Plastic does three jobs a settlement needs at once: it insulates electrical conductors, it makes comfortable insulating handles, and it seals joints as gaskets. Use polyethylene or PVC for flexible wire cover, Bakelite for rigid parts that must tolerate heat, polypropylene or Bakelite for handles, and rubber or soft polyethylene for gaskets. Never use celluloid for any electrical job, because it burns.
Part of Plastics and Polymers
Plastic insulation and tool parts are molded or extruded components that isolate conductors, grip tools and seal joints.
Match The Plastic To The Job
Plastics Manufacturing gives the pairings:
| Application | Why plastic works | Plastic |
|---|---|---|
| Electrical insulation | Best insulator available, flexible | PE, PVC, Bakelite |
| Tool handles | Insulating, comfortable grip, durable | Bakelite, PP |
| Gaskets and seals | Flexible, conforms to surfaces | Rubber, soft PE |
| Rope and cord | Rot-proof, strong | PP, nylon |
For depth on dielectric properties see Electrical Insulation.
Wire Insulation
Flexible conductors need a flexible cover. There are two routes.
- Extrude it. The wire passes through a die and the plastic closes around it. This is from general knowledge; the corpus describes extrusion generally in Extrusion Products.
- Dip it. A lacquer that dries to an insulating film can be built up by dipping and drying repeatedly, as Plastics from Scratch describes. See Lacquers and Coatings. For copper wire itself see Copper Wire.
Test insulation you did not make or that has aged: Insulation Materials explains how.
Rigid Parts: Switch Bodies, Terminal Blocks, Bobbins
These want a plastic that holds shape near warm conductors. Bakelite is the choice, molded in a heated press: Compression Mold. Salvaged phenolic switch plates and handles can be reused whole, which is easier than making new ones.
Handles
A handle needs grip, insulation and enough strength to take torque without cracking. PP is tough and forgiving; Bakelite is hard and heat-tolerant. Mold the handle onto a tang with a hole or notch so that it cannot spin. Buttons, Handles and Hardware covers molded fittings. A hafting alternative from the corpus is Tool Hafting.
Gaskets
A gasket must follow a surface and stay put under bolts. The corpus’s Gaskets and Seals gives 2-4 mm rubber sheet for flat gaskets, retightened after 24 hours as it settles; the same retightening applies to plastic that creeps. Rubber suits hot fluids; rubber processing is in Rubber Processing. Soft PE works for cold water.
Worked Example: Thickness Of A Bakelite Terminal Block
Plastics from Scratch gives a dielectric strength of 10-15 kV/mm for Bakelite. A block 3 mm thick between two terminals then resists 3 × 10 = 30 kV up to 3 × 15 = 45 kV, in a perfect, dry, defect-free sample. A real block is lower: moisture, cracks, voids and surface contamination make it break down at a fraction of that. At mains voltage the limit is not thickness but surface paths: leave a long surface path between conductors and keep the block dry.
Common Mistakes
- Celluloid for electrical parts — the flammability is disqualifying.
- Casein for electrical parts — it absorbs moisture and stops insulating.
- Handle that can spin on its tang.
- Reusing insulation without testing it.
Insulation failure is a shock and fire hazard
Test any insulation before it carries power, and never trust it near high voltage because it looks intact. Keep celluloid away from heat and electrical arcing, since it burns violently. Do not heat unidentified plastic to shape it. See Electrical Safety.
What’s Next
- Learn next: Electrical Insulation — how insulators fail
- Combine with: Safety and Quality — testing what you made