Open Molding

Open Molding Process

Open mold methods allow for a rapid product development cycle because the tooling fabrication process is simple and relatively low cost.

Foam or wood core and stiffeners are commonly used as reinforcement with open molding. These produce light weight, strong parts without increasing laminate thickness. This can reduce manufacturing cost for larger, rigid parts.

Hand Lay-Up

Hand lay-up is used in low-volume production of large parts. Pigmented gel coat is sprayed onto the mold for a high-quality surface. When the gel coat has cured, glass reinforcing mat or woven roving is placed in the mold, and catalyzed resin is poured, brushed or sprayed on. Manual rolling removes trapped air, compacts the composite, and thoroughly wets the reinforcement. Additional layers of mat or woven roving and resin are added for thickness. A catalyst or accelerator initiates curing in the resin system, which hardens the composite without external heat.


Spray-up offers greater shape complexity and faster production. Spray-up utilizes a low-cost open mold, room temperature curing resin, and is ideal for producing large parts in low to moderate quantities. Chopped fiber reinforcement and catalyzed resin are deposited in the mold from a chopper/spray gun.

Manual rolling removes trapped air and wets the fiber reinforcement. Woven roving is often added in specific areas for thickness or greater strength. Pigmented gel coats can be used to produce a smooth, colorful surface.

  • Wide range of part size potential
  • Parts have one finished surface and require secondary trimming
  • Best for low-volume, large and/or complex components
  • Best for production rates of <1,500 parts per year*
  • Lowest cost tooling option
  • Can accommodate single or multi-piece molds
  • Preferred method for prototype development – design changes are easy.

*Volume recommendations are averages and provided only as a general guideline. Actual volume efficiencies are a more complex matter requiring detailed statstics about the part to manufactured.

Center of Excellence for Low Pressure Molding