Silicone rubber transfer molding

MOULDING OF SILASTIC SILICONE RUBBER
Transfer Moulding
In transfer moulding, the unvulcanized rubber is placed in a
chamber (called a pot), usually at the top of the mould, and
the assembly is placed in a press. The press applies pressure
to a piston-Ike plug in the open end of the pot, clamping the
halves of the mould together and forcing the rubber to flow
through one or more sprues into the heated mould.
Transfer moulding is particularly useful in producing parts
whose shape is such that the moulds cannot provide good
flow and tend to trap air. It is the best method of moulding
parts that contain wires, pins and other inserts that require
precise positioning.In some cases, the pot is built into the
mould. In others, the pot is separate from the mould and is
positioned by pins or location marks. In the latter case, the
pot is usually removed immedately after the mould is filled,
to prevent vulcanization of the rubber in the pot. With this
system, the pot can be filled with enough rubber to load the
mould several times.
Moulding Time and Temperature
The time and temperature for transfer moulding are the same
as for compression moulding. The figures given in Table II
are typical; but for each specific moulding job, the most
favourable values of time and temperature can only be
determined by experimentation.
Shrinkage
Most products moulded with Silastic silicone rubber shrink
during vulcanization and oven cure. Thus, the finished parts
are smaller than the mould cavities in which they were
formed. The amount of linear shrinkage ranges from about 2
percent to 5 percent depending on the composition of the
moulding rubber, the moulding temperature, and the size and
shape of the part. There are two principal causes of
shrinkage:
• The coefficient of thermal expansion of silicone rubber is
greater than that of the steel mould. Therefore, when the
moulded part cools to room temperature, it is smaller than
the mould cavity at room temperature.
• During vulcanization and oven curing, decomposition
products of the vulcanizing agent and volatile components
of the rubber are driven off, decreasing the size of the
moulding.
• Moulded parts can be held within commercial tolerances,
by using rubber of consistent shrinkage and controlling the
processing variables.
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Fig. 10. Typical transfer
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