plastic welding can be done a few ways. Basically that's all I know in welding.
1. Solvent liquid welding using acetone to certain plastic polymers works great, other plastics not so great. Various ways to apply from wide brush for a patch to a Q-tip for hairline cracks.
https://www.hemmings.com/stories/2012/08/30/tech-tip-solvent-welding-plastics
Before you choose a material for your custom part, make sure you know acetone affects certain plastics. Choose wrong, and your product could even dissolve.
www.millerplastics.com
2. Heat-Shrink tubing layered enough to repair rod cables in areas that the rod would require massive amount of man hours to remove or cost prohibitive or areas that hot welding can't possible reach. First place heat tubes on each rung side of the rod. Different diameter heat tubes too to allow stacking and layering. Once on, evaluate next if Gorilla glue, super glue or Permatex can bond the 2 halves. Bond the rod back together and then start with the heat-shrink tubes to hold the joint together and keep working upwards till the layers are all shrunk and done.
3. Heat-gun or the hot air tool in your H-F link. Works like a heat shrink for thin faced sheets or using plastic 'weed-eater' string line, one can heat weld to bond things.
How to Weld Plastic
4. Hot forming tips, plastic welding much like soldering to a degree. Tips get messy, but can be cleaned. Years back, I had a heater leak in my truck. Had to cut the HVAC plastic housing apart just to get in there. Got the new heater in and then had to plastic weld like the image shown.
At times, a combination of a few of the 4 mentioned really does the job. I once had a display table that the top cracked in 1/2 from a blunt impact. I was able to fuse it back together using a combination of 1, 3 & 4.
For vehicle bumper plastics or formed body plastics, check out this technique too.
A manual for the Seelye Electric Welder that details how to weld plastic including HDPE utilized in our amphibious six wheeler bodies.
www.6x6world.com