By Jason Vander Griendt

January 14, 2018 at 6:52 am

Everyone has their own preferred method of building molds, as there are a lot of different ways of doing the same thing. However, in order to make a foam mold for fiberglass, some people prefer to use the lost foam technique. This technique has been in existence for quite a long time and is widely used especially for molds that don’t need to be too complex.

Basically, with the lost foam technique, all you have to do is to dissolve a part of the foam to maintain rigidity, leaving you with the remaining part of the foam when the piece is done. To clarify, what you do is; carve the foam into the desired shape, cover the carved foam with layers of epoxy and fiberglass, and then discard the foam.

This method is commonly used for nacelles and fuse glass but you can also create a hollow composite wing with this technique. Below are the steps involved in making a foam mold for fibreglass.

Before you get started though, you may like to have your mold mocked up as a CAD design. Making a CAD file is definitely help with! Get in touch using this link or by clicking the button below.

Talk to Us About Your Project buttonMold making and fiberglass layup

Once you’ve finalised your design with a CAD file, the steps below will help you to prepare your layup for mold making.

Step 1:  Carve out a big chunk of the foam to match the design, and then cut out the sides using a carpet or kitchen knife. Rough cut the sides with the knife to complete the shape according to your dimensions and sand or otherwise smooth out the foam to achieve the perfect shape. This step shouldn’t take you too long, especially if the mold you’re creating is for a straightforward shape.

Step 2: Once you’re satisfied with the shaping and final outline of your foam piece, you can wrap up the foam model and plug any holes with a layer of packing tape. This will stop the epoxy / fibreglass you will be applying from seeping into the foam. You can also apply wax or a fine layer of vaseline to ease removal of the mold from the tape later on.

Step 3: Create a mechanism to rotate the foam core so that you can apply the resin / fibreglass on all sides effectively. You can do this by creating a fixture with a nail holding the foam in place at each end. Alternatively something like a potter’s wheel can come in handy.

Step 4: When working with fibreglass, make sure to take adequate safety precautions. Begin by applying a layer of fiberglass cloth, using 3M #77 spray before applying the epoxy. Apply the cloth in about 3” overlapping strips to ease the placement. You can mix your epoxy by using the manufacturer’s directions as a guide.

Step 5: Apply the amount of layers of fibreglass cloth you need to get the desired rigidity and allow the epoxy to harden for about 24 hours or more.

Finishing processes

Step 1: When the epoxy is all dried, you can sand off any problematic areas and rough up the entire surface.

Step 2: Mix a blend of microballoons and epoxy to use as a filling agent and brush the outside of the entire mold. Give the mold at least a day in order for it to harden.

Step 3: Once the casting has completely dried, sand off all the raised aspects of the casting until you get a smooth surface.

Removal of foam

Step 1: Remove the foam after you’re done with the sanding as it provides a strong base to apply pressure. You can use acetone or any other suitable chemical to remove the solvent, which will pour out freely once you open an area of the fuselage.

Step 2: Remove the packing tape by pulling a corner of the wax from the wing bay located inside. This will leave you with a clean interior as the wax will also remove the remaining foam paste that failed to pour out.

Does this process sound too confusing or complicated? There are alternatives, for example depending on the part you’re trying to create, 3D printing the part instead could be a very viable option. Get in touch using this link or buying clicking the button below to find out more about how we can help your project succeed.

Find out how we can help buttonPainting

Once the cast has been finished by sanding it, spray with Kylon primer or a similar product of your desired color. After painting or in between coats, be sure to continuously wet sand the surface until you achieve the desired smoothness.

Quick tip: In order to increase accuracy, you can make use of cardboard templates or a 3D printed to-scale miniature of your end part, and remember that fiberglass is a lot weaker on sharp corners. Therefore, consider adding more layers of fibreglass cloth to reinforce the corners and help you sand all the corners when you’re finishing the part without weakening it.

Making a foam mold from fiberglass is a relatively straightforward process, but it requires a high level of patience and consistency.

Applications of this process

For a cool example of how people are applying these and similar processes, check out our book:

Designing, Prototyping and Manufacturing Your Product Idea book cover

If all this sounds too complicated for you, consider opting for a foam injection molding technique or 3D printing. It ensures top quality and you can easily outsource the whole process to experts without breaking the bank. If you’d like our help with your project get in touch for a quote here, or email

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Jason Vander Griendt

Jason Vander Griendt is a Mechanical Engineering Technician with years of experience working at major companies such as SNC Lavalin Inc, Hatch Ltd. Siemens and Gerdau Ameristeel. He is the CEO of JCAD – Inc., a company he started in 2006 after seeing a gap in the market for businesses who could assist clients through the entire product design and manufacturing process.

Jason has been featured in Forbes, has had his businesses analyzed and discussed in multiple start-up books, was a previous winner of the Notable8 Digital Innovator of the year award, and is a regular guest on business panels and podcasts. Email Jason at or follow him on LinkedIn.

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