How do I repair broken plastic so it actually holds?

Most plastic repairs fail not because the adhesive was wrong but because the joint had almost no area. A crack across a 2mm wall gives you a bond a couple of millimetres wide carrying a load that snapped the moulding in the first place. Any repair that lasts adds material behind the break.

Safety

What you'll need

• The correct adhesive for the plastic
• Two-part epoxy
• Fine brass or stainless mesh, or fibreglass tape
• Abrasive paper and isopropyl alcohol
• Clamps or tape
• A soldering iron with an old tip, for welding (optional)

Step by step

1. Identify the plastic first.The recycling mark decides everything. If it is polypropylene or polyethylene, adhesives will not work and you need welding or a mechanical fix. If it is ABS, PVC, polystyrene or acrylic, a solvent weld is available and is far stronger than glue.
2. Work out what load broke it.A snapped clip loaded in one direction needs reinforcement in that direction. A cracked housing that was over-tightened needs the load spread. Repairing without understanding the load reproduces the failure.
3. Dry fit and check the pieces still meet.Plastic that has been stressed sometimes distorts, and a break that will not close cleanly needs the parts held in a jig or taped in position.
4. Clean and key both faces.Isopropyl alcohol to degrease, then abrasive paper on the areas to be bonded and on the surrounding surface where reinforcement will sit. Key well beyond the break itself.
5. Rejoin the break to restore the shape.Solvent cement for a compatible plastic, or a small amount of superglue for a rigid plastic, just enough to hold the alignment. This step is about geometry, not strength.
6. Reinforce from behind — this is the actual repair.On the hidden side, bond a plate of similar plastic across the break, overlapping well onto sound material on both sides. Bonded area is what carries load, so bigger is better.
7. Or embed mesh in epoxy for an awkward shape.Lay a piece of fine brass or stainless mesh, or fibreglass tape, across the back of the break and bed it in two-part epoxy, working it in so no air is trapped. It behaves like reinforcement in concrete, taking the tension the epoxy cannot.
8. Consider stitching for a crack that must stay flexible.Small staples, or short lengths of paperclip, heated and pressed into the plastic across the crack on the hidden face, then covered with epoxy. This is standard practice on car bumper repairs and it works well.
9. Weld it if it is polypropylene or polyethylene.A plastic welding kit, or a soldering iron with a spare tip and a strip of the same plastic as filler. Melt both faces and the filler together and work them into one mass — this is a genuine weld and the only durable repair for these plastics. Ventilate well and never heat PVC.
10. Use a mechanical fixing where you can.A small nut and bolt with washers through a plate, or a self-tapping screw into a boss, is stronger and more predictable than any adhesive, and it can be undone. On a broken mounting tab, adding a metal bracket is usually the durable answer.
11. Let it cure fully before loading it.Epoxy 24 hours, longer in a cold room; solvent welds a day or more. Loading a joint at handling strength is what destroys otherwise good repairs.
12. Finish only after the strength is right.Sand and fill cosmetically once the structure has cured — with extraction or a mask, because the dust is fine and unpleasant, and old painted items may contain lead.
13. Know when to stop.Do not repair anything safety-critical: a helmet, a climbing or lifting component, a child's car seat, a ladder, a pressure fitting, or a part of an electrical appliance that provides insulation or mechanical protection. Also weigh the time honestly — many plastic parts are available cheaply as spares.

Tips

• Photograph the assembly before dismantling anything. Plastic housings are full of hidden clips.
• Roughen further than you think. Bond area is the whole game.
• Fine mesh from a kitchen sieve or a piece of window gauze works perfectly well as reinforcement.

Common mistakes

• Gluing the break and stopping there — The bonded area on a butt joint in a 2mm wall is a couple of square millimetres per centimetre of crack, loaded in peel — the weakest way to load an adhesive. It will break again at the same place, usually the first time it is used.
• Repairing a safety-critical component — A repaired helmet, ladder, child seat or lifting part looks intact and fails under exactly the load it exists to handle. There is no way to inspect a plastic repair for the strength that matters, so these parts are replaced, not mended.

If it doesn't work

Questions people ask