders, the liquid injection pump, a melt pump, a second side feeder, metal detection, automatic bagging, palletizing, blending silos and most laboratory instruments are all straightforward retrofits. The condition is that the mezzanine, the barrel ports, the utility connections and the electrical cabinet were sized for the final configuration during construction. What cannot be added easily is barrel length, water bath length, transformer capacity and cooling tower capacity, so those should be specified at the final target from day one.
How does a glass-fiber line differ from a standard filler line?
A glass-fiber line uses bimetallic barrel liners and powder-metallurgy screw elements to survive abrasion, introduces fiber through a dedicated side feeder placed downstream of the melting zone, and runs a deliberately gentle mixing configuration after the side port to preserve residual fiber length. The strand pelletizer needs tungsten-carbide or high-alloy blades and more frequent blade service. A high-filler mineral line instead emphasizes side feeder capacity, twin-shaft powder metering, additional vacuum capacity and a substantially larger dust collection system.
When is underwater pelletizing worth the extra complexity?
Underwater pelletizing earns its place with low-viscosity products that cannot hold a strand, with elastomers, TPE and hot-melt adhesives, with high-output single-product lines above roughly 1000 kg/h, and where a customer specifies spherical, dust-free pellets. It brings a process water loop, a heat exchanger, seal maintenance and longer changeover. For a job-shop compounder running frequent color and material changes, strand pelletizing usually remains the better economic and operational choice.
How many temperature control zones should the barrel have?
Most compounding machines use 6 to 12 zones, with one zone per barrel segment or per pair of segments plus separate control on the die and any adapter. More zones give finer control over the melting profile and are valuable for heat-sensitive systems such as PVC compound, flame-retardant grades and biodegradable polyesters. Zone count should be matched with proportional-valve cooling if the process needs to hold within about 1 to 2 degrees Celsius, because solenoid on-off cooling cannot achieve that band reliably.
Do I need both an atmospheric vent and a vacuum vent?
Two-stage venting is recommended whenever the feedstock carries a significant moisture or volatile load, which includes recycled material, undried filler and reactive systems. The atmospheric vent removes the bulk of the vapor at low cost, so the vacuum section only has to handle the remainder and can reach a deeper effective vacuum. For dried virgin resin with light additive packages, a single vacuum vent is normally sufficient.
What data interfaces should the line support?
Specify OPC UA as the primary interface for process data, recipe download and batch reporting, since it is the current direction of travel for plastics machinery connectivity. EUROMAP 63 is still present on older equipment and may be required for compatibility with an existing plant system. Agreeing the tag list, the sampling interval and the responsibility for the connection at the purchase stage avoids a costly integration project after commissioning.
How much floor space does a complete compounding line need?
A 100 to 300 kg/h line typically occupies a straight run of about 15 to 22 metres with a width of 5 to 7 metres, plus mezzanine height of at least 4.5 metres for the feeder platform. A 500 to 1000 kg/h line generally needs 25 to 40 metres of length, 8 to 12 metres of width and 6 metres or more of clear height. Add separate areas for raw material storage, finished goods, the utility plant room and the laboratory, and leave maintenance access along at least one side of the barrel for screw pulling.
Conclusion
Building a twin screw compounding plastic factory means specifying roughly thirty machines that must work as one continuous system. The extruder sets what the plant can make, but the feeders set how accurately it makes it, the vacuum and filtration set how clean the melt arrives at the die, the pelletizing choice sets the shape and dust level the customer receives, and the utilities set whether all of it still works at full rate on the hottest day of the year. Reading the plant as a material flow, station by station, is the only reliable way to avoid the gaps that appear when equipment is bought as a collection of individual machines.
The practical sequence is straightforward. Fix the target throughput and the product family first, because those determine screw diameter, specific torque, L/D and feeder count. Then choose the pelletizing method, since it dictates the cooling, drying and water treatment scope. Then size utilities against the real heat and dust load rather than a rule of thumb. Finally decide what to install now and what to reserve space for, installing everything on the melt path and everything that is hard to retrofit. Kerke, a Wanplas factory with more than twelve years of specialization in parallel co-rotating twin-screw compounding extruders, supplies these lines as integrated systems covering feeding, extrusion, pelletizing and auxiliary equipment, so that the interfaces between stations are engineered once rather than negotiated on site.
