Guide to Working with Plastics -- A Choice of Good Connection: Introduction

• Introduction
• Mechanical Devices: Joinery’s Nuts and Bolts
• Heat as a Means of Merging Thermoplastics
• Fast Welding with Heated Gas
• Hems or Seams in Flexible Film

A Choice of Good Connections -- Introduction

The methods for joining one piece of plastic to another are both reassuringly familiar to and startlingly different from methods for joining metal and wood—a reflection of the chameleon nature of these synthetic substances. Many plastics can be joined with mechanical fasteners as prosaic as stove bolts and rivets; others with adhesives as mundane as white glue. But sometimes neither of these two standard fastening techniques is as effective or as convenient as one of several methods peculiar to plastics.

Two of these methods take advantage of plastics’ most vexing weaknesses: their sensitivity to heat and to strong cleaning com pounds. Many a homeowner has discovered that polyethylene food wrapping instantly melts if brushed against a hot saucepan. But that same sensitivity to heat makes it possible to weld plastics at temperatures far below those of conventional metal welding, using a hot plate or a household iron for a welding tool.

Similarly, the solvents in window-cleaning compounds that soften the surface of shatterproof plastic glass, causing it to haze and crack, also yield a method for creating an invisible joint. Used in small quantities and in confined areas, certain solvents soften two plastic edges so that, when pressed together, their molecules mingle, then harden into a molecular bond.

Not all of these fastening methods, familiar or unique, will work on all types of plastics. The world of synthetics is too varied. There are some plastics, especially thin films, that are too soft to accept mechanical fasteners; others, such as polyethylene, have slick, non porous surfaces that reject adhesives, causing them to peel right off. Still other plastics, notably the polystyrenes, char when heated, making hot-gas welding impractical, choosing the right technique becomes even more crucial in joining two dissimilar plastics or in joining a plastic and, say, wood.

Narrowing the choice still further are practical concerns. You will want to consider the conditions under which the assembled parts must perform. Must the joint be watertight or airtight? Solvent- cementing and welding are fine, but screws and rivets won’t do. Must the joint be flexible enough to expand and contract with extreme temperature changes? Use a silicone adhesive and avoid white glue. Do looks count? The most attractive joint is produced by solvent cement, the least attractive by hot-gas welding. But a welded joint is rugged. Will the parts need to be disassembled? Nuts and bolts are the logical choice. These and other considerations are discussed in the section that follows, making it possible for you to join plastic parts by the method that best suits the physical or esthetic demands of the project.

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===Welding a strong joint. An invisible jet of hot gas from a plastic-welding tool melts a PVC filler rod between the edges of two PVC sheets. The edges of the sheets have been beveled to form a V joint. The joint edges and the rod are simultaneously heated with an oscillating motion of the welding-tool tip. As they soften, the rod is pushed gently into the joint, fusing the plastic surfaces. Several rods will be melted along the joint, overlapping each other until the V is filled.

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