Page 203 - Handbook of Plastics Technologies

THERMOSETS

THERMOSETS 3.73

3.2.2.9 Bulk Molding Compound (BMC). The entire bulk molding compound (BMC)
is premixed—resin, reinforcing fiber, fillers, catalysts, mold lubricants, colorants. Option-
ally, a thermoplastic resin may be added to reduce cure shrinkage and give a low-profile,
high-quality surface. To produce a stable doughy consistency, CaO or MgO may be added
to react with acid end-groups on the resin and produce gelation. A weighed amount of
doughy BMC is simply placed in the matched die mold, pressed, and cured. For good
moldability, the glass fibers are generally under 1 in long.
3.2.2.10 Sheet Molding Compound (SMC). Liquid resin is spread onto a moving web
of polyethylene film. Glass roving is chopped and spread over it. More resin is spread over
that, and a second polyethylene film is overlaid on it. The assembly is kneaded to remove
air and distribute resin and fibers more uniformly. Then, it is rolled up and stored until use.
For molding, a portion of the leathery sheet is cut, placed in the matched die mold,
pressed, and heated about a minute, until cured.
Glass fiber length in SMC is generally 1 to 2 in, which is less moldable than BMC but
gives higher strength properties.
3.2.2.11 Prepreg Process. Reinforcing fabric is impregnated with catalyzed resin and
advanced to the gelled B-stage. It has limited life, so it must be refrigerated and used up
fairly rapidly.
The preimpregnated fabric is cut to the desired shape and hand laid into or onto the
mold. Successive layers can be oriented to optimize properties. Then, the assembly is
matched die molded or vacuum bag molded until cured.
This gives high-strength products. Manpower and skill are expensive. It is used for
aerospace products such as radomes, brakes, and communication housings.
3.2.2.12 Preforms. A metal screen is made in the shape of the desired product. Glass
roving is chopped and blown onto the surface of the screen. Pulling air or even vacuum on
the backside of the screen helps to compact the chopped fiber onto the screen. A few per-
cent of binder resin is sprayed onto the chopped fiber preform to stabilize it. Then, it is re-
moved from the screen and placed in the mold. There, it is impregnated with catalyzed
resin, pressed, heated, and cured. For greater strength, the fibers can be braided onto the
perform to form a woven fabric reinforcement.
3.2.2.13 Foam Reservoir Molding. A sheet of open-cell polyurethane foam is impreg-
nated with catalyzed thermosetting resin. It is sandwiched between two layers of reinforc-
ing fabric. This is then molded at low pressure (<100 psi). This is sufficient to squeeze
liquid resin from the foam out to the fabric skins. When the assembly cures, it is a sand-
wich structure of low density, with good flexural rigidity and impact strength. This is use-
ful for making large fairly flat shapes such as vehicle roofs, decks, and hoods.
3.2.2.14 Continuous Lamination. Mass production of flat or corrugated sheet is ac-
complished by spreading liquid catalyzed resin on a web of release sheet, then chopping
glass fiber onto the wet resin, covering with a second sheet of release film, kneading be-
tween pressure rollers to squeeze out air and distribute resin and fiber uniformly, passing
through a heating zone to cure the resin, and cutting to the desired size. This is used for
production of glazing, paneling, roofing, hoods, and ducts.
3.2.2.15 Pultrusion. This process produces long, continuous products of constant cross-
section. It is a low-cost mass-production technique. Continuous filament, tape, or fabric is
pulled through catalyzed liquid resin. Next, it is squeezed to remove excess resin. Then, it
passes through a die to shape the cross section. Then, it passes through a heated die to cure
it. Finally, it is cut to length, stacked, and packed. Strength in the lengthwise direction is

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