Open moulding

Open moulding

Applied with a pressure roller, a spray device or manually. This process is generally done at ambient temperature and atmospheric pressure. Two variations of open moulding are Hand Layup and Spray-up.

Open moulding is by far the most common process used to fabricate composites parts accounting for over 40% of composites processed world-wide. It is a relatively simple process with low investment cost but a high degree of manual handling. Virtually all types of reinforcement can been used in open moulding which together with the use of core materials to create sandwich structures enables access to the widest range of mechanical and structural performance of any composites process. Unsaturated polyester resins dominate in this area but epoxy and vinyl ester resins are also common. Open moulding can be used for a very wide range of mouldings from caravan parts and cladding panels to boat hulls and radomes. Typical economic run lengths range from 2 or 3 individual parts up to several hundred.

An open-molding process is capable of producing PVC prototypes that have properties similar to those of injection-molded parts at lower cost and in only three days, according to ND Industries, Troy, Mich. Called Plastisol, the PVC material can sometimes be open-molded at lower cost than thermal injection molding, rubber molding, or die cutting.

open mold process can make product or process changes to reduce air emissions. Opportunities for reducing styrene emissions include:

  • Implement a controlled spraying program.
  • Improve raw material monitoring through better processing control.
  • Substitute low-styrene emission resins.
  • Upgrade resin and gelcoat application equipmen
Open contact molding in one-sided molds is a low-cost, common process for making fiberglass composite products. Typically used for boat hulls and decks, RV components, truck cabs and fenders, spas, bathtubs, shower stalls and other relatively large, noncomplex shapes, open molding involves either sprayup or hand layup. In an open-mold sprayup application, the mold is first treated with mold release. If a gel coat is used, it is typically sprayed into the mold after the mold release has been applied. The gel coat is then cured and the mold is ready for fabrication to begin. In the sprayup process, catalyzed resin (viscosity from 500 cps to 1,000 cps) and glass fiber are sprayed into the mold using a chopper gun, which chops continuous fiber into short lengths, then blows the short fibers directly into the sprayed resin stream so that both materials are applied simultaneously. To reduce VOC emissions, new piston-pump-activated, non-atomizing spray guns and fluid impingement spray heads dispense gel coats and resins in larger droplets at low pressure. Another option is roller impregnators, which pump resin into a roller similar to a paint roller. In the final steps of the sprayup process, workers compact the laminate by hand with rollers. Wood, foam or other core material may then be added, and a second sprayup layer imbeds the core between the laminate skins. The part is then cured, cooled and removed from the reusable mold. Hand layup and sprayup methods are often used in tandem to reduce labor. For example, fabric might first be placed in an area exposed to high stress; then, a spray gun might be used to apply chopped glass and resin to build up the rest of the laminate. Balsa or foam cores may be inserted between the laminate layers in either process. Typical glass fiber volume is 15 percent with sprayup and 25 percent with hand layup.