Welding is the process of uniting surfaces by softening them with heat. When welding thermoplastics, one of the key components is the material itself. For as long as plastic welding has been around many people still do not understand the basics, which is critical to a proper weld.
The number one rule of welding thermoplastics is you must weld like-plastic to like-plastic. In order to get a strong, consistent weld, it is necessary to make sure your substrate and your welding rod are identical; for instance, polypropylene to polypropylene, polyurethane to polyurethane, or polyethylene to polyethylene.
Here are some tips for welding different types of plastics and steps to ensure a proper weld.
Polypropylene (PP) is one of the easiest thermoplastics to weld and is used for many different applications. PP has excellent chemical resistance, low specific gravity, high tensile strength and is the most dimensionally stable polyolefin. Proven applications using PP are plating equipment, tanks, ductwork, etchers, fume hoods, scrubbers and orthopedics.
In order to weld PP, the welder needs to be set at approximately 572°F/300°C; determining your temperature will depend on which type of welder you purchase and the recommendations from the manufacturer. When using a thermoplastic welder with a 500 watt 120 volt heating element, the air regulator should be set at approximately 5 p.s.i. and the rheostat at 5. By doing these steps, you should be in the vicinity of 572°F/300°C.
Another fairly easy thermoplastic to weld is polyethylene (PE). Polyethylene is impact resistance, has exceptional abrasion resistance, high tensile strength, is machinable and has low water absorption. Proven applications for PE are bins and liners, tanks, laboratory vessels, cutting boards and slides.
The most important rule about welding polyethylene is that you can weld low to high but not high to low. Meaning, you can weld low density polyethylene (LDPE) welding rod to high density polyethylene (HDPE) sheet but not vice versa. The reason being is quite simple. The higher the density the more difficult it is to break down the components to weld. If the components cannot be broken down at the same rate then they cannot join together properly. Other than making sure your densities are compatible, polyethylene is a pretty easy plastic to weld. To weld LDPE you need to have the temperature at approximately 518°F/ 270°C, the regulator set at approximately 5-1/4 to 5-1/2 and the rheostat at 5. Like PP, HDPE is weldable at 572°F/300°C.
Tips for Proper Welds
Prior to welding thermoplastics, there are a few simple steps that need to be taken to ensure a proper weld. Clean all surfaces, including the welding rod, with MEK or a similar solvent. Groove the substrate large enough to accept the welding rod and then cut the end of the welding rod to a 45° angle. Once the welder has adjusted to the proper temperature, you need to prep the substrate and the welding rod. By using an automatic speed tip a lot of the prep work is done for you.
Holding the welder about an inch above the substrate, insert the welding rod in the tip and move it in an up and down motion three to four times. Doing this will heat the welding rod while heating the substrate. An indication the substrate is ready to be welded is when it starts getting a fogging effect — similar to blowing on a piece of glass.
Using firm and consistent pressure, push down on the boot of the tip. The boot will push the welding rod into the substrate. If you choose to, once the welding rod adheres to the substrate, you can let go of the rod and it will automatically pull itself through.
Most thermoplastics are sandable and the strength of the weld will not be affected when sanded. Using 60-grit sandpaper, sand off the top part of the welding bead, then work your way up to 360-grit wet sandpaper to get a clean finish. When working with polypropylene or polyethylene, it is possible to regain their glossy surface by lightly heating the surface with a yellow open flame propane torch. (Keep in mind that normal fire safety procedures should be followed.) Once these steps are completed you should have a weld that looks similar to the photo at bottom left.
Keeping the above tips in mind, welding thermoplastics can be a fairly easy process to learn. A few hours of practicing welding will give the “feel” for maintaining the right even pressure on the rod straight down into the weld area. And experimenting on different types of plastics will help master the procedure. For other procedures and standards, contact your local plastics distributor.