---
type: process
name: "Compression Molding"
category: "Forming"
subcategory: "Plastics and Rubber"
materials: ["Plastic"]
tolerances: "About ±0.005 in (±0.13 mm) on features contained within one mold half; ±0.010 in (±0.25 mm) or more on dimensions spanning the parting line, since flash thickness varies with charge weight. Rubber tolerance classes follow ISO 3302-1."
volumes: "100–50,000 parts per year; above that, injection or transfer molding usually takes over"
lead_time: "3–8 weeks for tooling; cycles of 1–10+ minutes depending on section thickness, plus deflash and any post-cure"
url: https://manufacturingprocesses.org/processes/forming/compression-molding
---

# Compression Molding

Compression molding presses a measured charge of rubber or thermoset into an open heated mold that closes to cure it into its final shape.

- **Category**: [Forming](https://manufacturingprocesses.org/processes/forming.md)
- **Family**: Plastics and Rubber
- **Materials**: Plastic
- **Typical tolerances**: About ±0.005 in (±0.13 mm) on features contained within one mold half; ±0.010 in (±0.25 mm) or more on dimensions spanning the parting line, since flash thickness varies with charge weight. Rubber tolerance classes follow ISO 3302-1.
- **Surface finish**: Reproduces the mold finish, from polished gloss to blasted matte; a flash witness line at the parting line is unavoidable
- **Typical volumes**: 100–50,000 parts per year; above that, injection or transfer molding usually takes over
- **Lead time**: 3–8 weeks for tooling; cycles of 1–10+ minutes depending on section thickness, plus deflash and any post-cure

## Overview

Compression molding places a measured charge of uncured rubber or thermoset compound into an open, heated mold and closes it under pressure, so the material flows to fill the cavity and cures in place. It is the oldest production molding process still in wide industrial use and remains the default for rubber seals, gaskets, bushings, and mounts, and for thermoset parts in phenolic, melamine, urea, and epoxy — electrical insulators, cookware handles, brake pads, and closures.

The two branches diverge in what they optimize. Compression molding rubber handles high-viscosity elastomer stocks that would scorch in a long injection runner. Compression molding plastic covers glass-reinforced thermosets such as SMC and BMC, where the reinforcement must not be broken up by shearing through a gate.

Tooling is simpler and cheaper than an injection mold because there is no runner, gate, or high-pressure injection unit — but cycles are measured in minutes.

## How it works

1. **Prepare the charge.** Rubber stock is calendered and cut, or extruded and slug-cut, to a preform of controlled weight. Thermoset molding compound arrives as pellets, granules, or a preformed puck; SMC is cut from sheet. Charge weight tolerance directly becomes flash thickness and part height variation, so it is weighed rather than estimated.

2. **Load.** The charge is placed in the open, heated cavity. Placement matters: the compound flows outward from where it sits, so a poorly placed charge produces knit lines, entrapped air, and uneven fiber orientation in reinforced compounds. Charge coverage of roughly 30–70% of the cavity area is typical for SMC.

3. **Close and pressurize.** The press closes and applies pressure on the projected area, commonly in the range of 300–2,000 psi (2–14 MPa) depending on compound viscosity and part complexity. Bulky phenolic charges sit at the high end; low-viscosity rubber compounds at the low end.

4. **Cure under heat and pressure.** Mold temperatures typically run 275–320 °F (135–160 °C) for SMC, 300–360 °F (150–180 °C) for phenolics, and 320–380 °F (160–195 °C) for most rubber compounds. Curing is heat-transfer limited, so cure time rises steeply with section thickness: thin rubber gaskets cure in 1–3 minutes while thick sections take 10 minutes or more. Bumping (briefly opening the press early in the cycle) vents volatiles and trapped air.

5. **Demold and deflash.** Thermosets are demolded hot, since they do not soften on reheating. Flash at the parting line is trimmed, tumbled, or cryogenically deflashed for rubber. Rubber parts for critical sealing applications are then post-cured in an oven to complete crosslinking and drive off residual volatiles.

### Where does transfer molding fit?

Transfer molding is the intermediate step: the charge is loaded into a separate pot and forced through a sprue into a closed cavity. It gives better dimensional control and allows encapsulating delicate inserts, at the cost of sprue scrap and a more complex tool.

## Design guidelines

### Wall thickness

Practical range is 0.060–0.250 in (1.5–6 mm) for most thermoset parts, and thicker for rubber. Uniformity matters less than in injection molding — there is no gate to freeze off — but cure time scales steeply with the thickest section, so a single heavy boss can double the cycle for the whole part.

### Draft

1–2° per side is the norm for thermosets; rubber can go to 0.5° or effectively zero, since a cured elastomer strips off tooling that a rigid part could never leave. Add draft on textured or deep-drawn features.

### Radii

Internal radii of at least 0.030 in (0.8 mm), and more on structural corners. Sharp internal corners in a filled thermoset are both stress risers and points where the flow front has to turn hard, which locally reorients fibers and creates a weak line.

### The parting line and flash

Every compression molded part has flash, because the mold closes on an excess charge and squeezes it out. Put the parting line where flash removal is easy and where a witness line is acceptable. Dimensions that span the parting line vary with charge weight and press closure, so keep tight tolerances inside a single mold half wherever you can.

### Undercuts, inserts, and ribs

Metal inserts, threaded bushings, and reinforcing frames are loaded into the cavity and encapsulated. Undercuts in rubber can often be stripped off directly because of the material's elongation; in rigid thermosets they need a split cavity or a loose piece. Ribs behave much as they do in thermoplastics — the rib and boss ratios tabulated on [/charts/injection-molding-design-guidelines](/charts/injection-molding-design-guidelines) transfer directly, because the sink-mark mechanism is the same shrinkage differential.

| Feature | Recommended | Limit | Why |
| --- | --- | --- | --- |
| Wall thickness | 0.090–0.190 in (2.3–4.8 mm) | 0.060–0.250 in (1.5–6 mm) | Cure time is set by the thickest section |
| Draft, thermoset | 1–2° per side | 0.5° | Rigid part must release from a hot mold |
| Draft, rubber | 0.5–1° per side | Effectively 0° | Elastomer elongation lets parts strip off |
| Internal radius | 0.060 in (1.5 mm) | 0.030 in (0.8 mm) | Sharp corners reorient fibers and concentrate stress |
| Rib base | 0.5–0.6 × wall | — | Same shrinkage-sink mechanism as thermoplastics |
| Tolerance, in one half | ±0.005 in (±0.13 mm) | — | Not affected by charge weight or press closure |
| Tolerance, across parting line | ±0.010 in (±0.25 mm) or more | — | Flash thickness varies with charge weight |

## Variants

- Compression Molding Rubber
- Compression Molding Plastic

## Cost drivers

Tooling is meaningfully cheaper than an injection mold: no runner system, no gates, no hot half, no injection unit, and lower clamp requirements per unit of projected area. Against that, the cycle is measured in minutes rather than seconds, charge preparation is a real labor step, and every part needs deflashing.

Volume breakpoints:

- Under 500 parts: single-cavity tooling, hand-loaded charges, hand deflash. Common for rubber prototypes and short-run seals.
- 500–50,000: multi-cavity tooling in a hydraulic press; charge preforms cut on a dedicated line.
- Above roughly 50,000 parts a year for a small rubber part, injection or transfer molding usually wins on cycle time and consistency.

Cost reduction:

1. **Reduce the thickest section.** Cure time follows the heaviest wall, and cure time is the cycle. Coring out one thick boss can cut minutes from every shot.
2. **Add cavities.** The press is heating and holding pressure regardless; a second, fourth, or sixteenth cavity spreads that fixed cycle across more parts.
3. **Control charge weight tightly.** Excess charge is both wasted material and thicker flash to trim; underweight charge is a short shot. Preform tooling pays for itself quickly.
4. **Design for automated deflash.** Parts that can be cryogenically tumbled avoid hand trimming entirely, which is often the largest labor line on a rubber part.
5. **Keep tight tolerances within one mold half.** Dimensions spanning the parting line carry the charge-weight variation and are far more expensive to hold.

## FAQ

### What is the difference between compression molding and injection molding?

Compression molding loads a measured charge into an open heated cavity and closes the press on it, so there is no runner, gate, or injection unit. That makes tooling cheaper and avoids shearing glass reinforcement through a gate, but the cycle runs minutes instead of seconds and every part carries parting-line flash.

### What pressure does compression molding use?

Commonly 300–2,000 psi (2–14 MPa) on the projected area, with bulky high-viscosity phenolic charges at the top of the range and low-viscosity rubber compounds near the bottom. That is far below injection molding's 5,000–20,000 psi, which is why compression tooling and presses are less expensive per unit of projected area.

### How long does a compression molding cycle take?

Cure is heat-transfer limited, so cycle time is set by the thickest section. Thin rubber gaskets cure in 1–3 minutes; thick sections take 10 minutes or more. Reducing the heaviest wall in the part is the single most effective way to shorten the cycle.

### What tolerance can compression molding hold?

About ±0.005 in (±0.13 mm) on features contained within one mold half. Dimensions spanning the parting line are looser — ±0.010 in (±0.25 mm) or more — because flash thickness varies with charge weight and press closure. Keep critical dimensions inside a single half wherever the design allows.

### Why does compression molding always produce flash?

The mold closes on a deliberately excess charge to guarantee the cavity fills, and the surplus escapes at the parting line. Charge weight is controlled to minimize it, but some flash is inherent. Place the parting line where a witness line is cosmetically acceptable and where trimming — ideally cryogenic tumbling — is easy.

### When should I use transfer molding instead?

When you need better dimensional control or you are encapsulating delicate inserts such as electrical terminals. Transfer molding loads the charge into a separate pot and forces it through a sprue into an already-closed cavity, which avoids the insert being displaced by the closing press. The trade is sprue scrap and a more complex tool.

## Alternative processes

- [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.
- [Liquid Silicone Rubber (LSR) Molding](https://manufacturingprocesses.org/processes/forming/liquid-silicone-rubber-lsr-molding.md): LSR molding injects two-part liquid silicone into a heated mold where it cures, producing flexible parts with a very wide service temperature range.
- [DMC and SMC Molding](https://manufacturingprocesses.org/processes/forming/dmc-and-smc-molding.md): DMC and SMC molding compression-molds a pre-mixed dough or sheet of chopped fiber and thermoset resin into a stiff, dimensionally stable part.
- [Reaction Injection Molding](https://manufacturingprocesses.org/processes/forming/reaction-injection-molding.md): Reaction injection molding mixes two liquid reactants that polymerize inside a low-pressure mold, producing large tough polyurethane parts on light tooling.

## Related processes

- [DMC and SMC Molding](https://manufacturingprocesses.org/processes/forming/dmc-and-smc-molding.md): DMC and SMC molding compression-molds a pre-mixed dough or sheet of chopped fiber and thermoset resin into a stiff, dimensionally stable part.
- [Liquid Silicone Rubber (LSR) Molding](https://manufacturingprocesses.org/processes/forming/liquid-silicone-rubber-lsr-molding.md): LSR molding injects two-part liquid silicone into a heated mold where it cures, producing flexible parts with a very wide service temperature range.
- [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.
- [Overmolding](https://manufacturingprocesses.org/processes/forming/overmolding.md): Overmolding molds a second material, usually a soft elastomer, directly onto an already-molded rigid substrate so the two bond permanently.

---

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

*Last updated: August 11, 2026*