PLASTIC WELDING: Plastic Welding Q & A
Q. What shielding gas do you use when welding PVC pipe?
A. You can use nitrogen or hydrogen, but this is not necessary for PVC. A clean and dry air supply is adequate. You can also use a PVC adhesive.
Q. Is nitrogen the only shielding gas ever used in plastic welding?
A. No, normally a clean and dry heated air is used to preheat the plastic materials. Nevertheless, nitrogen is the most common gas to reduce oxidation inside the weld as well as keep the gas flow more consistent. Another gas used is hydrogen.
Q. I see from the website that nitrogen is used to minimize oxidation of plastic and I am familiar with steel oxidation looks like, but what would plastic oxidation look like?
A. Generally all plastics will age under the influence of light, heat and oxygen. Virtually every plastic material is affected by reactions with oxygen. Oxidation breaks the long chains of the plastic molecules which causes a loss in plastic qualities, because these molecules are responsible for the plastic's properties.
When using hot air or hot gas to preheat plastic, it will be blown directly into the weld area. This causes oxidation and reduction of weld quality.
Q. How do you aliviate the porosity problem when welding PA? The welds start fine, then the bubbles start, which I know makes for a poor weld.
A. It depends what kind of PA you are using. There are different kinds. PA 6; PA 6/6; PA 11; PA 12 and so on. First you need to dry the rod material prior to welding. Porosity is actually moisture inside the material. Materials that store moisture are called hygroscopic. That means they will absorb moisture from the surrounding environment. You cannot see this, but as soon as you heat the plastic, you heat the moisture. The heated-up moisture will expand and will cause the porosity appearance.
Drying the rod material is the only way to get rid of this problem. PA 11 and PA 12 are normally not very hygroscopic, but PA 6 and PA 6/6 are. Drying PA material can be quite a challenge. First you need a drying system. An industrial dryer is perfect. But a regular kitchen oven will do the trick as well given more time. Set the device you are using to 120║ F for PA 6 and 140║ F for PA 6/6. Do not increase the temperature to speed the process, you might deform and damage the welding rod. The minimum drying time is 3 hours. This time can be much longer, depending on the drying system, rod quantity and local humidity. You have to experiment in your location to find the right time.
Other hygroscopic plastics include ABS; PMMA; PC and PU.
Q. I am fabricating covers out of plastic and am having trouble bending the plastic sheets. I use a tacking tip to heat the material, bend it by hand then secure it with clamps until cool. This works, but is very slow and does not always form a joint that is tight enough to get a good weld. I have learned that plastic is not very forgiving and proper fit is important. Do you have any suggestions on how I can improve my technique?
A. Working with plastics and plastic welding seems so easy, but there are many things to consider to produce quality parts and repairs. Unfortunately, this is something you cannot learn just by doing. To get the right start, I highly recommend a plastic course. There are many available, including ours, contact me for more information.
To fully answer your question, I would need more information, but basically to bend plastic heat the whole sheet and over-bend the material onto a form. Over-bending is necessary because of the so-called memory effect of plastics - plastic wants to return to its original shape.
For additional information on welding products advertised in the current issue of The Plastics Distributor & Fabricator Magazine, visit the following websites:
Plastic Welding Technology International Inc.
WEGENER, North America, Inc.
Winkleman Sales, Inc.
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