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Hot Gas Welding

Hot Gas Welding - Hot gas welding is one of the most widely used thermoplastic welding processes.
Principle of operation
Hot gas welding is a technique which basically consists of using a stream of hot gas to soften both filler rod and parent metal. The filler rod becomes tacky on its surface when suitably softened and the same applies to the surface of the material to be welded. The tacky surfaces bond together under pressure of shoe.

Equipment required
1. Welding Torches. There are two basically different types of torches, the gas operated type and the electrically heated type.

(a) Gas heated torches
Advantages
(i) Light weight,
(ii) Few replacements,
(iii) Independence of power supplies.

Disadvantages
(i) Cannot be used in confined spaces or on premises where open flames are not permissible.
(ii) Cannot be used for tank lining as the heating flames penetrate through the protective cover causing overheating of the plastic in the corners.
(iii) Bulky compared with electric torches.

Use
They are mostly used in the field or when electricity is not available. The gas heated torches use either propane or butane as fuel heating gas.

(b) Electrically heated torches
Advantages
(i) Less drag on the operator's hand (only one length of tubing and one cable carried).
(ii) Risk of explosion avoided.
(iii) No irritating gases emitted.

Disadvantages
1. Risk of holding a live tool in hand unless a low voltage supply is used.
2. Welding Tips (Flat tip or speed tip).
They are available in a variety of different sizes and shapes depending upon the nature of the welding job.

3. Heating Gas
Air is a suitable heating gas for PVC but Nitrogen is generally used for oxygen sensitive plastics such as polythene. Hydrogen may also be employed as a heating gas but generally com­pressed air provides a satisfactory and economical heating medium.,
4. Air regulator and pressure gauge
The degree of surface heat obtained in welding plastics depends not upon the rating of the heating element, but upon the flow of compressed air (or inert gas). Therefore, temperature control is permitted with the air regulator and pressure gauge. The faster the rate of flow of air, the lower the temperature will be.A small compressor provides the compressed air flow. The compressor may be integral with the torch.

Welding Procedure
Before welding, the edges of plastic materials must be beveled using a saw jointer, sander or block plane. Single- Vee butt joint with a 6()0 included angle and equal root faces and root gaps of 0.8-1.6 mm, depending upon sheet thickness, is probably the best type of joint.

The surfaces to be joined must be free from all contaminants i.e., dirt, dust, oil, moisture and loose particles of the material. Mechanical roughening and solvent cleaning of the surfaces are commonly employed. Solvents, however, may be undesirable with certain plastics as they may tend to soften the edges and cause poor welds.

Tack Welding is done to hold the sheets in proper alignment until the final weld is made. Tacking may be done by using a tacking tip on the welding torch.

 
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