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Perfecting plastics

Avoid common plastic repair mistakes
Tuesday, June 11, 2013 - 12:08
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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.

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