Plastics are shaped in three simple ways: compression molding, in which a charge is pressed in a heated two-part steel mold, casting, in which liquid resin is poured into a mold and cured slowly, and machining, in which finished stock is cut, drilled and turned. The right choice depends on the plastic: Bakelite and casein suit compression or casting, while thermoplastics can also be melted and injected.
Part of Plastics and Polymers
Plastic molding is the shaping of a plastic into a finished object by pressing, casting, injecting or cutting it.
Pick The Method By How The Plastic Hardens
| Method | How it works | Suits | Detail |
|---|---|---|---|
| Compression molding | Charge pressed in a heated mold | Bakelite, casein, thermoplastics | Compression Mold |
| Casting | Liquid resin poured and cured slowly | Phenolic and casein resins | below |
| Injection | Melt forced into a closed mold | Thermoplastics only | Injection Molding |
| Machining | Stock cut, drilled, turned | All three early plastics | Machining Plastics |
The scratch carrier, Plastics from Scratch, names compression molding as the standard method for Bakelite and casein, then casting and machining.
Casting Needs Patience, Not Pressure
For a phenolic or casein resin in liquid form:
- Mix the liquid resin (the carrier specifies a resol-type casting grade).
- Pour it into a mold at room temperature.
- Cure slowly: the carrier gives 60 °C for 24 hours, then 80 °C for another 24 hours.
- Slow curing prevents bubbles and cracking.
- The result is clear amber for Bakelite or translucent white for casein.
Casting needs no press but takes days. A mold for casting has to resist the heat and not stick, and the resin shrinks as it cures, so the pour should be topped up. The carrier’s wording for the casting resin mixes terms, and the safer route and reasoning are in Synthesis and Molding.
Mold Materials
Steel is best for hot pressing. Aluminium is easier to work. Wood is fine for casein and starch, and chars on hot plastic. See Mold Design.
Finish By Machining And Polishing
Moulded and cast parts are trimmed, drilled and polished. Casein and Bakelite machine well: Machining Plastics. For parts you will make most often, see Buttons, Handles and Hardware.
Worked Example: Charge For A Pressed Part
The carrier’s rule is charge weight = part volume × material density × 1.05, to overfill slightly. For a part of 20 cm³ and an assumed density of 1.35 g/cm³ (a typical filled phenolic, not a corpus figure): 20 × 1.35 = 27 g, and with the overfill 27 × 1.05 = 28.4 g.
Common Mistakes
- Casting too fast — bubbles and cracks.
- Mold without draft or release — the part sticks.
- Choosing injection for a thermoset.
Hot molds, resin fume
Heated molds burn and presses crush; liquid phenolic resin gives off phenol and formaldehyde while it cures. Work outdoors or with strong ventilation, use gloves and eye protection, and keep hands out of the press.
What’s Next
- Learn next: Compression Mold — the main method
- Combine with: Machining Plastics — finishing