Hot Air Welding Compared to Solvent Welding

Product Stories23 Mar 2021

In this technical article, Leister's roofing expert, Roland Beeler, compares plastic welding techniques: hot air welding vs. solvent welding. This much in advance: hot air welding has many advantages.

Authors: Roland Beeler, Head of Plastic Fabrication, Roofing & Flooring Business Lines and Silke Landtwing, Corporate Communication Manager, Leister Switzerland

We have compared the two processes since we are asked time and again what advantages hot air welding has over solvent welding. The advantages of hot air welding outweigh the disadvantages in terms of speed, welding material, quality/time, application, ergonomics, and safety.

Before we go into more detail about our comparison, we will introduce you to the two processes.

Plastic welding with hot air

In our case, hot air welding means welding and joining plastics. For example TPO, PP, PE, PVC, EVA, PIB, modified EPDM as well as bitumen, using environmentally friendly Leister hot air or hot wedge technology.

Let's take one example: Overlap welding roofing membranes. Leister offers various automatic roof welding machines for this application. Using hot air, the membranes consisting of the plastics listed above or of modified bitumen, are tightly joined together in an overlap welding process. First, the membranes are heated with hot air at a temperature of approx. 400 °C to 550 °C / 752 °F to 1022 °F (depending on the material) until they melt. They are then pressed together firmly, creating a molecular, homogeneous bond in a matter of seconds.

In the videos you can see the UNIDRIVE 500 semi-automatic welding machine and the VARIMAT V2 automatic welding machine joining roofing membranes.

Apart from welding roofing membranes, welding plastic with hot air is possible with various technologies. At Leister we offer the following: Hot air, hot wedge, various laser welding processes and infrared radiation.

None of these technologies use toxins, chemical additives or open flames.

Solvent welding

Solvent welding is a seam joining technique for thermoplastic membranes made of PVC. The clean edges of the PVC membranes to be joined are coated with a suitable solvent (also known as solvent welding agent), for example tetrahydrofuran. The swelling solvent dissolves the edges due to a chemical reaction and then bonds them together by applying uniform pressure.

Important: Evaporating swelling solvent is harmful to one's health. If used improperly, it causes severe eye irritation, drowsiness and dizziness. It is also explosive because both the liquid and the vapor are highly flammable. Therefore, solvent welding can only be carried out outdoors or in well-ventilated rooms. In addition, swelling solvent must be transported and stored like a hazardous substance. For this purpose, users must abide by the applicable country-specific safety guidelines on handling and transporting hazardous substances.

Factual comparison on solvent welding versus hot air welding

In the table below, we have provided a factual comparison.

See the comparison in the video


We have also provided a video comparison.


The comparison illustrates the advantages of hot air welding compared to solvent welding. We understand the argument that such an automatic welding machine can get pricey, however, it will reliably accompany you for close to a lifetime. The purchase will pay for itself sooner or later, depending on how often you use it.


If you have any questions about plastic welding, hot air welding and/or would like advice, our roofing expert, Roland Beeler, would be happy to help you.