Matching Material Matters

When welding thermoplastics, one of the key components is the material itself. For as long as plastic welding has been around many people do not understand the basics. Webster�s Dictionary definition of welding: to unite (surfaces) by softening them with heat.

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, Polyethylene to Polyethylene, etc.

Polypropylene is one of the easiest plastics 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 and scrubbers, and orthopedic. In order to weld PP the welder needs to be at approximately 572�F: determining your temperature will depend on which welder you purchase. Using a hot air thermoplastic welder (Fig. 1) 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 this you should be in the vicinity of 572�F.

Another fairly easy plastic to weld is Polyethylene. 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 welding rod to High Density Polyethylene 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. Polyethylene is impact resistant, 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. To weld LDPE you need to have approximately 518�F, regulator set at approximately 5 � to 5 � and the rheostat at 5. Like PP, HDPE is weldable at 572�F.

PVC, Polyvinyl Chloride, is a plastic that can be extremely temperamental, however, it is an extremely useful material. PVC has excellent resistance to corrosion, weather and impact, is an excellent thermal and electrical insulator and is self-extinguishing. Proven applications for PVC are plating equipment, tanks and liners, ductwork, etchers, fume hoods and scrubbers. When welding PVC, if you have your temperature too high the material will char, if it is too low it will not weld. Because of the chloride in the material it is necessary to break it down. To do this you need to score the surface area you want to weld and then using a thinner such as MEK, Methyl Ethyl Keytone, to clean the surface. Again, like PP and HDPE, PVC needs 572�F to weld. As a side note, PVC is the exception to the like-plastic- to-like-plastic rule. It is possible to weld PVC to acrylic and vice versa.

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 (Fig. 2) and cut the end of welding rod to a 45� angle. Once the welder has come to temperature, you need to prep the substrate and the welding rod. By using an automatic speed tip (Fig. 3) 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 Normal Fire Safety procedures should be followed. Once these steps are completed, you should have a weld that looks similar to the photo (Fig. 4).

Keeping the above tips in mind, welding thermoplastics can be a fairly easy process to learn. For other procedures and standards, contact your local distributor.

Written by Paige D. Bowen, Marketing Director for Seelye, Inc., manufacturers of a full line of hot air/gas welders.

For more information, contact Seelye, Inc., 333C Enterprise Street, Ocoee, FL 34761, 800-258-2936/407-656-6677, Fax: 407-656-5244, E-mail: seelyeinc@aol.com, Web: www.seelyeinc-orl.com.