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What is Open Molding

Learn About Hand Lay-Up and Spray-Up


Resin application
Heidi van der Westhuizen/E+/Getty Images

Open molding and hand lay-up are the most common and widely used methods of manufacturing fiber reinforced polymer composites. In open molding, a one-sided mold is used where the cosmetic surface is directly against the mold, and the structural FRP is built up behind the cosmetic surface.

With open molding, products are produced from the exterior to the interior of the part. The following is the order of material applied to the mold:

  1. Release Agent: This is a wax or non-binding polymer that is first coated onto the mold. This allows the finished cured part to easily pop out and release from the mold.
  2. Gel Coat: This is a pigmented polymer resin that is sprayed on to give the glossy high quality finish to the product.
  3. Barrier Coat: This is a coating that is applied behind the surface gel coat. This helps create a better surface finish and improves the properties of the gel coating by reducing cracking. This is very common in the marine industry.
  4. Skin Laminate: This is the first layer of reinforcement and it is usually kept very thin. This improves the strength of the gel coat and also helps prevent osmotic blistering.
  5. Coring Material: Cores are used to add thickness to a part, without adding a large amount of extra weight. With cores, the properties of a sandwich structure create increased stiffness, much like an I-beam.
  6. Bulk Laminate: This is the majority of the FRP part. Fiber reinforcement and resin is layered to provide the strength and structural properties of the product.

In open molding, there are two common methods of apply the reinforcement and resin, hand lay-up and spray-up.

Hand Lay-Up

Hand lay-up is the method of cutting lengths of fiber reinforcement off of rolls. The reinforcement most often comes in the form of chopped fiber, woven fiber, or stitched fiber. Once a layer is placed in the mold, resin is applied either by pouring on by hand, or it can be sprayed on with a mixing gun. The layers are consolidated and air bubbles are removed by using squeegees and hand rollers.


Spray-up is a method of spraying a mixture of chopped fiber and catalyzed resin onto the mold. A chopper gun feeds strands of fiber which are cut into small lengths, while simultaneously being mixed with the resin. Pneumatic air pressure send this fiber mixture out of the tip of the gun. The glass is then consolidated and air is removed much like in hand layup, most often with rollers.

Open molding can create very large parts. The only restriction in size, is the ability to handle the part and mold. Although labor intensive, closed molded parts are a low cost way of manufacturing composites. Emissions can become an issue however, as regulated and controlled by Federal, state, and local standards.

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