Introduction: Lashing Repair of Plastic Stuff
I have a plastic scoop shovel that I use for shoveling snow. A couple of years ago, it developed a fatal crack. From past experience with attempts to glue plastic used in tools, cars, and electronic housings, I knew that gluing wouldn't work.
I decided to try lashing. This repair is an unqualified success. It has held up well for two years and shows no signs of deterioration.
I used 100% nylon upholstery thread. Virtually any abrasion resistant heavy thread or cord that will not deteriorate under the environmental conditions it will be subjected to will work just fine.
Step 1: Stress Relieve the Crack
Drill a hole at the very end of the crack. If the crack is within a surface and doesn't break an edge, drill a hole at both ends of the crack. This will prevent the crack from "running" further.
The sharp point at the end of the crack focuses stress astronomically. If the crack is pulled apart, all the force is concentrated at a very tiny point. One pound of stress can concentrate to hundreds or thousands of pounds per square inch.
By drilling a hole, the stress is now distributed around the circumference of the hole and the psi is vastly reduced.
Step 2: Drill Holes for Lashing
The holes only need to be large enough to allow for easy threading. In this case, I used 3/32"
Spacing of holes and distance from edge is important:
1) The closer to the edge of the crack, the stronger the lashing will be. This is because there is less room for lashing to flex and stretch.
2) The closer together the holes, the more lashes you get, the stronger the repair.
HOWEVER, too close to edge or too close together will create a "tear out" situation. Think perforated paper. (This is exceedingly important in riveted aircraft construction. You want as many rivets as possible for maximum strength, but too many rivets actually weakens the joint. Aeronautical engineers get to figure out the optimum number.)
I leave approximately two hole diameters between holes and approximately two hole diameters from edge.
Step 3: Lash It Up
Refer to photos in Steps One and Two:
Keep your lacing as tight as you can. The more tension you can manage, the better.
I laced the crack in a way that allowed me to tie off in the middle somewhere rather than either end. I figured I would get tightest lashings at each end of crack this way. It is hard to keep maximum tension on thread while tying a knot.
Paint the knot with nail polish, solvent based model paint, or glue to prevent it working loose over time. Leave a generous "tail" for the same reason. If you leave tail long enough, you can thread it under stitches to hold it down for better appearance.