Get the Offline ArchiveFree lifetime updates

Compression Mold

4 min read

To compression-mould Bakelite, weigh a charge of moulding powder equal to the part volume times the material density times 1.05, load it into a steel mould preheated to 160-180 degrees Celsius, close the press at 10-30 MPa and hold for 1-5 minutes per millimetre of thickness. The press force is the pressure times the part’s projected area, which for an 80 mm by 50 mm part is 40-120 kN.

Part of Plastics and Polymers

Compression molding of thermosets is the pressing of a measured charge of powder in a heated steel mold so that it flows, fills the cavity and cures into a rigid part.

What You Need

  • A two-part steel mold machined to the final shape, with draft and rounded corners: Mold Design.
  • A press that develops the force: a hydraulic press (Hydraulic Press) or a screw press (Screw Mechanisms).
  • A way to heat the mold to 160-180 °C and measure it. The manufacturing carrier gives 150-180 °C. The scratch carrier’s 160-180 °C range is the one written for hexa-containing powder.
  • Moulding powder from Synthesis and Molding, weighed on a scale.

Method: The Carrier’s Six Steps

Step 1: Calculate the charge. Charge weight = part volume × material density × 1.05. The 1.05 overfills slightly so that the cavity fills and a thin flash escapes.

Step 2: Preheat the mold in an oven.

Step 3: Load the charge and close the mold in the press.

Step 4: Apply heat and pressure per the material requirements: 10-30 MPa.

Step 5: Hold for the cure time. The scratch carrier gives 1-5 minutes per millimetre of thickness. The manufacturing carrier gives 2-10 minutes in total at 150-180 °C. They differ widely for thick parts, so cure time is a number to find on test pieces.

Step 6: Open the mold, extract the part and trim the flash.

Worked Example: An 80 × 50 × 6 mm Insulating Block

  1. Volume = 8 cm × 5 cm × 0.6 cm = 24 cm³.
  2. With an assumed density of 1.35 g/cm³ (a typical filled phenolic, not a corpus figure): 24 × 1.35 = 32.4 g.
  3. Charge = 32.4 × 1.05 = 34.0 g.
  4. Projected area = 80 mm × 50 mm = 4,000 mm² = 4 × 10⁻³ m².
  5. Force at 10 MPa = 10 × 10⁶ × 4 × 10⁻³ = 40,000 N = 40 kN; at 30 MPa = 120 kN. That is about 4 to 12 tonnes-force.
  6. Hold time for a 6 mm part: 6 × 1 = 6 min up to 6 × 5 = 30 min by the scratch carrier, against 2-10 min by the other. Find the right value from test pieces.

For comparison, the corpus’s hydraulic press produces 157 kN from a 100 mm ram at 20 MPa, so this part is well within reach of such a press.

Reading The Result

  • Short, unfilled corners: too little charge, or mold too cold.
  • Thick flash: too much charge, or worn mold faces.
  • Blisters or swelling when ejected: gas not vented, or cured too fast; the standard remedy is to open the press briefly early in the cycle to let gas out (general knowledge).
  • Soft, tacky part: undercured.
  • Part sticks: no draft, rough mold surface or no release agent.

Common Mistakes

  • Cold mold — the powder does not flow before it cures.
  • Overfilling heavily.
  • Opening the press before the cure is done.
  • Reaching into a hot mold without tongs.

Hot press, crush and fume

The mold at 160-180 °C burns instantly, and a press at 40-120 kN crushes. Keep hands out when the press closes, use tongs, and wear eye protection. Curing phenolic gives off phenol, formaldehyde and, from hexa, ammonia (general knowledge): work outdoors or with strong ventilation.

What’s Next