Page 202 - Handbook of Plastics Technologies

THERMOSETS

3.72 CHAPTER 3

chopped fiber as a random mat, so strength is lower. The spray gun requires a modest cap-
ital investment, but the labor cost is lower. This technique is particularly common in build-
ing boat hulls.
3.2.2.3 Vacuum Bag Molding. To improve compaction of hand or spray laid assem-
blies, the wet product is covered with a release film such as polyvinyl alcohol, nylon, or
silicone rubber. The edges of the film are clamped and sealed to the edge flanges of the
mold. An internal vacuum is pumped, so atmospheric pressure presses the film down onto
the product. Hand roller pressure helps the process. This squeezes out air bubbles and ex-
cess resin and results in a denser product with a better inside surface. The extra work adds
to the cost, so this is used mainly in the aerospace and military fields, where the improved
quality is worth the cost.
3.2.2.4 Pressure Bag Molding. A further improvement over vacuum bag molding is to
place a rubber bag over the assembly, fasten it in place, and inflate it up to 50 psi air pres-
sure. This provides additional force to squeeze out air and excess resin and produces a
denser, more uniform product.
It can also be used to make hollow shapes such as tanks. The bag is covered with a pre-
formed glass fiber mat and placed inside a hollow mold. The glass fiber mat is impreg-
nated with catalyzed liquid resin, and air pressure inside the bag presses the assembly
against the interior surface of the mold. The mold can be heated to speed the cure reaction.
Here again, labor cost and slow process cycle are economic limitations. Also, the mold
must be sturdier to withstand the pressure.
3.2.2.5 Autoclave Molding. Another further improvement is to put the pressure bag as-
sembly into a heated autoclave and fill the autoclave with up to 80 psi air pressure. Here
again, equipment and labor cost are higher, but quality is higher too. Typical products are
rocket nozzles, nose cones, heat shields, electronics, and aircraft parts.

3.2.2.6 Centrifugal Casting. Cylindrical products such as pipe, tubing, and tanks can be
produced by centrifugal casting. Glass fiber mat is laid or sprayed onto the inner walls of a
rotating cylindrical mold. Catalyzed resin is sprayed onto the mat. Speed of rotation is in-
creased to densify the layup, and the mold is heated to speed the cure reaction. The pro-
cess can also make other shapes of simple rotation such as cones and parabolas. The
equipment does require some capital investment, but labor cost is low, and product quality
and uniformity are high.
3.2.2.7 Rubber Plug Molding. To make a hollow product, a metal mold contains a cav-
ity for the exterior of the product. Liquid silicone rubber is poured into the cavity and
cured. The silicone rubber plug is pulled from the mold and preferably covered with a
PTFE film. The glass fiber and liquid resin are built up on the surface of the silicone rub-
ber plug. This assembly is inserted back into the mold cavity. The mold and silicone plug
are heated, the silicone plug expands, and the resin cures. Then, the assembly is removed
from the mold, and the silicone plug is removed from the product.
3.2.2.8 Matched Die Molding. All of the above techniques are relatively specialized
processes for small production runs. For economical mass production of uniform quality
products, matched die molding is the standard technique. This is used for manufacture of
auto and appliance parts.
Matched steel dies are chrome plated and operated in heated compression presses. The
entire premix is placed in the mold and pressed and heated 1 to 2 min to produce complete
cure. Matched die molding is applied to bulk molding compound, sheet molding com-
pound, prepregs, and performs.

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