Page 199 - Handbook of Plastics Technologies

THERMOSETS

THERMOSETS 3.69

3.2.1 Molding Processes
Liquid B-stage resin is held in a closed cavity and heated to cross-link it to a rigid solid
product. This is done in a variety of ways.
3.2.1.1 Compression Molding. The original and classic method of producing thermoset
plastic products is by compression molding. A two-part steel mold is made with a cavity
representing the shape of the desired product. The cavity surface is chrome-plated to give
a smooth corrosion-resistant finish. The mold is mounted in a vertical compression press
with two horizontal platens. The mold half with the deeper cavity is mounted on the lower
platen; the other mold half is mounted on the upper platen. The mold is heated (143 to
232°C), originally by steam and more recently by electricity. The thermosetting resin is
measured into the lower mold cavity, either by weight or by volume, or preferably as a
cold-pressed preformed pellet. The press is closed to heat and compress the resin. If the
cure reaction releases water or other volatile by-products, the press is opened briefly to re-
lease the gases and then closed again at full pressure (1,000 to 12,000 psi). Molding pres-
sure is maintained until thermosetting cure is complete (1/2 to 5 min). Then, the press is
opened, and the molded product is ejected by the help of “knock-out pins.” After that, the
cycle is repeated.
For faster, easier, more uniform melt flow, the performs can be preheated to 82 to
138°C by microwave or infrared heaters. This reduces abrasion of the mold and produces
higher-quality molded parts.
Originally, the molding cycle was carried out manually. With progress, most molders
converted to semiautomatic operation: the operator loads the resin into the mold and takes
the product out of the mold, but he activates the process by simply pressing a button, and
the entire molding cycle proceeds automatically. More recently, many molders have con-
verted to fully automatic processes wherein loading, molding, and removing the molded
product are all done automatically on a preset cycle.
3.2.1.2 Transfer Molding. Transfer molding is intermediate between compression and
injection molding. Whereas the compression mold has just one cavity for the finished
product, the transfer mold also contains a preliminary auxiliary cavity. The resin is loaded
into the auxiliary cavity (“pot”), preferably as a preheated perform, where it is heated to
melt processing temperature. Then, a plunger forces the molten resin into the final mold
cavity, where it cures to the finished product.
This eliminates the need to “breathe” out gases. It operates at lower pressure and there-
fore only needs a lighter weight mold. It is more fluid, so there is less mold wear. It gives a
faster cycle and a more uniform product, to very close tolerances. There is less flash, so
less post-molding finishing is needed. It is particularly useful when making complicated
parts, thin walls, working with fragile inserts, and encapsulation.
The original process used a single ram, both to close the mold and to transfer the resin
from the pot to the mold cavity; this was a manual operation. The preferred process at
present uses one ram to close the mold and another auxiliary ram to transfer the resin from
the pot to the mold cavity; this is semiautomatic. A third process, which is used occasion-
ally, uses a screw preplasticator to warm the resin and feed it to the transfer pot; this is
completely automatic.
3.2.1.3 Injection Molding. Injection molding was first developed for metals and then
for thermoplastics. With further progress in both materials and equipment, it has been ap-
plied successfully to thermosetting plastics. It is used mostly for phenolics and is being
used more and more for polyester bulk molding compounds.
The conventional injection molding machine is modified by giving it a shorter barrel
and eliminating the 3-to-5 compression ratio of the screw. Granular resin in a conical hop-

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