Fiberglass Plug and Mold Making

Fiberglass Mold Making - Plugs

The first step in fiberglass mold making is to make a plug.  A plug for a fiberglass or composite mold is typically a representation of the finished part, and can be an actual part or a mock up of a part.  In planning for the mold certain factors must be taken into consideration; surface finish, draft angles, undercuts, and method of production.  A composite mold that is going to be used for vacuum infusion or closed-molding processes will require large flanges. Parts that are undercut or have negative draft will need to be done in two part molds for them to be able to be extracted from the mold. Composite molds that are designed to be used with carbon fiber parts often need to be made out of carbon fiber in order to match the coefficient of thermal expansion, especially if during the molding process the part and mold will be heated to cure and then cooled to demold, i.e. prepreg parts.  If the mold is to see high temperatures and a post cure is necessary on the plug then your selection of materials will change.


Plugs made from scratch can be done from a wide variety of materials using a wide variety of methods.  We have posted an example of making a plug from scratch and taking it all the way through making a mold here. In that example we used a foam core to make the body, mounted it to plywood to make the flange, and glassed over it to provide a stable surface to fair over.  We have used MDF to make plugs, high density foam, styrofoam, body filler, and other materials.  For large parts often a framework is built and then layered with foam, fiberglass, and body filler before priming and finishing.


To make a fiberglass mold from an existing part, we will normally mount the part on a backing board of some sort.  Generally speaking, we build all our molds with flanges, assuming that at some point we will want to use them in a closed-molding or vacuum infusion process. It is much easier to build flanges into the fiberglass mold from the start than to do it later.  Mold flanges should be a minimum of 6" with 8" being preferable.  Mount the part to the backing board by whatever means available and necessary.  Hot glue is one of our preferred go-to's as it sets up quickly and is usually removable, however, if you are dealing with elevated temperatures hot glue will not work.  Once the part is mounted on your backing board fill and fillet any gaps or transitions between the part and the backing board so that your plug becomes one unit. The surface quality of your plug and your backing board will be reflected in the gelcoat of the mold.  It will not get any better, only worse. Before proceeding, bring the surface finish up to the level you desire in the finished composite part.  With this type of plug we generally recommend using paste wax and PVA as the mold release.  The PVA will make a physical barrier in between the plug and the gelcoat of the fiberglass mold. This provides a little extra protection to the finished part you are using as the plug.


Once the plug is refined to the surface finish desired and mold release has been applied, it is time to begin constructing the fiberglass mold.  The materials typically used for this are: gelcoat, polyester resin (iso tooling resin), fiberglass cloth or fiberglass mat. When making a carbon fiber mold you will use: carbon fiber, epoxy surface coat, and epoxy. In this article we’ll give a brief overview of the steps for a gel coated fiberglass mold.  

First apply the gelcoat, (preferably with a spray gun to a thickness of 20mils wet (0.020in) ) over the surface of the plug.  Allow the gelcoat to cure until it is firm but tacky to the touch.  Next, apply one layer of fiberglass mat over the whole mold surface with polyester resin over the gelcoat, being extremely careful to remove all of the air and make sure that the laminate is void-free.  Allow this layer to cure fully to minimize any chance for surface distortion.  After this layer has cured, apply 2-3 layers of fiberglass mat at a time until you have built up the mold to your desired thickness.  For example, 9-10 layers of 1.5oz fiberglass mat will give you a mold thickness of around .38" (which on a small mold is perfectly acceptable).  On larger fiberglass molds, it may be more practical to add core materials to bring the mold up to the desired level of stiffness.

For more information about specific materials please visit this link