How-to: Welding plastic ventilation ductwork

Chemically resistant ventilation duct fittings

Have you ever wondered about the ventilation system that was used in the swimming pool or laboratory? These types of facilities are exposed to various highly toxic substances and chemical compounds. The ventilation system must be highly resistant to an aggressive and humid environment.

The most important aspect for the quality of the working ventilation system is an installation made of appropriate materials, observing the principles of correct assembly and using proven fasteners. Innovative materials which meet that specific requirement are plastics. Installation and connection of such a system may raise some doubts- nothing could be more wrong.

In this article, you will learn the techniques of welding plastic ventilation ducts and find out what are the stages of preparation of the surface of joined elements. 

The choice of welding method is determined by the form of the joined elements, applications, operating conditions, chemical structure and physical properties of the joined materials.

How to join plastic ducts and fittings

  • glueing,
  • hot gas extrusion welding
  • hot gas welding,
  • mechanical joining: flange connection using acid-resistant screws. This types of connection protect the components of the duct against displacement.

Preparation of the welding surface:

1. Welding preparation of elements includes operations and procedures such as: cutting out patterns from plates and hard foils, cutting pipes.
2. During using fittings with the same outer diameter are it must be chamfered first.
3. Over beveling, the entire wall thickness should be considered.
4. Before welding, make sure that there is no dirt, grease or oxide layer on any of the items. The material should be removed completely.

Preparation for joining plastic ductwork

It is essential that the surfaces to be welded are not damaged and cleaned
(e.g. dirt, grease, hand sweat and oxide layers must be removed)

Plastic welding
is described by plasticizing joined surfaces and the joining material (binder, welding wire) using heat and pressure. The surfaces are heated above their melting point and bonded together under pressure so as to form a uniformly homogeneous joint.

What is important during plastic welding?

  • The joining process should be carried out at room temperature to achieve the best results.
  • The condition for welding is that the welded ventilation parts are evenly fitted/arranged.
  • The standard rule is to keep plastic parts in a fixed place and position.
  • By minimizing the gap between the welded elements and levelling the space between the socket and the connector we can obtain uniform weld.

Plastic welding process

The welding process should be carried out by a qualified welder, who has experience in the field of welding materials.

What you will need during the process of plastic welding:

  • ventilation ducts and fittings 
  • If on-site modifications of the fittings are necessary: Handsaw or circular saw, jigsaw or bandsaw,
  • Extrusion welding device or hot gas stream welding device depending on wall thickness,
  • Scraper,
  • Crescent blade knife,
  • Appropriate Protective Clothing.

Hot gas welding equipment

We offer our clients a complete set of welding equipment - Autotherm3 - Wegener for rent.

Hot gas extrusion welding

In extrusion welding, the plasticized binder is extruded from the extruder. The weld is formed with a special head between the plasticized hot gas and the joined surfaces. This is a partly mechanical connection path for most thick-walled materials, where a large volume homogeneous weld is required. The plasticizing depth is 0.5-1.0 mm, which means that the welding time is shorter. Good weld quality means that it has: high mechanical strength and low internal stress. These properties affect the tightness of the installation. A characteristic feature of extrusion welding is the laying of only one filler stitch.

Hot gas extrusion welding

Extrusion welding is a more advanced and efficient method, it is possible to create very thick welds during a single run - 
using extrusion welding You can weld the material with the thickness up to 40mm.

Hot gas welding

A wire is a form of connecting material, usually made of the same material as the joined elements. The joining surfaces of the base material and the welding filler are plasticised by hot gas, generally air. The beak-shaped attachment at the end of the nozzle applies the necessary joining pressure. The nozzle guide evenly pre-heats and plasticises the base material and the welding filler.

Diagram of wire welding in a stream of hot air

Diagram of wire welding in a stream of hot air
a - welding wire; b -hot air; c -  weld; d -  joining elements; e - welding nozzle.

When welding elements are bigger than 2 mm thick, several layers of binder are successively applied, because it is impossible to fill the entire groove volume in one operation due to the limited heating time. The important things during hot gas welding are the right choice of hot gas temperature, uniform welding speed and pressure, so that a proper inflow and outflow occurs, which guarantees a good weld. The shape of the cross-section of the bars depends on the intended use. The most common welding wires are circular and triangular cross-section, the typical diameter range is 2 to 5 mm.

The number of adhesive layers of  welded elements

Dependence of the number of adhesive layers on the thickness (mm) of welded elements.

Oval rods are also available. Both the joining material and the welding wire must be plasticized to the same extent, otherwise, there will be no diffusion (the process of particle propagation) and the weld will be defective. It needs to be highlighted that the temperature of the material during welding depends on the gas temperature and welding speed. For example the time the hot gas will interact with the material. At higher welding speeds, the gas temperature should be higher than at lower welding speeds.

Comparison of Hot gas extrusion welding vs. Hot gas welding 

Hot gas extrusion welding Hot gas welding
Laying only one fill stitch. Precise pressure of the welding wire is necessary.
Usually, it is necessary to lay several stitches.
High aesthetics of conducted works. The important thing is the welding speed.
High power and efficiency of the welding machine. Slower process flow.
Independent exhaust temperature and mass control. It is widely used during welding smaller elements

The choice of the joining method depends on the form of the joined elements, application, operating conditions as well as the chemical structure and physical properties of the joined elements.

Chemically resistant plastic ductwork catalogue
Chemically resistant plastic ductwork catalogue
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Alnor Ventilation Systems
Krakowska 10 Avenue
05-552 Wola Mrokowska

tel. +48 22 737 40 00