Welding of Plastic
The concept of plastic welding is nearly eight decades old, when German chemists and technicians began a strict search for substitute materials in 1933 due to a shortage of non-ferrous metals.
Two years later, in 1935 polyvinyl chloride became available on an industrial scale. Shortly thereafter, it was discovered that plasticized PVC could be molded, extruded and heat-formed provided it was properly stabilized.
Subsequent research work (the United States 1937, Germany 1938, and Great Britain 1939) revealed the fact that PVC can be welded under simultaneous application of heat and pressure.
The first patents on plastic welding were awarded to the Dow Chemical Company on 5 November 1940 in the United States, and to IG Farbenindustekt on 19 August in Germany.
Plastic gas welding was widely practiced in Germany during World War II, when the Germans introduced it to select firms in Austria, Belgium, France, Denmark, Holland, Italy, and Norway.
Initial work in plastic welding mainly focused on PVC involuntarily.
Today, welding is an important fabrication technique, with a heavy emphasis on rigid PVC and, to a lesser extent, on polyethylene. Without welding technology, many products associated with plastic products can never be fabricated.
Any welding process with plastic, involves the use of energy, chiefly in the form of heat, for fusing together the sheet material.
Types of Plastic:
Plastic can be grouped as follows:
Common weldable plastics
|1- polyvinyl chloride(PVC)
Types of Joint and weld
Joint designs similar to sheet metal are used for welding plastics,
Usually, all types of welds known for the welding of metals can be used for the welding of thermoplastics. such as:
Butt welds, fillet welds, lap welds, Corner welds, Edge welds, and Strip welds, etc.
Beveling of Joint
Beveling is very essential when trying to obtain quality welds. Square butt, tee, and (over) lap-fillet welds, however, do not require beveling.
Additional plastic can be added to the weld if required, for this purpose, a round rod, oval rod, triangular rod, or a flat bar is used.
A flexible flat plastic strip is used to weld or repair tank linings.
Plastics Welding Processes
Following basic welding process are used to join thermoplastics
1-Heated tool welding 2-Hot gas welding
3-High frequency welding 4-Ultrasonic welding
5-Friction welding 6-Induction welding
Heated tool Welding of plastic
In this process, heat is applied to surfaces that connect to or are in contact with a heat source such as a power strip
heater, a hot plate, a soldering iron, etc. By doing this, once the plastic is softened, the heat source is removed and the components are quickly butt welded or welded together under slight pressure and held until the joint is cooled.
An important factor in heated tool welding is the interval between the removal of components from the hot tool and the application of pressure.
This duration should be kept as short as possible because a delay at this point will produce poor welds performance.
Applications of Heated tool Welding
Heated tool welding may be applied to a wide variety of thermoplastics and is chiefly used for low and high-density polythene, plasticized PVC, polypropylene, and to a lesser extent, acrylics.
Rigid PVC is not generally suited to heated tool welding, as the material tends to degrade thermally on contact with the hot metal surfaces.
Heated tool welding lends itself to sheet fabrication in polythene and is widely used in welding pipe and large tubing.
Hot Gas Welding of plastic
The hot gas welding technique used a stream of hot gas to soften the base metal and filler materials.
Usually, two types of welding torches are used in plastic welding techniques, (a) Electrically heated and (b)Gas heated.
To be continued