PTFE Tubing: Process Parameters And Their Impact
PTFE Tube extrusion is among the most difficult processes within the polymer space. All polymers have their peculiarities and these certainly play a part in both their processing and machining. But PTFE tube comes with a set of so many different process parameters, that finding a combination that works consistently is something that not every tube manufacturer is able to discover. We have undertaken so many trials on tubes, each time assuming that we have looked into all the aspect. However, even after years of manufacturing, a new parameter may present itself that had hitherto gone unnoticed.
We would like to take a look at some of these parameters and their effect on the end-product:
- HandlingHandling resin is among the most easily overlooked aspects of PTFE processing. While many resin manufacturers specifically lay out guidelines for limiting the shear on the resin before processing, these become even more important where tubing is concerned. Due to the structure of PTFE tubing, the fibrils that form during extrusion are paramount to the strength of the final tubing. Excessive shearing of the resin before extrusion can cause a poor formation of fibrils and seriously hinder the achievement of good final properties
- BlendingThe parameters within blending include the type of extrusion aid used (the surface tension of the aid needs to be less than that of PTFE, while also not having a volatility and/or flash point that can cause fires during sintering), the amount of extrusion aid used, the RPM of the blending process and the post blending storage of the fine power mixture. Since our unit is in India, we need to follow a slightly different process to that in colder countries. For starters, we need to artificially cool the resin to allow of a more easy mixture of the PTFE with the extrusion aid. Such nuances are only learnt through extensive trial and error. But unless the blending is done in the correct manner, the final extrudate will be either too soft or too dry. Furthermore, unless the blend is uniform, the preform billet will have uneven densities, causing issues during extrusion.
- PreformingPreforming is done purely as a means to create a shape that can be fitted into the extruder. Preforming has two functions: first, it gives shape and second, it removes any air pockets from within the material. The process needs to be done keeping in mind that too little pressure will not allow for an adequate venting of the air within the material. Air pockets result in bursts during the extrusion, which damage the tubing and render it unusable. Too much pressure and the extrusion aid may get squeezed out of the preform, causing the extrudate to be too dry and increasing the extrusion pressure required to form the tube.
- ExtrusionWhile extrusion is understandably the most important step, by the time the preform billet is loaded into the extruder, the preceding processes have already defined a lot of the tube’s final characteristics. Nonetheless, extrusion offers the tube it’s final shape and this process needs to maintain both adequate pressure on the billet while ensuring the concentricity of the final tube. If the pressure is too high or too low, the tube will experience either too much shear, or too little pressure to form a proper end-product respectively. Concentricity is dependent not only on the tooling within the extruder (which needs to be precise and offer the correct extrusion angles depending on the size of the tube being drawn), but also on the uniformity of the billet’s density (discussed above). Finally, the extruder itself needs to be capable of offering a uniform load, so as to ensure the billet is under constant and non-erratic pressure throughout the extrusion run.
- SinteringWhen heating the tube, the temperature needs to account for both a drying section as well as a sintering section. The drying section needs to be warm enough to evaporate all traces of vapour from the tube. At the same time, if it is too warm, there is a risk of the vapours igniting.Sintering needs to account for the fact that if the tube is heated too quickly, there is a chance of over-sintering. Also, although PTFE does not melt, it may under its own weight, elongate during sintering, causing dimensional deviations. Therefore the temperature has to be sent to ensure that the PTFE reaches its ‘gel state’ just before it leaves the sintering chamber, so it can cool down at room temperature.
Aside from the above-mentioned parameters, PTFE tube also undergoes pigmentation, addition of anti-static fillers and extrusion of specific profiles. Each of these needs to re-look at all of the above processes and understand how they need to be modified to allow for a proper end-result.