Page 126 - Module DMV20173

6.0 POLYMER PROCESSING

1. Transfer molding is a development of compression molding.

2. Transfer molding consists of melting the plastic resin in a separate chamber and then
injecting it into a closed mold.
3. It is in principle similar to the ram injection molding of thermoplastic materials (Figure

6.10). There are a number of advantages of transfer molding. These are:

a) Shorter molding cycle times - because when the material is going through the
runners and gates there can be large enough flow restrictions to add a sensible amount
of heat to the liquid resin, hence saving time in the chamber.

b) Tighter Dimensional Control - because the material is injected into a closed mold,
there is little or no flash and dimensions across the parting lines are held. There is less
flash and thus finishing costs are lower.

c) Insert molding - the plastic material enters the mold as liquid, compared to
compression molding, thus the pressure on delicate metal inserts is less and it is less
likely for pins or thin mold sections to bend.

d) Encapsulation Transfer molding is the only feasible method of encapsulating delicate
electronic components where maximum density is required with low molding pressures.

e) 'Family' Molds. Transfer molding is well suited for producing parts of different sizes

which must be assembled together. One shot gives several parts all connected together
by the runners and cured at the same time. In compression molding the parts would
likely be molded separately and cured for different times, which may result in slight
colour differences.

4. There are disadvantages with transfer molding :

a) Mold costs -generally mold costs for transfer molding is more than for a similar

compression molding because of their greater complexity.

b) Material costs -material costs of transfer molding is higher than for compression
molding due to the material left in the well, sprue, runners and gates which cannot be
reused. For small components this scrap material can be a very sizable factor.

BPLK 158 DME 1083