Transfer Molding

 Transfer molding is like to compression molding except that instead of the molding material being pressurized in the cavity, it is pressurized in a separate chamber and then forced through an opening and into a closed mold. The advantages (Benefits) of transfer molding are that the preheating of the material and injection through a narrow orifice improves the temperature distribution in the material and accelerates the cross linking reaction. As a result the cycle times are reduced and there is less distortion of the molding. The improved flow of the material also means that more intricate or complicated shapes can be produced.

Molding Sequence:

1.     The required amount of resin or the polymer is weighted and inserted into the transfer pot before the molding process.

2.     The transfer pot is heated by the heating element above the melting temperature of the polymer charge.

3.     The liquid charge is gravity filled through the sprue to the mold cavity.

4.     A “piston/plunger/ram and cylinder” arrangement is built in the transfer pot so that the resin is transferred into the mold cavity through a sprue.

5.     The plunger is also preheated in the transfer pot.

6.     The plunger is used to push the liquid polymer charge (Polymer melt) from the transfer pot into the mold cavity under pressure.

7.     The mold cavity remains closed as the polymer charge is inserted.

8.     The mold cavity is held closed until the resin gets cured which means the material sets hard to the cavity shape after a certain time (cure time) has elapsed.

9.     The mold cavity is opened and the molded part can be removed once it has hardened with the help of ejector pin.

10.  The sprue and gate attached to the molded part have to be trimmed after the process has been completed.

Types of transfer molding process

There are two types of transfer molding process.

1.     Pot transfer molding:

 The charge is injected from a pot through a vertical sprue channel into the mold cavity.

2.     Plunger transfer molding:

 A plunger injects the charge (Polymer melt) from a heated well through lateral channels in the cavity.

Advantages:

1.     Fast setup time and lower setup costs.

2.     Low maintenance cost.

3.     More intricate or complicated shapes can be produced.

4.     Plastic parts with metal inserts can be made.

5.     Design flexibility.

6.     Good dimensional control on the molded part.

7.     Uniform thickness (density) of parts.

8.     Large production rate.

9.     Reduce molding cycle time.

Disadvantage:

1.     War page is more of a problem because the flow of transfer materials is softer and shrinks more than compression grade materials.

2.     The scrap rate for transfer molded parts will usually be higher than that for compression molded parts because of the added scrap from the runner and cull.

3.     To prevent the mold from opening slightly resulting in heavy flashing of the parts, the clamping tonnage for transfer mold parts is greater than for compression molded parts. Thus a higher tonnage press is required.

4.     Air can be trapped in the mold.

5.     High tooling caste.

Applications

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

Mold

1.     Loose plate mold

2.     Integral mold

3.     Auxiliary mold

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