IH Hood Video
Several other posts have documented the work I performed on my 1993 International Truck Hood. I also took some video and recently got it all put together and posted on Youtube. Hopefully you can learn a couple of things.
The SMC hood was ground in preparation of work, and then I used epoxy and fiberglass to complete the work and restore the structure and shape back to original for this truck hood.
IH Hood Repair– Passenger Corner
One of the repairs on my International Hood was for the passenger corner. This portion of the hood was damaged before I got it, and was COMPLETELY MISSING!
Time, effort, epoxy, and fiberglass, allowed me to successfully completed the repair. This SMC hood will be repaired as good as new!
Damaged area to be repaired
The hood was placed upside to allow for work access. The repair area had several cracks and breakage areas. The repair began with surface preparation.
Surface Preparation on the repair area
I used a grinding disc to remove material on the front and back side of the repair to scarf the repair into the large area. Good surface preparation gives us a clean area that can hold a bond and create a transition area. I also drilled some holes in the end of the cracks to stop their propagation.
The back side was prepared too
The back side was prepared for repair by grinding the surface to allow for good bonding by the epoxy resin and glass reinforcement.
Beginning to add material
I started to add epoxy and fiberglass back onto the fender. I had to gradually move back out to where the existing fender used to be. I worked both sides bit by bit and allowed it to start curing before adding more.
Getting the shape back
I actually went a bit beyond the shape I needed. This allowed me to get the surface planes in the right spot, and later return and trim back the proper hood edge.
Back side getting rebuilt
The back side of the repair also got transitioned into the hood and out to meet the front repair area.
Grinding the repair
After most of the rebuilding was complete, some grinding got it back into shape to allow an evaluation of the repair progress.
Grinding on the back
The back side was cleaned up as well, getting the appropriate part thickness back to where it was originally intended. A nice transitioned surface was created, and all sharp edges were removed.
Bodyfiller for cosmetics
I used some bodyfiller to smooth the repair and restore the cosmetics of the hood. I could also have used epoxy, and it would have bonded better. Polyester bodyfiller is less expensive, easier to apply, and easier to sand. I smoothed the surface and feathered it back into the surrounding area to allow for a consistant surface.
Application of Primer-Surfacer
Urethane Primer-Surfacer is applied to the whole area to allow for removing the sanding/grinding scratches and preparing the surface for paint application.
The repair is complete, and the hood looks back like it was original. This repair, and many others, can be detected with some investigation of the back side of the repair area. This will be a very durable repair and will last as long as the rest of the composite body panel.
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.
Polypropylene Fiber Reinforcement
One of the press releases I recently came across discussed the commercial release of Polypropylene fiber for use as a reinforcement in composites. The one I saw is sold under the trade name Innegra S Fiber by Innegrity LLC.
Polypropylene’s low density is a huge weight advantage, especially as compared to glass. Measured at 0.84 grams per cubic centimeter, it can be compared to glass which is at 2.55 grams per cubic centimeter, Kevlar (aramid) which is 1.44 grams per cubic centimeter, Carbon which is 1.76 grams per cubic centimeter, and UHMWPE which is 0.97 grams per cubic centimeter.
This material exhibits high toughness, which will make it great as a potential replacement for aramid reinforcements in applications such as bulletproof vests and mass transit. The low cost of polypropylene and its huge cost benefits because it is more of a commodity material will bring it into many new applications. It will be exciting to see new materials like this find commonplace usage and application.
Cutting Layup Reinforcement
When working with reinforcements in the form of a woven mat, cutting is necessary to allow for proper orientation, workability, and strength. The most common way to cut these mats is with industrial scissors. Other methods include rotary cutters, die cutters, and electric shears, but a good pair of oversized, resharpenable, thru-hardened shears (scissors) are the best way to get started.
Woven mats can be cut to size in the dry stage -before the resin is applied- or in the wet stage, when resin is flowing freely. There are benefits and drawbacks to both, and operators usually find their own preferred technique. Cutting mats in the dry stage requires that it gets put together correctly when it is in the wet stage. Handling dry fiberglass is typically more itchy than wet fiberglass, which is sticky. Scissors used to cut wet reinforcement must be properly cleaned in order to be used again.
Many claims are made about the difficulty of cutting kevlar reinforcement. This can be remedied by using a dedicated pair of sharpened scissors only for kevlar. Kevlar is a material that requires a different angle on the blade in order to cut it. I have demonstrated to fellow workers how a fresh pair of scissors will cut kevlar all day, then cut a bunch of fiberglass. But when going back to the kevlar, the scissors will not cut it. Carbon Fiber falls into the category side of fiberglass where it will dull the blades and not go back.
Reinforcement to resin ratio
Reinforcement and resin are mutually beneficial to each other. The reinforcement is the strength in the system and the resin is the binder that holds the reinforcement together and shapes the product. The ratio is important in creating the optimal characteristics of cost, quality, and weight of the final product.
The reinforcement may consist of glass fibers (fiberglass), carbon fiber, kevlar, as well as a myriad of other natural and manmade fibers. The resin may consist of thermoset polyester, thermosetvinylester, thermoset polyurethane, epoxy, as well as any thermoplastics. As these are combined to create a product, the ratio used can create a wide range of properties.
The process used and processing goes a long way towards the actual resin to reinforcement ratio. Hand layup is extremely operator dependent where a good, careful laminator can achieve a 30-40% glass loading depending on the design of the glass and the time allowed.
Sprayup processing will allow for glass loading up in the 25 to 35% range. This process is usually a faster pace production process where more advanced equipment is used, though it is generally hand-operated and again operator dependant.
Resin Infusion processing can achieve reinforcement ratios towards 60% depending upon the reinforcements used and the processing. Some reinforcements have voids in them for the resin flow, which remain full at cure and lower the reinforcement ratio.
Infusion processing will have a ratio that is more consistant across the entire part because of the lower interaction with operators and the application of resin.
Vacuum Bagging processing will allow for the highest of ratios, which may reach 75% especially with autoclave operations. Taking very much more resin from the laminate will allow the reinforcement to separate itself and lead to failure.
There are also several more closed mold processing operations including thermoset injection molding, thermoplastic injection molding, compression molding, etc. that I will not get into here.
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