Transfer Moulding :
Transfer molding is similar to compression molding in that a carefully calculated, pre-measured amount of uncured molding compound is used for the molding process. The difference is, instead of loading the polymer into an open mold, the plastic material is pre-heated and loaded into a holding champer called the pot. The material is then forced/transferred into the pre-heated mold cavity by a hydraulic plunger through a channel called sprue. The mold remains closed until the material inside is cured.
Transfer molded parts inherently have less flash (excess material that runs along the parting line of the mold) than their compression molded counterparts because the mold remains closed when the plastic enters the mold cavity. However, transfer molding still produces more waste material than compression molding because of the sprue, the air holes and the overflow grooves that are often needed to allow air to escape and material to overflow.
One of the key advantages of transfer molding over compression molding is that different inserts, such as metal prongs, semiconductor chips, dry composite fibers, ceramics, etc., can be placed/positioned in the mold cavity before the polymer is injected/drawn into the cavity. This ability makes transform molding the leading manufacturing process for integrated circuit packaging and electronic components with molded terminals, pins, studs, connectors, and so on.
In the composite industry, fiber-reinforced composites are often manufactured by a processed called Resin Transform Molding (RTM). Layers of textile preforms (long fibers woven or knitted in patterns) are pre-arranged in the mold. The resin is then injected to impregnate the performs. Vacuum is often used to avoid air bubbles and help draw the resin into the cavity. In addition, the resin used has to be relatively low in viscosity.
- Loading a preform into the pot takesless time than loading preforms intoeach mold cavity.
- Tool maintenance is generally low, although gates and runners aresusceptible to normal wear.
- Longer core pins can be used and canbe supported on both ends, allowingsmaller diameters.
- Because the mold is closed before theprocess begins, delicate inserts andsections can be molded.
- Higher tensile and flexural strengths areeasier to obtain with transfer molding.
- Automatic de-gating of the mold’s tunnelgates provides cosmetic advantages.
- Molded parts may contain knit lines in back of pins and inserts.
- The cull and runner system of transfer molding leaves waste material, but this scrap can be greatly reduced by injection molding with live sprues and Runnerless Injection Compression (RIC).
- Fiber degradation of orientation occurring in the gate and runner system reduces the molded part’s impact strength.
- Compared to compression molding, high molding pressures are required for the transfer process, so fewer cavities can be put into a press of the same tonnage.