Making a Small Mold
There are many methods and materials that can be used for making a fiberglass composite mold.
I found a video from Eastbay Composites that demonstrates a method for making small molds in a very quick an inexpensive fashion.
As you see from watching this, the basic construction materials are tooling gelcoat and inexpensive bondo for the support structure.
This mold construction technique certainly has drawbacks, but also has several advantages. None of the materials required spray equipment, which is messy. Brushes are inexpensive and easily disposed. A disadvantage of this method is that surface finish might suffer from a lack of consistency on the coating thicknesses for the gelcoat and the release agents.
Using bondo as the support structure is quick and easy compared to glass and resin, but can cause problems as well. It is more likely to crack, and can warpage issues as it cures and may be hotter in some areas than others. Bondo may have some difficulty maintaining dimensional tolerance as it shrinks during cure. Industrial resins used to create molds have minimal shrinkage in their chemistry and are placed on slower to minimize heat from the chemical reaction.
For quick, inexpensive and easy parts, this method from Eastbay Composites may work well for you. What has not been discussed yet is that the mold model can be the most difficult part. Off the shelf items are easy, but custom ones may be difficult. Creating a shape or surface is time consuming and tenuous. After that is complete, you can make your mold and final production parts!
Model and Sculpture
Composites have great effectiveness for creating low-volume and one-off production runs. A quick layup can be shaped over a rough foam carving before being sanded, smoothed, and painted. Autobody filler (Bondo) is shares the same chemistry as the polyester and vinylester resins and bonds very well to surfaces that are either just hitting cure stage or are sanded with sandpaper rougher than 80 grit. The range of shapes, details, and size are almost limitless from here. This is why fiberglass is very commonly used in Hollywood movie sets, amusement parks, and sculptures.
These structures do need proper support from moving over time. Even if the structure is not meant to be weight-bearing they need to at least support themselves. There is some amount of thermal expansion that happens with temperature changes which can lead to crack development. The complexity of the geometry and thickness of the laminate are variables to consider when figuring howmuch additional support must be added. Building a frame with welded steel tubing or screwed dimensional lumber is one way to support a fiberglass structure. And another school of thought is to create supporting ribs with flat stock to hold the shape. Both of these supporting structures are commonly attached via resin and glass tabbing.
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