This page describes some quick and dirty repair techniques for working with plastic models and model trains. It's started out pretty small, but I'll add to it as I either find or remember some tricky thing that I did to get around some broken or damaged part.
A common assembly technique for large scale trains is to use a "self tapping" type of screw to make a threaded hole in a plastic part. I usually refer to self tapping screws as "self stripping" screws. Even the high quality engineered plastics are much softer than these screws and after a screws has been run in and out several times, the threads in the plastic part will often let go. Sometimes you can just use a slightly larger screw, but sometimes you have to fix the stripped threads.
A stripped threaded hole in a plastic part can often be repaired to be "good as new" by pouring a little plastic resin, such as Alumilite, into the hole and letting it set. Then the newly plugged hole is drilled out with a small (#70) bit and the old screw can be driven back in. This patch seems to provide good results as the screws can be torqued very tightly and hold well.
Regular epoxy doesn't work very well to repair a hole with stripped threads because epoxy is usually too hard and brittle. It'll either crack when the new screw is driven in or it will apply so much force to the plastic around it that the plastic will break. The casting resin is softer and takes self tapping screws much better.
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Many injection molded plastic parts use a post as an attachment point. Most often, the post will have a screw hole in the end. Almost as often, the post will split. Depending on what the kind of plastic is used and the extent of the damage, you may be able to repair it.
If the post is split and all the pieces are still there, then get some good quality CA adhesive (SuperGlue) and a small tie wrap. Glue the piece(s) back on and then tighten the tie wrap around the post as quickly as possible and as tightly as possible. If it will fit, leave the tie wrap in place.
If the plastic doesn't glue well or if the broken piece is missing or too badly damaged to reuse the repair is more difficult. Find some kind of tubing, a soda straw will do, that is the same diameter as the post and slip it over the post to at least the height of the original post. This tube will serve as a form. Then pour the plastic resin of your choice in the tube to fill it to the top. Alumilite is good because it is quite strong. Wait until the plastic resin sets and then file the post back to the correct height. Then drill a pilot hole in the end and tap the screw back in. If the form will fit, leave it in place, it will serve to strengthen the repaired post.
Resin is not an adhesive and it may or may not adhere well to the broken plastic that is already there. It may be necessary to drill small holes or cut small slots into what is left of the post to allow the resin to mechanically grab the old post material. In one case, I put Zap-A-Gap CA on the old plastic and then poured on the resin. It worked although there is no real reason why it should have.
This page has been accessed times since 21 Feb 01.
© 2001-2007 George Schreyer
Created Feb 21, 2001
Last Updated September 21, 2007