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Understanding plastic repair with adhesives

Tuesday, May 1, 2018 - 07:00
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To that end, it makes good business and economic sense to broaden shop offerings by using an adhesive solution for plastic repair. “Why not explore adhesive solutions offered by the local distributor instead of spending a few thousand dollars or more on a new piece of equipment that may end up eventually just collecting dust?” Creegan says, comparing the latest “shiny, new equipment” to a treadmill or other piece of equipment that ends ups as a clothing rack or just collecting dust. 

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Don’t skimp on surface and application prep

One of the most important steps in the repair process is cleanliness. Technicians need to ensure the surface of the plastic parts to be repaired are thoroughly cleaned front and back using adhesion prep/cleaner specified by the adhesive manufacturer before starting the process.   

“Cutting corners on proper surface preparation can cause the adhesive to fail,” warns Douglas Craig, structural adhesives applications engineering manager & collision repair industry liaison for LORD Corporation. “Cleaning can be done with prep/cleaner products specially formulated to remove all waxes, silicones, dirt, and road oils. Dust and debris can be removed with compressed air.” (Dirty shop air can also cause plastic repair failures, adhesive or welded.) 

Craig offers these tips to help collision repair technicians with proper surface prep: 

  • Thoroughly clean the surface! Preferably first with a surfactant (soap) and water then followed up with a cleaner specifically designed to remove waxes, silicones, and road “grease.” Do not allow cleaners to soak into the plastic, especially composite panels such as SMC or other FRPs. 
  • Make sure that all surfaces are dry before proceeding to the next preparation step.  
  • Backing patch: A backing patch is needed if the damage penetrates through the part, such as a bumper cover.  
  • Sanding and abrasion: Abrade the surface and apply a surface modifier, if needed. Scuffing the substrate with a scratch-pad, sanding with a variety of grit ranges, or grinding are all techniques that can create more surface area. An abraded surface typically has double or triple the surface area compared to the original flat, smooth surface but always be cautious to sand “slow” so the plastic does not “burn” or the bond will actually be weaker.  
  • Surface modifier or adhesion promoter: Use the recommended surface modifier or adhesion promoter before applying the adhesive. Surface modifiers and adhesion promoters must be fully flashed or cured. 

After completing thorough surface preparation of the part to be repaired, technicians need to make sure the adhesive itself is properly prepped. The ratio in a two-component adhesive cartridge is critical because it will otherwise be off ratio, Craig notes.  

“If the plungers are not leveled first, the mixture will be off ratio likely resulting in a failure,” he says. “You also need to purge a few inches of material from the mixer nozzle before beginning the application.” 

Warranty considerations

When deciding which process or combination of processes to use for plastic repair, warranty issues need to be considered. Adhesive repair manufacturers have their own product warranties. A plastic welder manufacturer also may warrant their repairs against failure, but typically their process requires adhesive to prepare the repair for the next step – refinishing.  

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