The first step in any plastic repair project is identifying the type of plastic that needs repair, and that may not be as simple as it sounds. The list of plastics manufacturers use to make household and automotive goods stands at nearly 20 types, and there's no reason to suspect it won't keep growing. Most plastics can be welded with specialty welding rods, but to repair others, it's better to use epoxy filler. Procedures vary according to the plastic and repair method, but a few guidelines apply across the board.
Polypropylene, polyethylene and polyvinyl chloride -- PP, PE and PVC respectively -- are examples of thermoplastics, which are common around the house. Items made from one of these plastics are usually semi-rigid or rigid, and may include toys; plastic container; and appliance, door and window trim. You can repair a crack in a thermoplastic by fusion welding it.
Procedure for Fusion Welding
Clean the area around the crack with soap and water to remove dirt and grease.
Create a V-groove along the crack on one side of the plastic surface using a rotary tool and a teardrop cutting accessory. The groove needs to extend for the length of the crack and must be at least 1/8 inch wide at the top.
Set the temperature of an airless plastic welder high enough to readily melt an 1/8-inch polypropylene welding rod. Align the rod in the groove and run the tip the welder along it to melt it and fuse it into the plastic. Work the crack in sections no longer than 2 inches each. After the plastic in each section cools, go back over it with the welder to flatten it more.
V-groove and fill the crack on the opposite side of the plastic in the same way. Allow the plastic to cool completely.
Grind the repair with a rotary tool and an aluminum oxide grinding wheel attachment.
Alternate Repair Method
Some plastics, such as cross-linked polyethylene -- or PEX, and polycarbonates, don't respond well to heat, and it's difficult to repair cracks by fusion welding. These plastics, which you might find on plastic furniture or appliances, can be repaired with two-part epoxy resin.
Epoxy Repair Procedure
Prepare the back of the plastic item by scuffing it with 50-grit sandpaper to ensure an effective bond. Cover the front of the item -- which is the visible part -- with tape or something else to act as a backing for the adhesive. If it's a large crack, you may want to tape a piece of thin-gauge metal or plastic over the crack.
Mix enough epoxy resin with the hardener that comes with it to fill the entire crack. Mix the resin according to the instructions and be sure to use all of it during its open time, which is specified on the container.
Spread the epoxy with a plastic putty knife, completely filling the crack. Scrape off as much excess as you can before it begins to stiffen.
Remove the tape or other material you used as backing when the resin stiffens but before it hardens completely. If desired, you can sand down the epoxy with a rotary tool and a grinding wheel when it's completely hard.