THE ROLE OF THE

72 The Role of the Chemist in Automotive Design
piston or plunger will push the granules into the hopper when the barrel is withdrawn
into a heated barrel. A screw within the barrel is rotated thereby feeding the pellets
through the screw’s grooves. The barrel has heated zones that assist in the process;
however, a great deal of heat and pressure is generated as the screw flights decrease
toward the end of the screw toward the mold. This mechanical action will heat the
plastic as the screw rotates.
The sprue is the channel from which the resin will flow through the gate and
into the mold. Runners are perpendicular to the direction of draw, and the sprue is
parallel to the direction of draw. This portion of the system is often recycled because
typical OEMs allow up to 15% of regrind material (the use of polymer salvaged from
the runner or sprue and not used in the original part). When injection molding takes
place, automotive thermoplastics must be cooled under the T m or T g before ejection.
Injection mold machines are rated by their ability to mold polystyrene in a single
shot [3]. Mold pressures can range from 55 to 275 MPa and cycles as low as 15 s. It is
possible to injection mold a thermoset resin, although it is very difficult. Compression
molding or transfer molding is used in these cases:
• Compression molding is an older method of polymer processing that is still
currently used for parts such as polyester thermoset engine covers or polyamide
transmission seals. The resin will be placed in half of the mold (generally
the female half shown in Figure 5.4). The male half of the press will
compress the resin to a pressure of about 15 MPa. The resin will be heated
simultaneously in both halves of the mold, which will cause it to begin to
cross-link.
• Transfer molding combines injection molding and compression molding.
• Extrusion is a method of producing a length of material with a uniform
cross section [5]. Extruders can be heated and cooled as required. A screw
provides pressure to the material after it is fed into the barrel through the
feed hopper. The pellets are melted and pass through a breaker plate into
the die. The material is then forced out of the die; its cross section is determined
by the shape of the die [5].
Upper mold
half (male)
Ejector
PIN
Lower half of
mold (female)
FIgure 5.4 Compression-molding diagram.
P
Flash