When was the last time you were confronted with repairing a plastic part on a vehicle? Chances are, with today's cars, it was within the past week. Plastic has been used in bumpers and car interiors for decades. Saturn introduced the first quality, high-production thermoplastic door panel 10 years ago. Cars produced today contain plastic in almost every major system, from electrical and power train to fuel, chassis and even the engine. As the saying goes, this is not your father's Oldsmobile.

These innovations have led to increased fuel economy due to lighter weight and more aerodynamic design. Unfortunately, the amount of damaged plastics that end up in landfills due to the difficulty or impossibility of repairing them offsets the environmental gains. With so many different types of plastics being used, how do you know what to do with them once they reach your service bay? What happens when the crack won't sand, the adhesive won't stick, or the paint starts to bubble off a week after the customer has left your shop?

Decisions, Decisions

First, it helps to know what can be repaired, what can't be repaired and what shouldn't be repaired.

“Core supports simply can't be repaired because of what they're made of and the tolerances they go to,” said Charles Wilhite, an I-CAR instructor and ASE certified master collision repair/refinish technician.

In the Volkswagen Jetta, for example, the core support serves as the headlight mounting panel, the grill mounting panel, the radiator support and the fan shroud.

“It presents a huge economy for the manufacturer,” said Wilhite. “What that means, though, is that every dimension is critical. Particularly in places where they're likely to be broken, it's too difficult to repair them without changing thicknesses, so it's not practically repairable. It's a standard thermoset plastic, but it won't fit when I'm done.”

Other parts that should not be repaired include gasoline lines, radiator tanks, composite bumper reinforcements, composite suspension parts (springs), brake lines, battery cases and composite radiator supports. One simple guideline suggests that any pressurized container should not be repaired, nor should any part that affects passenger safety, which includes many interior panels and details. Even a small amount of dye or adhesive on a steering wheel or dashboard could affect the way an airbag functions, for example.

Some other factors to consider when determining whether or not to repair a plastic part include safety, location and extent of damage, access to both sides of the damaged part, if disassembly is required, long-term durability of repair, texture duplication, availability of replacement parts, technician skill and training, and customer expectations.

Name That Plastic!

So, you're confronted with a part you feel safe in repairing. To know where to begin, you have to determine what kind of plastic you're working with. Plastic is a term used to describe the physical properties of a material, indicating it can be formed or molded. Plastic also refers to a large group of materials with widely different properties. There are two main groups: thermoplastics and thermoset plastics.

Thermoplastics are manufactured by melting the plastic into a liquid state and forcing the material into a mold. Once cooled, the parts are ready for service or for secondary operations such as trimming or machining. Many of the plastics dealt with in collision repair fall into this group, such as urethane, polyethylene, polypropylene and vinyl. Parts made from thermoplastic material may be reheated and reshaped during repairs. Since they melt, they may be fused, or welded, back together.

Thermoset materials are formed by a chemical reaction and remain firm once cured. Two-part paints, some urethanes, epoxy and polyester resins fall into this group.

A third group, composites, refers to a material made from two or more dissimilar materials. The resulting material has distinctly different physical properties. The most common example of this is fiberglass. Soft glass fibers are combined with a plastic resin to form very strong parts. The glass fibers are strong in tension only, while the resin has good compressive strength, but is very brittle. Together they form a very strong composite material. Recently, metal and plastic used together to form a new kind of lightweight material has been developed. Manufacturers are also making use of blends of different types of plastics, combining them to make a sort of thoroughbred that combines the best qualities of each component.

Test Time

Several years ago when manufacturers began using plastics more frequently, the type of plastic was often identified by a symbol on the back of a part. However, fewer new parts are appearing with symbols because they are often composites made from an amalgam of different plastics.

There are a variety of established testing methods for determining how to repair your plastic part. One formerly recommended way was to cut off a sliver of the plastic from an unnoticed section of the part and burn it to see what color the smoke was. This method is not to be used anymore because of the dangers of inhaling toxic smoke and fumes. Another way of determining plastic type was to float the item in water; however, due to the blending of plastics this method no longer truly determines anything.

According to Martin Dewey, a training specialist for Sherwin-Williams, the most reliable way of typing your plastic is to turn to the back side of the part and sand a small section. Dewey wrote and teaches one of Sherwin-Williams plastic refinishing courses.

“If the plastic makes a gummy smear, don't sand it. If it turns to fine powder, it most likely can be sanded,” he said.

If you have purchased a replacement part, primers are another factor that must be tested. Not all primers are yet effective. Because some plastics have a higher level of release agent in them (the chemical that enables them to fall out of their mold), they resist paint and adhesives. Dewey suggests if you have a primed part, press some solvent on an inconspicuous area. If nothing comes off, it should hold paint well. If some primer does come off, for the safest result you should remove the primer and treat it as raw plastic.

As long as you have these basic tests, it doesn't matter what kind of plastic you have, you know how to work with it.

Performing the Repair

“Perhaps it sounds elementary, but the very first thing you should do is clean, clean, clean,” said Dewey. Specifically, clean the item three different times, both with a water-based cleaner and an alcohol-based cleaner. He also recommends using special adhesion promoters to make the paint stick. Also, use a primer you trust, and always test it before painting the whole piece.

Sounds like a lot of work? Better that you should clean thoroughly than have the paint peel off a week after your customer leaves your shop.

“If you don't clean enough, the release agents will still be there and will still act to release the paint,” Dewey said. “And the back of the part needs to be cleaned just as thoroughly, otherwise those release agents from the back will spread like germs to the front of the piece while you're moving it around.”

Once the piece is clean there are two basic options: welding or adhesive repair.

“Welding or melting plastic is one good method of repairing some kinds of plastic, particularly thermoplastics,” said Wilhite. “Thermoset materials do not melt when heated and so may not be welded. However, thermoset materials can be softened with heat and reshaped.”

A large range of adhesive products is available to bond parts. Some are applicable to both thermoplastics and thermosets, while some are only used with one or the other. Reinforcements may also be used to make stronger repairs. Inserting fiberglass strands in the adhesive is a common technique. Steel fabric, much like screen door fabric, may be used when welding to make a very strong repair.

Adhesive repairs have recently become very popular due to great advances in adhesive technology, said Dave Robinette of Farmers Insurance.

“In the past, adhesives were formulated for specific uses. Failures were common if plastic and adhesive were not strictly matched. Now, adhesive companies have improved their products to be more versatile and user-friendly,” he said. “Less emphasis is placed on plastic identification, and more on the plastic's physical characteristics.”

Many of the adhesive makers have training available. Some even have in-depth courses available, explaining the use of their product to ensure successful repairs. Companies including 3M, SEM, Lord and Duramix teach repair classes, while other product manufacturers will almost always send a representative out to train you in using their product. Urethane Supply Co. has a wealth of information on its Web site and publishes a newsletter with “how to” articles.

Plastics are challenging, but they can be repaired. Awareness and training are the keys. With their use increasing every year, the time for that training is now.

RATE THIS ARTICLE What do you think of this article? Your input will help AutoInc. develop additional articles on this subject. Share your thoughts! Your name Your e-mail address

MOST ACCESSED ARTICLES Fuel Injection Service, Not Just Cleaning The Art of Extraction EGR Systems: Operation and Diagnosis Proactive Target Marketing:_Rethinking Your Business Strategy Engine Performance: HO2S Diagnostics MOST E-MAILED ARTICLES Developing Employee Potential How Critical Thinking Can Help Your Business How to Diagnose the Ford Glow Plug What to Look for When Shopping for the Right Shop Management Software Putting a Price Tag on Complaints

AutoInc. Web Site | ASA Web Site | How Will States Receive Insurer-Owned Shops? | Hydrogen, Horsepower and the Silent Power of the Fuel Cell | The Plastics Question: To Repair or Not to Repair? | The Role of the Shop Owner in Technician Training | Guest Editorial | Tech to Tech | Tech Tips | Shop Profile | Net Worth | Stat Corner | Chairman's Message