Compression molding makes a plastic part by placing a measured charge of powder, granules or a preform in the lower half of a heated mold, closing the mold under pressure so the material flows into the cavity, and holding heat and pressure until a thermoset cures or a thermoplastic cools. It is simpler than injection molding, needs no runner or gate, and is the natural method for Bakelite.
Part of Plastics and Polymers
Compression molding is the shaping of plastic by pressing it between the halves of a heated mold, used mainly for thermosets.
Simpler Than Injection Because The Material Starts In The Mold
In injection molding the melt travels from a cylinder through a runner and gate. In compression molding the charge is already in the cavity, so there is no plunger, runner, gate or injection cylinder. That is why Plastics Manufacturing calls it the simpler alternative and recommends it for thermosets like Bakelite. A thermoset cannot be melted and injected, because it would cure in the cylinder.
The Five Steps
- Place a measured charge of powder, granules or a preformed shape in the bottom half of a heated mold.
- Close the top half under pressure from a press, lever or heavy weights.
- Let heat and pressure make the material flow and fill the cavity.
- Hold heat and pressure until a thermoset has cured, or until a thermoplastic has cooled.
- Open the mold and remove the part.
The Bakelite specifics, with the pressure and temperature the scratch carrier gives, are in Compression Mold.
What You Need
- A two-part mold with a deep enough lower half to hold the loose charge, which is bulkier than the part: see Mold Design for draft and corners.
- A press that develops the force: a lever press, a screw, or a hydraulic jack (Hydraulic Press).
- A way to heat the mold evenly and measure its temperature.
- A scale, since the charge has to be weighed.
Weigh The Charge
Too little charge gives a short part. Too much forces a thick flash out of the parting line and may prevent the mold closing. Charge weight is the part volume times the material density, with a small overfill.
Worked Example: Pressing Force
A flat part with a projected area of 100 mm × 100 mm = 0.01 m². If the process calls for 10-30 MPa (the range Plastics from Scratch gives for Bakelite), the force is 10 × 10⁶ Pa × 0.01 m² = 100 kN up to 30 × 10⁶ × 0.01 = 300 kN. That is about 10 to 31 tonnes-force. For comparison, the corpus’s hydraulic press makes 157 kN from a 100 mm ram at 20 MPa; the pressing force of this part at 20 MPa is 20 × 10⁶ × 0.01 = 200 kN, which exceeds it, so a larger ram or a smaller part is needed.
Compression Versus Injection
| Compression | Injection | |
|---|---|---|
| Material | Thermosets or thermoplastics | Thermoplastics |
| Runner and gate | None | Required |
| Waste | Flash around the edge | Gate and runner |
| Machine | Press and heated mold | Cylinder, plunger, clamp |
| Detail on complex parts | Limited | Better |
Common Mistakes
- Opening a thermoset early — an uncured part is soft and deforms.
- Uneven mold temperature — one side cures first and the part warps.
- Trapped air and steam — blisters; the standard remedy is to open the press briefly early in the cycle so gas escapes (general knowledge, not a corpus figure).
Hot press, crush and fume
A heated mold under a press can burn and crush. Keep hands out of the mold area while closing, use tongs for hot parts, and wear eye protection. Uncured phenolic gives off phenol and formaldehyde as it cures, so work outdoors or in strong ventilation.
What’s Next
- Learn next: Compression Mold — the Bakelite cycle
- Combine with: Injection Molding — the thermoplastic alternative