Page 200 - Handbook of Plastics Technologies

THERMOSETS

3.70 CHAPTER 3

per is gravity-fed into the screw, which feeds it forward, heating it by both conduction and
friction, to a temperature of 66 to 93°C. As molten resin builds up at the front of the screw,
this pushes the screw back until it accumulates enough material to fill the mold. Then, the
screw plunges forward rapidly, pumping the molten resin through the sprue and runners
into the mold cavity or, very often, multiple cavities. In the mold at 177 to 204°C, the resin
cures rapidly. While the resin is curing, and while the mold is opening, discharging the
molded product and closing again, the screw is already plasticating another batch for the
next shot. This is a faster cycle than compression or transfer molding. It is also fully auto-
matic.
Bulk molding compound is doughy in nature, so it is fed by plunger instead of by grav-
ity. Since it is already viscous, it does not need much preheating to make it flow. It may be
injected by screw or plunger.
Since a hot runner produces considerable thermoset scrap, it may be replaced by a
“cold runner” that keeps the molten resin at 66 to 99°C, still usable for the next shot. An-
other innovation is injection-compression molding, where the mold is left about 1/4 in
open during injection and then clamped shut to finish the cycle by compression; this re-
duces flow orientation and improves impact strength.
3.2.1.4 Liquid Resin Molding and Resin Transfer Molding. Liquid resin, such as ep-
oxy and polyester, curing agents, and catalysts, can be mixed at room temperature and in-
jected into a lightweight mold at low pressure (25 to 50 psi), where they are warmed and
held until cured. This is useful when working with very delicate inserts and heat-sensitive
components such as electronic systems. The process is called liquid resin molding.
It is also useful for making fabric-reinforced laminates. The multiple layers of fabric
are hand-laid into the mold in a predetermined pattern to optimize strength properties.
Then the mold is closed, and the liquid mixture is injected to impregnate the fabric, often
using vacuum assist to replace air thoroughly by resin. The assembly is held in the press
and heated until cured. The products are similar to those made by perform press molding.
The process is called resin transfer molding.

3.2.1.5 Reaction Injection Molding (RIM). In reaction injection molding, liquid polyol
and polyisocyanate are metered into a mixing head, which delivers the mixed liquid into a
lightweight mold, where it polymerizes and cures rapidly to a finished polyurethane
molded product. This is particularly useful in the auto industry, where it is used to mold
large body parts very economically.
The polyol and polyisocyanate are stored in large tanks. Catalysts, foaming agents, and
other additives are generally dissolved in the polyol. Temperature must be controlled
closely (30 to 38°C) to ensure proper metering and mixing. Viscosities are under 3000 Cp.
The liquids are fed to a high-pressure (2000 to 3000 psi) impingement mixer, which
pumps the mixed liquid rapidly into a lightweight stainless steel or aluminum mold. The
polymerization reaction is very rapid and exothermic, so the mold must be cooled to keep
it at 40 to 80°C. The foaming agent evaporates, creating a pressure of 30 to 100 psi in the
mold.
The resulting polymers range from slightly foamed elastomers to fairly low-density
rigid structural foams. These typically may have an 0.25-in foam core surrounded by a
0.0625-in solid skin, a simple approximation to sandwich structure. Typical products
range from shoe soles to beer kegs to auto front ends to as large as a 600-lb boat hull.
Low capital investment, very fast cycle, and low energy requirements combine to
make this process very economical. In addition to polyurethanes, the process has been
used for nylon 6 monomer casting and has been suggested for epoxy and other thermoset-
ting resins.

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