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Material properties, types of damage and decision data

Material properties, types of damage and decision data

Plastic parts roughly comprise three component parts: plastic, reinforcement fibre and filling material.

Around 15 different types of plastic are currently used in the auto industry, most of them on the outside of our cars. Types of plastic are normally divided into two categories, namely thermoplastics and thermosetting plastics.

Thermoplastics soften or dissolve when affected by solvent and/or heat, and can be glued, welded or heat formed. As a rule they comprise: 70 - 90% plastic raw material, 10 - 30% reinforcement fibre and 10 - 20% filling material. In addition, thermoplastics have the property of attempting to regain their manufactured moulded form if heated to an adequate temperature.

Thermosetting plastics are of the type that do not significantly soften or dissolve when affected by solvent and/or heat. They can be glued but not welded or heat formed. Thermosetting plastics will not be discussed in more detail in these instructions.

Plastic identification



It is therefore thermoplastics which can be welded. In order for welding to be possible the correct filler material must be used, i.e. if cast iron or aluminium is to be welded then welding wire intended for cast iron or aluminium must be used.

The same principles apply for welding thermoplastics: the correct filler material must be used, i.e. the plastic type must be identified in order to obtain the correct filler material = correct welding wire.

There are currently around ten thermoplastics used in the auto industry, ABS - PP - PE etc. For Saab the number of plastic types (and welding wire) is five. These are sufficient for the repair of most plastic parts, especially those most prone to damage. See .

The content of a plastic part, i.e. the plastic type is stamped into the plastic part with later car models. The marking itself can look like this: >PP / EPDM<. This means that identification is easy to make.

Plastic parts from older car models pose small problems for us when we have to identify the correct plastic type. If there is no marking, try the following: identify the part in our materials compilation for the model's plastic parts. A code is given for each part. If a part is not in the list we advise against welding it in. Note that plastic parts of some models, i.e. bumpers, are not weldable. The weldable plastic types are stated in the table, see .

Warning

Some models have polyurethane plastic parts. These are marker PUR. Polyurethane is not weldable and must not be heated under any circumstances as it gives off dangerous gases. The gases can cause trouble in the form of asthma and allergies. Always check the part's marking if possible.


Types of damage

The different types of damage which can arise for repair are primarily scratches, cracks and small holes in the plastic part. In addition, distortions and “buckles” can be straightened with good results by means of heat treatment to the damaged part.

“Repair or replace?”

When it comes to the decision over repairing or replacing the decision is whether the time expenditure for the procedure is reasonable from a cost standpoint.

A general guideline can be to repair cracks that are no longer than approx. 50 mm and scratches that are no longer than 150 mm. If there is a hole in the plastic its diameter should not exceed approx. 30 mm.

Tip: Even if the component is replaced the discarded one should be kept for practice or to replace material that is missing from components that can be repaired. This especially applies to mounting lugs for components such as grilles and headlamp inserts.