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Preheat the welding gun for at least 2 minutes. Secure the plastic together by tack welding the ends. Trim the end of a welding rod with angle cutting pliers. Insert a welding rod into a speed nozzle on a welding gun. Move the tip of the welding gun slowly over the plastic for a speed weld. Swing the welding gun back and forth if you’re doing a pendulum weld.
Every type of plastic melts at a different temperature, so setting your welding gun up correctly is important. The temperature you need will be somewhere between 200 and 300 °C (392 and 572 °F). Anything beyond that range either burns the plastic or doesn’t melt it enough.  For example, set the welding gun to 300 °C (572 °F) when working on propylene and polyurethane. Adjust the heat setting to about 275 °C (527 °F) to work on PVC. Set the heat to about 265 °C (509 °F) for polyethylene. Before you start the actual weld, pin loose pieces of plastic together by melting the ends of the joint. Fit a tack welding nozzle on your welding gun, then apply a little bit of heat. Wait for the plastic to begin melting, joining the pieces of plastic together. This will keep the plastic from moving as you complete the weld.  A tack welding nozzle looks like a tube with a fin on the end. Press the fin against the plastic to heat it and melt it together. All you need to do is melt the plastic a little to ensure it can’t come apart. If you need to, tack weld spots every 1 ft (0.30 m) along the joint for additional security. Trimming a rod is very simple. Hold the pliers diagonally toward the end of the rod. Then, snip it to hone the rod to a point. If you don’t have pliers, use a trimming knife to scrape the rod to a point.  By giving the rod a pointed end, you increase the chances of getting a smooth, steady weld without a big bubble of plastic where you start. You can let the welding gun cool off before changing nozzles and inserting the welding rod. However, remember to let the gun heat up again before starting the weld. A speed nozzle has an opening to hold the welding rod as you melt it onto the joint. If one didn’t come with your heat gun, you can buy one separately. After fitting the nozzle on your plastic welding heat gun, feed the rod into the second opening on top. Put the clipped end in first so you can use it when starting the weld.  Make sure you don’t touch the tack nozzle if it is still hot. Either wait for the nozzle to cool or swap the nozzles carefully with a pair of pliers. With a speed nozzle, you will need to feed the rod into the opening while you weld. You could also hold the rod horizontally over the joint and melt it with a technique called pendulum welding, where you sweep the gun or torch back and forth. It takes a little longer but is great for tight spots. Start at the top of the crack or area you wish to join. Hold the gun downward at about a 45-degree angle, touching the edge of the nozzle to the plastic. Then, heat the plastic until you see it beginning to melt. As you push the welding torch along the joint, feed the welding rod into it with your free hand.  The key to success with welding is consistency. If you move as a deliberate pace, you can melt the plastic and welding rod just enough to bind them without burning them. If you notice the plastic burning or changing colors, move the torch at a faster pace. Don’t let it linger on the plastic or else you will end up applying too much heat. Hold the nozzle about 2.54 cm (1.00 in) above the crack, tilting the gun at a 45-degree angle. Then, position the welding rod at a 45-degree angle from the opposite side. As you hold the rod in place, sweep the nozzle back and forth 3 or 4 times to melt it. Keep doing this as you move down the plastic to complete the weld.  Pendulum welding is useful if you don’t have a plastic heating gun with a speed nozzle. You can do it with a basic propane torch. This is also an effective way to fill a tight joint you can’t easily reach with a speed nozzle. This part is a little trickier than with speed welds since you have to control both the torch and the welding rod at the same time. Move the torch constantly to prevent the plastic from burning. Swing it back and forth across the joint at a consistent pace to lightly heat and melt the plastic.