Plastic repair necessities

Jan. 1, 2020
While a tech might doubt it at first, with some guidelines to help one identify and work with these different types of plastic, it is easy to develop the skills to repair them.

Doesn’t it seem that more plastic is being used with each new automotive model year?  We see plastic bumper covers, header panels, interior parts and other plastic and composite body parts. Being able to repair these varied types of plastic parts has become a necessary skill for shops and technicians. While a tech might doubt it at first, with some guidelines to help one identify and work with these different types of plastic, it is easy to develop the skills to repair them.

Figure 1

So what do we need to know about plastic? First, plastic comes in two basic categories, thermoset and thermoplastic. Thermoset is a polymer material that irreversibly cures through heat, chemical reaction or irradiation. Once hardened, a thermoset cannot be reheated and melted back into a liquid form. Thermoplastic is a polymer that becomes pliable or moldable after reaching a specific temperature and returns to a solid upon cooling.

Knowing whether a plastic is a thermoset or a thermoplastic is necessary to choose the proper repair technique. A thermoset plastic does not melt when heated, and therefore cannot be repaired by welding. (Welding plastic will be covered later in this article.) 

Figure 2

There are several other terms that you'll also see that refer to plastic types, such as rigid, flexible, reinforced plastic and composite.  Rigid plastic (Fig 1), such that is used for interior door panels, is plastic that holds its shape and cannot be flexed. It is often, but not always, made from a thermoset plastic and thus requires a specific repair procedure. Flexible plastic, used for parts such as front bumpers, is a polymer that can be flexed (Fig 2) and often be repaired by welding or using adhesives.

Reinforced plastic is enhanced by the addition of high strength fibers embedded in the components. Fiberglass is a reinforced plastic, as is sheet moldable compound (SMC); however, even though their repair techniques are very similar, the rosins needed for SMC are different than those required for repairing fiberglass.

A composite plastic (such as Kevlar) is formed when a combination of materials are bonded together to form an overlapping structure that is better than the individual components. Although the repair procedure for a composite is similar to the procedures for fiberglass and SMC, again the rosins used for its repair should be specifically designed to repair the composite material.  

Figure 3

Repairing plastic parts is no different than working with other potentially hazardous materials, whether the parts are rigid or flexible and whether they are made from thermoset or thermoplastic. Therefore, as with other materials, the technician should be continually concerned with assuring personal, environmental and workplace safety. Common safety precautions include the following:

  • Read, understand and follow all safety precautions set by the manufacturer.
  • Read, understand and follow the MSDS related to the material being used.
  • Wear safety glasses at all times.
  • Wear the recommended respirator, gloves and other safety clothes and equipment as outlined in the MSDS.
  • Always work in a well-ventilated area.
Figure 4

Plastic identification
With all these different types of plastic, how can one be sure which type it is, so that he or she uses the proper repair procedure? The easiest and most accurate way is to identify the plastic by using its ISO code. This code should be stamped into each plastic part during manufacturing (Fig 3). Other ways of identifying plastic (although they are less accurate) are such tests as the sand test (Fig 4), in which an area is sanded. If the plastic smears, melts or gets waxy, it is likely not to be a polyolefin. If when sanded the plastic produces dust, it is likely that it is a polyolefin.

Another identification method, the float test, involves removing a small piece of the plastic in question

Figure 5

and placing it in water. If the plastic part floats, it is likely to be a thermoplastic; if its sinks, it may not be a thermoplastic (Fig 5). Both of the tests described above are limited, and again, the most accurate way to identify a plastic is by using the ISO code. Depending on the repair system you are using, it will tell you which type of plastic it is suited for repairing.

Repair techniques
Plastic repair may only require a single-side repair, such as a scratch or gouge (Fig 6); or it may need a two-sided repair if the damage has penetrated to the plastic or it has been ripped (Fig 7). As the name implies, a single-side repair means that the repairs are done from one side only, generally the exposed side; and that the repair could be done with the part in place (Fig 8). The two-sided repair involves first mending the back or inside of the part, and then cosmetically repairing the outside. This type of repair always requires that the part

Figure 6

be removed from the vehicle. Both single- and two-sided repairs can be performed using an adhesive or by using plastic weld.

Figure 7

Plastic welding
Plastic welding can be accomplished by using either a hot air welder (Fig 9) or an airless welder (Fig 10). Both systems use a filler rod, which is matched to the type of plastic being repaired. The thermoplastic part softens and a softened filler rod is added (Fig 11). When the part cools, the fused repair becomes strong and can be sanded smooth and cosmetically repaired. To properly perform this welding procedure, the part first must be thoroughly washed (Fig 12) with soap and water, and then cleaned with plastic, wax and grease remover (Fig 13). The area to be plastic welded needs to be ground into a V-shape (Fig 14) to accept the filler rod. If a two-sided repair is to be made, the repair can be "tacked" into place to hold its shape as the filler rod is added on the opposite side (Fig 15). As with all types of welding, the filler should have full penetration. Therefore, if a two-sided repair is being completed, filler rod may need to be applied to both sides to accomplish proper penetration. After the weld has cooled, it can be sanded and cosmetically repaired.

Figure 8

Adhesive repair
When repairing plastic with adhesive, begin as with any repair: the part should first be washed with soap and water and then cleaned with the appropriate wax and grease remover. In the case of plastics, when raw plastic is being cleaned and a standard wax and grease remover is used, it may swell the plastic; therefore, only clean using a wax and grease remover specifically designed for cleaning raw plastic.  First prepare the site by removing the paint and beveling the edge (Fig 16) for a single-sided repair. The adhesive may be applied directly to the prepared site (Fig 17), smoothed, and allowed to cure (Fig 18). Then after the repair has cured, the site should be cosmetically sanded and repaired (Fig 19).

For a two-sided repair, a backing is applied. If the technician uses a self-adhesive backing pad, the repair surfaces should be cleaned and sanded. Then the backing pad is applied (Fig 20) according to the manufacturer’s recommendation. The backing material that is used depends on the method of the repair. If an adhesive is being used, the backing material could be (Fig 21) of various types: sheet plastic with adhesives (red), fiber reinforcement (white and black), and steel reinforcement. Even aluminum tape with

Figure 9

adhesive is sometimes used (Fig 22). If a patch is not used, reinforcement must be added to the back side of the repair. A temporary patch such as aluminum tape is placed on the outside so the adhesive bond does not fall through. Then the adhesive is applied, and the reinforcing is applied into the soft adhesive bond.

Make sure that the bonding plastic comes through the mesh of the reinforcement (Fig 23). When the backside plastic has cured, the temporary patch should be removed from the front. Then the front side can be repaired as described above for a single-sided repair.

Figure 10

Depending on the type of plastic that is being repaired, an adhesion promoter may be necessary. Polyolefin, a plastic commonly used in the manufacture of flexible bumpers, requires an adhesion promoter. This product often comes in a spray can, and should be applied according to manufacturer’s recommendation, prior to applying adhesive bonds. 

Figure 11

Fiber-reinforced plastic repair
As with other plastics, the damage to fiber-reinforced plastic can vary, as will the repair method. In some instances, the surface might be repaired using only shallow, one-sided cosmetic repairs. Or the job could require a more extensive two-sided repair procedure when the damage has penetrated the part. First, as with other plastic repairs, it's important to identify whether the part being repaired is a fiber-reinforced plastic, such as fiberglass, where a single filler has been used; or if it is an SMC, where more than one filler may have been used, in which case special rosins are required. If it is a true composite repair, where multiple layers are involved, different rosins may be necessary than in either of the two previous situations.

Figure 12

Once the type of material to be repaired is identified and the correct repair rosins are selected, the

Figure 13

procedure for single-side repair is:

  • Wash with soap and water to remove water-soluble contaminants.
  • Clean with the appropriate wax and grease remover to remove the non-water-soluble contaminants.
  • Remove paint from the surrounding area with a DA and 80-grit sandpaper.
  • Bevel the damaged area, allowing for sufficient bonding of fiber agent.
  • Mix the filler according to the manufacturer’s directions, and apply.
  • After a repair area has cured, level it by sanding, and then finish sand.
  • Apply primer filler as needed and block sand for proper contour.
    Figure 14
    Figure 15

    The repair procedure for a two-sided repair:

  • Wash the part with soap and water.
  • Clean with a wax and grease remover.
  • Remove paint with a grinder and 80-grit sandpaper.  The paint should be removed to approximate 3 inches beyond the repair area on both sides.
  • Remove sanding dust.
  • Cut enough fiberglass matting to make three layers that are each slightly larger than the area to be repaired.
  • Prepare and mix the rosin according to the manufacturer’s directions.
  • With a paint brush, saturate the first layer of fiberglass reinforcement with rosin.  Also paint rosin on the side of the
    Figure 16 FRP area to be repaired.  Place the saturated reinforcement on the backside of the repair area and tap into place with a brush.  Make sure that the saturated reinforcement makes tight contact with the FRP.
  • Repeat the process above until three layers of the reinforcement have been placed on the back of the area being repaired.  Allow to catalyze.
  • After the back has cured, grind the front of the reinforcement with 50-grit sandpaper, and then clean.
  • Repair the front with polyester plastic body filler, sanded and primed as needed
  • Figure 17

    This repair can also be completed with chemical adhesives, which are designed for an FRP part. The

    Figure 18

    adhesive is substituted for the rosin, but the process to follow is the same as previously explained. The procedure for repairing sheet moldable compound (SMC) is also the same as above, although the adhesive used should be specifically designed for the repair of SMC.

    The repair of plastic in its various forms is no more difficult to master than other skills most collision repair technicians have already developed. By properly identifying the substrate being

    Figure 19

    repaired, the proper repair materials, and the appropriate techniques, technicians will learn to accomplish relatively simple and quick repairs. Applying filler rod with plastic repair is no more difficult and only slightly different than applying filler rod when brazing or tape welding. Applying the

    Figure 21

    adhesive is similar to the application of plastic body filler, and of course the cosmetic repair uses the same skills used when applying plastic body filler. Many plastic parts that are often discarded can easily and profitably be repaired.

    Figure 20

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    Figure 22
    Figure 23