Open Mold / Hand Lay Up

Posted on June 16th, 2014 by admin

Sintex – Wausaukee Composites utilizes multiple processes to manufacture fiber reinforced plastics (FRP). SWC’s extensive expertise in Open Molding/Hand Lay Up allows us to produce innovative and cost-effective solutions for our OEM customers. SWC specializes in open mold parts for:

Large Hoods
Medical housings and tables
Complex Shapes

Hand lay up or open molding is frequently used to create complex, large or complicated components. The process is best utilized for low volume requirements where a one sided finish is required. With hand lay molding, Sintex – Wausaukee Composites can accommodate single or multi piece molds and present the least costly tooling options to our customers. Open molding is the preferred method for prototype development and is commonly used in applications where the number of parts needed are less than 1,000 per year.

SWC often uses the Spray-up open-molding fabrication process. This is where resin and reinforcements are sprayed onto a mold. The resin and glass may be applied separately or simultaneously “chopped” in a combined stream from a chopper gun. Workers roll out the spray-up to compact the laminate. Wood, foam or other core material may then be added, and a secondary spray-up layer imbeds the core between the laminates. The part is then cured, cooled and removed from the reusable mold.

Why Open Mold Glass Reinforced Composites?


SMC parts offer superior mechanical and surface appearance. SMC benefits offers very high volume production ability, excellent part reproducibility, and is cost effective due to low labor requirements and reduced scrap production. SMC structures made of composites have a long life and need little maintenance. Composites are corrosion resistant with matrices and binders that prevent environmental attack on fibers and increase resistance to corrosive environments.


Composites combine high strength and low weight, are non-corrosive and have thermal and electrical insulation properties. Composites can be machined like wood using diamond-tool equipment.

Dimensional Stability

Composites retain their shape and size when they are hot or cool, wet or dry. Composites can be a better choice in applications where demanding tight fits or component shape and size require products that do not vary.

Design Flexibility

Composites can be molded into complex parts that would require sub-assemblies if done in metal. Reducing the number of parts in a machine or a structure saves time and cuts down on the maintenance needed over the life of the item. Components can be designed with rigid, semi-rigid, or foam materials. Components can be made fire retardant, or include EMI/RFI shielding.

Excellent Cosmetic Finish

Composites can be designed with a high level of surface detail. Parts can be offered in various thickness either coated with high gloss gel coats or with a paintable surface.

General Design Guidelines

Minimum Radius6 mm (0.236 in)
Minimum Draft 2 degrees
Minimum Thickness 3 mm (0.125 in)
Maximum Thickness Unlimited
Typical Reinforcement Loading 33%
Density (GP resin) 1578 kg/m3 (.057 lb./in3)
Die Lock (Reverse Draft) Yes

Selection Guidelines

Complex Components
Varied Part Size – Very Small to Very Large
Multi Piece Tooling
Low Cost Prototypes
Low Tooling Costs
Allows for Die locks & Reverses
Ideal for Low Volume Parts (less than 1,000 per year)
Sintex - Wausaukee Composites

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