Hot Gas Welding / Hot Air Welding

HOT GAS WELDING / HOT AIR WELDING:

1. Applies only to thermoplastics

2. Hot air/ Hot gas (Nitrogen) is used

3. The principle of welding with hot air is to make plastic flow at the conjunction of two pieces (or two pieces and a welding rod of parent material) such that under a small amount of pressure the material will intermix and fuse together to give a homogeneous bond.

4. Materials to be welded must be of the same type or similar flow behaviour.

5. The temperature of the hot air should be sufficient to make the plastic begin to flow

6. Temperature required varies with plastics, usually between 270 Deg-c  350Deg-c under 3 to 4 lbs/sq in pressure.

PREPARATION STAGES

Supply air to hot air gun should be free water or oil.

The material to be welded together and the welding rod must of the same grade of plastic.

Some plastic need special attention

PMMA, PC, ABS, easily absorb moisture and therefore they should be stored at warm and dry conditions prior to welding.

PE & PP may quickly oxidise. Therefore the surfaces to be welded, including the welding rod should either be mechanically scraped or cleaned with solvent such as Trichloro Ethylene or Acetone.

Square butt joints

The square-groove is a butt welding joint with the two pieces being flat and parallel to each other. This joint is simple to prepare, economical to use, and provides satisfactory strength, but is limited by joint thickness. The closed square butt weld is a type of square-groove joint with no spacing in between the pieces. This joint type is common with gas and arc welding.

V-joints

Single butt welds are similar to a bevel joint, but instead of only one side having the bevelled edge, both sides of the weld joint are beveled. In thick metals, and when welding can be performed from both sides of the work piece, a double-V joint is used. When welding thicker metals, a double-V joint requires less filler material because there are two narrower V-joints compared to a wider single-V joint. Also the double-V joint helps compensate for warping forces. With a single-V joint, stress tends to warp the piece in one direction when the V-joint is filled, but with a double-V-joint, there are welds on both sides of the material, having opposing stresses, straightening the material.

J-joints

Single-J butt welds are when one piece of the weld is in the shape of a J that easily accepts filler material and the other piece is square. A J-groove is formed either with special cutting machinery or by grinding the joint edge into the form of a J. Although a J-groove is more difficult and costly to prepare than a V-groove, a single J-groove on metal between a half an inch and three quarters of an inch thick provides a stronger weld that requires less filler material. Double-J butt welds have one piece that has a J shape from both directions and the other piece is square.

U-joints

Single-U butt welds are welds that have both edges of the weld surface shaped like a J, but once they come together, they form a U. Double-U joints have a U formation on both the top and bottom of the prepared joint. U-joints are the most expensive edge to prepare and weld. They are usually used on thick base metals where a V-groove would be at such an extreme angle, that it would cost too much to fill.

Tee-Joints

The Tee Weld Joint is formed when two bars or sheets are joined perpendicular to each other in the form of a T shape. This weld is made from the resistance butt welding process. It can also be performed by Extrusion welding. Usually two flat pieces of poly are welded at 90 degrees to each other, and extrusion welded on both sides.

Application

1. Containers 

2. Tanks for storage chemicals; 

3. Ventilation ducting; 

4. Tubes; 

5. Repair works.