Plastics Fabrication
With hot air tools
by Dave Rothbard

oday, there exists a desire — from either a creative or a business perspective — to manufacture products with rigid thermoplastic, using a process commonly referred to as plastic fabrication. This term describes a variety of procedures and processes. This article focuses on describing the process of the joining/welding of rigid thermoplastics using hot air tools, specifically hot air portable extrusion welders. There is another popular fabrication process using hot air hand tools that lends itself to certain situations and applications, but that’s a subject for future consideration.

The three main variables that must be controlled in the portable extrusion welding process are heat, speed and pressure. The process involves a handheld or automated robotically held extrusion welder that uses a welding rod. This welding rod is drawn into the welder, macerated, melted and screw-extruded into a weld bead (commonly referred to as “the melt”). The bead exits the welder into a profile created by a polytetrafluoroethylene (thankfully shortened to PTFE) welding shoe attached to the front of the tool. At the same time, hot air (commonly referred to as the pre-heat) is blown onto the substrate being welded. Just enough force is exerted, by physically pushing and/or using the weight of the welder, to interlock the molecules of the melt and the preheated substrate while they are both in a melted state, creating a molecular bond upon the cool down of the material. It is important that the cool down process occurs uniformly so as not to create a vacuole (void) inside the weld bead due to the thermal expansion properties of heated thermoplastic and subsequent contraction upon cool down.

The temperature of the pre-heat and the melt are controlled with varying levels of precision depending on the capabilities of the extrusion welder, with higher quality welders offering digital and individual controls for each. The speed is limited to:

  • The output/size of the welder
  • The amount of melt produced
  • The physics of the welding process (you cannot go too fast or the pre-heat will not be sufficient to melt the substrate)
  • The correct amount of pressure must be applied to force the melted material together, but not so much as to push the melt away from the substrate

The thickness of the material being welded determines the required size and capacity of the extrusion welder, while some thermoplastics such as polyvinylidene fluoride (PVDF) and polyvinyl chloride (PVC) require more precise temperature control and resistance to corrosion of the internal components of the extrusion welder.

Without droning on at length about the properties of thermoplastic at the molecular level, a key rule of thumb is: When joining two pieces of plastic and creating a molecular bond or weld, both pieces of material must be of the same plastic chemistry. Even then, within the same type of thermoplastic, variables such as the production process, batch and possible pigment added can have an effect on the welding process, with the closer the match resulting in the strongest possible the weld.

The most common types of thermoplastic being welded in the field using hot-air portable extrusion welders are polypropylene (PP), polyethylene (PE), PVC and PVDF. Each one of these chemistries has subgroups of various types, creating an alphabet soup of acronyms, and although I have minimum word count to achieve for publication, I am remaining true to the simplicity theme and limiting this discussion to broad terms such as PP, PE, PVC and PVDF.

Each thermoplastic material has unique welding characteristics and specific temperature ranges required for the preheat and the melt to achieve a proper weld. PP and PE are the most common and the most forgiving with a comparably wide range of melt temperatures and a similar pre-heat temperature. They are also the least corrosive to the internal parts of the hot air extrusion welder.

Precise digitally controlled spiral tank welding with Leister’s powerful and ergonomic WELDPLAST S2.
Precise digitally controlled spiral tank welding with Leister’s powerful and ergonomic WELDPLAST S2.
PVC, on the other hand, has a narrow melt temperature range of approximately +/- 10 degrees F. This requires a tool with a precise digitally controlled temperature capability. In addition, the delta between the melt temperature and the preheat temperature for PVC is wide, requiring a tool that is not only precise in the control of the temperature, but also able to control the melt and pre-heat temperature individually and able to physically separate the two as much as possible. It is difficult to maintain large amounts of PVC at a stable melt temperature in the barrel of the welder over time, so there is a limit to the capacity of PVC extrusion welders. Since PVC is corrosive to the internals of an extrusion welder, the internal parts of the tool that come into contact with the PVC require a special coating and it is necessary to “rinse” the extrusion welder with PE or PP prior to letting it idle or shutting it off. Suffice it to say that PVC is more difficult to weld than PE or PP. PVDF has a high melt/preheat temperature and is an extremely hard material requiring ample pressure during the process, but not so much as to push the melt away from the substrate. Uniform cool down consideration is important due to the high melt temperature.
Pipe and tank welding is effortless with Leister’s lightweight and agile FUSION 1.
Pipe and tank welding is effortless with Leister’s lightweight and agile FUSION 1.
Finally, and circling back to the beginning of the extrusion welding process, it is extremely important to weld with a clean substrate that is free of oxidation. Thermoplastic oxidizes quickly when exposed to ultraviolet light/sunlight. To mitigate oxidation and clean the substrate prior to welding, it is vital to scrape the material just prior to the welding process by taking a thin layer of material away. This should be done mechanically with a razor-sharp scraping blade or burr bit, not with a solvent or abrasive such as sandpaper. Otherwise, you run the risk of introducing a foreign substance which will add an additional barrier to the material being welded.

So ends this high-level description of the hot air extrusion welding process. I hope you found it informative and useful.

Dave Rothbard is a product specialist for Leister Technologies, LLC. For more information, contact Leister Technologies, LLC at 1275 Hamilton Parkway, Itasca, IL 60143-1150 USA; phone (855) 534-7837 or (630) 760-1005, fax (630) 760-1001, david.rothbard@leister.com or www.leister.com.