Most plastic patio chairs are not built to take repairs. While some have removable legs or other pieces, most are molded units that have few, if any, moving parts. On the other hand, it is a shame to throw out a chair from a matched set because of a small crack or a molded bracket that doesn't hold properly. There are some general repair techniques listed below that will help you make minor repairs to plastic patio furniture.
Some plastic patio chairs have legs that slip into a molded bracket. If the leg breaks, you can purchase a replacement and simply insert it in the bracket. Make sure that the replacement part is the same size as the broken one. In some cases, you can shorten a leg by cutting the bottom off, but this is not always true, and may not work for you. If at all possible, use authorized replacement parts.
Repairing Leg Brackets
One common problem is for the molded bracket to crack or get snapped off. When this happens, the repair can be accomplished with a couple of screws. Fit the leg into the bracket as it should be, and then insert one screw on all accessible sides of the leg, penetrating through the cracked bracket or the outside edge of the seat. This same method can be used to tighten worn joints that have become wobbly and can prevent more serious breakage from happening.
Using JB Weld
Some broken patio furniture can be repaired using an epoxy putty like JB Weld. Use a vise or C-clamps to hold the broken pieces in place and apply the putty to the cracks. Work it into the cracks and make a small mound around the entire damaged area. The putty will be warm when it is activated and begins to set within minutes. For best results, allow it to fully set for 4 to 6 hours before using the chair.
Many problems associated with plastic chairs can be fixed with screws or bolts, or by using some sort of epoxy or glue. Other damage may require redesigning the chair to replace a damaged seat or back. If you have a damaged chair, and a reasonable idea of how it can be repaired, it may be worth a try. With plastic, there is no guarantee that any repair will last for long.
Worst Case Options
Plastic furniture is probably going to break eventually. If you are concerned that you will not be able to replace your particular brand or style later, purchase extra pieces to begin with. If all else fails, plastic is relatively inexpensive, and you can replace the damaged chairs, avoiding the hassle of making repairs, and the risks of the repairs not holding. The decision is up to you, but you should keep in mind that once the original design has been compromised, the chair is not likely to ever be as good as new again.