• Perfecting Plastics - Avoid Common Plastic Repair Mistakes

    plastic repair
    • 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 tried and true and also extremely reliable. But, the key to successful plastic repair is using those adhesives in the exact manner for which they are 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, often times because they strayed from the instructions. The drawback of 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 are some of the most common plastic repair errors that technicians make. 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 the instructions are followed and the technicians 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 like spider cracking, peeling paint, broken tabs, etc. It is especially important to clean the back side 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 like 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 and 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 for use to emphasize “no liquid cleaners on bare plastic”.
    • Poor Repair Taper

      Poor Repair Taper:

      It’s very important when repairing a deep 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 should be deep enough to expose about ¼ inch wide strip of the patch on the back side 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 for the adhesive to bond to.

    • Poor Sanding or Surface Prep

      Poor Sanding or Surface Prep:

      Because most technicians use a die grinder with a carbide bit, a grinding disc or a file belt tool to cut a repair taper into the plastic (photo 2), it is then critical to rough up the surface of the plastic to give it some “tooth” for the adhesive to grab on to. Die grinders and discs will remove plastic material aggressively which creates smooth plastic within the scratches. For best adhesion to plastic the surface should be “fuzzy” not smooth.

    • Sanding taper

      To create this fuzzy surface you must always sand the taper at a slow speed with 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.

    • P80 grit on a D.A.

      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.

    • P180

      To avoid deep scratches from showing through the repair at the featheredge, you can sand around the outer ring of the repair area with a finer grit such as P180 to refine the P80 grit scratches.

    • Blocked components in cartridge

      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 it passes through it. One common mistake is to attach the mixing tip to the cartridge before checking to see if both 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 adhesive stuck in the opening which blocks one of the components from entering the tip (photo 6).

      • First, check the cartridge to make sure adhesive is being freely dispensed from both ports
    • Clear components on cartridge
      • Then always dispense some adhesive out of the cartridge, to ensure that the ports are clear prior to attaching the tip (photo 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.

      The ability to perform plastic repairs is becoming more important as 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.

    • plastic repair tools

      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.

      For more information, visit 3MCollision.com/Perfecting_Plastics

      Written by Shawn Collins, Senior Technical Service Engineer for 3M. Shawn was an ASE Certified Master Collision technician for 26 years and an I-CAR Instructor for 19 years. He teaches more than 50 different training programs and is 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.