Compression Molding of SMC (sheet molding compound)
This molding technique involves compressing material containing a temperature-activated catalyst in a heated match metal die using a vertical press. The matched metal dies define the outer surface of the part being made.
The molding process begins with the delivery of high viscosity uncured composite material to the mold. Mold temperatures typically are in the range of 275- 350 Deg F. As the mold closes, composite viscosity is reduced under the heat and pressure approximating 1000 psi. The resin and the isotropically distributed reinforcements flow to fill the mold cavity.
While the mold remains closed, the thermoset material undergoes a chemical change (cure) that permanently hardens it into the shape of the mold cavity. Mold closure times vary from 30 seconds up to several minutes depending on part design and material formulation. When the mold is closed the pressure exerted by the press tonnage forces the resin, gel coat and UV inhibitor throughout the reinforcement. The resulting part is extremely strong, durable and consistent.
Spray lay-up is an open mold process that uses mechanical spraying and chopping equipment for depositing the resin and glass reinforcement. Chopping equipment consists of a spray gun attachment, which chops glass fiber into predetermined lengths and projects it to merge with the resin mix stream. This causes the resin and chopped glass to be deposited simultaneously to the desired layer thickness on the mold surface (or on the gel coat that was applied to the mold.
The mold (female mold) is first coated with a wax to ensure removal after curing. A layer of gel coat is then sprayed on to the mold to form the outermost surface of the products. The polyester resin is applied with a spray gun that has a glass chopper attachment.
Layers are built up and rolled out on the mold as necessary to form the part. The spray gun has separate resin and catalyst streams that mix as they exit the gun.
Drawing 1 - Spray Lay-up
Each manufacturing technique has unique capabilities and limitations. Overall a compression-molded part is the most desirable because you are guaranteed a consistently strong and durable product each and every time. Unfortunately, for low-volume production, compression molding is not effective. Currently the spray-up process is the best alternative for low-volume parts.