Page 204 - Handbook of Plastics Technologies

THERMOSETS

3.74 CHAPTER 3

outstanding. Typical products include fishing rods, flag poles, tent poles, antennas, golf
club shafts, hockey sticks, bows and arrows, ski poles, fence posts, ladders, light poles,
pipe supports, and tool handles.
3.2.2.16 Filament Winding. The strongest plastic products ever made, competing with
or exceeding metals, are made by filament winding. Continuous filament or fabric goes
continuously through a catalyzed liquid resin bath and is wound onto a mandrel in the
shape of the desired product. The winding pattern is calculated and controlled to produce
maximum strength. When it has been wound layer-upon-layer up to the desired thickness,
the assembly is oven cured. The mandrel may remain as part of the finished product; more
often, it is designed so that it can be collapsed and removed from the cured filament-
wound product. Most products are simple cylindrical shapes, but more complex shapes
can be produced by thoughtful design. The most common products are pressure pipes and
pressure tanks. Some more specialized products include rocket motors, railroad hopper
cars, turbine blades, helicopter blades, and plastic housing.

3.2.3 Pouring Processes
Liquid A-stage thermosetting resins can be poured into finished form and simply cured in
place. These processes are often called casting and sometimes distinguished as potting,
encapsulation, and dip coating.
3.2.3.1 Casting. Epoxy, polysulfide, polyester, polyurethane, and silicone A-stage liq-
uids can be poured into a mold, cured, and removed from the mold as finished products.
Typical products are simulated wood frames, figurines, and furniture decoration; electrical
and electronic products; and solid polyurethane rubber tires for industrial equipment.
Small-scale production is manual, whereas larger production runs can be automated.
For some products, rubber molds make it easy to remove the products. Where gas bubbles
may disfigure the product or ruin electrical performance, vacuum degassing can prevent
this problem. For penetration into fine details, vacuum and pressure impregnation are
helpful.
Conversely, pouring a foamable plastic is useful for gap filling, light weight, and ther-
mal insulation. Use of hollow glass or plastic spheres can encapsulate closed-cell bubbles
in syntactic foam, which is useful for low dielectric constant and loss and for compressive
resistance such as deep-sea immersion.

3.2.3.2 Potting. Electrical and electronic assemblies are often insulated and protected
against mechanical abuse and environmental attack by placing them in a shell, filling the
space by casting liquid A-stage thermosetting resins, and curing them in place. When the
cured assembly is left in the shell, the process is called potting.
3.2.3.3 Encapsulation. Similarly, when the electrical/electronic assembly is cast and
potted, and the shell is then removed, the process is called encapsulation or embedment.
3.2.3.4 Dip Coating. Electrical and electronic products may be insulated and protected
by a conformal coating. This is produced by dipping the product into a thixotropic A-stage
thermosetting resin, rotating it to ensure uniform coverage and thickness, and curing it to a
finished coating which coats the entire product.

3.2.4 Powder Coating Processes
When metals are coated with polymer solutions, the solvent brings problems of flamma-
bility, toxicity, environmental pollution, and cost. As an alternative, powder coating simply

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2006 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.