Page 155 - Handbook of Plastics Technologies

THERMOSETS

THERMOSETS 3.25

Matched die molding. This uses matched male and female dies in a hydraulic press.
The material may be introduced in several ways. (1) Glass fiber mat is impregnated with
catalyzed resin and then pressed; this only permits small simple shaping such as cafeteria
trays. (2) Glass fiber perform is made by spraying chopped fibers onto a screen prototype
of the desired product then placing the impregnated perform in the mold and pressing it;
this can make anything from small products up to boat hulls and auto bodies. (3) Premix is
doughy molding compound of polyester, catalyst, short fibers, and fillers; this permits
more versatile shaping.
Mat and perform use long glass fibers, which give better properties; premix uses
shorter fibers that give easier processing. Concentrations are typically 35 to 40 glass fiber
+ 30 to 65 CaCO filler.
3
High-temperature initiators are typically benzoyl peroxide, t-butyl peroctoate, or t-bu-
tyl perbenzoate. Molding pressures are 100 to 300 psi. Molding temperatures are 107 to
121°C for mat and perform, and 135 to 149°C for premix. Cure times are 2 min or longer.
Bulk molding compound (BMC). Polyester and low-profile resin, organic peroxide
initiator, 0.25-in glass fiber, calcium carbonate filler, mold release agent, and alkaline
thickener are blended into a doughy mass (Table 3.22). This is compression molded and
cured. High flow permits complicated features such as ribs, bosses, and inserts. Major ap-
plications are electrical parts, dinnerware, small tools and appliances. Bulk molding com-
pound can also be injection molded, but this requires more skill and care than simple
thermoplastic injection molding.

TABLE 3.22 Bulk Molding Compound
Formulation

Polyester 32.9%
Dicumyl peroxide 0.8%
Glass fiber 14.5%
CaCO 3 49.3%
Zn stearate 0.8%
MgO 1.7%

Sheet molding compound (SMC). (1) Polyester resin, low-profile resin, organic per-
oxide initiator, filler, mold release agent, and alkaline thickener form a paste (Table 3.23),
which is spread onto a moving continuous carrier film. (2) Glass fiber roving is chopped
(e.g. 1 in long) and spread over the paste. (3) Another layer of paste, with a top carrier
film, is laid on top of the glass fiber. (4) The entire sandwich, encased between the two car-
rier films, is passed between pairs of rolls that knead and squeeze it to complete impregna-
tion of the paste into the chopped fiber. (5) The sandwich is rolled up and stored until it is
used. Shelf life is limited, so it is best made in house where it can be used more promptly.
Compression molding temperature is typically 135 to 149°C. Major uses are automo-
tive body parts, business machine housings, and seating.
Pultrusion. Continuous reinforcement (fiber or fabric) is pulled continuously through
liquid catalyzed resin to impregnate it and then through heated dies to shape and cure it.
Properties are excellent in the machine direction but limited in the transverse direction.
Filament winding. The highest modulus and strength of any plastic material are
achieved by filament winding. Continuous roving is pulled continuously through liquid

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