Plastic repair options for today's vehicles

June 1, 2018
With composite use in vehicle construction on the rise, here’s what technicians need to know about new repair options and how to do them right.

Editor’s note: This is the second part of a two-part series on understanding plastic and composite repair with adhesives. See Part One to learn how to make successful plastic repairs with adhesives, plus why shop management should carefully consider the cost of equipment and necessary training before investing in the next “cool tool” for the shop.

As automakers are expanding their use of composite plastics such as carbon fiber, fiberglass and sheet-molded compound (SMC) in vehicle construction, there is also the need for body technicians to understand their repair options and how to do them properly. 

Tighter government regulations on emissions are driving demand for automakers to use carbon fiber to build high-strength, lighter-weight vehicles that can meet U.S. Corporate Average Fuel Economy (CAFÉ) standards and European Union (EU) carbon dioxide gas emissions requirements. There is also a rising demand for carbon fiber-reinforced plastics (CFRP) in luxury cars, race cars and other high-performance cars.  

“Vehicles’ structures have regularly changed, but it didn’t become overly significant to the collision repair industry until about the 1990s,” points out Kevin Creegan, Sales Manager for LORD Fusor Aftermarket Repair Adhesives in the US and Canada. “This is when collision repairers began to deal with repairing vehicles built with unique structural elements and different materials such as high-strength steel (HSS), ultra-high-strength steel (UHSS), composites and other mixed materials.” 

Cover the damaged area in the composite panel with masking tape. This protects the damaged area from absorbing the plastic and rubber cleaner and eliminates wicking of the cleaner through the fibers into the composite.

Carbon fiber in the global automotive composites market is forecasted to grow at a compound annual growth rate (CAGR) — a measure of growth throughout multiple time periods – of 10.4 percent from 2016 to 2021, says global research company Lucintel in its March 2017 report, “Growth Opportunities for Carbon Fiber in the Global Automotive Composites Market.”  

With the growth of light weighting, many of the major vehicle manufacturers, including BMW, Mercedes, Toyota, FCA, Ford and General Motors (GM), have put focus on incorporating carbon fiber into mass production vehicles. Recent news reports indicate that the next generation of General Motors pickups will be made mixed material, with a high-strength steel cab safety cage and aluminum doors. Higher-end versions of the GMC Sierra and Chevrolet Silverado trucks may feature carbon fiber.  

Creating a high-strength bond 
This growing use of composites by original equipment manufacturers (OEMs) also means both seasoned and new technicians will need to receive training to understand proper repair methods – and options – for structural and cosmetic repairs.  

When vehicles manufactured with composites were first introduced, the mindset was typically to replace the parts, Creegan says, as the repair capability wasn’t yet developed. Initial attempts with resins and fiberglass matting would either completely fail or read through was common. Technicians would replace large component pieces to deal with small, locally damaged areas. “This just isn’t efficient and is too costly, especially for high-volume vehicles,” Creegan points out. “It’s easier to fix a small, damaged area on panels and hoods than replacing the whole part.” 

The reinforcing patch on the back side of the repair can be made with either a matching piece of composite material or formed from fiberglass cloth.

Now, repair adhesives specifically formulated for composites are providing technicians with an economical, alternative solution to simply replacing these parts or using welds and rivets. The introduction of new products and procedures is now allowing for robust repair.  

“The bond created when using repair adhesives on composite substrates is as strong – if not stronger – than rivets and welds,” Creegan points out. “Using repair adhesives also offers advantages such as lightweighting, bonding dissimilar materials, aesthetics and reducing noise vibration and harshness (NVH).” 

Although the process for using repair adhesives is pretty straightforward, technicians need to first determine the type of repair needed – structural or cosmetic – before proceeding. 

A cheat sheet to composite repair with adhesives

Two types of damage:

Cosmetic damage: 

  • Minor damage on Class A side only  
  • Only in the resin rich outer surface  
  • Not entering fiber-rich body of plastic (scratches/gouges) 

Structural damage: 

  • Any damage that penetrates into the fiber-rich body 
  • Key elements (cosmetic):
  • Proper taper 
  • Choose proper adhesive 
  • Cure adhesive fully 
  • Finish sand and feather edge 
  • Key elements/steps for a successful repair (structural)
  • Reinforcement on back side is required 
  • Proper taper on front side is required 
  • Pyramid patch on front side 
  • A pyramid patch is comprised of adhesive and fiberglass cloth and duplicates the fiber-rich center of composites; repair area expands and contracts similarly to composites 
  • The use of fiberglass cloth in the pyramid patch reduces read-through 
  • Proper adhesive  
  • Saturation roller 
  • Heat cure 
  • Proper procedures = No read-through
Source: Fusor 003 Composite Repair Clinic

“A structural repair is when there is any kind of cracking of the plastic, fine ‘spider webbing’ cracks, or a hole,” Creegan explains. “Anything disruptive to the composite behind the actual surface requires a true repair. A cosmetic repair is just an extension of the structural repair process such as a scratch, gouge or damage that doesn’t get into the fiber of the substrate or fiber vest.” 

Repaired part performance is always a concern, especially in situations in which the mechanical integrity of a part depends upon the continuity of the varied layers and orientation, of the “cloth” in the part. Structural components such as a pillar, as compared with simple bolt or bond-on parts such as fenders or even roof panels (which are more of a cosmetic treatment), require a far different method of damage identification and repair that is beyond the scope of this article.  

Work as much fiberglass cloth into the repair as possible while attaining a thickness similar to the original panel to control expansion and contraction in hot and cold weather. This will control visible “bullseyes” and “read-through” in the finished repair.

However, it should be noted that these types of repairs include replacing multiple layers of “pre-preg” material (pre-impregnated with the adhesive or resin and generally shipped on dry ice to retain their “freshness” as they have a very short life from manufacture to end use), orientating those layers in a precise fashion. Then, heat is used to cure the materials, while also holding all under a vacuum. This intricate process is why companies such as Lamborghini hire Boeing-trained composite repair specialists to make repairs on their structural assemblies. Standard composite parts made from SMC and similar “plastics” are produced with chopped fibers in the mix, not oriented layers as would be seen in a structural component. Some “hang-on” carbon fiber panels do use oriented weave material, but it is primarily to better function in the molding process and strengthen key mounting locations instead of the load-carrying qualities of true structural assemblies. 

Breaking down the repair process 
If a composite has been compromised, back-up reinforcement is needed. This can be accomplished by applying an initial patch to the damaged area and grinding out any broken material or fabricating a missing piece. 

“You are essentially making a patch over a hole,” explains Douglas Craig, Technical Application Engineer & Collision Industry Liaison, Structural Adhesives Tech Service, LORD Corporation. “A pyramid patch may be used to fill in the hole. You’re still pyramiding, but also have any one edge of the repair that is missing.” 

The most important points to remember when developing a patch is to use multiple layers of fiberglass repair cloth and saturate it with the plastic panel repair adhesive. The use of fiberglass cloth is critical to controlling the expansion and contraction in hot and cold weather. Using a repair adhesive with low viscosity is desirable because it can be difficult to saturate a cloth with thicker materials. 

After applying several layers of heavily saturated repair cloth, carefully roll them to eliminate any air pockets and apply heat to set and harden the repair.

“The key is getting in as much cloth as possible and really saturating it,” Craig says. “It may seem like only one or two pieces are needed, but it becomes thin when you start stacking it. The more fiberglass you get there while attaining a thickness similar to the original panel, the better the repair will be.” 

Everything then must be carefully rolled together to eliminate any air and sharp edges. This will eliminate a “halo effect” or “read through” at the edges of the repair. It also provides strength so the patch is as robust as the panel, providing a high-quality finished repair with maximum strength characteristics. Leaving air pockets also may cause future layers to pop or result in paint issues, Craig warns.  

Curing for a proper repair 
After ensuring several layers of heavily saturated repair cloth have been applied and carefully rolled to eliminate any air pockets, heat can be applied to set and harden the repair. 

“Extra layers [of repair cloth] may be added, if needed,” Craig notes. “When the technician is happy with the overall fill of the repair – the patch has been built up to the proper level – a full heat cure needs to be done at 180 degrees F for 60 minutes using a heat lamp.” 

A repair may be made on or off the vehicle, even on a vertical surface. The patch design and shape may vary, but the important detail is to ensure ample fiberglass is embedded in the adhesive.

He stresses that the 60-minute cure time begins when the surface reaches 180 degrees F, not when the heat is first applied. Craig likens it to putting unbaked cookies in an oven, turning it on and counting the time toward the bake time needed for the cookies to turn out as expected. 

“You have to wait until the panel reaches the right temperature before starting time,” he quips. “The heat causes a chemical reaction in epoxy. At the end of the 60 minutes, we are doing what may have taken a longer period of time but it has to be done from the proper temperature and for the right amount of time to turn out well – just like baking cookies.” 

This step ensures the epoxy has been completely cured and makes the repair undetectable in the future. “If you skip the full heat cure, the repair may ‘halo’ in the future,” Craig cautions. “It will cure on its own, but we know the repair is done and will never show through by curing at this temperature for this amount of time.” 

When OEMs call with problems from repairs done in the assembly plant, Craig reminds them of the time and temperature that must be used to ensure a full cure.  

“I ask them to check their repair line when we get a phone call,” he says. “We’ve often found that someone is applying heat and just walking away. If you do the repair right – this means using the proper time to fully cure it – you’ll never see it. If you don’t adhere to it, you will see the repair later on because it has been done incorrectly.” 

Once the structural repairs have been completed, technicians should be at the point where a cosmetic top coat just needs to be added. They can switch from a heat-set plastic panel repair adhesive to plastic finishing adhesives. After putting on the final glaze, it’s ready for primer and paint.  

Adapting to the repair 
In addition to following proper procedures, creativity and artistic skills also come into play when making repairs. “You have to adapt to the repair,” Creegan remarks. “Although the basic technique used is identical, each repair is different.” 

Sand to shape the surface so it will mate with the panel being repaired. Make sure that the composite panel has been sanded wherever adhesive will be applied in the repair area.

One such example is a vehicle with a 6-inch crack that shoots off to the left or right. Two separate patches would be needed for this repair, but the cloth has to be crisscrossed. “There will be a weak spot if you try to abut them,” Creegan adds.  

The part of the vehicle and size of patch needed also factor into the repair approach. Structural damage to a fender requires a reinforcement – a.k.a. “backer” – patch to be created first before the pyramid patch can be built.  

Composite plastic substrates

Composite plastics include the following: 

  • fiberglass 
  • sheet-molded compound 
  • carbon fiber 
  • fiberglass-reinforced plastics (FRP) 
  • Metton LMR (liquid-molding resin) 
  • GTX nylon blend 
  • resin transfer molding (RTM)

“A backer patch is nothing more than fiberglass cloth and a product such as Fusor 100 EZ or a piece of fiberglass or comparable substrate, such as an existing piece of the fender material, but is important to the repair,” Creegan says. “It also needs to be the same shape as the repair area.” 

Technicians also need to be adaptable as to whether a patch is created on or off a vehicle. In the case of a fender, the patch will need to be built directly on the vehicle if the fender is not removed for the repair.  

“Sometimes you will work on the vehicle and sometimes you’ll work off of it,” points out Julie Shepherd, Global Business Manager for Aftermarket, LORD Corporation. “You may need to build the patch smallest to largest or largest to smallest.” 

The size and shape of the repair will dictate whether the repair is made on the vehicle or off, Shepherd notes. A patch can usually be built up and then laid over any repair the size of a hardball or smaller, while anything larger may need the patch to be built on the vehicle.  

“Small repairs are easier to build off the vehicle using the pyramid patch method,” she says, “while bigger patches are easier to build on the vehicle so the fiberglass cloth can be layered and placed where necessary. The main point is that technicians need to adapt to what is called for with the repair.” 

Heat-cured adhesives to advance vehicle repair 
Although many technicians historically have been wary of repairing composite panels because of read-through and other issues with making repairs, adhesives designed specifically for composites are making repair jobs more efficient and providing more options for body shops and technicians.  

Causes of read-through in composite repairs
  • Sharp edges = Stress concentration 
  • Taper to eliminate sharp edges and stress concentration 
  • Temperature changes 
  • Expansion/contraction increase stresses  
  • Lack of full heat cure 
  • Proper heat cure of adhesive eliminates shrinkage after painting 
  • Lack of fiberglass cloth in repair 
     
  • Fiberglass cloth needed so repair acts like sheet-molded compound (SMC)

“Historically, technicians have had problems with making composite repairs, such as not getting enough fiberglass material built into the repair or the adhesive shrunk after the repair was complete,” Craig says. “Some technicians have used polyester-based body filler without a cloth, but it’s not as hard so it doesn’t even begin to duplicate the base material. It may seem more complicated and messy to use a fiberglass product, but it never comes back to haunt you.” 

Employing repair methods such as a wet layup can make a repair too rigid. When resin and adhesive are mixed in a can, it may be difficult to ensure it’s done so properly and consistently.  

To that end, an adhesive heat-set formula for structural repairs that is automatically mixed when dispensed from a cartridge eliminates these types of problem.  

“When the final heat cure has been done, the repair is not going to change a little bit during the new few hours or days,” Craig maintains. “It will never change. It will never move. You will never see it. It’s just done.” 

To learn more about repair adhesives and to sign up for training, visit www.fusor.com and www.i-car.com. Go to https://youtu.be/vzAKI5WxJWw for a step-by-step video of how to perform a composite structural repair using repair adhesives.

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