One common complaint we hear from people who avoid restoring cars built from about the mid-1970s on is that those cars contain increasing amounts of plastic ? plastic interior trim, plastic bumpers, plastic engine covers ? and that repairing automotive plastics is impossible. Yet, as we see from all those plastic welding setups advertised in our favorite restoration supply catalogs, fixing broken plastic pieces is absolutely possible.
Not only possible, but it?s also possible to do without those setups, which can sometimes cost big bucks. In fact, repairing plastics can actually cost next to nothing. I?d read about the solvent-welding technique in the past ? in fact, Scotty Lachenauer wrote a good article on solvent-welding plastics back in Hemmings Sports and Exotic Car #59, July 2010 ? but a more recent post on Hackaday persuaded me to investigate the process a little further. As it so happened, I had a broken piece of A-pillar interior trim from one of the HMX?s donor cars (as seen above), and I had committed to giving a presentation on automotive plastics repair for our most recent Hemmings Mediterranean cruise, so I gave the process a shot.
Step one is identifying the plastic. There?s all sorts of plastics out there, and they all respond to different solvents. Urethane Supply Company has a pretty comprehensive guide to identifying plastics, should the piece you?re repairing not have any identification marks on it. Plastic interior trim is more likely than not to be some sort of polyethylene, such as HDPE. Once you?ve identified the type of plastic, round up some scrap plastic of the same type. Fortunately, I had another broken piece of interior trim from the same car in the same nutmeg color.
Next, you?ll want to find a solvent that corresponds to your particular plastic, which may take some research. Nerf gun modders have actually put together an extensive list of which solvents work on which plastics, but if you?re in doubt, common acetone ? available at any hardware or home improvement store (look in the paint supplies aisle) ? seems to work on most automotive plastics.
The scrap plastic will dissolve quicker in the acetone with more surface area, so I chopped it up into smaller pieces and placed it with some acetone into a glass jar, put a lid on it, and set it aside. This batch took about an hour to get soft and an overnight soak to fully dissolve. Once it?s fully broken down, the plastic separates out into a goop or slurry in the bottom of the jar, with excess acetone floating on top.
While the acetone?s doing its job, prep the piece you?re repairing by fitting the broken pieces together and using some sort of tape to loosely hold the two pieces.
On the opposite side, Vee out the crack to allow the repair to penetrate deeper into the plastic. Some sources claim this isn?t necessary for the solvent welding process because it works via capillary action, but I found that it doesn?t hurt either.
After that, simply brush the dissolved slurry into the Veed-out crack. The acetone in the slurry vaporizes fairly quickly, leaving behind the plastic in the slurry, which bonds on a molecular level to the surrounding material. According to Lachenauer, this bond is about 95 percent as strong as the original plastic.
I found that the thinner the slurry, the more coats it took to build up a level repair. However, thin layers are actually preferable to slopping on one thick layer because thicker coats are more prone to cavitation ? tiny bubbles of acetone trapped within the hardened plastic weaken the bond. Once the acetone vaporizes completely (usually within a minute or two), you can then sand or paint the repair as you would the base plastic.
Note that I only did the repair on the backside of the piece of trim; that?s because I didn?t want to deal with replicating the grain on the front side, and the repair is indeed rather strong. As it turned out, that capillary action I mentioned before actually worked to bring some of the slurry through to the front side, though it doesn?t look too bad. Were both sides free of texture, I would go ahead and duplicate the repair on the front of the piece, then sand it smooth, as Lachenauer did, creating a stronger and almost invisible repair.
I still have quite a bit of slurry left over (I probably needed only a quarter of the scrap plastic I cut up for this repair), so you can see how little scrap material and acetone this sort of repair requires, and how easily I saved this piece of plastic trim from the landfill. With a little imagination and practice, one could not only perform more intricate and extensive repairs, but also (with the use of a flexible rubber mold) replicate broken plastic tabs and conceivably cast new plastic pieces at home.
Source: http://blog.hemmings.com/index.php/2012/08/30/tech-tip-solvent-welding-plastics/
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Public release date: 27-Aug-2012
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