Plastics machinery series
Twin-screw compounding line
the machine that makes masterbatch and compounds
Almost every masterbatch and compound used by moulders and extruders has passed through a co-rotating twin-screw extruder. This article opens the line unit by unit, from the feeders to the pelletizer, with the specs to check, how to size a line and what to maintain.
Marketing Department, Advanced Chem Solutions · 22 Oct 2026 · 14 min read
Machine overview
A compounder does not make parts. It takes carrier resin, pigments, additives and fillers, melts them, mixes them, removes volatiles and cuts the result into new pellets that all carry the same formulation. The output is either masterbatch (a concentrate let down at a few percent on the moulding or extrusion machine) or a compound (a ready-to-use recipe such as talc-filled PP or FR PP, used as is).
It differs from a single-screw extruder in three ways. First, the two screws intermesh and turn in the same direction (co-rotating intermeshing) and wipe each other clean (self-wiping), so material does not hang up. Second, the screws are built from individual screw elements on splined shafts and can be rearranged for each formulation. Third, the machine is starve-fed: the feeders set the throughput and the screw speed sets the mixing energy. Being able to adjust the two independently is the main reason compounding is done on this machine.
- Main loss-in-weight feeder — weighs its hopper continuously and trims auger speed to hold the set kg/h; multi-ingredient recipes use several
- Main motor — supplies the drive power, variable speed
- Gearbox — reduces speed, splits torque equally to both shafts and takes the back pressure from the die on a thrust bearing
- Barrel segments — bolted in series, each with its own heaters and cooling channels
- Co-rotating screws — screw elements on splined shafts, arranged for the job
- Side feeder — a small twin screw that pushes filler or glass fibre into the side of the barrel once the polymer has melted
- Atmospheric vent — lets out air carried in with powders and fillers
- Vacuum vent — pulls moisture, monomer and volatiles before the die, via a knock-out pot to the pump
- Screen changer and die plate — filters contamination, then splits the melt into strands
- Water bath — cools the strands until they can be cut
- Air knife — blows water off the strand surface before cutting
- Strand pelletizer — cuts strands into pellets, then on to the classifier and packing
Unit by unit
1. Feeding
Formulation accuracy is decided at the feeders, not at the screws. A starve-fed machine has no mechanism to correct a wrong ratio. There are two feeding strategies:
- Premix blends everything in a mixer (for example a high-speed or tumble mixer) and meters it with a single feeder. Simple and few feeders, but pellets and powders differ in size and density, so the blend can segregate on its way to the throat.
- Split feeding uses one feeder per ingredient, for example carrier pellets · a pigment or additive premix · filler via the side feeder. More accurate and faster to change over, but needs more feeders. Very minor ingredients are usually pre-blended with polymer powder first so the feeder runs in its accurate range.
| Part | Job | What to check on the spec / at acceptance | Symptom when worn or failing |
|---|---|---|---|
| Loss-in-weight feeder | Meters a constant kg/h by measuring weight loss over time and correcting auger speed | Accurate kg/h range (turndown) · auger type suited to pellets, powders or poor-flowing powders · load cell resolution · isolation from machine vibration | Feed rate fluctuates, colour or test values vary between bags, powder bridges in the hopper |
| Feeder refill system | Tops up the feeder hopper in cycles | Refill speed · how long the feeder runs unweighed during refill | Recipe drifts in step with the refill cycle |
| Side feeder (side stuffer) | Pushes talc, calcium carbonate or glass fibre into the barrel after melting | Screw size versus filler rate · venting of entrained air · position on the barrel | Powder backs up out of the opening, actual filler content below set point (ash value off) |
| Liquid injection pump | Adds oils or liquid additives straight into the barrel | Gravimetric or positive-displacement pump · non-return injector · injection point | Blocked injector, actual rate differs from set point, greasy pellet surface |
2. Drive
The best single number for a twin-screw extruder's capability is specific torque in Nm/cm³: torque per shaft divided by the cube of the centreline distance (Md/a³). It lets you compare machines of different sizes. Current machines are typically around 10–18 Nm/cm³; older machines are usually lower typical reference range — confirm with the machine maker and the TDS. Higher torque means more throughput at the same speed, and the ability to run viscous or highly filled recipes without raising screw speed until the melt overheats.
| Part | Job | What to check on the spec / at acceptance | Symptom when worn or failing |
|---|---|---|---|
| Main motor | Drives the screws | kW · speed range · speed control · motor cooling | Abnormally high current, hot motor, noise |
| Gearbox | Reduces speed, splits torque to two shafts, takes back pressure on a thrust bearing | Specific torque (Nm/cm³) · torque per shaft · maximum speed · lubrication and oil cooler | Whining, high oil temperature, metal particles in the oil, vibration |
| Safety coupling (torque limiter) | Disconnects the drive on overload to protect shafts and gearbox | Trip value · reset method · interlock with the motor | Frequent trips mean the recipe or settings are at the torque limit, or a foreign object entered |
3. Process section
The barrel is made of several segments; its total length divided by screw diameter is the L/D. Segments with openings (feed port, side feeder, vents) can be moved. Screw channel depth is expressed as Do/Di, outer diameter divided by root diameter. A higher value means deeper channels and more free volume for fluffy powders and fillers, but a thinner shaft that carries less torque. Most machines sit around 1.5–1.8 typical reference range — confirm with the machine maker and the TDS.
| Part | Job | What to check on the spec / at acceptance | Symptom when worn or failing |
|---|---|---|---|
| Barrel segments | Enclose the screws, control temperature zone by zone | Number of segments (= L/D) · wear- or corrosion-resistant bore (e.g. nitrided or bimetallic liner) · movable port positions | Clearance to the screw grows, less pressure build-up, higher melt temperature |
| Heaters and cooling channels | Heat at start-up, remove excess shear heat while running | Heater power per zone · cooling valve per zone · thermocouple position | Zone will not hold, temperature swings, failed heater, scaled-up cooling channels |
| Splined shafts | Transmit torque to every screw element | Spline type · rated shaft torque | Worn splines, elements hard to remove, twisted or broken shaft after overload |
| Screw elements | Convey, melt, mix, build pressure | Element types available · wear- or corrosion-resistant steel · Do/Di | Rounded flight tips and kneading disc edges, poorer mixing, incomplete degassing, back-up at the side feeder |
| Atmospheric vent | Releases air carried in with powders | Position · opening size | Powder blowing out, bubbles in pellets |
| Vacuum vent + vent stuffer + pump + knock-out pot | Pulls moisture and volatiles; the vent stuffer (a small side twin screw) pushes rising melt back into the barrel | Achievable vacuum · knock-out pot size · filter before the pump | Melt rising into the vent (vent flooding), blocked lines, pump damage, bubbles or odour in pellets |
Screw configuration
Each element type does a different job, so choosing and ordering the elements is part of the formulation, not just machine set-up.
- Conveying elements — wide pitch conveys fast and runs partially filled, used under feed ports and vents; narrow pitch builds pressure before the die.
- Kneading blocks — discs offset by a stagger angle. Forward 30–60° still conveys and shears moderately · 90° (neutral) does not convey, holds material back and shears more · reverse stagger opposes flow, fills the screw ahead of it and puts in the most energy.
- Disc width — wide discs shear hard and break agglomerates (dispersive mixing); narrow discs split and fold the flow often (distributive mixing).
- Mixing elements — toothed or slotted elements give distributive mixing at low shear, suited to shear-sensitive additives and to glass fibre that should not be chopped short.
- Reverse elements — used as a melt seal before the vacuum vent; the screw runs full here so air cannot leak in from upstream.
Dispersive mixing breaks pigment or filler agglomerates into smaller particles and needs shear or elongational stress higher than the agglomerate strength. Distributive mixing spreads particles evenly without making them smaller. Good masterbatch needs both. Residence time is short, typically about 10–60 seconds typical reference range — confirm with the machine maker and the TDS, so heat-sensitive additives degrade less than in machines with long hold-up.
4. Downstream: die and pelletizing
The pelletizing system is chosen by throughput and material. Strand pelletizing is low in investment and easy to clean between colours, so it is common for masterbatch. Underwater pelletizing suits high throughput and materials whose strands break easily. Die-face cutting (water-ring or hot-face) is used for highly filled grades whose strands are brittle.
| Part | Job | What to check on the spec / at acceptance | Symptom when worn or failing |
|---|---|---|---|
| Screen changer | Filters contamination and unmelted material | Slide-plate or continuous type · screen mesh · pressure rating | Pressure before the screen keeps rising, throughput drops |
| Die plate | Splits the melt into strands | Number and size of holes · heating | Blocked holes, strand breaks, uneven strands, die drool breaking off as black specks |
| Water bath | Cools strands until they can be cut | Bath length · water temperature · strand guide rolls | Strand breaks, moisture inside pellets, voids |
| Air knife | Blows water off the strands | Air pressure · position | Wet pellets that clump in the bag |
| Strand pelletizer | Cuts strands between a rotor and a bed knife | Strand take-off speed · pellet length · knife material | Tails, fines, twins, angled cuts |
| Underwater pelletizer | Cuts at the die face under water, giving round pellets | Water loop · centrifugal dryer · start-up procedure | Die holes freezing off at start-up, tails, agglomerates |
| Water-ring / hot-face pelletizer | Cuts at the die face and throws pellets into a water ring or air stream | Material suitability · knife speed | Pellets sticking together, fast knife wear with fillers |
| Classifier | Removes longs, twins and fines | Screen opening · capacity | Fines and off-size pellets reach the customer and upset their feeding |
| Dryer and packing | Removes surface moisture, then weighs and bags | Final moisture · scale accuracy | Wet pellets, bags off weight |
Machine types
| Type | Principle | Strengths | Limits | Best for |
|---|---|---|---|---|
| Co-rotating intermeshing twin screw | Self-wiping screws, interchangeable elements, starve-fed, high speed | Both dispersive and distributive mixing, easy recipe changes, short hold-up, mid-barrel filler feeding and degassing | Higher investment and element cost than single screw, needs accurate feeders, element wear must be tracked | Colour and additive masterbatch, filled and glass-fibre compounds, FR, upgrading recycled pellets |
| Counter-rotating twin screw | Screws turn in opposite directions, forming closed chambers that push material forward, low speed | Positive conveying, good pressure build-up, low shear | Low speed, less dispersive mixing | PVC from dry blend, e.g. pipe and profile |
| Single screw with mixing section | Flood-fed, throughput tied to screw speed | Simple, builds pressure well | Poor at breaking pigment agglomerates, limited distributive mixing | Re-pelletizing clean regrind, recipes using pre-dispersed ingredients |
| Internal batch mixer + discharge extruder | Mixes batch-wise with two rotors in a closed chamber, then a discharge extruder or roll mill forms pellets | Very high shear, takes high filler and rubber loadings | Batch-to-batch variation, more dust and manual work, needs a pelletizing step | Rubber, highly filled compounds, some black masterbatch |
| Reciprocating single-screw kneader | The screw rotates and oscillates past kneading pins in the barrel | Low, uniform shear, good distributive mixing | Lower throughput per size, needs a discharge extruder to build pressure | PVC and cable compounds, highly filled grades, heat-sensitive materials |
How to size the line
Start from the hardest recipe you need to run, not the easiest, and work through these steps:
- Set the kg/h target per recipe Separate the highly filled recipes, the hard-to-disperse pigments and the recipes that need heavy degassing; these three groups decide machine size.
- Find which limit the recipe hits first Torque-limited means torque runs out first because the recipe needs a lot of mixing energy (colour masterbatch, high filler in low-MFI resin); you need high specific torque. Volume-limited means the screw channels fill first because powders are fluffy with low bulk density; you need a higher Do/Di, or move the filler to a side feeder.
- Estimate power Shaft power (kW) ≈ specific energy (kWh/kg) × throughput (kg/h). General polyolefin compounding typically needs about 0.1–0.3 kWh/kg; hard-to-disperse recipes need more. Worked example: 500 kg/h × 0.2 kWh/kg = 100 kW at the shaft, then add margin for losses and future recipes.
- Understand torque versus speed Power (kW) = total torque (Nm) × speed (rpm) ÷ 9,550. At the same torque, higher speed gives more power, but also more shear heat. Many plants aim to run at about 70–90 % of maximum torque to leave headroom for feed fluctuations.
- Scale up from a lab machine Throughput rises with screw diameter to a power of roughly 2.5–3 (Q₂ = Q₁ × (D₂/D₁)^x), but barrel surface area grows more slowly, so large machines remove less heat through the barrel. Recheck melt temperature after scale-up. Development machines of about 16–30 mm run from a few to several tens of kg/h; production machines of about 40–75 mm run from several hundred to a few thousand kg/h, depending on the recipe.
- Choose L/D by the zones you need Feed, melting, side feed, mixing, degassing, pressure build-up. General masterbatch and compounding lines typically use L/D of about 32–48; heavy devolatilization or reactive processing needs longer.
- Make sure the downstream keeps up Bath length, pelletizer size, classifier and packing must handle the maximum kg/h. An undersized downstream becomes the bottleneck before the screws do.
Numbers in this section are typical reference ranges — confirm with the machine maker and the TDS. The most reliable method is to trial your actual recipe on a machine of similar size before deciding.
Maintenance checkpoints
A twin-screw line rarely fails suddenly; it wears slowly until pellet quality slips. The fastest way to catch it is to log torque, die pressure and melt temperature for each recipe as a baseline and compare every time the recipe runs again.
| Frequency | Checkpoint | What to look at | What happens if neglected |
|---|---|---|---|
| Every shift | All feeders | Actual kg/h versus set point, alarms, powder bridging | The recipe drifts unnoticed until the customer finds it |
| Every shift | Torque, die pressure, melt temperature | Compare with the baseline for the same recipe | Screw wear or a blocked screen goes unnoticed until scrap piles up |
| Every shift | Vacuum vent and knock-out pot | Vacuum reading, liquid level in the pot, melt in the vent | Bubbles or odour in pellets, pump damage |
| Every shift | Die face, water bath, air knife | Blocked holes, die drool, water temperature and level | Uneven pellets, black specks, wet pellets |
| Every shift | Safety | Pelletizer guards and interlocks, emergency stops, gloves and face shields for hot melt | Hands in the cutter, burns from melt spurting from the die or a vent |
| Weekly | Pelletizer knives | Sharpness, rotor-to-bed-knife gap | Tails, more fines |
| Weekly | Vacuum lines, filters, classifier screens | Deposits, blockage | Vacuum not reached, fines in the bag |
| Weekly | Dust around feeders and blending | Dust extraction working, no powder build-up on floor and frames | Cross-contamination between recipes, combustible-dust risk |
| Monthly | Feeder calibration | Check load cells with test weights and do a catch test (collect for a fixed time and weigh) | The feeder is off while the screen still shows the right number |
| Monthly | Gearbox | Oil level and temperature, filter, oil cooler, oil analysis at set intervals | Bearing or gear damage, long downtime |
| Monthly | Barrel cooling, heaters, thermocouples | Zone cooling valves, scale, failed heaters | Zones out of control, overheated melt |
| Yearly or at the maker's hour interval | Screw and barrel wear measurement | Pull the screws, measure element outer diameter and kneading disc edges, measure barrel bore at several points with a bore gauge, compare with new values | Poorer mixing, back-up at the side feeder, vent flooding, rising melt temperature |
| Yearly or at the maker's hour interval | Gearbox oil, coupling, safety coupling | Oil change, alignment check, test the safety coupling trip and interlocks | Broken shafts when a foreign object gets in |
| Every time the machine is opened | Lock-out/tag-out (LOTO) | Isolate and lock motor and heater power, release pressure before opening the die or screen changer, never stand in front of the die at start-up | Pressurised melt spurting out, screws turning with hands inside |
Masterbatch and additives on this machine
On this machine masterbatch is not added, it is made, so the quality the end user sees starts here. Where each ingredient enters matters: pellets and pigments go in at the main feed so they pass all the kneading zones; fillers and glass fibre go in through the side feeder after melting to reduce screw wear and keep fibres long; volatile or heat-sensitive additives go in further downstream, or the zone temperatures are kept low.
Dispersion and let-down ratio A more concentrated masterbatch uses a lower let-down and costs less per part, but each pellet must then spread through far more base resin on an injection or single-screw extrusion machine, which mixes much less than a twin screw. Agglomerates not broken during compounding show up later as colour streaks, specks, gels in film, or UV and antistatic performance that varies from part to part. Common dispersion tests are film extrusion with speck counting and the filter pressure value test of EN 13900-5. Most functional masterbatches are let down at about 0.5–3 %, while flame retardants need high loadings of around 20–30 % typical industry range — confirm by trial on your own machine. The cost logic of let-down is explained in Why a pricier masterbatch can be cheaper, and you can run your own numbers in the cost per good part calculator.
Why toll compounding works What makes two lots of masterbatch differ is the formulation, feeding sequence, feed positions, screw configuration and zone temperatures, not the machine itself. If these are written down precisely and followed, the same recipe run on twin-screw lines of similar specification gives similar results. ACS works this way: it designs the formulation and the production conditions, and production runs on partner compounders' twin-screw lines, while Circular recycled grades are made in the company's own plant (see how recycled pellets are made). The product list is at Products & TDS.
Problems no additive will fix Black specks from degraded material hanging up on worn elements or a dirty die face, bubbles from a blocked vacuum vent, and recipe drift from an uncalibrated feeder are all mechanical and must be fixed at the machine. More antioxidant or a different carrier only helps partly and mostly hides the symptom.
Related process articles
A compounder is a specialised extruder; screw melting, dies and cooling are explained in Extrusion. If the problem shows up in the finished part, see black specks · colour streaks · gels and fish-eyes.
FAQ
Twin-screw compounder questions we hear often
What should I check when buying a used twin-screw extruder?
Ask for the screw and barrel wear records, check the gearbox by listening to it and sending an oil sample for analysis, and look at the specific torque, because older machines are usually lower. Count the spare screw elements included, check that the feeders can still be calibrated and that control-system spares are available, then run your hardest recipe on the actual machine before you agree.
How do I know the screws and barrel are worn?
Compare with the baseline for the same recipe at the same speed and kg/h. Typical signs are higher melt temperature, fluctuating die pressure, powder backing up at the side feeder, melt rising into the vacuum vent and poorer dispersion. When several appear together, pull the screws and measure them against new values.
Why do pellets turn yellow or smell when I raise screw speed?
Higher speed adds shear heat, so the real melt temperature can run well above the barrel set points even though residence time gets shorter. Measure melt temperature at the die with a melt probe, then adjust speed, screw configuration or zone temperatures. Antioxidants help partly but do not fix the cause.
How can I change colour or recipe faster on a twin-screw line?
Sequence production from light to dark, purge with carrier resin or a purging compound, clean the feeders, vents and die face, change the screen and, if possible, dedicate separate lines to black and white. Self-wiping machines change over faster than machines with dead spots.
Caution
General knowledge for understanding only. Numbers are typical reference ranges. Selection, installation, operation and maintenance must follow the machine maker's manual and instructions and your plant's safety rules.
Let-down ratios and technical data on this page are typical reference values, not a guarantee of properties. Confirm by trial in your own process before production use.
