Better adhesives and repair materials have made it easier to work with plastics during repairs. With some training and expertise, plastic repairs can become a viable profit center for your repair facility.
The current trend with the use of advanced plastics and composites poses a whole new set of challenges to repairers. Carbon fiber has recently been introduced for both cosmetic and structural parts. For most structural applications, damaged carbon fiber parts are removed and replaced. For cosmetic carbon fiber parts, there may be repair procedures available. Blends of different types of plastics used for bumper covers and exterior body panels have created some adhesion problems with repair materials. Plastic repair product makers have addressed these considerations to make repairs to plastic look more attractive.
The decision to repair collision-damaged plastic body parts, or to replace them with new parts is not an easy one to make. There are several factors that can influence the decision, including the type of plastic, whether there is access to both sides of the part or not, whether the part contains any texture that cannot be duplicated during the repair process, and whether there are repair processes developed for the repair. The skill level of the technician repairing the part also should be considered. Ultimately, the decision to repair or replace a plastic part should be made between all involved parties, including the vehicle owner, the repairer and the insurer.
Advantages of Plastic
One of the biggest advantages of using plastic for exterior body panels is its ability to be molded into intricate shapes. Parts can be made lighter, yet maintain style characteristics. Another advantages is, depending on the type of plastic, it has some flexibility and dent resistance.
You may remember the commercials made a few years back showing a shopping cart smashing into a side door without any evidence of damage. Other types of plastic aren't quite as flexible. The key point is plastics can be made to perform a specific task, be it for structural or cosmetic purposes.
The first step for repairing plastic is determining the type. Generally, plastics used for automotive applications can be broken down into two categories; fiber-reinforced and non-fiber-reinforced. Other classifications commonly used by adhesive product makers are rigid, semi-rigid and flexible. Fiber-reinforced plastics are also known as composites. A composite is defined as two materials, that when combined, make a stronger material.
In the case of fiber-reinforced plastics, plastic resins are combined with fibers acting as reinforcement. The fibers may be fiberglass, carbon fiber, or other fabric or cloth. Some examples of fiber-reinforced plastics include sheet-molded compound (SMC), fiberglass reinforced polyester (FRP), reinforced reaction injection molded (RRIM), carbon fiber reinforced composite (CFRC) and resin transfer molded (RTM). Damaged fiber-reinforced plastics typically can be identified by visually inspecting for the fibers in the area of the damage.
The other category is non-fiber reinforced plastic. These plastics do not have any reinforcement fibers. Non-fiber-reinforced plastics may be broken down into smaller categories specific to the manufacturing process. It is important to correctly identify the specific type of non-fiber-reinforced plastic so that the correct repair procedure or process can be determined. The easiest way to identify the type of plastic is to look for the International Organization for Standardization (ISO) code on the back of the part.
The ISO code is a set of letter codes that identifies the type or manufacturing process of the plastic used to make the part. Another way to identify the type of plastic is checking the vehicle maker service information. As an example, DaimlerChrysler has a repair deck that identifies the types of plastic, as well as the recommended repair products and techniques (See figure 1).Initial Repairs
Damaged non-fiber-reinforced plastic that is in the form of a dent can often be returned to the correct shape using a heat source. Heat is applied until the plastic softens. When soft, the damage can be pushed out. This will remove much of the damage, but some filling may be required. This process also relieves the stress placed on the plastic from the damage. On fiber-reinforced plastics, the damage is commonly stress-relieved by grinding the area of damage, allowing the part to return to the undamaged state.
Urethane Plastic
Urethane plastics are commonly used on low production domestic vehicles. Urethane plastic is called a thermosetting plastic, meaning it cannot be re-melted. Urethane plastics may be yellow or grey in color. Urethane plastics are generally quite flexible. The repair processes may consist of using plastic welding techniques or using adhesives. Urethane plastic does not require an adhesion promoter. Some examples of urethane plastic ISO codes include polyurethane (PUR), reaction injection molded (RIM) and thermoset polyurethane (TPU).
Polyolefin Plastic
Many of the non-fiber-reinforced plastics used for bumper covers are plastics called polyolefin, or simply olefins. These plastics are thermoplastics and are formed by melting pellets and forming the molten solution into a mold, also called injection molding. Polyolefin may have different plastic types mixed together to combine the characteristics of each to perform a specific requirement. Polyolefin will typically smear when sanded, instead of creating dust. Some typical examples of polyolefin plastics include polyethylene (PE), polypropylene (PP) and thermoplastic elastomer polyolefin (TPO). Polyolefin plastics are repairable using both plastic welding techniques and plastic repair adhesives. If plastic repair adhesives are used, most product makers recommend applying an adhesion promoter or using a flame treatment.
Plastic Welding Repair Process
There are two types of plastic welding devices commonly used during plastic repairs. Hot air welders use compressed air that is heated up to 590°C (1,100°F) to melt a filler rod (see figure 2). Airless plastic welders have an electric heating element that heats a metal shoe and a melt tube for melting the filler rod. There also may be a larger shoe without a melt tube that is used for melting plastic around reinforcement screen, or for melting a universal ribbon filler material (see figure 3).
The filler material used for plastic welding is generally based on the type of plastic being welded. When plastic welding, there are two processes, fusion welding and adhesion welding. Fusion welding is the most common on polyolefin and thermoplastics. Adhesion welding is commonly used on urethane plastics that will not melt. The process is similar to using hot-melt glue. The base material should not be heated with a plastic welder.
Following the welding operation, the repair area will have to be filled with repair adhesive or filler. Most adhesive product makers recommend against using a polyester-based filler material on flexible plastics.
Adhesive Repair Processes
Damage to non-fiber-reinforced plastics that protrudes through to the backside of the part may be repairable using plastic repair adhesives or with plastic welding techniques. When a part is in an undamaged state, the part is at the maximum strength for which it is intended. After it has been damaged, the part will only be as strong as the bond of the materials that are used during the repair. This is called a secondary bond. Repairing a rip or tear to a part with an adhesive will require a two-sided repair. A taper is made around the area of damage. The more surface area that the adhesive has to adhere to, the stronger the bond will be (see figure 4).Pinning holes are drilled using a 3 mm (1/8-inch) drill bit. The pinning holes should not be drilled above the edge of the repair taper. The holes must be drilled in the correct area to allow the repair material enough surface area to sufficiently bond to itself, similar to a plastic rivet.
When using adhesives to repair damage to polyolefin plastics, repair product makers have different recommendations to help ensure adhesion of the repair material. The most common is the use of an adhesion promoter. Typically, an adhesion promoter is applied to the surface with an aerosol can or brush, and allowed to dry.
Flame treatment is another method to promote adhesion (see figure 7). Flame treating changes the polarity of the plastic to create a molecular bond between the repair material and the part. The polarity of an atom is like the positive and negative end of a magnet. Flame treating makes the atomic atoms in the part attracted to the atomic atoms in the repair material, which results in a molecular bond. When flame-treating a part, a propane or butane torch should be used.Adhesive Repairs to Fiber-Reinforced Plastics
Two-sided repairs to SMC, FRP and CFRC typically require access to the backside of the part (see figure 8). The damage may be repaired using a backer plate of a similar material, or may be made using a dual taper on both sides of the damage. If a backer plate is used, the backside of the damage is prepared for adhesively bonding a piece of scrap material over the damage. The repair will require the use of a reinforcement fabric. Fiberglass fabric is the most common, although carbon fiber or Kevlar® fabrics or fibers may be used as well (see figure 9). The reinforcement fabric is saturated with a repair adhesive and compressed down into the taper.
Some adhesive product makers recommend curing the adhesive with heat. After the repair material has cured, it is sanded to the contour of the part. If additional filling is necessary, a filler material or additional repair adhesive can be applied to the repair area.
Refinishing Plastics
After repairs have been made to a plastic part, refinishing generally follows. There are a few things that need to be considered. Flexible parts (see figure 10) generally require the use of a flexible additive to the refinish materials. This allows the paint film to have some flexibility when installing the parts and if the part is bumped or bent. Most refinish product makers have specific recommendations and procedures that are designed specifically for plastic parts. It is important to follow the refinish product maker's recommendations and procedures and keep all of the products within the system when refinishing plastic parts.Training
The key to making long-lasting plastics repairs requires following the repair product maker's procedures and recommendations. Most repair product makers provide training programs for the use of their specific products. I-CAR also has two four-hour repair programs specifically on the repair of plastics, Plastic Welding Repair (PLA01) and Plastic Adhesive Repair (PLA02). Visit the I-CAR Web site (www.i-car.com) to see when a plastics repair program is scheduled, or to request one in your area.
Current government regulations requiring better fuel economy have made vehicle makers look at using lightweight materials where traditionally heavier materials were used. Plastics have been a choice for many of these parts. In the not-so-distant past, plastic repairs were looked at as a last resort to replacing a part with a new one. With the development of different plastics came the concerns of repair materials not adhering to some of the plastics. Plastic repair product makers have addressed much of the concern with the development of better adhesives and repair materials, procedural steps to the repair and providing training. With some training and expertise, plastic repairs can change from a hindrance to a profit center for your repair facility.
