Grinding Tools
During my recent International Delivery truck hood project, I did some grinding on the composite SMC truck hood prior to performing extensive repairs. It is important during any composite repair exercise to have good surface preparation. There are many ways to accomplish this depending upon the work area, tools available, and the work to be performed.
The following is how I did it.
Composite Grinding Tools
Beginning with personal safety, I used a dust mask to prevent inhaling the dust into my lungs. I used OSHA Z87 approved safety glasses to protect my eyes. A pair of earplugs protected my ears. Abrasion/dust resistant gloves for my hands, and a light jacket to keep the dust from my skin.
For removing the layers of SMC to feather edge the surfaces, I used several abrasive tools. A five-inch pneumatic grinder with 80 grit sandpaper worked well on the large areas to quickly remove large amounts of material. A right-angle die grinder with Roloc 3″ or 1.5″ sanding disks worked well for the concave areas and other difficult access areas. A straight die grinder with a fluted burr worked to get into the detail areas of the front grill and other cracks that needed material removed.
All of the tools I used were air powered, so I had a good air supply that would keep up with me. The large amount of dust that is produced from this sort of work presents explosion hazards when using electric tools. Large dust piles can also spontaneously combust, so care must be taken during dust storage and disposal.
A air blow gun was also used in this process to clean the dust from the part and from my clothes. It is important to keep personal safety equipment such as safety glasses and earplugs in place when using the blowgun. A small vacuum can also be substituted with care taken to prevent dust ignition/explosion.
After all of the material is removed from the SMC hood, all of the dust is blown off to leave a part with a bonding surface that is clean, dust-free and ready to be repaired with epoxy and fiberglass.
Fiberglass Fertilizer Box
Composites’ resistance to rust and harsh chemicals along with the ability to form complex one-piece shapes allows for its use as a fertilizer box on an old corn planter.
This piece was removed from the original International Harvester corn planter, but not because of any problems with the fiberglass! You can see the weathered appearance of the unit from the many years of weather exposure. Glass fibers cover the surface as the resin has eroded away.
The design of this piece included a top lid and a mounting/feed base. Made entirely of one piece, it has remained structurally sound and durable for its purpose. The harsh fertilizer chemicals and weather conditions have not affected this piece to the point of failure.
Removable Blazer Roof
One of the original composites applications in the automotive industry was removable roofs on Chevrolet/GMC K10 Blazers in the 68-72 years, which continued on the Blazers until about 1991. The one-piece composite roofs were durable and relatively lightweight, and way too complicated to be made of steel. Their resistance to rust has left us in the northern states with only the tops left to the Blazer.
This composite part has some mold and mildew on it, but is still mostly as structurally sound as the day it was manufactured. It still has the side windows installed and has clearly sunk down into the grass and dirt, but I am sure it could be lifted out and sanded and painted and re-installed on a vehicle. Good as new!
Hood Repair– A big hole!
My IH SMC truck hood project involved repairing a large hole. This was one of my largest hurdles to the whole project. This is what I initially saw.
The Big Hole
The picture is showing the hood upside-down on a work table. The hole is the result of some sort of long-ago impact the shattered the composite material and did significant damage. My first reaction was to just repair it from the backside. This hope was lost when I quickly discovered that there was not any access to the backside of the repair. At least not without cutting some support structures out of the way. Which would mean that I would need to rebuild those after I repaired the hole. Not impossible, but it seemed like a lot of work.
I began this repair like any other, with the surface preparation. The hole got larger as I removed all of the damaged material and created a transition area.
Damage is removed and transition area prepared
The edge of the hole was a razor’s edge as I transitioned out to the existing finished surface. I also used some sandpaper to sand around the inside of the hole to allow epoxy to adhere to the inside surface when I put in my patch.
My strategy was to create a thin layer of composite using epoxy resin and 3oz Chopped Strand Mat. The layup was done on a piece of plastic and allowed to cure. Once cured, I cut it to be about 1/2 inch larger than the hole. Then I mixed up some thickened epoxy and applied it around the perimeter of my patch and placed it in the hole. I had already placed a sheet metal screw in my patch, and attached a wire to a support to hold the patch in place until cure.
Patch in the hole
In the picture you can see the patch, the wire, and the support. The tension on the wire held the patch in place until the patch was cured in place. Next I worked to add material from the front to fill in the low spot. This returned strength to the laminate and restored the surface profile back to where it should be. I again used 3oz Chopped Strand Mat and started with small diameters and worked out to larger ones until the surface at the correct level.
Most of the Glass and Resin have been applied
Now most of the surface has been filled in, and it can be ground to remove any high spots, air bubbles, and allow for adhesion of the next layer.
Grinding the surface
Now I can fill the low spots with thickened epoxy or bondo and sand the surface to match the surrounding areas and get a nice finish.
This is how a repair should be made. A nice transition in the existing laminate will keep the repair from cracking or breaking in the future. After the bodywork is complete nobody will know it was repaired.
Working with Chopped Strand Mat
One of my projects is repairing a composite hood for a large International Medium-Duty truck. This hood is made of SMC, and is damaged in various locations to various degrees. In doing the repairs, I am using epoxy resin and glass reinforcement. The fiberglass reinforcement I am using is referred to as Chopped Strand Mat, and is the 3 oz. per square foot version.
Chopped Strand Mat
The chopped strand mat is made up of random glass fibers held together with a light binding material. A good pair of scissors can cut the glass into manageable sizes, though the scissors will be dull when you are finished. The chopped strand mat can also be torn by hand. This leaves a “feathered” edge to create nice transitions between the patched areas and the existing surface. Pieces that are cut will leave a silhouette that will show and may require other methods to make it disappear.
The first thing that I checked was to make sure the glass was compatible with the epoxy resin. The package told me so, but I also did a small test sample to ensure that the epoxy would mix with it, was workable, and would harden properly. Testing away from the actual part can save many potential headaches and pitfalls.
Applying the glass and resin is relatively easy. Thoroughly mix some resin and apply it to the properly prepared surface (clean, dry, and sanded). Tear off an appropriate size piece of glass and place it on a piece of cardboard. Use a cheap paintbrush to get the glass wet with resin. Flip the glass over, and wet the backside. The glass will go from white to transparent on the cardboard. Now lift the glass from the cardboard and apply it to the area with the resin recently-applied resin. Use the brush to push it down against the surface to get as much contact area as possible.
Additional layers can be applied on top to build thickness, making sure to create a nice transition and limiting air bubbles in the laminate.
Stopping a Crack
Composites can be very impact-resistant compared to other materials. Based upon their makeup, different composites materials will offer different degrees of resistance to impact. Once the threshold to impact resistance is passed, cracking will occur. Localized impact can form a crack in the weakened material, and vibration and additional loading can keep the cracks spreading. There is a relatively simple solution to stopping this.
I am currently working on repairing an SMC composite truck hood, which has various forms of damage including cracks. I want to repair the cracks, but also must keep them from spreading. So the solution is …
Drill a Hole!
A hole that is drilled at the end of a crack in the material will stop it from spreading. There is no place for it to restart. It cannot travel any farther because there is not any high-stress area that is weaker than the surrounding surface.
In my example, I am working on a complete repair, not just stopping the crack from spreading. I drilled the hole, reinforced the back side, and sanded the surface to accept filler. The filler will fill this low spot over the crack as well as the hole that was drilled. It can be filled right back in but will still retain its crack resistance!
Another way for cracks to start is from high-stress areas such as square corners. When making a hole in composites, it is important to always avoid sharp corners. Round holes are the best, but if the opening must be rectangular, the corners should have some radii incorporated into the corners.
A cracked laminate is a sign of failure, and it must be addressed before it gets worse.
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