Transfer and Injection Molding Presses

Transfer Molding Press

Application

Santec's Transfer Moulding Presses are extensively used for manufacturing engine mounting bellows, electrical switch gears, DMC & urea parts dead end, Rubber molded parts & polymer insulators. These machines are suitable for production, where the following features are desired:

Mid range dimensional tolerances
Low to medium volume production
Small to medium sized parts
Delicately shaped parts
Low to medium durometer materials
Insert molding
Colored and translucent compounds

Process Description
The process of transfer molding involves pushing unvulcanized rubber through sprues into the heated mold. Rubber is placed in a chamber at the top of the mold then transfered in a press. During the transfer, rubber is forced through the mold to prevent trapped air (bubbles) while forming the final product. The mold is tightly held closed while the material cures. As the result of this process, the transfer molded parts formed are more consistent /uniform than compression molded parts.

There are two variants of the process: (a) pot transfer molding, in which the charge is injected from a "pot" through a vertical sprue channel into the cavity; and (b) plunger transfer molding, in which the charge is injected by means of a plunger from a heated well using lateral channels to feed the rubber into the mold cavity. In both cases, scrap is produced each cycle in the form of the leftover material in the base of the well and lateral channels, known as cull. In addition, the sprue in pot transfer is scrap material. Because the polymers are thermosetting, the scrap cannot be recovered.

Transfer molding is closely related to compression molding, because both the processes are applied on the same polymer types (thermosets and elastomers). Similarity to injection molding can also be observed, in the way the charge is preheated in a separate chamber and then injected into the mold. Transfer molding is capable of molding part shapes that are more intricate than compression molding, but not as intricate as injection molding. Transfer molding also lends itself to molding with inserts, in which a metal or ceramic insert is placed into the cavity prior to injection, and the heated plastic bonds to the insert during process of molding.