Highline Products

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.


  • Compression-molded box pads are twice as stiff as spray-up boxes. They are also superior in every area of strength, offering 50-60% greater flexural strength, 20-30% greater tensile strength and 30-70% greater impact strength than spray-up products.
  • Consistency-- wall thickness is consistent throughout the molded part, whereas sprayup construction is difficult to control and is dependent on the operator.
  • UV Protection- Compression-molded box pads have integral molded U.V. protection throughout the entire product; not just on the gel coat surface, which is the case with spray-up.
  • Speed of Production.
  • Class A Surface- Both inside and outside surfaces are smooth.


  • High initial capital investment- match metal molds are very expensive thereby limiting compression molding to only high-volume parts.


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


  • Low start up cost-- molds and equipment are relatively inexpensive.
  • Variable sizes-possible to modify mold sizes, thus enabling a wide range of available sizes.


  • Only one side of the product is smooth.
  • UV protection and gel coat is only on the top surface.
  • Inconsistent product thickness--the product is only as good as the operator.
  • After manufacturing, part must be trimmed which leaves raw edges that are susceptible to environmental ingress.


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.

  Compression Molded Spray-Up
Thickness Tolerance 1/1000th"=.001 1/16" to 1/4" Varies
Flexural Strength 25,000-30,000psi (ASTM-D790 Test Method) 17,000-18,000 psi
Flex Modulus 1.2-1.7psi x 106 (ASTM-D790 Test Method) .59-.65 psi x 106
Tensile Strength 12,000-13,000psi (ASTM-D638 Test Method) 9,100-10,500psi
Impact Strength (Notched Izod) 20-30 ft lbs/inch (ASTM-D256 Test Method) 15-17 ft lbs/inch
Glass Length (longer is stronger) 2"-3" Random 1" Random
Glass Fiber Orientation Random Non-Random
Color Throughout Gelcoat surface only
UV Protection Throughout Gelcoat surface only
Resin Type Resilient Isophthalic Non-Isophthalic
Water Absorption Non-Existent 1.50%
Spalling, Chipping, Splitting, Flaking Rarely Variable
Blooming None Variable

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