Compression and Transfer Moulding

Definition

Compression moulding is a method where a pre-heated polymer charge is placed into an open mould cavity, then closed with a top plug to apply heat and pressure until the material cures. Transfer moulding is an evolution of this process where the material is pre-heated in a separate chamber and then "transferred" or forced into a closed mould cavity through channels, making it ideal for complex parts.

Main Content

1. Compression Moulding Mechanics

It primarily uses thermosetting plastics that require heat to undergo a chemical reaction (cross-linking) to set.
The mould remains under constant pressure throughout the entire curing cycle to ensure the material fills every detail of the cavity.

2. Transfer Moulding Mechanics

This process separates the charging phase from the moulding phase, allowing for delicate inserts (like metal pins or wires) to be placed in the mould without being moved by the force of the initial charge.
The material is pushed through a "sprue" or "runner" system into a closed mould, which provides better dimensional control.

3. Comparison of Material Flow

Compression moulding relies on the material flowing directly under the weight of the mould halves.
Transfer moulding relies on hydraulic pressure to inject the material into the cavity, which allows for higher precision and the use of smaller, more intricate mould designs.

Working / Process

1. Material Loading

In compression moulding, the measured "charge" (pellets or pre-form) is placed directly into the heated open mould.
In transfer moulding, the material is placed into a "pot" or transfer chamber located above the mould cavity.

2. Mould Closure and Pressure

The hydraulic press forces the mould shut, compressing the material into the shape of the cavity.
In transfer moulding, a plunger forces the material from the pot through channels (sprues) into the already closed mould.

COMPRESSION MOULDING         TRANSFER MOULDING
   [Plunger]                    [Plunger]
      |                             |
  [Charge]                      [Pot/Charge]
      |                             |
  [Cavity ]                     [Channels]
                                    |
                                [Cavity ]

3. Curing and Ejection

Heat is applied to both processes to initiate cross-linking (curing) of the thermoset plastic.
Once the cycle time is complete, the mould opens, and the finished part is ejected using pins.

Advantages / Applications

Compression moulding is highly cost-effective for large, bulky, and simple parts like dinnerware, electrical housings, and heavy-duty automotive components.
Transfer moulding is essential for complex shapes with deep sections or delicate metal inserts, such as electronic connectors and encapsulated microchips.
Both processes offer high material utilization with very little waste compared to other manufacturing methods.

Summary

Compression and transfer moulding are fundamental manufacturing processes used to produce high-strength thermoset plastic components by applying heat and pressure to raw material. Compression moulding excels in simplicity for large parts, while transfer moulding provides superior precision for complex geometries containing inserts. Important terms to remember: Thermosetting polymers, curing, mould cavity, sprue, and cross-linking.