Plastics machinery series
Sheet extrusion line
Melt, filter, spread through a flat die, then cool and polish on rolls
Sheet lines make PP, PS/HIPS and PET sheet for thermoforming, as well as thicker sheet for industrial parts. This page opens the line unit by unit, from the dryer to the winder, with what to check on the spec sheet, how wear shows up, how to size the line and what to inspect in maintenance.
Marketing Department, Advanced Chem Solutions · 22 Oct 2026 · 16 min read
Machine overview
A sheet line is an extruder coupled to a flat die and a roll stack. Dried, blended pellets are melted, filtered and pushed into the die at steady pressure. The die spreads the melt into a wide curtain; the rolls squeeze it, cool it and give it its surface. The sheet is then gauged, edge-trimmed, pulled and either wound or cut to length.
Sheet quality comes down to three things: a steady melt flow (screw and melt pump), even distribution across the width (die) and uniform cooling (rolls). Almost every sheet problem traces back to one of the three.
- Dryer / crystallizer — dries PET before melting. PP normally needs no drying; PS/HIPS is dried when pellets or regrind carry moisture.
- Gravimetric blender / dosing unit — weighs virgin resin, regrind and masterbatch to the recipe before the feed throat.
- Main extruder — vented single-screw or twin-screw; melts and builds pressure. Includes motor and gearbox.
- Co-extruder — smaller extruder feeding a skin layer or another layer of a multilayer sheet.
- Screen changer — filters dirt and unmelted material; screens can be changed while running.
- Melt pump (gear pump) — delivers a fixed volume per revolution and isolates the die from screw surging.
- Feedblock — combines the melt streams into A/B/A or more layers before the die.
- Flat die — spreads the melt into a wide curtain; thickness is trimmed with the flex lip and die bolts.
- Roll stack / calender — squeezes, cools and polishes the sheet.
- Thickness gauge — scans across the width and feeds automatic profile control.
- Edge trim + haul-off — cuts the thick edges for regrind; the pull rolls hold line speed and tension.
- Winder or cutter-stacker — winds thin sheet; thick sheet is cut to length and stacked.
Unit by unit
Each table answers four questions: what the part does, what to check on the quotation or at acceptance, and what you see on the sheet when it wears or fails. Use it as a checklist when comparing quotations or chasing a problem on the floor.
1. Material preparation: drying, crystallizing and blending
PET picks up moisture from the air. Melted wet, it hydrolyses: chains break, IV drops and the sheet turns brittle. PP and PS absorb little water, but regrind that was washed or stored outdoors can still be wet. The commonly quoted PET drying window is 160–180 °C for 4–6 hours with dry air at a dew point of −40 °C or lower, to reach roughly 50 ppm moisture or less (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 |
|---|---|---|---|
| Desiccant dryer | Blows hot, very dry air through the pellets in the drying hopper | Airflow, dew point actually achieved, hopper size vs output, regeneration system | Dew point creeps up; PET sheet turns hazy, bubbly or brittle; IV drops |
| Crystallizer | Crystallizes amorphous PET such as regrind and flake before the dryer | Temperature, agitator, residence time | Pellets fuse into lumps in the dryer, stop flowing, output fluctuates |
| Gravimetric blender | Doses virgin, regrind and MB by weight | Number of components, load-cell resolution, hourly capacity | Colour drifts in bands, MB use above recipe, load cell out of calibration |
2. Melting unit: main extruder and co-extruders
Most sheet lines run single-screw extruders. When moisture or volatiles must come out, a two-stage screw with a vent connected to a vacuum pump is used. The first stage melts; a deeper decompression section lowers the pressure under the vent so vapour can escape; the second stage rebuilds pressure. The second stage must pump more than the first, otherwise melt rises into the vent (vent flooding). Vented screws are therefore longer, typically L/D 30:1 or more (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 |
|---|---|---|---|
| Screw | Conveys, melts, mixes and builds pressure | Diameter, L/D, flight design (barrier, mixing section), material and hardfacing | Less output per rpm, higher melt temperature, more gels |
| Barrel and heaters | Heats and controls temperature zone by zone | Number of zones, heater type, fan or water cooling, liner material | A zone that won't heat or won't hold; screw-to-barrel clearance grows |
| Vent and vacuum pump | Pulls moisture and volatiles out of the melt | Vacuum level, condensate trap, a vent port you can clean | Bubbles or haze in the sheet, blocked vent, vent flooding |
| Motor and gearbox (drive) | Turns the screw at the required torque | Motor power, maximum torque, gear ratio, thrust bearing | Noise, vibration, hot gear oil or metal particles, unstable motor current |
| Co-rotating twin-screw for PET | Melts and devolatilizes through several vacuum vents, with no pre-drying | Number and vacuum level of vents, screw element layout, IV control | IV drops when vacuum weakens; sheet yellows or turns brittle |
| Co-extruder | Feeds the skin, colour or functional layer | The lowest output it can run steadily, not just the maximum, because skins are thin | Uneven skin thickness, colour drift, matte bands |
3. Filtration and pressure: screen changer, melt pump, feedblock
Every screw delivers melt with a small pulse at screw frequency. Lines that need tight thickness tolerance therefore put a melt pump between extruder and die. The gear pump delivers a fixed volume per revolution, and the control loop adjusts screw speed to hold the pump inlet pressure constant. The feedblock combines the layers before the die: the layer sequence is set by a selector plug, and the thickness ratio by each extruder's output.
| Part | Job | What to check on the spec / at acceptance | Symptom when worn or failing |
|---|---|---|---|
| Screen changer (slide plate / piston / continuous) | Swaps a clogged screen without stopping the line; continuous types never interrupt flow | Filter area, number of screen cavities, venting after a change, seals, suitability for regrind or PCR | Leaks at the seals, pressure spikes or air bubbles after a change, contamination getting through |
| Melt pump | Meters a constant volume and isolates the die from screw surging | Displacement per revolution, maximum inlet/outlet pressure, pressure control | Pump speed must rise for the same output (worn gears); thickness varies along the length |
| Feedblock | Builds A/B/A or more layers before the die | Number of layers, how the sequence is changed, adjustable flow channels | Skin layer not covering the full width, wavy layer edges, black specks from dead spots |
4. Flat die
Most flat dies use a coat-hanger manifold: a triangular channel designed so that the flow path and resistance from the inlet to every point across the width are about the same. After the manifold, a choker bar (restrictor bar) makes coarse adjustments across the width, and the lip is then trimmed finely with die bolts. For sheet, the lip opening is set close to the final thickness because the sheet is drawn very little, unlike cast film, which is drawn down heavily.
| Part | Job | What to check on the spec / at acceptance | Symptom when worn or failing |
|---|---|---|---|
| Coat-hanger manifold | Spreads the melt evenly over the full width | Designed for your resin and viscosity range; polished, hardened flow surfaces | Thick centre and thin edges (or the reverse) even after bolt adjustment; dead spots degrade material |
| Choker bar / restrictor bar | Coarse flow adjustment across the width | Adjustment range, number of adjustment points | Coarse profile out of shape, forcing the lip to be bent too far |
| Flex lip and die bolts | Fine adjustment of the lip gap point by point | Bolt spacing, adjustment range, lip material and plating | Die lines from a nicked lip, wavy thickness profile, seized bolts |
| Automatic die bolts (thermal actuators) | A heater in each bolt expands or contracts it to push the lip on command from the gauge | Number of actuators, response speed, mapping to gauge positions | Some positions don't respond (failed heater), profile hunts back and forth |
| Die heaters and deckles | Zone temperature control; deckles reduce width for a product change | Number of zones, thermocouple positions, deckle range | Edges abnormally thick or thin, material stagnating behind the deckle and degrading |
5. Roll stack and cooling
The standard stack has three rolls. The melt curtain enters the nip between the first and second roll, where a small, even bead of melt should sit across the full width: the rolling bank. Too large a bank causes chatter marks and frozen-in stress; no bank at all means the rolls are not fully contacting and the surface will not be smooth. Each roll has its own temperature control unit (TCU). Unpressurized water works up to roughly 90 °C; above that, pressurized water or oil is used (typical reference range — confirm with the machine maker and the TDS).
Amorphous PET (APET) has to be quenched quickly below its Tg, around 70–80 °C, to stay clear; cooled slowly, it starts to crystallize and turns hazy. PP needs roll temperatures that give the same crystallization on both faces, or the sheet curls.
| Part | Job | What to check on the spec / at acceptance | Symptom when worn or failing |
|---|---|---|---|
| Chrome-plated rolls | Squeeze, cool and transfer a gloss, matte or embossed finish | Diameter, face width, plating thickness, finish, deflection compensation (crown), internal water channels | Marks repeating once per roll revolution, matte bands, flaking chrome |
| Nip / roll-gap system | Sets the gap between rolls and the nip force | Hydraulic or servo, resolution, left/right parallelism | Sheet thicker on one side, tilted rolling bank |
| Temperature control units (TCU) | Hold the surface temperature of each roll | Temperature range, heating and cooling capacity, flow rate, accuracy | Uneven roll temperature across the width, curl, gloss variation |
| Cooling rolls / conveyor | Support the sheet until it is cool enough to cut or wind | Length of the cooling section, free-running rollers, level | Scratches; thick sheet warps after cutting |
6. Downstream: gauging, trimming, pulling, winding
A scanning gauge traverses the width, measuring by absorption of beta radiation, X-rays or infrared, and feeds the profile to the automatic die-bolt control. Beta gauges contain a radioactive source and need licensing and handling under your country's radiation law; X-ray gauges also need radiation-safety measures.
| Part | Job | What to check on the spec / at acceptance | Symptom when worn or failing |
|---|---|---|---|
| Scanning thickness gauge (beta / X-ray / IR) | Measures thickness or weight per area across the width | Measurable thickness range, scan speed, suitability for your resin and layers, licences required | Drift, readings that disagree with a micrometer, profile shifted in position |
| Automatic profile control | Drives each die bolt from the measured profile | Mapping of bolts to measurement points, neck-in compensation | Corrections in the wrong place, profile hunting |
| Edge trim knives | Remove the thick edges caused by neck-in | Knife type, positioning, edge extraction | Ragged or fibrous edges, plastic dust on the sheet |
| Edge recycle | Grinds the trim and feeds it straight back to the throat, or stores it for blending | Granulator, conveying fan, control of the return ratio | Regrind ratio swings, and thickness and colour swing with it |
| Haul-off / pull rolls | Pull the sheet at a constant speed matched to the roll stack | Rubber-covered rolls, speed synchronization | Sheet stretches or sags, thickness varies along the length |
| Corona and laminating station | Raise surface energy for printing, or laminate a protective or barrier film | Corona power and width, ozone extraction, film tension control | Ink won't adhere; laminated film wrinkles or traps bubbles |
| Winder (shaft / turret) | Winds thin sheet; a turret changes shafts automatically without stopping the line | Maximum roll diameter, tension control, changeover speed | Coned or telescoped rolls, uneven roll edges, blocking |
| Cutter-stacker | Cuts sheet too thick to wind into panels and stacks them | Cut-length accuracy, blade type, stacking system | Out-of-square cuts, varying length, surface marks |
Line types
No configuration is best for every job. Choose by resin, thickness range, number of layers and how the sheet reaches the thermoformer.
| Type | Suits | Advantages | Limits |
|---|---|---|---|
| Single-screw, no vent | Dry PP and PS/HIPS, pre-dried PET | Simple, easy to maintain | PET needs a dryer and crystallizer; volatiles cannot be removed |
| Vented single-screw (two-stage) | Pellets or regrind with a little moisture or volatiles | Devolatilizes in one machine | Vent flooding must be controlled; longer screw |
| Co-rotating twin-screw with vacuum vents | PET including flake and regrind, without pre-drying | Removes the dryer and crystallizer; good colour and additive mixing | Vacuum and IV must be watched closely; more complex machine |
| Mono-layer | General sheet, single colour | Fewer machines, fast product changes | Additives must go through the full thickness |
| Multilayer (A/B/A or more) | Gloss or coloured skins, regrind or PCR core, barrier sheet | Additives only in the skins; high regrind in the core with a clean surface | Needs co-extruders and a feedblock; layer ratio must be controlled |
| Vertical, horizontal or inclined roll stack | Depends on resin, thickness range and die position | Vertical is compact; horizontal lets the die point down into the nip; inclined gives easier access | The wrong choice means a long die-to-nip distance or hard cleaning; discuss with the machine maker |
| Roll-fed or cut sheet | Thin sheet wound for thermoforming; thick sheet cut to panels | Rolls feed the thermoformer continuously | Thick sheet cannot be wound; panels need storage space |
| Inline with the thermoformer or offline | One high-volume product, or many products and sizes | Inline uses the heat left in the sheet and needs no roll storage | Inline machines stop together; offline sheet must be reheated |
A commonly quoted split for thermoforming: thin gauge below about 1.5 mm, usually roll-fed; heavy gauge above about 3 mm, usually cut sheet (typical reference range — confirm with the machine maker and the TDS).
How to size the line
Start from the sheet you need and work back to the die, the rolls and the extruder. Do not start from the screw size.
- Define the product Net width, thickness range, resin, number of layers, gloss or matte finish, and delivery form (rolls or panels).
- Find the die width Die width = net width + trim on both edges + neck-in between die and rolls. If you run several widths, check the deckle range.
- Calculate output kg/h = gross width incl. trim (m) × thickness (m) × line speed (m/min) × 60 × density (kg/m³). Example: PP sheet 1.0 m gross, 0.8 mm thick, density about 900 kg/m³, at 6 m/min → 1.0 × 0.0008 × 6 × 60 × 900 ≈ 260 kg/h.
- Check the cooling limit Thick sheet is limited by how much heat the rolls can remove (roll size, wrap length and TCU capacity), not by screw output. If the rolls cannot keep up, line speed has to come down. Thin sheet is usually limited by extruder output instead.
- Choose extruders and split the layers Output per extruder = total output × layer ratio. A co-extruder must run steadily at the lowest output of the thinnest skin, not just reach the maximum. Pick screws that run in the maker's recommended speed range, not flat out all the time.
- Dryer size (for PET) Hopper capacity ≈ output × drying time. Example: 300 kg/h × 5 hours ≈ 1,500 kg of pellets in the hopper.
- Downstream Maximum haul-off and winder speed, the largest roll diameter the thermoformer accepts, or the cut length of the cutter-stacker.
The examples above are arithmetic to show the method and use typical reference ranges — confirm with the machine maker and the TDS.
Maintenance checkpoints
A sheet line runs for days without stopping, so small problems left alone turn into full rolls of scrap. The most dangerous point is the nip between rolls, which can pull in a hand or arm in a fraction of a second.
| Frequency | Checkpoint | What to look at | If neglected |
|---|---|---|---|
| Every shift | Guards, emergency stops and trip wires at the roll stack | Test that the rolls really stop, guards are closed, interlocks are not bypassed | Hands or arms drawn into the nip |
| Every shift | Die lip | Build-up; lines on the sheet in the same position | Die lines; build-up breaking off as black specks |
| Every shift | Pressure before and after the screen, melt-pump inlet pressure, vent port | Pressure trending up or oscillating; melt rising in the vent | Screen clogs and ruptures, contamination in the sheet, bubbles |
| Every shift | Roll surfaces and every TCU temperature | Scratches, deposits, actual vs set values | Marks repeating every revolution, gloss variation, curl |
| Weekly | Gravimetric blender | Check load cells with test weights; weigh the real recipe against the setpoint | Over- or under-dosing MB without noticing; colour drift |
| Weekly | Dryer | Dew point, air filters, inlet air temperature | Wet PET, IV loss, brittle sheet |
| Weekly | Edge trim knives and screen changer seals | Sharpness, leaks | Ragged edges; leaking melt that degrades and burns |
| Monthly | TCUs and water system | Scale, pumps, filters, water quality | Uneven roll temperature; blocked roll channels |
| Monthly | Thickness gauge | Standardize against reference samples or a micrometer | Automatic control corrects to a wrong reading |
| Monthly | Barrel and die heaters and thermocouples | Heater current; readings vs an independent instrument | Zones running hot or cold with no alarm |
| Yearly | Screw, barrel, melt pump | Measure screw-to-barrel clearance; compare output per rpm with the new-machine figure | Output loss, high melt temperature, thickness variation |
| Yearly | Die and roll surfaces | Strip and clean the die, polish the lip, inspect the chrome, re-plate when needed | Permanent lines on the sheet, loss of gloss |
| Yearly | Gearbox and beta gauge | Oil, vibration; source leak test as required by law | Major gearbox damage; breach of radiation rules |
- Never clean rolls or pull sheet at the nip while the rolls are turning.
- Never scrape the die lip with steel tools; use brass or copper so the lip does not nick.
- Never work on heaters, the die or the nip hydraulics without lock-out/tag-out, power isolated and pressure released.
- The die and barrel run above 200 °C; wear heat-resistant gloves and a face shield when opening the die or the screen changer.
Masterbatch and additives on this line
Masterbatch enters at the gravimetric blender above the throat of each layer's extruder. In multilayer sheet, put additives only in the layer that needs them, such as colour, UV or antistatic in the skins, and calculate the let-down against that layer: actual use relative to the whole sheet = let-down × layer fraction. A single screw has limited distributive mixing; an MB whose carrier MFI is far from the base resin shows up as colour streaks. Use a screw with a mixing section and choose a carrier that matches the base resin.
- Colour and white MB Poorly dispersed TiO₂ makes screen pressure rise fast and causes lines on the sheet. After an MB change, watch the screen-pressure trend.
- Antistatic For electronics trays and sheet that attracts dust. Choose between migrating and permanent types according to service life. See dust and static on parts.
- Nucleating agents and clarifiers For PP sheet that needs stiffer or clearer cups. They shift the crystallization temperature, so roll temperatures and the forming window need re-tuning. See nucleating agents.
- Slip and antiblock For thin sheet that blocks on the roll, or cups that are hard to denest. See film blocking and slip.
- Regrind and edge trim Keep the ratio constant hour by hour. Re-melted PET loses IV; re-melted PP can yellow and embrittle. At high ratios, review the stabilizer package. See inconsistent recycled resin.
To be straight about it: lines from a nicked lip, thickness bands from a seized die bolt, gloss bands from a failed TCU or black specks from a dead spot in the feedblock are machine problems, and no additive will fix them. When you weigh a formulation change, count cost per good sheet, not price per kilogram — try your own numbers in the cost-in-use calculator. If you want help separating a formulation problem from a machine problem, the ACS team can look at your data.
Related process articles
The three-zone screw, the die and the parameters to control are covered in Extrusion; heating the sheet and forming trays and cups is in Thermoforming. Common sheet defects: gels and fish-eyes · sharkskin (melt fracture) · black specks · colour streaks
FAQ
Sheet extrusion line FAQ
What should I check when buying a used sheet line?
Check the four most expensive repairs: screw and barrel (measure clearance and compare output per rpm), the die lip and flow channels (nicks, corrosion, die bolts that still turn), the chrome on the rolls (marks, flaking plating) and the gearbox (noise, vibration, oil). Ask for a trial run with the resin you will actually use and measure the thickness profile across the width. If the line has a beta gauge, check the licence and the paperwork for transferring the radioactive source.
The sheet is uneven across the width. Where do I start?
Look at the pattern first. Thick or thin spots that repeat in the same position usually point to a die bolt or the lip. Thick centre and thin edges (or the reverse) across the whole sheet point to die zone temperatures and the choker bar. One side thicker than the other points to roll-gap parallelism and roll temperatures. Variation along the length points to feeding, melt-pump inlet pressure and the haul-off. Also confirm that the gauge agrees with a micrometer before letting the automatic control correct to it.
Does PET always have to be dried before sheet extrusion?
On a single-screw line, yes, and amorphous regrind or flake must go through a crystallizer first or it will fuse into lumps in the dryer. A co-rotating twin-screw with vacuum vents is designed to remove the moisture inside the extruder without pre-drying, but the vacuum must be held at all times and the sheet IV checked regularly, because moisture that gets through lowers IV and makes the sheet brittle.
How do I change colour or resin on a sheet line quickly and with little scrap?
Sequence jobs from light to dark colours, and from lower to higher melt-temperature resins when possible. Reduce the material held in the system by running down the hopper and blender before the change. Use a purging compound suited to the resin and temperature, and raise screw speed during the purge to increase shear. Colour hangs up longest in the screen changer, the feedblock and the die edges; if streaks persist for a long time after a change, these may need to be stripped and cleaned.
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.
