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Material properties, damage types and decision data

Material properties, damage types and decision data

Roughly speaking, plastic parts have three constituents: plastic, reinforcing fibre and filling material.

Presently, 15 different types of plastic are used in the automotive industry. The majority are mounted externally on our cars. Normally plastics are divided into two types, namely thermoplastic and thermosetting plastic.

Thermoplastics soften or dissolve when they comes into contact with solvents or are heated. They can be bonded, welded and thermoformed. As a rule they consists of: 70 - 90% resin, 10 - 30% reinforcing material and 10 - 20 % filling material. Thermoplastics also have the property that when heated to a suitable temperature, they try to reattain the form they were given when moulded.

Thermosetting plastics are such that they do not soften considerably or dissolve when heated or in contact with solvents. They can be bonded but not welded or thermoformed. For this reason, thermosetting plastics will not be covered in detail in these instructions.

Identification of plastic



Thermoplastics can be welded. You must use the right materials for welding plastic, just as you would use welding wire for cast iron or aluminium when welding cast iron or aluminium respectively.

The same principle applies when welding thermoplastics: you must use the right welding materials, i.e. you must identify the type of plastic to then be able to select the right welding material = welding rod.

Presently, there are roughly ten types of theromoplastics used in the automotive industry; ABS, PP, PE, etc. Saab uses five types of plastic (and welding rods). These are sufficient to repair the majority of plastic parts and especially those most susceptible to damage. See .

On more recent car models the contents of a plastic part, i.e. the type of plastic, is stamped on the part. The actual marking can look like this: >pp / EPDM<. This facilitates identification.

Plastic parts on older models can cause problems when trying to identify the type of plastic. If there is no stamp; look up the details in our register of materials used in the plastic parts of different models. The code for each part is stated here. If a part is not included in this register we advise not to weld it. Note that plastic parts of certain models, e.g. the bumpers cannot be welded. The types of plastic that can be welded are given in the table, see .

Warning

Certain models have parts made of polyurethane plastic. These are marked PUR. Polyurethane cannot be welded and must under no circumstances be heated since it produces dangerous fumes. These fumes can cause allergic reactions and asthma. Whenever possible, check the marking on the part.


Type of damage

The types of damage concerned for repair are primarily scratches, cracks and small holes. In addition, skewness and “dents” can be fixed well by heating the damaged part.

“Repair or replace?”

When it comes to deciding whether to repair or replace a part, consider whether the time involved to rectify the damage is worth it with regards to the cost.

A rule of thumb is not to repair cracks longer than 50 mm and scratches longer than 150 mm. If there is a hole in the part it should not be larger than 30 mm in diameter.

Useful tips: Even if you decide to replace a part you should try to salvage the part removed from the car so that you have something to practice on or to replace material missing from other parts that can be repaired. This is especially relevant for the attachment lugs of headlamp inserts and grilles, for example.