Step 1: Scrap Material
Step 2: Safety While Heating PVC
Vinyl Chloride, one of the components of PVC, is carcinogenic. When it is locked up in the polymer, however, it is much safer to be around. In my years of experience working with PVC, I have not noticed any adverse effects on my health from being around it.
Always work in areas with good ventilation. If you do get caught in a cloud of smoke, hold your breath and move to clean air.
When heating PVC with a gas stove or propane torch, try not to let it burn. Smoke from burning PVC is bad. With experience one burns it less and less. Don't panic the first time you do burn some. It scorches, but doesn't immediately burst into flame. Move the material away from the flame and try again. Don't breathe the smoke. Smoke avoidance comes naturally for most people.
While heating PVC over a gas flame, keep the plastic an appropriate distance from the flame to avoid scorching the surface before the inside can warm up. It takes time for heat to travel to the center of the material being heated.
Keep the plastic moving, and keep an eye on the state of the plastic. When heated, the PVC material is flexible, like leather. Beyond this stage, you risk scorching it.
A word from James, the plastic engineer -- "Just a word of warning, PVC can handle some high heats but if it catches fire, you wont be able to put it out, it does not need oxygen to burn so don't do this inside".
I do work inside, but my house is made of cement and has good ventilation. MAKE SURE THAT YOU HAVE GOOD VENTILATION. PLAY WITH FIRE -- CAREFULLY.
Step 3: Heat Forming the Handle
I used a piece of 1 inch diameter pipe to form the softened material around. I wrapped it with a mail order catalog to protect my hands from the heat, and to help keep the handle uniformly cylindrical.
Heat, wrap with the catalog, and squeeze with your hands. Hold it until the plastic cools and rigidifies.
On the first attempt, I got the handle at an angle that was not to my liking. I decided I wanted it to run parallel to the blade direction.
Step 4: Modifying the Handle-blade Angle
I wanted a little jog at the joint area, so I raised the handle on a book while pressing the blade down to the table. That made the blade and handle directions parallel.
Step 5: Shaping the Tip of the Trowel
The scrap plastic was too rounded to make a good trowel, so I penciled in the modification and formed it with a miter saw and a course file. I sharpened the edge of the blade a little with a file to help it dig into the dirt easier. There was no point in getting it too sharp, because it is plastic and will dull faster than a metal blade.