PLASTIC WELDING: Plastic Welding Q & A

As we have covered many of the key aspects of welding thermoplastics, our series will now address reader questions. If you have a question regarding an aspect of welding plastics, please forward it to me at abramer@drader.com or to the magazine at pdfm@plasticsmag.com.

Q: How do I minimize weld stress when adding fittings to rotationally molded parts?

A: Rotational molded parts are usually pretty stress-free. This is because the resin is normally in powder form. Nevertheless, the parts shrink when they cool down. This shrinkage will add a certain amount of stress to the parts. The amount of stress depends on a number of factors: shape, material thickness and cooling time to mention a few.

Normally, there shouldn’t be a big problem with stress, but material stress will always occur when plastic welding is involved. The most stress occurs where the material changes from hot to cold. This could be 1/2" to 1" or so away from the actual weld area.

The amount of stress will also depend on the welding system you use.

Stress and other problems occur when you use a grinder as a spin-welding unit. This might be a cheap and fast solution, but there is no control over the 3 necessary weld parameters:

In this case the temperature is generated by friction. When is the adequate weld temperature achieved? Well, who knows? Generally, the temperature is too high. You can see this by rising smoke and floating material. This adds a large amount of stress to your weld.

It is a wild guess to say when the material reaches its weld temperature. If you wait too long or too little the plastic will get too hot or stays too cold. Some people cool down the weld with compressed air or cold water. This will shrink the weld too quickly: this is the point when the stress problems occur.

So, you have to guess the weld temperature. Then you have to guess the time when the weld temperature is achieved. Now you have to guess the right amount of pressure in order to bring the molecules of the molded part and a fitting together.

If you use a true spin welder, these issues will be reduced because you have better control over the three welding parameters. You can also spend a little bit more time and use an injection welder or variety of hot gas welders. By taking care when cutting an opening in a tank and with control over the temperature, time and pressure, you can minimize the stress you add to the tank.

A: PVC comes basically in two forms: Rigid (PVC-U) and flexible (PVC-P). The material’s behaviors are quite different.

PVC-U is considered a rigid material. The application for this material should be analyzed before choosing it for a project. Solvents easily penetrate PVC-U and it gets brittle in cooler environments etc. Welding is a little bit more difficult than PE or PP materials. There are many areas that are perfect for PVC-U, but you need to understand the customer’s needs and the material’s properties, to choose the right plastic. Temperature, solvents or other fluids, fumes, etc. will have an impact on the material.

When working with sheet material, the sheets are already stressed due to the manufacturing process. Normally, we work with extruded sheets, so all the molecules are orientated in the flow direction. Generally you cannot see the flow direction on the finished sheets. This will cause a problem if you did not temper the sheet before cutting the part. You will observe that one side will be shorter than the other side. Tempering the sheet releases the stress in the plastic. Tempering or annealing, is heating up the plastic carefully in an oven and cooling it down very slowly. Once the material is cool, most of the extrusion stress will be relieved. The PVC-U will still be quite brittle, especially in a cool environment, but not as much as without tempering.

PVC-P is used mostly for gaskets or flexible joints. Because of it flexibility its can be tricky to work with. A heated wedge can be very helpful to tack-weld parts together or to perform overlap welds. When using PVC-P welding rod, it helps to use a stick welder and a roller to apply the necessary pressure and to guide the welding rod. Nevertheless, PVC-P will get very brittle over time, especially when heat and/or chemicals are involved. To give the PVC flexibility, certain additives are mixed into the resin. These additives disappear with time, so PVC-P needs to be replaced periodically. The time frame depends on the environment and application.

For more information, click on the Author Biography link at the top of this page