Perfecting plastics

Jan. 1, 2020
Throughout more than 20 years of teaching plastic repair, I have witnessed the evolution of repair methods and products.

Throughout more than 20 years of teaching plastic repair, I have witnessed the evolution of repair methods and products. The adhesive products used for plastic repair have never been more user-friendly and have made repairs very simple and reliable. Over time, the repair methods have been tweaked and adjusted to the point where they are extremely reliable, but the key to successful plastic repair is using those adhesives in the exact manner for which they were designed.

This is one segment of the repair where following the product maker’s instructions to the letter is directly linked to success. This is no place for freelancing or bench-top chemistry. One of the main reasons technicians are reluctant to repair plastics is that they have had a bad experience or failure in the past, oftentimes because they strayed from the instructions. The drawback to instructions is that they only instruct technicians what to do, but it may be just as important to tell technicians what not do to. Below is a list of the most common plastic repair errors that technicians make, and avoiding these errors will vastly improve their chances for success. Due to the variations in products, this information may be somewhat general, but will apply to most products. If you follow the instructions and are still having problems, these suggestions may help.

Cleaning mistakes
It’s difficult enough to get adhesives to bond to some plastics, but it’s nearly impossible if the plastic is not squeaky clean. The first step in a plastic repair is to clean the entire part (front and back sides) with soap and warm water. Cleaning the entire part will allow you to thoroughly inspect it for hidden damage such as spider cracking, peeling paint and broken tabs. It is especially important to clean the backside of the bumper because it may be coated with a mold release agent that was used to prevent the plastic part from sticking to the injection mold at the factory.

Next, the part should be cleaned with a plastic cleaner. This is where things can get tricky. If the adhesive maker recommends a specific cleaner, use it according to instructions; if not, an isopropyl alcohol cleaner will clean without leaving a residue. Solvents such as lacquer thinner or reducer are never recommended. Once the initial cleaning is done, it is very important to avoid using liquid cleaners of any kind on the raw, exposed plastic in the repair area where adhesive will be applied. Because you have already cleaned the part before you sanded it down to bare plastic, there is no need to clean it again with anything other than compressed air. Some cleaners may take hours to completely “off-gas” out of the bare plastic, so failure will occur when a technician wipes the bare plastic with a liquid cleaner then minutes later applies the adhesive – trapping the solvents underneath. The trapped solvents eventually escape, usually when the part is baked, causing the adhesive to come off in one big sheet. This is similar to applying wax over a fresh paint job, which also traps solvents. This is very common but also very simple to correct. Several adhesive makers have recently modified their directions to emphasize “no liquid cleaners on bare plastic.”

Poor repair taper
It’s very important when repairing a deep

Figure 1

gouge or a tear that goes all the way through the plastic, to make a wide, gradual taper. A proper taper or “U” groove on the cosmetic side of a bumper (Fig. 1) should be deep enough to expose about a ¼-inch wide strip of the patch on the backside of the part. Changes in temperature will cause the adhesive to expand and contract and pull away from the sharp edge of a “V” groove, causing a ghosting line to appear. With a gradual taper, there is no sharp edge where the plastic will separate from the adhesive. Also a “V” groove is too narrow to hold enough adhesive for a strong repair. The gradual taper allows more surface area for the adhesive to bond to.

Figure 2

Poor sanding or surface prep
Because most technicians use a die grinder with a carbide bit or a grinding disc to cut a repair taper into the plastic (Fig. 2), it is then critical to rough up the surface of the plastic to give it some “tooth” for the adhesive to

Figure 3

grab on to. Die grinders and discs will remove plastic material aggressively, which creates smooth plastic within the scratches. For best adhesion to

Figure 4

plastic, the surface should be “fuzzy,” not smooth. To create this fuzzy surface, you must always sand the taper at a slow speed with

Figure 5

a dual action sander, removing all shiny areas. Remember, adhesives will not adhere to smooth or melted plastic, no matter how small that area is (Fig. 3). Follow the product maker’s recommendations for which grade abrasive to use, but most adhesive makers require sanding plastic with P80 grit on a D.A. before applying an adhesive. The P80 grit will give you that rough or fuzzy surface for the adhesive to bond to (Fig 4). To avoid deep scratches showing through the repair at the feather edge, you can sand around the outer ring of the repair area with a finer grit such as P180 to refine the P80 grit scratches (Fig. 5). It’s also important when sanding the finishing filler products to use light pressure on the sander and keep moving to avoid heat build-up, which can cause the edge of the filler to roll back, instead of feathering.

Adhesion promoter mistakes
If the adhesive maker requires an adhesion promoter, use it correctly and read the instructions. It’s critical to follow coat thickness and flash time guidelines to avoid failures. One adhesive maker uses an adhesion promoter that contains a chemical molecule that etches or bites into the plastic. This creates a chemical bond to the plastic. Attached to that molecule are varying-length strands of plastic that create a “fuzzy” layer. If you looked at that layer under a microscope, it would look something like a scuff pad. When adhesive is applied, it will bond onto this fuzzy surface for a strong mechanical bond. With both a chemical and mechanical bond, the chances for good adhesion vastly increase. Spraying too much adhesion promoter on the repair area is a common mistake. If it is sprayed on too thick, the promoter will pool up and flow out smooth, instead of fuzzy. If you are having problems getting a fine featheredge, you may have applied the adhesion promoter too heavily or may not have allowed enough time for it to flash off.  Only use the adhesion promoter specifically recommended for that adhesive. You can also greatly enhance adhesion by firmly scraping the adhesive onto the repair area before continuing to add more adhesive into the taper; this is called a “tight coat.”

Incomplete mixing
Most adhesives are packaged in dual cartridges that use a static mixing tip to mix the two components of the adhesive together. The tip contains an internal auger that mixes part A and part B together as they pass through it. One common mistake is to attach the mixing tip to the cartridge

Figure 6

before checking to see if part A and part B are coming out of the ports unobstructed. Whether the cartridge is new or partially used, it is common to have some hardened

Figure 7

adhesive stuck in the opening, which blocks one of the components from entering the tip (Fig. 6). First, check the cartridge to make sure adhesive is being freely dispensed from both ports, and then always dispense some adhesive out of the cartridge to ensure that the ports are clear prior to attaching the tip (Fig. 7). If the adhesive doesn’t cure properly, there’s a good chance the tip was clogged and the chemicals did not mix at the correct ratio.

Figure 8

The ability to perform plastic repairs is becoming more important as the insurance companies continue to emphasize cycle time, severity and repair versus part replacement. Many shops are embracing this trend as they see the opportunities for repairing most other parts on the modern vehicle dwindling. Most accidents involve bumper damage, so if you aren’t repairing plastic, you are throwing away a lot of repairable parts. In many cases, the bumper you scrap will be picked up by a bumper company to be repaired by a much lower skilled worker than your technicians. Keeping that repair in-house has many advantages, including avoiding blending adjacent panels, repairing at a better gross margin than replacing, faster cycle time and having more control over the quality of the repair. The key to successful plastic repairs is to strictly adhere to the instructions from the product makers and be thorough in completing the steps they outline. Technicians are conditioned to complete repairs as quickly as possible, but because plastic repairs are extremely process-dependent, they need to slow down and be methodical. Attempting to reduce repair time by rushing the repair will be disastrous. The repair time savings will be realized by making the repair correctly the first time.

 Shawn Collins is a Senior Technical Service Engineer for 3M. He was an ASE Certified Master Collision technician for 26 years and has been an I-CAR Instructor for 19 years. He teaches more than 50 different training programs and is both a Steel and Aluminum Welding Qualification Test Administrator. He was named the 2009 I-CAR instructor of the year and received the I-CAR Tech Center Award in 2011.

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