Compression Molding Large Vehicle Parts
Compression molding with composite resins can yield very durable, strong, and well-engineered parts.
Placing reinforcements (usually fibered glass) and resin in a heated metal mold before compressing it under high temperatures will result in a finished piece with very unique properties. The parts made in these matched metal molds have very repeatable properties, high strength to weight ratios, and are corrosion and chemically resistant. The parts can be painted, and have many positive design attributes.
Ashley Industrial Molding is a company that manufactures such parts. These large parts are made with compression and RIM molding for customer such as John Deere and Case tractors. They have a good video demonstrating their equipment, processes, and finished 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.
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.
Trimming composite parts
I read an article on waterjet cutting of laminates recently, which demonstrated several advantages of using computer-controlled equipment for trimming and adding features to composites structures. Robotic-guided CNC routers have also been used with much success for large 3D shaped parts with much success. While technology is great, it is expensive. With the rest of composites processing being relatively inexpensive, the trimming operation usually follows suit. Most bimetal saws that cut steel can handle fiberglass, but for large production runs where they will wear down, diamond coated tools offer longer life.
Making a cutting guide for a production composite part is relatively easy. I always called it a “splash”, and it can be done similarly to building a part. Just wax up the gelcoated surface of the finished part, lay down a coat of gelcoat, then build a 3/16 laminate. Then mark out the desired cutout shape and allow for the tool setback for the router collar or saw guide. An aluminum channel edge should be added for wear resistance. Drill bushings can be located on this splash to mark where holesaw work needs to be done as well.
Using a guide collar on a router is essential. I was in a shop where they had a 1/4 inch steel plate guide that was the exact size of the hole, and required running the router bit against it to make the cut. There was TONS of wear on the guide and they were going though router bits like crazy. I set them up with a router collar and compensated for the offset on the jig, and it was a whole new world for the trim operators. The router bits lasted several times longer and the cuts were much faster to make.
Damaging the laminate is one of the dangers when cutting or machining composites. Delamination is the biggest risk, ans is diagnosed by a separation of the layers of fiberglass. The resin bond becomes too weak to hold them together. Delamination can be caused by several other things too, but very often it can happen around machined areas from dull tools or operators that are pushing tools too quickly along a cut.
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