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!

Making Holes

If you are working with fiberglass parts, you may need to attach other parts, pieces, and features mechanically with fasteners.  Bolts and rivets are the most common mechanical fasteners used to accomplish this.

Composites with a nice, decorative gelcoat finish such as boats and RV’s require special care to make holes in them for placing bolts and rivets.  Disturbing the area around your hole in a gelcoated surface can lead to very expensive repairs by a fiberglass expert.

You can make holes yourself, but it requires extra care and attention.  I found a great Youtube video that demonstrates this from user CenturionCrew.

Of course the biggest mistake that can be made is improper placement of the hole.

Following the instructions in the video and drilling a nice slow speed hole is the best way to be successful.  He also mentions the caution that must be noted to stop the drill chuck from contacting the gelcoated surface.  One tip that I have is to place a small piece of rubber hose over the drill bit to contact the gelcoat before the drill chuck.

One other note with holes (all shapes and sizes) in cored composites fiberglass pieces.  If there is a layer of balsa or foam core in the cross section, extra precautions should be exercised.  One is to coat the inside surface of the hole with gelcoat, resin, or silicone to keep moisture and UV out of the core.

Another concern is compression of the core with mechanical fasteners.  Balsa and foam cores typically are low in density, and are not meant to be highly compressed.  If you are going to bolt something on, and it is going to be really tight, it is best to use a metal sleeve in the hole that is the same thickness of the fiberglass part.  Large washers or backer plates should also be used to distribute the load across a larger surface.

5 Axis Filament Winder

Filament winding is a process that can be used to create round fiberglass shapes with exceptional strength characteristics.  Used for piping, tubing, and tanks, filament winding is normally an automated process that has computer-controlled equipment to place glass and resin around a mandrel- the piece that functions as the mold.

There are many variables that can be modified for filament winding, and these will affect the strength characteristics of the finished piece.  The angle of the glass, number of passes (thickness), use of glass mat, and type of resin will affect the finished product strength characteristics.

I found a short video that demonstrates the equipment and the process.

As you can see, the glass is applied in a consistent manner across the part, allowing for uniform strength characteristics.  As you can see, this is yet another process that the composites industry uses to create useful products with advantages over those of competitors.

Repairing the Inner Fender

One of my recent projects involved the repair of a 1993 International Medium-Duty truck hood made from SMC.  There were several areas needing attention, and one of them was the driver’s side inner fender.  This piece had formerly been attached with button-head pop rivets.  This design is common to composites, and allows for easy replacement of the separate fiberglass pieces.  The pop rivets had come loose over time, allowed to move around, and cause severe damage to the extent that the riveting flange was broken off.  My only solution was to bond the two pieces together.

Material Fatigue in the corner

The loose panel flexed so much and for so long that it fatigued the material and failed in the corner of the inner fender next to the attachment to the rest of the hood.  To repair this, I removed the area with the rivets, ground down the surfaces of both pieces on both sides, and reattached them with fiberglass and epoxy resin.

Prepared glass and resin

I wanted to place epoxy and fiberglass on both sides of the repair area to ensure a good, solid bond that would hold very well.

Epoxy Resin and Fiberglass applied

After the area was prepared, I applied epoxy resin to the surface to ensure good adhesion.  I had a low spot that was a gap, so I mixed some microfiber and epoxy to make a paste and fill this gap.  A stronger bond is produced when the fiberglass is not spanning an open gap between the two pieces.  I placed two layers of 3oz Chopped Strand Mat over the paste and worked the air out to make a nice consistent repair.  I then ground down the surface to make a nice-looking, consistent repair.

The rear of the inner fender had similar problems.  A hole had emerged in the black SMC piece.  I ground down both surfaces and placed some fiberglass across the area to bond it together.