Chapter 11 Plastics and Composite Repairs

Objectives List typical plastics and composite applications in vehicle construction Identify automotive plastics through the use of international symbols (ISO codes) and by making a trial-and-error weld Describe the basic differences between welding metal and welding plastic

Objectives Outline the basics of hot-air and airless welding Repair interior and unreinforced hard plastics Perform two-part adhesive repairs Repair RRIM and other reinforced plastics

Introduction Composites and plastics refer to materials synthetically compounded from crude oil, coal, natural gas etc. Do not occur in nature and are manufactured More and more plastic is used in automobile manufacturing Fuel saving and weight reduction programs by auto makers make plastic parts more common

Types of Auto Plastics Thermoplastics can be repeatedly softened and reshaped by heating with no change in their chemical makeup –Can be welded with a plastic welder Thermosetting plastics undergo chemical change by heating, a catalyst, or ultraviolet light –Cannot be welded, but can be repaired with flexible parts repair materials Compound plastics are blends of plastics

Plastics Safety Wear rubber gloves when working with fiberglass resin or hardener Use protective skin cream on exposed body areas Safety glasses are a necessity Work in a well-ventilated area Wear a respirator to avoid inhaling sanding dust and resin vapors PVC-type plastics produce a poison gas when burned – keep them away from heat and flames

Plastic Identification Can identify unknown plastics with ISO codes –Molded into plastic parts If part is not identified by a symbol, body repair manual will give the information To do a floating test, cut a small piece of plastic off part and drop it into a container of water –Thermoplastics float, thermoset plastics sink Make a trial-and-error weld on a hidden area, possibly using a piece of damaged part as the welding rod

Plastic Part Removal Fasteners such as screws, clips and adhesives can be used to secure plastic parts If plastic part has only minor damage it can often be repaired by welding More commonly, plastic parts are repaired using a two-part plastic repair adhesive Repairing the part saves time

Principles of Plastic Welding Plastic welding uses heat and sometimes a plastic filler rod to join or repair plastic parts Outer surface of welding rod becomes molten while inner core remains semisolid Force rod into joint to create a good bond Apply pressure to welding rod with one hand while applying heat with tool Use a constant fanning motion with hot air from welding torch

Hot-Air Plastic Welding Hot-air plastic welding uses a tool with an electric heating element to produce hot air Most hot-air welders use a tip working pressure of around 3 psi Hot-air torch is used with welding rod, which must be made of same material as part It may be acceptable to weld thin plastics if they are supported from underneath while welding

Figure When welding plastic, you must use the proper combination of heat and pressure. Hand pressure is needed to force the plastic rod down into the weld joint. Fan the welder over the joint to heat the rod and base material properly.

Figure Note the construction of a high-speed plastic welding tip.

Airless Plastic Welding Airless plastic welding uses an electric heating element to melt a smaller diameter rod with no external air supply Airless welding with a smaller rod helps eliminate panel warpage and excess rod buildup Be sure rod is same material as damaged plastic Good practice to run a small piece through welder to clean out tip before beginning

Plastic Welding Methods Plastic welding rods are often color coded to indicate their material, but coding is not standard Too much heat will char, melt or distort plastic; too little will not provide penetration Too much pressure distorts weld Angle between rod and base material must be correct If torch movement is too fast it will not permit a good weld; if too slow it will char plastic

Plastic Tack Welding On long tears where backup is difficult, small tack welds can be made to hold the two sides in place before doing the permanent weld For larger areas, a patch an be made from a piece of plastic and tacked in place

Welding A Plastic V-Groove Prepare rod by cutting end at a 60° angle, hold nozzle about ¼ to ½ inch above, parallel to base Hold rod at a right angle to the work, position cut end of the rod at beginning of weld Direct hot air from tip alternately at rod and base material Keep rod lined up and press it into seam When it has cooled and does not pull loose, cut rod with a sharp knife or scissors

Plastic Speed Welding Plastic speed welding uses a specially designed tip to produce a more uniform weld Rod is preheated as it passes through a tube in speed tip Base material is preheated by a stream of hot air passing through a vent in tip Pointed shoe on end of tip applies pressure to rod, and smoothes it out Average welding speed is about 40" per minute

Controlling Speed Welding Rate Angle between torch and base material determines speed welding rate Hold torch at 90° angle when starting and a 45° angle after If welding rate is too fast, bring torch back to a 90° angle Speed welding must be kept at constant rate Clean shoe on speed tip with a wire brush to remove any residue

Airless Melt-Flow Plastic Welding Place flat shoe part of tip in V-groove Hold in place until rod begins to melt and flow out around shoe Small amount of force is needed to feed rod through preheat tube Move shoe slowly, crisscrossing groove Work melted plastic into base material Complete a weld length of about 1" at a time

Plastic Stitch-Tamp Welding Plastic stitch-tamp welding uses pointed end of welding tip to help bond plastic rod and base Used on hard plastics like ABS and nylon After completing weld using the melt-flow procedure, remove rod Turn shoe over and slowly move pointed end of tip into weld area to bond rod and base material Smooth weld with flat part of shoe

Single-Sided Plastic Welds Single-sided welds are used when part cannot be removed from vehicle Align break using aluminum body tape V-groove damaged area 75% through base material, beveling torn edges Place shoe over V-groove; when it is filled, stitch-tamp rod and base together Resmooth weld area using flat shoe; cool with a damp sponge

Two-Sided Plastic Welds Two-sided plastic weld is strongest weld Align front of break with aluminum body tape, V- groove 50% of the way through back Weld back side of panel; when finished smooth the weld with shoe Quick-cool weld with a damp sponge V-groove front deep enough to penetrate first groove, then weld the seam, filling groove

Repairing Vinyl Dents Vinyl is a soft, flexible, thin plastic often applied over a foam filler Vinyl parts are dash pads, armrests, inner door trim, seat covers, etc. Dash pads or padded instrument panels are expensive and time consuming to replace, therefore are good candidates for repair Vinyl dents can often be repaired by applying heat

Figure Study the construction of a typical dash pad. Note the steel reinforcing bar behind the dash panel.

Heat Reshaping Plastic Parts Deformed plastic parts can often be straightened with heat Wash with soap and water, clean with plastic cleaner, then dampen with a sponge Apply heat directly to distorted area until opposite side becomes uncomfortable to touch Use a paint paddle, squeegee or wood block to reshape the piece Quick-cool area by applying cold water with a sponge

Plastic Bumper Tab Replacement When a bumper cover has been torn away from its mounting screws the mounting tabs are often broken or torn away Mounting tabs must be repaired with a two-sided weld to provide enough strength If material is thermoplastic, either the hot-air or airless method may be used For thermosetting plastic, use airless welding If welded properly, repaired piece is as strong as original undamaged part

Plastic Adhesive Repair Systems Adhesive repair is often preferable to plastic welding, producing a stronger repair Cyanoacrylates (CAs) are one-part fast curing adhesives used for rigid and flexible plastics –CAs do not work equally well on all plastics Two-part adhesive systems consist of a base resin and a hardener and are an acceptable alternative to welding for many plastic repairs Identify type of plastic using a flexibility test

Use the Correct Adhesive! Mixing product lines is not acceptable Most product lines have two or more adhesives designed for different types of plastic Product line usually includes an adhesion promoter, a filler and a coating agent Some product lines are formulated for a specific base material Product line might use a single flexible filler for all plastics, or different fillers for different plastics

Two-Part Plastic Adhesive Repairs Clean part with soap and water and plastic cleaner; it should be at room temperature Mix the two parts of adhesive thoroughly Apply material within time guidelines, using heat if indicated by manufacturer Follow cure time guidelines Support part adequately during cure time to ensure damaged area does not move

(C) Cut and fit the fiberglass cloth over the tear in the bumper. Figure Note the methods to prepare a large tear in a plastic bumper cover for repair.

Reinforced Plastic Repairs Reinforced plastic parts, including reinforced reaction injection molded (RRIM) polyurethane, are used many unibody vehicles Combinations of damage can occur on a single vehicle Four types of repairs: –Single-sided, two-sided, panel sectioning, full panel replacement Carefully examine vehicle to select repair method

Figure Study the reinforced plastic repair chart. It gives recommended repairs and materials.

Reinforced Plastic Adhesives Two-part adhesives have a base material and a hardener that are mixed to cure adhesive Work life or open time is the time when adhesive can be disturbed and still bond Cure time of some adhesives can be shortened with application of heat Temperature and humidity affect work life and cure time If you move adhesive as it starts to harden, it will be less durable

Reinforced Plastic Fillers and Glass Cloth Cosmetic filler is typically a two-part epoxy or polyester used to cover up minor imperfections Structural filler is used to fill larger gaps in panel structure and add rigidity to part As a rule of thumb, heat material to a surface temperature higher than any temperature it will be subject to when on road Choose unidirectional or woven glass cloth or nylon screening and allow adhesive to fully saturate weave

Reinforced Plastic, Single-Sided Repairs If break is clean and all reinforcing fibers are in place, a single-sided repair should be adequate Bevel deep to penetrate fibers in panel Clean with soap and water, and grease remover Remove paint and scuff area Bevel damage and apply filler and cure as recommended

Two-Sided Repairs of Reinforced Plastic Two-sided repair is normally needed on damage that passes all the way through Backing strip or backing patch is bonded to rear of repair area to restore strength Pull rod can be used to hold a patch in place Sheet metal can be attached to back side of panel with sheet metal screws Can apply fiberglass patch to outer side only

Repairing RRIM Reinforced reaction injection molded (RRIM) polyurethane is a two-part composite plastic –Part A is the isocyanate –Part B has reinforced fibers, resins and a catalyst RRIM products are stiff yet flexible and can absorb minor impacts without damage Gouges and punctures can be repaired using structural adhesives If damage extends through panel, a backing patch is required

Spraying Vinyl Paints Vinyl repair paints are usually ready for spraying as packaged When applying vinyl paints by siphon feed guns, normal air pressure is between 40 – 50 psi There is no retardant for vinyl paints; if blushing occurs allow the initial coat to set, and reapply Vinyl and soft ABS plastics should be cleaned, let to dry, and then treated with vinyl prep Wipe off vinyl prep right after it is applied

Summary Plastics and composites refer to a wide range of materials synthetically compounded from crude oil, coal, natural gas, etc. Bumpers, fender extensions, fascias, fender aprons, grille openings, stone shields, instrument panels, trim panels, fuel lines, and engine parts can be constructed of these materials Two general types of plastics are used in automotive construction: thermoplastics and thermosetting plastics

Summary (continued) Thermoplastics can be repeatedly softened and reshaped by heating with no chemical change Thermosetting plastics undergo a chemical change and are hardened into a permanent shape that CANNOT be altered by reapplying heat or catalysts

Summary (continued) Can identify a plastic by international symbols, or ISO codes, which are molded into plastic parts –If there is no ISO code, refer to the body repair manual for information Plastic welding uses heat and sometimes a plastic filler rod to join or repair plastic parts –Types of plastic welding include hot air and airless