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.

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Using mostly carbon fiber and kevlar reinforcements, Roush Racing fabricates many different components for the racing industry using epoxy resin systems.  Ranging from the front noses for the NASCAR Car of Tomorrow car to drag racecar bodies and small ductwork, Roush Racing’s composites shop does all sorts of fabrication.

The race shop includes two different fabrication processes.  Wet layup vacuum bagging is used for some parts, while others are made using prepreg material that goes into an autoclave.

The video tour is well done, and includes everything from the Eastman material cutter to the fabrication process, bagging process, and trim.  We also get to see some of the finished parts after they are demolded and trimmed.

These parts are very expensive to manufacture, due to the high cost of materials and labor.  Tooling and equipment costs for this type of process are somewhat reasonable, with the exception of the autoclave and the automated cutting table.  Composites fabrication of this caliber is labor intensive, but can produce very unique parts that are lightweight and strong.

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FRP rebar can replace steel rebar and provide equal physical strength while eliminating the corrosion problems of having steel embedded in concrete.

Prefabricated bridge decks made of FRP in a factory can be quickly and easily installed in the field to save time and money during the bridge construction project.  The large panels are shipped in and laid down, ready to use very quickly.  Concrete cure and inspection times are reduced.

Concrete crack repairs can be made to bridges and concrete structures by wrapping them with composite materials to seal these cracks and hold the concrete together, protecting it from further damage.

This technology has been in development for many years, and proper materials, construction techniques, and design guidelines have been established to allow for many successful projects to be completed.

Economics play a considerable role, and this cost comparison constantly changes with the prices of materials on both sides.  Composites have the advantage of typically requiring fewer road closures and construction time in the field.

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There are many advantages to vacuum bag molding, a few of which include:

• Improved resin/glass ratio
• More consistency across the laminate and part -to -part as compared to open layup
• Containment of air emissions from the resins

As compared to hand layup and chop layup, there are a few disadvantages, including

• Higher consumable material cost
• Higher capital equipment cost
• Difficulty with superior surface finish

Some parts are more suitable for vacuum bag molding than others.  It also depends upon which process it is being compared with.

Vacuum bag molding requires an extremely tight seal between the mold and the bag.  Molds with multiple pieces or holes for inserts can be difficult to complete a seal.

Parts that are overly large and complex can present challenges with placing resin and reinforcement before the cure cycle starts.  The bag must be completely sealed and under full vacuum before the curing cycle of the resin begins.

The basic premise of vacuum bag molding is that the air is removed from the bag, allowing the atmosphere (air on the outside of the bag) to push the bag onto the part on the mold, compressing the layers of resin and reinforcement.  Many misinterpret the process as “sucking the extra resin out.”  We are merely allowing the laminate to be compressed by the weight of the air above us in the atmosphere to consolidate it before cure.  The excess resin is usually absorbed by extra layers of sacrificial material inside the bag.

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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.

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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.

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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.

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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.

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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.

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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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