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Reader’s Choice: Plastic Repair

“I repair classic cars to newer ones. With all the different kinds of plastics out there, how do I determine what kind I’m working on and the ideal method of repair?”

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Mitch Becker has been a collision industry trainer for 30 years and an I-CAR instructor for more than 25 years. Contact him at (763) 585-6411 or [email protected]

“I repair classic cars to newer ones. With all the different kinds of plastics out there, how do I determine what kind I’m working on and the ideal method of repair?”

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Asked by: Rick Graves, owner, Graves Auto Body, Duluth, Minn.

Question answered by: Mitch Becker


Plastic repair has evolved to match the changes and amount of plastics vehicle manufacturers are now using. The cost of plastic parts on many vehicles has led to improvements in procedures and equipment to meet the repair industry’s needs. With the addition of much better materials in rods, the industry now has better tools and information for plastic welding.

Adhesive repair systems now have adhesion promoters that deliver a strong bond to ensure a durable repair.

Both plastic repair and adhesive repair processes have changed to meet the challenge of making a lasting, durable and more flexible (when required) repair while keeping costs down. The cost of these parts has forced the industry to realize that to simply replace instead of repair is no longer a valid business decision.

The use of combinations or blends of plastics in manufacturing has required technicians to learn to identify plastics and to use proper products and procedures. Failure to follow the most basic steps will lead to redos. Not following directions will also cause your reputation to take a hit in front of the customer.

this bumper is pretty beat up, but it can be repaired with today’s advanced plastic repair technologies.Types
The types of plastics and types or severity of damage that can be repaired have also changed. Deciding what repair procedure to use is up to the person doing the repair. Is one procedure better than the other? That depends on the types of plastics being repaired. Each type of procedure depends on the training and experience of the person doing the repair. It comes down to preference. What are you comfortable with?

The two types of repair addressed here will be plastic welding and adhesive bonding. On many plastics, you have a choice as to which type of repair to perform. On others, such as SMC plastics, you’re limited in choice. You always have a choice of which product manufacturer to use.

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I won’t go through step-by-step procedures as each company is different. But I will stress that you follow the instructions diligently. By failing to properly shape or clean, you’ll compromise other steps, and that will lead to failure.

Training will help you avoid the pitfalls of repair failures. I-CAR PLA03 is an excellent source for procedures as well as company training from product manufacturers. State Farm’s Hi-Tech Toolbox has excellent videos on plastic repair procedures as well as the most common reasons for failures.

Adhesion Promoters
Adhesion promoters for adhesive repairs have reduced the issues long associated with the repair of plastics classified as polyolefin. New rods used in welding plastics have improved the adhesion on blends of plastics and polyolefin. Although solvent welding is still being practiced on older plastics on classic cars, the new systems are considerably better and more safe for technicians.

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Two Categories
Plastics can be divided into two major categories:

1. Thermoset or thermosetting plastics. Once cooled and hardened, these plastics retain their shapes and cannot return to their original form. They’re hard and durable. Most repairs will involve an adhesive type process.
2. Thermoplastics. Less rigid than thermosets, thermoplastics can soften upon heating and return to their original form. They’re easily molded and extruded into films, fibers and packaging. Examples include polyethylene (PE), polypropylene (PP) and polyvinyl chloride (PVC).

First, you need to identify the composition or type of plastic being repaired. Most components have an ISO code stamped on the back. This code is put there for recycling purposes, but you can use it to identify what type of plastic you have for repair purposes.

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Then comes determining the plastic’s physical properties. Is the part rigid, semi rigid or flexible? These properties are fairly simple to determine just by trying to bend the plastic.

Most plastic components have an ISO code like this one stamped on the back that will help you identify the type of plastic.Two Methods
There are two common ways to repair plastics today:

1. Heat: Using either a hot iron (airless) or hot air system. This includes nitrogen welding. Other sources can be laser, frequency
or ultrasonic.
2. Adhesive: Using adhesives to bond to existing plastic. Solvent welding is a form similar to using adhesive.

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Both procedures have reinforcement available to either melt or bond into the plastic to give a strong, longlasting repair.

Plastic welders have been around for a number of years. The airless kind uses a hot iron to melt and shape plastic. A hot air welder works by passing compressed air over a heating element and heating the air to around 345ºC (650ºF) to melt the base plastic and filler rod/ribbon together. If a reinforcement is required on the backside to strengthen the plastic, it will be melted in to the base plastic and filler rod. This type of welder does not use a flat shoe or feeder tube-type tip. A V-groove is cut into the part, the rod is laid into the V-groove and the two are melted together. Whenever using this type of welder, it’s important to have airflow over the element at all times, no matter if it’s preheating, welding or cooling.

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A nitrogen hot air welder uses compressed nitrogen gas to eliminate oxygen from the weld area. The nitrogen acts as a shielding gas and allows for a contaminant-free weld with less smoke, which creates a stronger weld. This type of welder can also switch to compressed air so it doesn’t waste the nitrogen when preheating or cooling down the heating element. A fusion weld is made when the welding rod and plastic melt and mix together. This type of weld can only be done on thermoplastics. Thermoplastics, such as polypropylene/ thermoplastic polyolefin (PP/TPO), which is used to make most bumper covers today, work well with this type of welder.

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Weld Kit
Your plastic welding kit will include all of the components you need, including the heat gun and the various welding rods for the previously mentioned plastics. If you can’t identify the plastic, clean an area that is not visible to the finish side and find a rod that applies closest to the plastic being repaired. Heat the rod to the original plastic to check adhesion. Many kits now have a universal rod that will work with most plastics.

The weld kits come with a chart that tells you what temperature to use for the different types of welds and plastics. Reference your setting before you begin to tack weld, and set the heat gun accordingly.

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Shops that want to attempt these types of systems and train their employees should look into the cost of the machines and training availability. Many shops say it’s one of the best investments they ever made.

Adhesive Repair Systems
Adhesive repair systems do not require heat to bond. The adhesives bond to the plastics by linking to the repair area surfaces. Depending on the type of repair being done, a different adhesive may be used on the backside versus the front. This gives strength while maintaining the flexibility the plastic requires.

In the past, adhesives were limited due to the “slippery” nature of some polyolefin plastics. Many systems now offer adhesion promoters to help the repair material link to base plastic. Many technicians try to use a generic adhesion promoter with specific brands, but this will most likely cause
a failure.

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A successful adhesion process is dependent on very specific procedures being followed, from the prep of the part to be repaired to the grinding and shaping of the repair area. Each step is critical to success. A technician who doesn’t follow the procedures will run into complications, if not failures. Once a technician understands how to use these products, it becomes relatively easy to duplicate results.

Adhesive systems offer the widest variety of plastic repairs. From PUR to SMC plastic, the procedures are fairly similar. Adhesion promoters may not be required on all as the adhesive can bond to some plastics without them.

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Although adhesive plastic repair doesn’t have the same initial investment cost as heat repair, the cost of materials must be factored in. Both heat and adhesive plastic repair require a certain amount of training and practice to be efficient. A shop must decide if one or the other or both are an option for them. To not repair is not an option.

Classic Cars
On classic cars, the ISO code may not be there or may not be visible. There are some physical tests that can be done to help with identification. One is the sand test.

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  • TPO — Sands in chunks, melts like butter with high-speed grinders, feels like wax and gets stringy when hot.
  • Thermoset — Flexible or rigid, sands powdery, does not melt with high-speed grinders and easy to repair.
  • SMC — Sheet-molded compound; Corvettes have it and looks like fiberglass with white powder.
  • Polypropylene — RV water tanks, dirt bike fenders. New plastic rods make the repair of these plastics feasible.

Hot Staples
Another new weapon you can add to the plastic repair arsenal is hot staple kits. These staples melt into plastic to give good strength and hold the form well. They’re easy to use and inexpensive. This tool offers a variety of uses.

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Charts
Depending on the type of system you’re going to use, many companies offer charts on their websites with suggestions for materials and their proper use.

If you use adhesive repair products, identify plastics to be sure all the steps and materials such as adhesion promoters are used during the repair. Wall charts provided by certain companies become a necessity to ensure the proper use of materials.

So the question is, which system should you use? Sit down with your technicians and ask them. They’re the ones who will be using the materials. Have them take some training to make sure the repairs they’re doing are top quality. The investment will be worth it. Some shops have found it pays to be involved in both adhesive plastic repair and heat repair.

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Mitch Becker is a technical instructor for ABRA Auto Body & Glass. Contact him at (763) 585-6411 or [email protected].

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