Extrusion and Hot-Air Hand Welders

A technician is using a green LEISTER hot air welding tool to fuse or repair plastic joints on a large black industrial tank inside a workshop.

With plastic welding, workpieces made of thermoplastic are joined inseparably to one another using a combination of thermal energy and pressure. Central factors are welding speed and the length of the welding process. Plastic welding is used in many areas: For the processing of tarpaulins and plastic sealing sheets, on the roof, in earthworks, hydraulic engineering, or tunnel construction, for floor coverings, in vehicle repairs, and in equipment construction.

Welding Seam Geometries Galvanic Tank

Various seam geometries are used in the welding of galvanic tanks.

An illustration displays a large plastic tank with highlighted weld seams, surrounded by diagrams of various thermoplastic welding joint types labeled A–E, such as fillet welds, corner welds, and butt welds, showing typical applications of plastic tank fabrication and repair.

A. Fillet Weld

The fillet weld is one of the most frequently-use seam geometries. It is produced by welding two workpieces that meet in a T-joint.

B: Interior Corner Seam

Interior corner seams are generally used on difficult-to-reach locations. Free forms and spline-shaped weld seam geometries are welded most efficiently like this.

C: Corner Seam Appearance

The outer corner seam is a fillet weld in which the weld seam runs along the edge of the workpieces which are standing together. Consequently, the weld is made along the outer longitudinal side (edge).

D: X-Seam (also Double-V Seam)

The double-V seam is also known as an X-seam. It is a type of butt weld and consists of a combination of two V-seams on each of the two sides of the components to be joined.

D: V-Seam

In order to achieve the V-shaped angle that is typical for the V-seam, the workpieces are either beveled or positioned at an appropriate angle to each other.

E: Lap Seam

Lap seams are mainly used for plastic sheets. Here, the sheets are arranged on top of each other and the weld seam is laid on the upper exposed material edge.

Thermal Joining of Plastics

Plastic welding requires a correspondence between the three welding parameters temperature, pressure and speed. In contrast to other joining methods, welding can achieve high resiliency and a strong, homogeneous welding seam. Plastic compounds are extremely robust and perfectly sealed when processed correctly. They can also be repaired without a loss of strength.

Hot Air Hand Welding

Hot Air Hand Welding is used primarily for areas that are difficult to access and for short seams. This welding process is preferred for processing amorphous plastics, in particular PVC. Particular attention must be paid to uniform pressure and constant speed when welding by hand. During the welding process, press the welding rod vertically onto the seam. The force required depends on the selected base material and the size of the welding rod

The heat escaping from the tubular nozzle should be applied alternate to the welding rod and joint in an oscillating motion in the direction of welding until the seam end is reached. When achieved correctly with the right temperature and appropriate pressure, a welding seam is formed on both sides of the weld bead in the form of a uniform double bead.

Diagram illustrating the welding process, showing the welding rod, contact pressure, nozzle, pendulum motion, preheating, and welding direction on the base material.

Hot Air Speed Welding

Hot Air Speed Welding requires a speed welding nozzle that matches the shape of the filler material. The process is faster, more uniform, and thus more efficient than free hand welding. In addition, larger cross-sectional areas of the welding rod can be processed in a single operation. This results in less residual stress and less welding work. Hold the welding tool with one hand and press the welding rod into the nozzle with the other hand. The nozzle shape divides the hot air, which thus melts both the base material and the filler material. The latter is led through a preheating chamber and plasticized shortly before the two materials meet. The tip at the end of the nozzle is responsible for the welding force. If necessary, rework the resulting weld seam after the welding process with an appropriate scraper.

Schematic diagram illustrating a plastic welding process, showing the welding rod, nozzle, draw welding nozzle, preheating airflow, contact pressure, and direction on the base material

Hot Air Extrusion Welding

Hot Air Extrusion Welding is preferred over hot air hand and draw welding for wall thicknesses from around 6mm. Compared to manual welding, extrusion welding is expected to result in shorter working times, higher mechanical strength and lower residual stress. This leads to greater process safety and efficiency. This requires a welding shoe that matches the welding geometry and a filler material made of the same plastic as the base material, plasticized in the extruder. First, the joining surfaces are heated with hot air to a thermoplastic state. The extrudate is then immediately applied to the surface or into the joint using the welding shoe. Different pressure is required depending on the working position. The welding speed is determined by the amount of extradent as well as the dimensions of the weld seam.

Welding Parameters for Hand Welding

Based on DVS 2207-3

Welding parameters for hand welding

PDF

Welding Parameters for Extrusion Welding

Based on DVS 2207-4

Welding parameters for extrusion welding

PDF

Welding Errors

In addition to a failure to adhere to the welding parameters, the following errors can lead to cavities, vacuoles and poor weld quality:

Excessively high temperature
Residual moisture in the welding filler
Excessively high air humidity
Wet hands
Excessively cold welding shoe
Low-quality plastic

Diagram showing the cooling process of a hot weld seam, illustrating how cooling leads to negative pressure, formation of a solid edge layer, and the creation of cavities or vacuoles within the seam.

Base material and welding filler made of polyolefins can absorb moisture. The thicker the seam, the more frequently these phenomena occur. For this reason, you should store materials in a dry place and in their original packaging. You should avoid temperature differences between the welding parts to prevent the formation of condensation. Very thick welding seams must be welded in several work steps.

Vacuoles are caused by the excessively fast cooling of large weld seam cross-sections.

Rough surfaces on the seam can therefore be because...

...the welding shoe is too short.
...the welding shoe is too cold.
...the surface over which the welding shoe glides is too rough.

Image showing a person using a plastic extrusion welding gun on a transparent panel, with side-by-side comparisons of poor and optimal weld seam quality, indicated by a red cross for the defective seam and a green check mark for the proper seam.

Your contact

Leister Academy

The Leister Academy is your central hub for effective knowledge transfer, training and professional development. Here, we provide Leister employees, distributors and customers with practical know-how and a continuously developed training program to ensure a high level of competence and safety in the use of our Leister products.

Contact