Guide to Working with Plastics -- A Choice of Good Connection: Heat as a Means of Merging Thermoplastics

Because of the low melting point of many thermoplastics, welding them with heat produces none of the showy pyrotechnics of metal welding. Nor does it require the same kind of heavy equipment. But like metal welding, joining plastic by means of heat produces a bond that is waterproof, airtight and, in some cases, fully as strong as the nonwelded areas of the material.

=== Virtually any thermoplastic can be welded. Thermosetting plastics, on the other hand, merely char or decompose if subjected to welding heat. The plastics most commonly welded are polyethylene, polypropylene and polyvinyl chloride (PVC). Polyethylene and polypropylene resist solvents and thus cannot be joined with solvent cement—a generally faster and neater technique. PVC pipe is joined with solvent cement, but heat welding produces stronger joints in PVC sheet and film. Nylon, which can be cemented only after special surface preparation, is often welded instead.

Like metal welding, plastic welding re quires that the edges of parts be heated to a liquid and then held together until the molten material fuses and sets. Because the strength of a welded joint depends on molecular fusion—the uniform comingling and coalescence of the materials that are being joined—only identical thermoplastics can be successfully welded to each other.

The amount of heat needed to accomplish the necessary fusion—and the length of time that the plastic must be subjected to it—vary with the type and thickness of the plastic. Most can be sufficiently liquefied in less than half a minute at temperatures ranging from 430 to 560° F. Because these temperatures are relatively low, the heat for plastics welding can be generated by almost any heating tool or appliance—including a house hold iron or a kitchen range. However, welding is also done with certain specialized tools, such as hot-gas welding guns.

The handiest tool for heating the ends of pipes, rods and small sheets of plastic is a hot plate—preferably one with a thermostatic control and a smooth aluminum or steel top. To keep the plastic from sticking, coat the top of the hot plate evenly with a thin layer of permanent nonstick spray, available at hard ware stores. Never use wax, oil or silicone; they can be picked up by the plastic and will fatally weaken any weld. For the same reason, be sure to clean any excess melted plastic off the hot plate after each use, either by carefully wiping it with a rag while it’s still hot or by scraping it off with a knife or a razor blade after it has cooled.

To melt the ends of small parts, heat can be most precisely applied with a hot scrap of metal. Commonly, a small piece of aluminum or steel sheet is heated on a range burner or a hot plate, or with a propane torch, and then touched briefly to the parts to be melted, Of course, the open flame itself must never contact the plastic—the material would ignite rather than melt. For the flexible seams needed to join thin plastic sheets or films, an ordinary household iron performs admirably, although a special tool such as a heating wand can speed up the job considerably.

No matter which heating method you use, plastic edges to be joined must be scrupulously clean and must be cut to fit each other precisely. Then, to maintain the precise fit, the joint should be aligned with a jig as it cools.

A simple two-part jig for flat or round objects can be made from two pairs of plywood strips, each pair nailed together at a 90° angle to form an L-shaped sup port (Step 2, opposite). In use, the two parts are positioned about an inch apart and the plastic sections being joined are seated in them, with the seam line in the open space. The jig aligns the joint and also supports the plastic sections while the weld is cooling.

A more complex two-part jig for precise alignment of small rods and tubing is mounted on the jaws of a vise; as the jaws of this jig are closed, the cylindrical pieces of plastic are moved smoothly together.

Because of the low temperatures involved, most plastics welding can be safely done in any room of the house. The worktable can be protected with a sheet of cardboard or plywood. Good ventilation, however, is essential. Any plastic, when melted, gives off noxious fumes, and the fumes of PVC can be deadly. If you are welding PVC, use a fan or two to disperse the fumes; avoid leaning close to the work, where the vapors are most concentrated.

The Handy Hot Plate for Minor Welding

1. Melting the edges. Set a thermostatically controlled hot plate to a temperature of about 500°. Hold the pieces that you are welding against the plate, at right angles to its surface, without applying pressure. When the lower edges of the plastic become hot enough that they are clear and pliable about 1/8 inch up from the surface of the hot plate, the thin layer that is in direct contact with the plate will be liquid and ready for joining.

2. Joining the pieces. Lift the two pieces of plastic straight up from the hot plate and place them in the two-part jig, butting them against the jig’s L-shaped back. Immediately slide them toward each other until their molten surfaces touch. Press them together just enough to form a small bead of plastic along the joint (inset). Hold the pieces in position until the plastic cools. Then smooth off the bead with 280- to 400-grit wet abrasive paper or a sharp knife.

When fusing pipes and rods, hold them in alignment by pushing them into the V formed by the bottom and the back of the jig.

A Double-faced Heater Made of Scrap Metal

1. Melting the plastic. Heat a small scrap of aluminum or steel on a hot plate or over a gas flame. Using pliers, lift the hot metal and clamp it between the jaws of a vise. Touch the two pieces of plastic to opposite sides of the metal, holding them there until they soften and become clear to a depth of approximately 1/8 inch.

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2. Mounting the jig. Insert bolts through the bolt-holes in the upper and lower halves of the jig sections, securing them with wing nuts. Loop a hose clamp through each saw kerf, and attach the two jig sections to the two jaws of the vise. Then loosen the wing nuts, and clamp a length of plastic tubing or rod in each of the two jig sections. Finally, loosen the clamps and adjust the positions of the jig parts until the ends of the plastic are perfectly aligned; tighten the clamps.

Joining Rods or Tubes in a Jig-equipped Vise

1. Melting the jig-held plastic. Open the jaws of the vise until the ends of the tubing or rod are about ½ inch apart. Then insert a piece of scrap metal, heated as in Step 1, between the ends of the plastic, and tighten the vise until the ends rest against the hot metal. Leave the metal in place until the plastic is soft and clear to a depth of about 1/8 inch.

2. Fusing the ends of the plastic. Open the vise just enough to free the softened ends of the plastic from the hot metal. Remove the metal and immediately close the vise, bringing the ends of the plastic together; apply lust enough pres sure to form a small bead of plastic at the joint. Let the plastic cool and harden, then remove it. Smooth off the bead as in Step 2.