Transfer molding
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Transfer molding, like compression molding, is a process where the amount of molding material (usually a thermoset plastic) is measured and inserted before the moulding takes place. The molding material is preheated and loaded into a chamber known as the pot. A plunger is then used to force the material from the pot through channels known as a sprue and runner system into the mold cavities. The mold remains closed as the material is inserted and is opened to release the part from the sprue and runner. The mold walls are heated to a temperature above the melting point of the mold material, this allows a faster flow of material through the cavities.
The moulds in both compression and transfer moulding remain closed until the curing reaction within the material is complete. Ejector pins are usually incorporated in to the design of the molding tool and are used to push the part from the mold once it has hardened. These types of moulding are ideal for high production runs as they have short production cycles. Transfer moulding, unlike compression moulding uses a closed mould, this means smaller tolerances and more intricate parts can be achieved. The fixed cost of the tooling in transfer moulding is greater than that in compression moulding and as both methods produce waste material, whether it be flash or the material remaining in the sprue and runners, transfer moulding is the more expensive process.
Transfer moulding (TM) (or resin transfer moulding, RTM) differs from compression moulding in main due to that in TM the resin is inserted into the mould (or tool) which contains the layers of fibres or a preform, whereas in compression moulding prepregs or moulding compounds are in the mould which is then heated and pressure applied. No further pressure is applied in TM.
In RTM the resin is injected or drawn into a mould, which contains the fibres, from a homogeniser under low pressure. The mould can be made from composites for low production cycles or with aluminium or steel for larger production. The differences between the two types being that the metal has better heat transfer, hence quicker cycle times, lasts longer, deforms less, but at a higher cost. The main problem with this production route is that air can be trapped in mould and hence allowing this air to escape. A number of solutions to the problem exist including extending one level of reinforcement beyond the cavity (with a 25% resin loss), appropriate vents and creating a vacuum in the mould (which also improves quality). Larger structures, better properties (less movement of fibres), increased flexibilty of design and lower cost are some of the advantage this process has over compression moulding due in main to the low pressure injection. Other benefits include rapid manufacture, not labour intensive, ability to vary reinforcements easily or include cores such as foam and produce low and high quality products.
In the semiconductor industry, package encapsulation is usually done with transfer molding due to the high accuracy of transfer molding tooling and low cycle time of the process.
