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!

Cutting Epoxy/Fiberglass with Ring Saw

Cutting and machining of cured composites parts can be challenging.  Cured composites can wear cutting blades out very quickly.  They are also very dusty when cut without using some sort of cutting fluid.

Many of these problems can be eliminated with the proper equipment.  I found a very interesting video of a fiberglass-reinforced epoxy resin block being cut using a water-cooled ring saw.  This saw is called the Revolution XT and it uses some neat technology to get a relatively quick and clean cut without making dust.  It looks like it would also allow some good freehand movement to cut curves and angles in composites parts.

Cirrus Composite Airframes

I found a very interesting Youtube peek inside of the Cirrus airplane assembly factory in Minnesota.  While it would be cool to learn more about the actual composites fabrication of the individual parts, there is some great information about part bonding, inspection, and final assembly of the Cirrus aircraft.  They use fiberglass and carbon fiber reinforcements to create a very strong and durable fuselage, and it is great to see in action.

Personal jets made of composite materials offer many advantages and unique properties.  Design of complex shapes and anti corrosion of aluminum are two advantages.  Disadvantages include repeatability and upgrading/modification.

Composites that are properly designed and fabricated can be used in many applications where safety is a big concern.  Proper design and inspection during production can create an airplane that can be easily maintained and have a very long life.

Great factory tour courtesy of Aero-TV:

Panel Stiffness

Composites structures have requirements for stiffness to provide support and stability.  Tests can be completed to identify the stiffness of a given area on a composites structure, i.e. how much it will bend for a given force.

The required stiffness of a section of a composites part depends upon the overall design and service expectations.  Several factors, including the life expectancy of the object, the load rating of the composites surface, the inter-laminar bond strength, will help determine the threshold requirements.  Testing is very important to determine the life expectancy of the part and whether it meets the requirements of its’ job.

Panel stiffness can be modified to meet these requirements using two basic methods.  One way to increase the stiffness of an unsupported composites panel is to reduce the size of the panel through additional support structures.  The other way is to increase the panel thickness across the same area.  Choosing which method to use depends upon the engineering of the part and determining which method is acceptable with the surrounding part layout.  If there is room to add supports, this is likely a quick and easy option.  If there is room to increase the thickness, adding new or additional coring materials may be a simple solution.

Building a strong and stiff composites structure can be accomplished with the extremes of  building a robust “skeleton” with small open areas and a thin skin, or having a basic, limited “skeleton” with a heavy duty cored laminate that supports itself.

A combination of the two usually works out best.

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.