Sheet molding compound processing machine

Figure 5. Sheet molding compound processing machine [from Ref. 30].

Sheet Molding Compound, SMC, is a variant of BMC. SMC achieves higher strength and stiffness of the molded part through reinforcement with long glass fibers, a process which minimizes fiber breakdown, and because the glass fibers are partially oriented in the SMC machine and may be further oriented in the compression mold. 

Sheet molding compounds provide a high strength-to-weight ratio, with low cost and easy, rapid processing. They are used for automotive body parts such as front ends and fender extensions and for other structural parts such as business machine and air-conditioner housings. They (along with RIM polyurethanes and polyureas and injection-molded thermoplastics) are in the thick of the competition for increased use in auto body panels (doors, hoods, fenders, etc.). They have recently been adopted for the body panels in the GM minivans. 

Blends of PCTG with PC or SMA are typically injection molded into business machine, appliance and automotive parts but can also be extruded. Eilms of thickness 0.025 mm can be extruded and sheet thickness up to 3.2 mm is possible when conventional extrusion equipment is used. Typical processing conditions vary depending on film or sheet thickness. PP, HOPE or an acrylic compound is used for purging. 

Reinforced PPS compounds may be extruded into profile shapes or sheets under carefully controlled processing conditions using a 24 1 to 28 1 LID standard metering screw with 3 1 compression ratio and a heated die clamp. Typical barrel temperature set points are about 285 to 310°C (545 to 590°F). Some extruded rods and sheets of reinforced PPS compounds are used as stock for machining small quantities of parts, as is compression-molded stock. Thermoforming of extruded sheet may be performed using conventional techniques at temperatures above the crystalline melting point of PPS of 285°C (545°F) [133]. 

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