Page 127 - Robot Builder's Bonanza

96 WORKING WITH PLASTIC

How to Bend and Form Plastic

Many kinds of plastics can be formed by applying low localized heat. A sure way to bend sheet
plastic like PVC and acrylic is to use a strip heater, available at ready- made plastics supply
outlets. Look through your Yellow Pages for local stores, or see Appendix B, “Internet Parts
Sources,” for leads on online retailers that carry plastics and plastic tools.
When you plug in the tool, a narrow electric element applies a regulated amount of heat
to the plastic. When the plastic is soft enough, you can bend it into just about any angle you
want.
Plastic heaters are easy to use, but getting good results takes lots of practice. Try with some
scrap pieces until you get the hang of it. None of the projects in this book require plastic bend-
ing and forming, but feel free to try it on your own if you want to experiment. A couple of tips
to get you started:

• Be sure that the plastic is pliable before you try to bend it. Otherwise, you might break it
or cause excessive stress at the joint (a stressed joint will looked cracked or crazed).
• Bend the plastic past the angle that you want. The plastic will “relax” a bit when it cools
off, so you must anticipate this. Knowing how much to overbend will come with experi-
ence, and the amount will vary depending on the type of plastic and the size of the piece
you’re working with.
• Use the right heat setting for the plastic. Don’t apply too much heat, or you’ll be sorry.
Expanded PVC has a very low melting point— about 165 to 175°F. This stuff emits a very
noxious and corrosive gas (hydrogen chloride) when it burns, so treat it with care!

How to Smooth the Edges of Plastic

After cutting, your plastic parts may need a bit of smoothing to remove any rough edges. As
with wood, you can apply a light sanding with a fine- grit aluminum oxide (not garnet) sandpa-
per. For soft plastic like PVC, use the sandpaper dry. For hard plastic, you can use the paper
dry or get it wet with water.
You can also shape the plastic— to remove sharp corners in square bases, for example— by
using a very coarse sandpaper.

Recommended Grits: Aluminum Oxide
Use Very Fine Fine Coarse Extra Coarse
(160–200 grit) (120–150 grit) (50–60 grit) (30–40 grit)
Shaping • •
Smoothing • •

Grit represents the coarseness of the surface of the sandpaper. The higher the number, the finer the grit.
How to Glue Plastic

When building bots, my preference is always to use mechanical fasteners . . . nuts, screws,
that type of thing. The reason: It’s not uncommon to want to disassemble a robot, either to

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