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!
Working with Bondo
Commonly referred to by the trade name of “Bondo,” polyester body filler is used by autobody shops to achieve cosmetic repairs on automobiles. It is relatively inexpensive, easy to work with, and achieves good results when used properly. It is basically polyester resin that is highly filled to create a thick paste which can be turned into a solid with the addtion of a hardener.
I have been using autobody filler since age 14, and am finally getting good at it! Just kidding. It is more art than science, and practice makes perfect. Basically, a surface needs to be properly prepared to achieve good adhesion. Then the filler is applied after being mixed with hardener, and allowed to cure afterwords. Some fillers can be sanded as soon as 20 minutes after application.
Basic setup and tools
The basic setup is a working board surface to mix the bondo and hardener. The mixed material is applied with a squeegee or putty knife to fill in low areas. They also aid in cleanup, along with some paint thinner to clean the tools.
Hardener added
Mixing the hardener starts the application window from where the material goes from liquid to solid. The hardener is a peroxide paste that directly affects cure time along with temperature. Too little hardener can also cause poor physical strength characteristics along with difficult sanding properties.
Uniformly mixed
The hardener must be uniformly mixed before it is applied. This prevents lots of problems down the road.
Applied to surface
The body filler can be applied with putty knives or squeegees to achieve the desired affect. This polyester bodyfiller will shrink during cure, so filling high is typical. The flip side is that all of the high spots must be sanded back down, which is wasting filler and time, so it is a fine line to walk.
Working with body filler can be frustrating for the beginner who must practice lots of patience. There are many important aspects that must be followed, just like all composites materials processes. Autobody filler requires proper surface preparation, mixing and metering of materials, and attention to details in order to avoid future problems.
A couple of other practices I recommend is to always wear a dust mask, keep the body filler dry, and only apply bondo over sanded bondo. Some like to just build layers without sanding the lows, and this can lead to adhesion problems.
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