Transfer Molding – Working , Advantages and Disadvantages

Transfer molding process combines the principle of compression and transfer of the polymer charge. In the transfer molding, polymer charge is transferred from the transfer pot to the mold. The mold is cooled and molded part is ejected. The schematic of transfer molding process is
shown in figure
In this process, the required amount of polymer charge is weighted and inserted into the transfer pot before the molding process. The transfer pot is heated by the heating element above the melting temperature of the polymer charge. The liquid charge is gravity filled through the sprue to the mold cavity. A “piston and cylinder” arrangement is built in the transfer pot so that the resin is squirted into the mold cavity through a sprue. The plunger is also preheated in the transfer pot. The plunger is used to push the liquid polymer charge from the transfer pot into the mold cavity under pressure. The mold cavity remains closed as the polymer charge is inserted.

The mold cavity is held closed until the resin gets cured. The mold cavity is opened and the molded part can be removed once it has hardened with the help of ejector pin. The sprue and gate attached to the molded part have to be trimmed after the process has been completed.

This is used for mass production. It has short production cycle and smaller tolerances and more intricate parts can be achieved. It produces more waste material; therefore it is the more expensive process. The mold cavity can be made from metals such as aluminum or steel for
larger production.

Process Parameters

  • Heating time
  • Melting temperature of the charge
  • Applied pressure
  • Cooling time

Materials Used
Generally, thermoset plastics (such as epoxy, polyester, phenol-formaldehyde, vinyl ester, silicone) are processed by transfer molding process, but certain thermoplastic materials can also be processed.

Applications
This process is widely used to encapsulate items such as integrated circuits, plugs, connectors, pins, coils, and studs. It is suitable for molding with ceramic or metallic inserts which are placed in the mold cavity. When the heated polymer fills the mold it forms bonding with the insert surface. Transfer molding is also used for manufacturing radio and television cabinets and car body shells.

Advantages

  • Fast setup time and lower setup costs
  • Low maintenance cost
  • Plastic parts with metal inserts can be made
  • Design flexibility
  • Dimensionally stable
  • Uniform thickness of parts
  • Large production rate

Disadvantage:

  • Wastage of material
  • Production rate lower than injection molding
  • Air can be trapped in the mold
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