BMW Body & Paint Technical Training

This reference manual is part of a training information system designed to assure uniform procedures and information for participants at the BMW Group University Body & Paint Training Center. Technicians must refer to official BMW AG service publications available in the Integrated Service Technical Application (ISTA), including Service Information, Repair Manuals, Technical Reference Information, and Specifications.

The information contained herein is intended solely for participants in this training course conducted by the BMW Body & Paint Training Group or its approved vendors. For changes or additions to technical data, refer to the current information issued via ISTA and Service Information Bulletins.

Introduction

Automobile design and manufacturing have evolved significantly, impacting vehicle repair methods. Plastics are increasingly prevalent in vehicles, comprising up to 50% of volume and 10% of weight, contributing to greater vehicle efficiency. This increased use of plastics means a higher chance of component damage. Modern repair techniques allow for efficient and high-quality repairs, often offering advantages over replacement, including maintaining repair quality, faster repair times, higher profit per repair, lower overall repair cost, and increased customer satisfaction.

This manual focuses on plastic materials used in BMW vehicles and the assessment and repair of plastic vehicle components.

Definition of Plastic

Plastic is a synthetic or semi-synthetic organic compound that can be molded into solid objects. Typically organic polymers of high molecular mass, plastics may also include natural organic materials. Plasticity is the ability to deform irreversibly without breaking. Plastics are used in place of materials like glass, wood, and metal in construction and decoration, often known by trademark names such as Bakelite, Vinylite, or Lucite. In the automotive industry, plastics enhance safety and efficiency, allowing for complex shapes and lightweight components. Advancements in vehicle safety, such as seatbelts and airbags, are enabled by plastics. The recyclability of plastics also supports sustainability goals.

History

Before man-made plastics, Mesoamericans used natural rubber from 1600 BC. Natural rubber was later used by Aztec and Maya cultures for waterproof containers and textiles.

Parkesine

Invented in 1862 by Alexander Parkes, Parkesine was the first man-made plastic. It was transparent, moldable, and retained its shape when cooled. Parkes claimed it was a less expensive alternative to natural rubber.

Rayon

Developed in 1891 by Louis Bernigaut, Rayon was a modified cellulose substitute for silk. It was shiny but highly flammable and later replaced by more stable materials.

Bakelite

Leo Hendrick Baekeland invented Bakelite in 1907, a fully synthetic, thermosetting plastic. Known as "The Material of a Thousand Uses," it was used in electrical insulators, radio and telephone casings, jewelry, and more.

Cellophane

Invented in 1908 by Jacques Edwin Brandenberger, Cellophane was intended as a coating to make cloth more resistant to staining. Its US rights were sold to DuPont in 1923. Cellophane is 100% biodegradable and popular for food wrapping.

Types of Plastics

Plastics for body components are categorized by their mechanical and thermal properties into:

• Thermoplastics
• Thermosetting plastics
• Elastomers

Thermoplastics

Thermoplastics do not undergo chemical change when heated and can be molded repeatedly. Their properties are temperature-dependent; they soften and liquefy when heated. Examples include coolant expansion tanks and outer body skin components.

Common Thermoplastics:

Thermosetting plastics

Thermosetting plastics harden and change little with temperature. They can be molded once, after which they remain solid due to an irreversible chemical reaction. They are used in electrical components and vehicle body parts, often reinforced with fibers, such as sheet molding compound (SMC).

Common Thermosetting Plastics:

Elastomers

Elastomers are stable, malleable, and flexible plastics, often referred to as "rubber." Examples include door seals.

Identification

Identifying plastic components is crucial for repair. Plastic ID tags, often found on the component, provide information. For example, "PP+EPDM" indicates a blend of polypropylene and EPDM. "PPGF" indicates polypropylene with glass fibers. "Blends" combine properties of different plastics, similar to alloys. Trade names are also used but may not directly indicate the plastic type.

Methods for Identifying Plastics:

• Sanding: Thermoplastics become rubbery when sanded; thermosets produce dust.
• Heating: Thermoplastics soften above 200°C.
• Burning: Tests for smoke color, odor, burn speed, and melting behavior can identify plastics.

Properties of Plastic

Plastic properties vary greatly depending on raw materials and manufacturing processes. Some plastics are soft and supple, while others are hard and brittle. The following table compares properties of polypropylene with steel, aluminum, and carbon fiber:

Thermal Expansion

Thermal expansion describes how a material's dimensions change with temperature, indicated by its thermal expansion coefficient.

Yield Strength

Yield strength is the tension at which a material transitions from elastic to plastic deformation. If tension is less, deformation is reversible (elastic).

Tensile Strength

Tensile strength is the maximum tensile force a material can withstand before constriction and eventual breaking. The yield strength ratio can be determined from yield and tensile strength.

Modulus of Elasticity

The modulus of elasticity (E-module) describes the relationship between tension and stretching. A higher E-module indicates a firmer material, while a lower E-module indicates a more flexible material.

Elongation at Break

Elongation at break indicates the percentage change in a component's length after breaking due to tensile force.

Corrosion

Corrosion is a material's reaction to its environment. Plastics do not corrode like metals but can degrade or age due to heat, light, and chemicals. An example is plastic outdoor furniture cracking from exposure.

Manufacture

Injection Molding

Injection molding produces high-quality plastic parts quickly and accurately. Plastic granules are melted and injected under pressure into a mold, which is then cooled to harden the part. Injection molding machines consist of a material hopper, injection ram or screw-type plunger, and a heating unit. Molds are clamped to the platen, and plastic is injected through the sprue orifice. Presses are rated by tonnage, indicating clamping force.

Diagram: Plastic Injection Molding Process showing pellets, heaters, nozzle, split mold, ejector pins, and screw/tube assembly.

Injection Molding Cycle

The injection molding cycle includes mold closing, injection of liquified material, holding pressure to compensate for shrinkage, screw retraction for the next shot, and part ejection after cooling.

Wall Thickness

Proper cooling is crucial to prevent defects. Wall thickness significantly impacts cost, production speed, and quality. Typical injection molded part wall thickness ranges from 2mm to 4mm (0.080" – 0.160"). Thin wall injection molding can produce walls as thin as 0.5mm (0.020").

Types of injection molding processes

• Reaction injection molding (RIM): Uses thermosetting polymers with a curing reaction in the mold. Produces strong, flexible, lightweight parts like bumpers and spoilers.
• Thin wall injection molding: Mass-produces thin, light plastic parts to save costs and reduce cycle times. Wall thicknesses are less than 0.025” (0.62mm). Used in food packaging, automotive parts, mobile phone housings, and medical devices.
• Liquid Silicone Rubber (LSR) Injection molding: Produces pliable, durable parts with high purity silicone. Ideal for high-precision applications like automotive seals, electrical connectors, and infant products. Silicone rubber can be overmolded onto other plastics.

Safety

General protection and hygiene measures apply to plastic repairs. Eating, drinking, and smoking are forbidden during work. Avoid eye and skin contact with chemicals, as adhesives can cause irritation. Wear personal safety equipment, including safety goggles, chemical-resistant gloves, and protective clothing. If chemicals contact eyes, rinse immediately with water and seek medical attention. If skin contacts chemicals, rinse with water and remove contaminated clothing. Individuals allergic to isocyanates should avoid handling the product. Respiratory protection may be necessary if ventilation is inadequate; consult the manufacturer's safety data sheet for guidance.

Images: Black gloves, safety goggles, respirator masks, yellow earmuffs, white protective suit.

Damage Assessment

Damage analysis

Plastic repairs are performed on painted outer bodyskin components. Unpainted surfaces cannot be restored to a high quality with standard methods.

Severity of the damage

Repairs are considered for slight to moderate damage, including minor deformations, abrasions, cracks, or holes in the outer body skin, provided underlying structures are undamaged.

Deformations

Small dents and deformations can be repaired by reshaping, provided the plastic has no cracks that would become visible after painting.

Abrasions and scratches

Minor abrasions and scratches can be filled using plastic bonding. Repair is not possible if material thickness is significantly reduced, exposing underlying components.

Cracks and holes

Small cracks and holes (up to 25 mm) can be filled by bonding or welding. Repairs extending to the component's edge are not permitted by BMW.

Repair Options

The following table outlines plastic repair possibilities. Always verify repair permissibility using the ISTA workshop information system.

Economic efficiency

When considering "Exchange or repair?", evaluate repair time versus the cost of a new part. The repair cost (excluding paintwork) should not exceed 50% of a new part's cost, unless the new part is unavailable or has an unacceptable delivery timeframe. Country-specific hourly rates also influence cost-effectiveness.

Warning symbol: The latest information on the ISTA workshop information system must be respected for all repairs.

Repair Methods

Replace

Fasteners

BMW uses various joining and fastening methods for plastic components, including rivets, clips, tabs, and adhesives. Handle plastic body components with care during disassembly and assembly to prevent damage. Damaged plastic fasteners must be replaced.

Images: Various plastic clips and fasteners.

Bumper covers are fastened using retaining brackets bolted to the body, into which plastic clips engage. Follow vehicle-specific repair instructions for correct pulling force and direction when removing or installing bumper covers. Damaged bumper tabs can now be repaired using Plastic Fusion welding machines.

Diagrams: A) Retaining bracket, B) Bumper cover tabs and retaining bracket, C) Repair instructions noting pulling direction.

Adhesives

Plastic components can be secured using liquid adhesives or tape with a removable backing. BMW offers repair kits for components like PDC/PMA sensor mounts and headlight brackets. Adhesives also secure plastic structures to the CFRP Life module in BMWi vehicles. Consult vehicle-specific repair instructions and the electronic parts catalog for consumables.

Warning symbol: Always follow ISTA procedures for surface preparation, as an improperly prepared surface is the primary cause of insufficient adhesion.

Images: PDC/PMA sensor holder repair kit, plastic components bonded to CFRP Life module.

Repair

Plastic outer body skin components can be repaired up to a certain damage pattern without complete replacement. Different repair methods are available, and a thorough assessment of the damage is needed to select the most suitable method. Combining methods is often practical.

Warning symbol: The quality of the repair result has top priority. If in doubt, replace the faulty body component.

The permissibility of plastic welding depends on the type, severity, and location of the damage. Conditions for repair are found in the ISTA workshop information system. Fluid tanks, such as fuel tanks, cannot generally be repaired.

Plastic Bonding

Plastic bonding is a repair method for thermoplastics and thermosetting plastics on the outer bodyskin, requiring no precise plastic identification.

Work Preparation

Sand the edges of the damage to a wedge shape from the rear. Use a low-speed DA sanding tool with P80-P120 grit abrasive. Drill a 6 mm hole at each end of a crack to prevent extension. Clean the area with a solvent cleaner and diluting agent, then treat with plastic primer, respecting air drying times.

Image: Sanding the edges of a plastic component.

Procedure Overview

Begin restoration on the inside of the component. Incorporate fiberglass mesh into the plastic adhesive for reinforcement. The front can only be worked on after the adhesive on the inside has hardened. Infrared heat (60°C-70°C) can reduce hardening time to approximately 15 minutes.

Diagrams: A) Incorporating fabric mesh (rear side), B) Filling in the damage (front).

On the front, the adhesive acts as a filler compound; account for shrinkage. Submerge the mixing nozzle in the adhesive during application to avoid air pockets. Smooth the adhesive from the center outwards. Allow the adhesive to cure completely and cool before smoothing.

Tools & Consumables

Images: Teroson PU 9225 plastic repair sealant, Araldite 2000 adhesive manual gun, mixing nozzles.

Index: 1 - Repair Adhesive, 2 - Cartridge gun, 3 - Mixing nozzle

Cleaner

Clean the bare plastic substrate with ColorSystem Aerosol Antistatic Cleaner or ColorSystem Plastic Cleaner and allow to air dry.

Plastic primer

ColorSystem Polyolefin Adhesion Promoter Aerosol is used with Teroson PU 9225. The primer has an air drying time of 10 minutes at 68°F. It is essential for creating a sufficient bond.

Plastic adhesive

A two-component, polyurethane-based adhesive used for joining and as a filler. It can be easily sanded and overpainted. Working time is approximately 10 minutes. Partially used cartridges can be reused, but the mixing nozzle must be renewed.

Fabric mesh

Used to reinforce repair areas for holes and cracks. Warning symbol: Do not use metal reinforcements on outer body skin components, as it may impair passive pedestrian protection.

Plastic welding

Plastic welding is a repair method for thermoplastics.

Work preparation

Repairs are carried out from the inner to the outer side of the component, often requiring disassembly of the damaged part. Thoroughly clean the repair area. Sand edges to a wedge or "V" shape on inner and outer sides. Remove paint from the damage area. Use a low-speed sanding tool with P80-P120 grit abrasive to prevent overheating and smearing.

Image: Plastic welding procedure.

A one-sided repair is insufficient; both sides must be welded. The repair begins on the inside, fusing plastic and filler material with hot air. Overlap filler material layers. A single weld seam is usually sufficient on the outer side. Sand off protruding filler material and prepare the part for refinishing.

Tools & Consumables

Images: Plastic Fusion welder, flat welding strip.

Index: 1 - Plastic welder, 2 - Filler material (PP + EPDM)

The plastic welder produces hot air up to 480°C. The compressor provides air, heated just before the nozzle. Warning symbol: Never switch on the welder's heating stage if the compressor is off, to prevent damage. Allow the compressor to run for a minute after switching off the heat to cool the welding head.

Filler material consists of round or flat polypropylene (PP + EPDM) strips. Fuse base and filler material uniformly for optimal bonding.

Repair Steps

For proper fusion, both the welding rod and the part must be molten. Melting Flow Index (MFI) measures material flow ease. Plastic Fusion uses high welding temperature and low air volume for controlled welding. Key points include the welding bath/pool, welding rod angle, and pressure on the rod. Set welding temperature to 480°C. Adjust torch angle and distance for simultaneous melting of filler and repair material. A proper weld pool appears as rolled plastic near the weld pool.

During welding, the filler rod angle should be less than 90° (acute angle). Downward pressure on the filler rod is important for weld pool pressure. Incorrect angle or pressure will prevent successful weld pool creation.

Diagram: Welding Rod & Weld Pool showing filler rod, weld pool, and filler rod angle less than 90°.

Repairing a Hole or Crack

Remove coatings and expose bare plastic substrate using P80-P120 grit sandpaper. Sand edges to a wedge or "V" shape. Start welding on the back side, overlapping welding strips by 50%. Optionally, perform a perpendicular weld for added strength. Repeat for the front side. Allow to cool, then sand with P240-P320 for filler application.

Images: Sanding a hole, welding a hole, sanding a bumper tab area.

Repairing a Bumper Tab

Remove coatings and expose bare plastic substrate using P80-P120 grit sandpaper.

Make a slight groove into broken edges. Start welding on one side, ensure fusion, and allow to cool. Heat the filler rod to allow bending. Bridge the broken bumper tab and weld the remaining side. Use a flat vice-grip to smooth the repair area while warm. Repeat on the underside of the bumper tab. Remove excess material and test fitment.

Reshaping

Deformations in plastic parts can occur due to damage or the repair process. Apply heat (up to 60°C / 140°F) and pressure to remove deformations. Monitor heat application to avoid damaging paintwork. Press out the softened damage, hold pressure, and cool with water. Molded plastics have memory, making it easy to return them to their original shape.

Warning symbol: Always use personal protective equipment (PPE) when working with heat.

Image: Aluminum reshaping tool.