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.
Hood Repair–Driver’s Fender Corner
One of my recent projects was the repair of a large SMC fiberglass truck hood. It required several areas of repair, one of them including an outside wheel opening on the driver’s side of the hood. This area had a couple of large cracks from some sort of impact trauma.
Fender corner damaged by cracks
I began the work by preparing the area by grinding away the outside layer of existing material. This helps us get a good mechanical bond and a clean surface. It also allows for a feather edge transition between the repair area and the existing composite.
Fender getting epoxy and glass
The ground-out area is within the black outline created by a primer layer, and the underlying SMC is white in color. This area was ground with 40 grit sandpaper and followed by a healthy dose of compressed air to remove the dust from the process. I then wet-coated the areas with epoxy resin before turning to the glass and wetting that out. The soaked glass was applied to the back side of the fender to add strength back to the cracked laminate. A thickened epoxy paste was added to the front to fill the cracks that were ground out, as well as filling the holes drilled in the end of them.
Driver's fender from the backside
The glass applied to the area was applied down with a strong bristle brush to get maximum surface contact and remove air bubbles between the layers. The strength of the glass and epoxy will restore this area of the hood back to its original specification as long as a good bond is made to the composite laminate.
Trimmed after cure
As the resin and glass cure, there is a time window when the resin is in the solid phase, but not rock hard. This is when it is appropriate to take a utility knife and trim the edge of the loose glass. If it is tried too soon, it will move the glass on the laminate repair, and if it is too late, the knife will not cut through. If this time frame is passed, it still can be cut with a saw blade or grinder, but is much messier. This is also a good time to wipe up any resin that is draining from the repair.
The repair area can now be sanded and body finished before paint and primer! This resin will need to be covered to prevent UV damage in the future.
SMC Truck Fender
SMC (Sheet Molding Compound) works well for heavy duty truck parts, especially ones requiring complexity, strength, and impact resistance. One of the projects I am working on is a heavy truck with these SMC parts, including a hood and a driver’s and passenger’s truck fender extensions. Each of these SMC parts is very complex in geometry because of its complex application. The following is a picture of one of the SMC fender extensions.
SMC Truck Fender Extension
This SMC part is all one single molded piece, having very complicated geometry. It can also be noted that there are not any undercuts, i.e. the mold can open and close without having to move around the part. It has a mostly constant cross section, is not supported by any metal struts, and is attached to the cab by three bolts. The geometry and details to match the cab and hood styling are molded-in so that the part can be painted and bolted on the truck.
If a part needing this much complexity was manufactured with sheetmetal, it would have many more pieces of the assembly and much more bracing. Metal would also not have the resistance to corrosion and impact that is enjoyed by this composite part.
This fender extension is from a truck that has been on the road since 1993, with over 270,000 miles on it. This part has been in the wild for over 15 years, and has been exposed to tons of road salt, debris from the tires, and lots of other environmental exposures. It has survived well, only needing an update in paint to refresh its look.
Another great application of composites!
The Green Aspects of SMC
Sheet Molding Compound (SMC) is used to create many composite parts especially for the transportation industry, and contributes heavily to a positive environmental impact. SMC has been developed over the last 25 years to replace steel/sheet metal mostly in transportation applications. It is widely used in many heavy duty semi truck hoods, agricultural equipment, and pickup trucks, SUV’s and muscle cars.
The main goal of this substitution is to reduce weight, which improves fuel efficiency. Other positive side effects include fewer assembly operations, additional design freedom, dent and impact resistance, and the elimination of corrosion. Several “green” resin formulations have been introduced that make use of bio resins, which use much more renewable resources such as soy products. The fillers and reinforcements in this material can also be made from recycled and renewable materials.
SMC has overcome several hurdles in order to get to its present use and application. General acceptance and education had to be proven to the OEM manufacturers and consumers. Paint application and adhesion was one large consideration that had to be proven out. There were issues with popping and blistering from the SMC surface. Making sure the SMC parts held dimensions and aesthetics was also an important milestone.
SMC has become widely used today for many applications, and will find its way into many more. The weight saving aspects are paramount for reducing fuel consumption. The anti-corrosion and dent resistance are loved by consumers.
Sponsor
