Guide to Working with Plastics -- A Choice of Good Connection: Fast Welding with Heated Gas

A hot-gas welder, originally designed for such industrial uses as repairing plastic ductwork and storage tanks, also makes short work of household repairs, joining the broken parts of a plastic fan blade, patching a hole in a plastic garbage can, and restoring similar objects to active life. Welding can also be used to make fluid- proof liners for planters or custom- shaped storage tanks for household or hobby chemicals. The welder directs a jet of hot air or gas over a plastic filler rod, which softens and flows into the joint between the parts or around the edge of the patch, fusing the pieces together. A small home welder is not expensive, but you can often rent one from a tool-rental agency or plastics supplier.

The business end of a hot-gas welder consists of a nozzle-like handpiece containing a powerful stainless-steel heating element. Pressurized air or gas enters the handpiece through a hose and is warmed as it passes the element. It exits through one of several interchangeable tips that spread or concentrate the stream of air or gas, as the particular weld requires.

For most plastics welding, compressed air is satisfactory. It may be supplied by the type of compressor used for home spray painting—one that can deliver air at a rate of 3 cubic feet per minute under at least 15 pounds per square inch (psi) of pressure. Or, if a compressor setup is impractical, you can substitute bottles or tanks of pressurized air.

For polyethylene, use pressurized nitrogen gas, available in tanks or bottles. Air’s oxygen would react chemically with heated polyethylene, breaking it down and weakening the weld.

Whatever the source of air or gas, the pressure will need to be reduced to working level, between 1½ and 3 psi, by a valve in the connecting hose. Adjusting the pressure also changes the tempera ture of the air or gas exiting from the tip. At low pressures, the air or gas passes the heating element slowly and may be heated to 6000 F.; at higher pressures, the gas blows past the element quickly, in some instances reaching only 400°.

The right temperature for most plastics welding is between these extremes. Try welding a scrap of the same kind of plastic as what you plan to join. Check to see if it’s properly heated, then adjust the gas flow. After welding, switch off the heat and let the gas flow continue for a few minutes to cool the stainless-steel heating element and keep it from burning out prematurely.

Although there are many ways to make a welded plastic joint, good preparation of the plastic and a good welding technique are essential. Clean the edges of the plastic thoroughly and roughen them with sandpaper. To start the weld, position the pieces carefully and, if necessary, hold them together with a push stick or clamps; then tack the joint in a few spots with concentrated blasts of hot gas.

The filler rod for the weld must be the same type of plastic as that being welded. You can use a strip cut from a plastic sheet, but extruded round and triangular rods, available in rolls of several sizes and lengths, are more convenient. When melting the rod into the joint, hold the welding tip ½ to 2 inch above the plastic, and move it back and forth to distribute the heat evenly. Heat the rod only enough to liquefy its surface; its core should remain stiff enough to be pushed into the joint. Depending on the thickness of the stock, several rods may have to be laid in to fill the joint.

A slightly different welding technique is used for speed welding, a variant process that is particularly useful for projects involving many joints. In speed welding, a special type of slotted welding tip dispenses heated filler rod or filler strip directly onto the joint. This eliminates the need to fan the tip over the filler in order to heat it evenly. Also, because it’s in effect one-handed welding, freeing one hand to hold the plastic in position, the weld can often be made without preliminary clamping and tacking.

Whatever the welding mode employed, joining plastics with heat is generally a safe procedure. Except at the end of the welding tip, the temperatures involved are moderate; no special protective clothing or gear is required, and the work surface needs no special insulation. Do not, however, aim the tip at your skin at close range. Also, be sure that the welding area is well ventilated: Melting plastics release noxious fumes; those of PVC are particularly dangerous to inhale.

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=== A choice of welding tips. By shaping and directing the emission of air orgasm various ways, different tips increase the versatility of the hot- gas welder. The tacking tip, for spot welding a joint to hold it in place temporarily, has a sharply protruding end that focuses air or gas on a tiny spot. The all-purpose round tip, which is appropriate for most welding tasks, delivers a broader, more uniform flow than the tacking tip does. The flat and L tips are helpful for reaching into narrow spaces and tight angles. The round and flat dispenser tips, designed especially for speed welding, lay softened ribbons of filler rod or filler strip directly on the joint.

=== The welding apparatus. Air or gas from a compressor or a pressurized tank is pumped through a hose to a pressure gauge and valve. A second hose carries the air or gas from the valve to the handpiece; it’s heated there, then exits through the welding tip. The heating element is heated by an electric wire that runs through the same hose as the air or gas. A second electric cord activates the compressor.

=== Four basic welds. For a butt weld—the commonest way to join two pieces of plastic end to end—the edges of the joint are beveled at a 30-degree angle, creating a 60-degree V groove. A 1/64-inch space at the bottom of the groove allows the molten plastic to penetrate the joint, and one or more filler rods are laid into the groove to fill it. When speed-welding a butt joint, you need not bevel the edges, but a 1/64” gap must be left between the pieces of plastic so that the filler strip penetrates.

For an overlap weld, often used in patching large holes, one piece of plastic is laid on top of another, and the right angle where they meet is filled with molten plastic. The work is then turned over and welded on the other side.

For a corner weld, layers of filler are similarly laid into the right angle formed by the two pieces of plastic. For a stronger corner weld, butt the two pieces of plastic together in a T and weld on both sides; the excess plastic on the outside of the T is then trimmed back to the edge of the melted filler rods reinforcing the joint.

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=== Judging weld quality. The three butt welds illustrated show the effect on a joint of differing amounts of heat. In the weld at top, near right, filler rods have not been heated enough to lose their shape and have fused incompletely. Correct this defect before adding any more filler rods; reheat the weld, and either move the welding tip more slowly along the joint or decrease the gas pressure so that the gas leaving the welding tip is hotter. The gently rounded top of the filler material in the weld at far right indicates that just enough heat was used to fuse the rods to each other and the sides of the joint, creating a strong bond. In the discolored bottom weld, either the tip was moved too slowly or the gas pressure was too low; the filler rods overheated and charred before they could fuse. This joint cannot be salvaged; cut away the burned material and replace it with new filler rods.

Using a Filler Rod to Seal a Butt Joint

1. Setting up the weld. Place a piece of plywood on the work surface to help the plastic retain heat during welding. File or sand the two edges to be joined to 3Q0 angles and butt them together, with a hairline gap at the bottom of the V; clamp the plastic to the plywood. Fit a tacking tip into the handpiece and turn on the heat and the gas or air; let the tip warm up for two minutes. Direct the heated tip at a single spot on the joint, holding the end of the tip lightly against the plastic. When the plastic softens, move the tip about 1 inch along the joint, fusing the spot. Repeat this tacking procedure at 3-inch intervals along the joint. After the last tacking has cooled for about one minute, remove the clamps.

2. Welding the joint. Replace the tacking tip with a round tip, using work gloves to remove the tacking tip if it’s still hot. Allow the new tip to heat up; then, holding the handpiece in one hand and a filler rod in the other, set the rod in the groove at one end of the joint. Holding the tip ¼ to ½ inch above the rod, move the tip back and forth in a fanning motion across the joint, so that both the rod and the edges of the groove melt equally. As the rod melts, it will turn clear; when it reaches this stage, push it gently down into the groove. Move the handpiece along the joint, continuing to melt both the rod and the groove until the entire joint is filled.

If you run out of filler rod before the weld is complete, taper the end of a new rod and soften it with the welder. Lap the tapered end over the end of the rod in the groove and continue.

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3. Cutting off the rod. Warm the filler rod about ¼-inch beyond the end of the joint, then bring the welding tip close to the rod and, without fanning the tip, apply a steady stream of hot air to one spot on the rod. Meanwhile, pull gently on the rod; it will thin and then separate.

Continue to lay filler rods into the groove until the surface is slightly higher than the surrounding plastic. When the welded joint s barely warm to the touch, in three to four minutes, cut or file off the ragged ends of the rods.

A Strong Angles Joint Made with a Wooden Jig

1. Starting a corner T weld. Construct a right- angled jig by nailing together two boards slightly smaller than the plastic sheets being joined. Clamp one sheet upright, against the outside of the jig, aligning the bottom edge with the jig bottom. Place this assembly on top of the other plastic sheet, 1/8 to ¼ inch from its edge, forming a T joint with one short arm. Using the jig as a brace, clamp this assembly to the work surface. Begin welding as in Steps 1 through 3, laying filler rods into the short arm of the T. Allow the weld to cool for three to four minutes and then remove the clamps.

2. Completing the T weld. Carefully reposition the work so that the jig rests against the outside joint, in the short arm of the T. Prop the jig up with a scrap of plastic to bring it even with the horizontal plastic sheet. Secure the new arrangement to the work surface with clamps. Weld the inside joint as in Step 1. When the weld has had time to cool, remove the clamps and the jig. Using a file or a saw, trim away the excess plastic on the short arm of the T, leaving only the fillet of plastic that forms the outside joint. Also cut or file away the ragged ends of the filler rod, as in Step 3, above.

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Hole Patches, Big or Little

===Plugging a small hole. Fit a round welding tip into the handpiece, and hold the end of a filler rod just outside the edge of the hole—in this case, a hole in an automobile windshield-washer reservoir made of polyethylene. Using a fanning motion, soften the filler rod and the plastic surface beneath it. As the two soften, press the rod over the hole. Cut off the excess filler rod as shown, Step 3.

For a slightly larger hole, lay the molten rod across the opening in several passes, side by side. Or cover the opening by working in a spiral pattern, from the edge of the hole in to the center.

===Patching a large hole. Cut a patch from plastic of the same type and thickness as the plastic in the object being repaired, making it at least ¼-inch larger than the size of the hole. Lay the patch over the hole and hold it in place temporarily with a wooden stick while you spot-weld the patch to the object (Step 1). Exchange the tacking tip, used in spot welding, for a standard round tip, and weld filler rod into the joint around the edge of the patch (Step 2). When you change tips, be sure to protect your hand from the tip’s hot metal.

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Using a Speed-welding Tip

1.Tacking the joint. Hold or clamp the plastic sheets in position, leaving a 1/64-inch gap between them. Then attach a high-speed tip to the welder; in this example the tip is designed for a flat filler strip, commonly used for the butt joint illustrated. Insert the strip into the slotted feeder arid hold the heated welding tip almost upright, 1/8 to 1/4 inch above one end of the joint. When both the tip of the filler strip and the plastic beneath it are melted, use the curved nose of the feeder to push the molten strip into the molten plastic, tacking the end of the joint.

2. Filling the joint. Move the welding tip, still held upright, along the joint, feeding and pressing more filler onto the plastic until you have fused about an inch. Then drop the handpiece to about a 45° angle and pull the tool slowly along the joint, allowing the filler to feed out of the slotted welding tip in a continuous ribbon. As you work, continue to press together the molten filler and the molten plastic beneath it.

3. Finishing the weld. At the end of the joint, tilt the hand piece forward and dig the tip of the nose into the filler strip, severing it. Lift the tip back and away from the weld. Immediately remove the leftover filler strip from the hot tip, before it has a chance to harden and stick to the tool.