---
type: process
name: "Plastic Extrusion"
category: "Forming"
subcategory: "Plastics and Rubber"
materials: ["Plastic"]
tolerances: "±0.005 in (±0.13 mm) on small dimensions held against a calibrator; ±0.020–0.060 in (±0.5–1.5 mm) on free-extruded surfaces of large profiles; angles ±1–2°. Bow and twist are specified separately per unit length."
volumes: "Quoted by weight — minimum runs of several hundred to a few thousand pounds; annual volumes reach millions of feet"
lead_time: "3–6 weeks for a custom profile die plus calibration tooling, including one or more die correction iterations; days per production run thereafter"
url: https://manufacturingprocesses.org/processes/forming/plastic-extrusion
---

# Plastic Extrusion

Plastic extrusion pushes molten polymer through a shaped die and cools it, producing continuous profile, sheet, pipe or film of constant cross-section.

- **Category**: [Forming](https://manufacturingprocesses.org/processes/forming.md)
- **Family**: Plastics and Rubber
- **Materials**: Plastic
- **Typical tolerances**: ±0.005 in (±0.13 mm) on small dimensions held against a calibrator; ±0.020–0.060 in (±0.5–1.5 mm) on free-extruded surfaces of large profiles; angles ±1–2°. Bow and twist are specified separately per unit length.
- **Surface finish**: Smooth gloss as extruded; matte or embossed using a textured calibrator or embossing roll; sheet gloss is set by the polishing stack
- **Typical volumes**: Quoted by weight — minimum runs of several hundred to a few thousand pounds; annual volumes reach millions of feet
- **Lead time**: 3–6 weeks for a custom profile die plus calibration tooling, including one or more die correction iterations; days per production run thereafter

## Overview

Plastic extrusion pushes molten polymer continuously through a shaped die and cools it into a constant cross-section: pipe, tube, window profile, wire insulation, sheet, and film. It is the highest-volume plastics process by weight and the only economical way to make a part whose length is arbitrary and whose cross-section never changes.

Almost every thermoplastic extrudes — rigid and flexible PVC, HDPE, LDPE, PP, ABS, polycarbonate, nylon, TPE — and co-extrusion combines two or three of them in a single pass. That is why a window profile can carry a rigid PVC body, a soft TPE gasket, and a UV-stable capstock all formed at once.

Tooling is a die plate plus calibration and cooling hardware, roughly an order of magnitude cheaper than an injection mold and typically 3–6 weeks out. Minimum orders are quoted in pounds of resin rather than in pieces, because startup scrap while the line stabilizes is the real setup cost.

## How it works

1. **Feed and melt.** Pellets drop into a single-screw extruder, typically 24:1 to 32:1 length-to-diameter with a compression ratio around 2:1–4:1. The screw conveys, compresses, and melts the resin through a combination of barrel heat and shear work. Hygroscopic resins — nylon, PC, PET, ABS — are dried first, exactly as for injection molding.

2. **Melt through the die.** Melt temperatures follow the resin: rigid PVC 330–390 °F (165–200 °C), HDPE 350–420 °F (175–215 °C), PP 400–480 °F (200–250 °C), ABS 410–480 °F (210–250 °C). The die converts round barrel flow into the profile cross-section, and its channels are balanced so every part of the section leaves at the same velocity. Unbalanced flow is what makes a profile twist.

3. **Calibrate and cool.** The hot extrudate is drawn immediately into a vacuum calibration sizer that pulls it against a fixed cooled surface, then through a water tank. Calibration is what makes tight tolerances possible — free-extruded profiles that touch nothing but air hold far less.

4. **Haul off.** A caterpillar puller draws the profile at constant speed. The ratio of haul-off speed to die output, the draw-down ratio, is the operator's main dimensional control, and it interacts with die swell — the elastic recovery that makes the extrudate expand as it leaves the die. Die swell can be substantial for polyolefins, which is why a die opening never matches the finished profile shape.

5. **Cut or coil.** Rigid profiles are flying-saw cut to length; flexible tube and wire insulation is coiled.

### How are sheet and film different?

Sheet uses a wide flat (coat-hanger) die feeding a three-roll polishing stack, which sets both thickness and surface gloss. Blown film extrudes an upward annular bubble that is inflated and cooled in air, then collapsed and wound. Both share the same first three steps and differ only in downstream equipment.

## Design guidelines

### Uniform wall

This is the whole discipline. Thick sections cool more slowly, shrink later, and drag the surrounding profile out of shape — the extruded equivalent of a sink mark, except it warps the entire cross-section rather than marking one face. Hold wall thickness within about ±10% across the profile, and where you cannot, core out the thick region rather than leaving it solid.

### Minimum wall

Rigid PVC and ABS profiles are practical from about 0.030 in (0.75 mm), with 0.050 in (1.3 mm) a safer design target. Below that the hot profile cannot support itself between die and calibrator and collapses under the calibration vacuum. Flexible materials tolerate thinner sections.

### Radii

No sharp corners, inside or out. Use at least 0.015–0.030 in (0.4–0.75 mm) on internal corners; larger is better. Sharp internal corners are stress risers in the part and hot spots in the die where melt hangs up and degrades.

### Symmetry and unsupported legs

Make the section symmetric about at least one axis where you can — asymmetric sections need more die-balancing iterations and are more prone to twist. Avoid long, thin, unsupported legs; a leg longer than about 10× its own thickness will flutter and wander. Tie it back with a web or thicken its tip slightly.

### Hollows

Hollow and multi-hollow profiles need a mandrel supported by a spider or bridge, which leaves weld lines where the melt recombines. They cost more to tool but hold better tolerances, because the internal surface is supported by an internal calibrator. Keep internal webs the same thickness as the outer wall.

### Tolerances

Specify tolerance only where it matters, and understand where it comes from. Dimensions held against a calibrator can reach ±0.005 in (±0.13 mm) on small sections. Free-extruded outside dimensions on large profiles are more like ±0.020–0.060 in (±0.5–1.5 mm), and angles run ±1–2°. Bow and twist are separate callouts specified per unit length. For matching polymer stiffness and strength to the requirement, see [/charts/material-properties](/charts/material-properties).

### Secondary operations

Anything that is not a constant cross-section — holes, notches, end machining, bends — is a separate operation on cut lengths. Design those features to locate off a surface the extrusion actually holds, such as a calibrated internal bore, rather than a free-cooled outside face.

| Feature | Recommended | Limit | Why |
| --- | --- | --- | --- |
| Wall uniformity | Within ±10% across section | 2:1 thick-to-thin | Thick sections shrink last and warp the profile |
| Minimum wall (rigid) | 0.050 in (1.3 mm) | 0.030 in (0.75 mm) | Thinner sections collapse under calibration vacuum |
| Internal radius | 0.030 in (0.75 mm) or more | 0.015 in (0.4 mm) | Sharp corners are die hot spots and part stress risers |
| Unsupported leg | ≤ 5 × its thickness | about 10 × thickness | Long legs flutter and lose position |
| Hollow web thickness | Equal to the outer wall | — | Mismatched webs cool at different rates and pull the section |
| Tolerance, calibrated | ±0.005 in (±0.13 mm) | — | Only surfaces touching the calibrator hold this |
| Tolerance, free surface | ±0.020–0.060 in (±0.5–1.5 mm) | — | Nothing constrains a free-cooled surface |

## Cost drivers

Extrusion inverts the injection molding cost structure. The die is comparatively cheap, so the recurring costs — resin, line speed, and startup scrap — dominate. Resin is usually the majority of the delivered cost of a commodity profile, which makes cross-sectional area (pounds per foot) the single most important design variable.

Volume breakpoints are expressed in weight, not pieces. A custom profile normally carries a minimum run of several hundred to a few thousand pounds, because a line takes time and material to stabilize regardless of how much product you want.

Cost reduction:

1. **Take area out of the cross-section.** Every square inch of section is pounds per foot forever. Core out solid regions and replace them with ribbed or hollow sections that carry the same load.
2. **Widen tolerances on non-critical surfaces.** Every dimension held against a calibrator adds tooling and slows the line. Free-extruded surfaces are essentially free.
3. **Co-extrude instead of assembling.** A soft TPE seal co-extruded onto a rigid profile removes a separate gasket, an adhesive, and an assembly step.
4. **Consolidate into fewer profiles.** Two similar profiles mean two dies, two setups, and two rounds of startup scrap. One profile serving both applications, trimmed differently, usually wins.
5. **Design so the die can be balanced in one iteration.** Symmetric, uniform-wall sections with generous radii reach a sellable profile faster; complex asymmetric ones can take several die corrections, each of which is chargeable.

## FAQ

### What tolerance can plastic extrusion hold?

Dimensions held against a vacuum calibrator on a small section can reach ±0.005 in (±0.13 mm). Free-extruded surfaces on large profiles are more like ±0.020–0.060 in (±0.5–1.5 mm), and angles run ±1–2°. Bow and twist are separate callouts specified per unit length.

### What is the minimum wall thickness for an extruded plastic profile?

About 0.030 in (0.75 mm) for rigid PVC and ABS, with 0.050 in (1.3 mm) a safer design target. Below that the hot profile cannot support itself between the die and the calibrator and collapses under the calibration vacuum. Flexible materials tolerate thinner sections.

### Why does an extrusion die not match the profile shape?

Two effects intervene. Die swell — elastic recovery as the melt leaves the constraint of the die — expands the extrudate, and the haul-off then draws it back down. The die is cut to compensate for both, which is why custom profiles usually need one or more die correction iterations before first article approval.

### How much does an extrusion die cost compared with an injection mold?

Roughly an order of magnitude less. An extrusion die is a plate with a machined orifice plus calibration and cooling hardware, not a two-plate pressure-containing mold with an ejection system. The trade is that per-part cost is dominated by resin weight, so cross-sectional area matters far more than it does in molding.

### Can extruded profiles combine hard and soft materials?

Yes. Co-extrusion feeds two or three extruders into one die, so a rigid PVC window profile can carry a flexible TPE gasket and a UV-stable capstock in a single pass. It removes a separate gasket, its adhesive, and an assembly step, and it is standard practice in construction and automotive sealing profiles.

### How are holes and cutouts added to an extruded part?

As secondary operations on cut lengths, since extrusion can only produce a constant cross-section. Punching, drilling, notching, and end machining are done downstream, either in line with the puller or offline. Locate those features off a calibrated surface such as an internal bore, which the extrusion holds far better than a free-cooled outside face.

### Why do extruded profiles twist, and how do I prevent it?

Twist comes from unbalanced flow through the die and from non-uniform cooling, both of which are worsened by uneven wall thickness. Hold wall within about ±10% across the section, make the profile symmetric about at least one axis, and avoid heavy solid regions next to thin ones.

## Alternative processes

- [Pultrusion](https://manufacturingprocesses.org/processes/forming/pultrusion.md): Pultrusion pulls continuous fiber through a resin bath and a heated die, curing it into a constant-section composite profile of unlimited length.
- [Metal Extrusion](https://manufacturingprocesses.org/processes/forming/metal-extrusion.md): Metal extrusion pushes heated billet through a shaped die, producing continuous aluminum or copper profile of constant cross-section.
- [Roll Forming](https://manufacturingprocesses.org/processes/forming/roll-forming.md): Roll forming passes coil stock through a sequence of contoured roller stands that bend it progressively into a constant cross-section profile.
- [CNC Machining](https://manufacturingprocesses.org/processes/cutting/cnc-machining.md): CNC machining removes material from solid stock with a programmed cutting tool, producing accurate parts directly from a CAD model with no tooling.

## Related processes

- [Pultrusion](https://manufacturingprocesses.org/processes/forming/pultrusion.md): Pultrusion pulls continuous fiber through a resin bath and a heated die, curing it into a constant-section composite profile of unlimited length.
- [Blow Molding](https://manufacturingprocesses.org/processes/forming/blow-molding.md): Blow molding inflates a heated plastic tube or preform against the walls of a mold with compressed air, producing hollow one-piece containers and tanks.
- [Injection Molding](https://manufacturingprocesses.org/processes/forming/injection-molding.md): Injection molding forces molten thermoplastic into a steel mold under high pressure, producing high-volume identical plastic parts.
- [Metal Extrusion](https://manufacturingprocesses.org/processes/forming/metal-extrusion.md): Metal extrusion pushes heated billet through a shaped die, producing continuous aluminum or copper profile of constant cross-section.
- [Thermoforming](https://manufacturingprocesses.org/processes/forming/thermoforming.md): Thermoforming heats a plastic sheet until pliable and pulls or presses it over a single-sided mold, producing large thin-walled parts with low tooling cost.

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*Source: [Manufacturing Processes](https://manufacturingprocesses.org/processes/forming/plastic-extrusion)*

*Last updated: August 11, 2026*