From: Devinder Grewal
Date: Tue, 13 Mar 2001
Subject: plastic repair

Long answer to the plastic repair question. I'm writing this off the top of my head so please correct if I've left out something. Hopefully it's not too technical.

The best way to repair plastic (both thermosets and thermoplastics) is to use solvent welding. The most common plastics in automobiles are PVC (polyvinylchloride or just vinyl), ABS (acryl-butyl-styrene), Nylon (and all its trade-name derivatives), and acrylic (PMMA). Since it is difficult to identify the kind of plastic without doing FTIR (=expensive), I will just list a few common solvents for these plastics and you can try each.

A nice feature of the modern recycling laws is the marking of each plastic part, even the really small ones. On my motorcycle, when the speedo housing cracked, I looked inside and it was labelled ABS. I was able to fix that in a few seconds with ABS pipe cement. I'm listing the actual names of the plastics in case you have a marked part.

For PVC and ABS there are commercially availible solvents. All hardware stores will carry these. If you've ever joined household plastic pipe then you've used these solvents. It turns out that alot of the rigid plastics in cars are ABS and these days ABS solvent ("cement") comes in both white and black. PVC cement is clear solvent. The active ingredient in both is MEK (methyl ethyl ketone). I've fixed countless plastic parts with one of these two solvents. In theory, a solvent welded joint should be as strong as new.

Nylon is also very common in cars but I don't know of any good commercially availible solvent welding system for it. Most likely you have a nylon part if the above "cements" don't work on it. There is usually no harm in trying them first, but be careful trying solvents on cosmetic portions.

Acrylic (also known an PMMA sometimes - polymethmethacrylate) is not very common in automobiles, but it can be easily joined using a solvent. I don't remember the name but it can be purchased at any plastic specialty store (like TAP plastics in the SF bay area). Oh the tradename in the US is Plexiglas - I can't remember the euro tradename for it.

Bakalite is used mainly where heat and electrical resistance is required (commonly its used to make printed circuit boards). Not much use these days since it's very brittle. Bakalite had its heyday in the 30's to 60's. You have to glue this one.

Lastly we come to Teflon (PTFE - ploytetrafluoroethane) and polyethelene (PE). PE is used to make most plastic drink bottles and that six-pack packaging. These parts, if labeled, are labeled PTFE, PE, LDPE, HDPE or UHMWPE. I group these together because you can't glue them or weld them very easily. Nothing really sticks to them. They can be heat welded though, but this is difficult to do as a repair.