The production of a good plastic weld usually look like it’s pretty easy to do. However, it this really the case? Or is it that there are various hidden failures that can happen which nobody can see? Experts should be able to tell the difference between good welds and bad ones. When you’re aware of what types of failures are possible, then you’re going to have a better idea of plastic welding and what makes up a good plastic joint versus a bad one. As I’ve alluded to in my previous articles, you need to consider a number of things before you do any welding. As a refresher, I’d like to quickly review some of those crucial considerations. If you have read my previous pieces, then use this one as a starting point.
You should know that plastics come in two primary groups. Thermosets aren’t weldable, but thermoplastics typically are weldable. Don’t even bother trying to weld thermosets, because they just don’t melt. For the most part, you can’t weld different thermoplastics together. You’ll only successfully weld the same materials together. Be cautious when dealing with Polyurethanes or Polyamides (you might know these by their common trade name of Nylon). These materials are available in different grades, and these grades won’t fit together automatically.
Regardless of whatever kind of welding system you have, you always need to consider the following 3 parameters:
1) Pressure: If the pressure applied to the plastic during welding is either too low or too high, then the actual molecular mixing isn’t going to be possible.
2) Temperature: Every thermoplastic is going to have a particular weld temperature range. You need to stay in that range.
3) Time: Any material is going to need time to get up to its weld temperature. It will also need time for cooling down, too.
So, what exactly is a failure of a plastic welding joint? We’ll go over weld failure charts later.
The most important thing to consider is the specific location where a weld is going to be made. There is quite a difference between ‘indoor shop welds’ and ‘outdoor welds’ during the wintertime. Weld temperature ranges change when the temperature of the surrounding environment goes below room temperature. You need to make sure that the weld section is protected when conditions are cold, wet, or just otherwise irregular. Sometimes you can help by using a tarp or a tent with a heater. If your weld gets moisture inside of it, then voids are going to happen inside the plastic material bonds.
A good weld is going to illustrate a perfect mixture of the molecules of any involved welded plastic materials. If any of the previously listed welding parameters doesn’t stay within the specifications, then the fusion simply won’t be completed. Oil and dust in the weld or other sources of contamination are going to disrupt the mixing process. Contagion can happen if the welding rod and surface aren’t prepared correctly.
Damaged material, grease, dust, and oxidation all need to be removed ahead of the welding. The one exception is if a welding tool makes use of a heated tip for pre-heating and then penetrating the parent material. The tip will melt and push the surface off to the side. With the appropriate pressure, a molded welding rod will get injected underneath the surface and into the weld area. The result is an excellent mix of molecules that creates a robust weld.
The proper surface preparation is crucial to prevent failures in the weld at the very start. A scraping blade is an ideal tool for this. Solvents and sanding paper aren’t recommended since the holdovers of the solvent or the dust would get mixed into the inside of the weld. If the weld involves an angle, that is also crucial. If you pick the wrong angle, then the filler material isn’t going to be capable of filling the entire weld area properly. Cracking lines and voids will happen, and you’ll wind up producing a weak joint.
You have to choose the appropriate welding system for your application. It won’t matter what type you take, so long as you always keep the three parameters of pressure, temperature, and time in mind.
When you use a hot air welding tool, then you can expect oxidation to happen automatically. This event isn’t ideal for the production of a quality weld. You can use a gas like nitrogen to minimize the oxidation problem, but it won’t eliminate it. One other problem could be air supply that isn’t dry, clean, or oil-free. If your airline shows these symptoms, then particles are going to get blown right into the weld. They work as a shield between the various molecules, preventing the important process of mixing.
To make a strong weld, you know that the material has to be prepped first. Some plastics, such as Polycarbonate or ABS, are hygroscopic, which means that they absorb any moisture from their surrounding environment. Such hygroscopic materials have to be dried out before you use them for any welding purposes. Estimate the material which you need to weld and then only dry this particular amount. That is going to save you time since the drying time will depend on the rod quantity and material.
What plastic is heated, it will expand. It shrinks when it’s cooled down. If a material isn’t properly fixed into place, then it’s going to bend towards the welded side. A weld from both of the sides is ideal for preventing a cracking area that keeps the desired shape. If this isn’t possible, then prepare a weld angle that means the welding rod is able to reach the other side. In order to keep a straight shape, use a fixture. This unit should be positioned until such time that the weld can cool down completely.