Hot gas welding of thermoplastics
TWI Bulletin, May/June 1989
Martin Watson BSc, PhD, MWeldl, is Leader of the Plastics Joining Section at The Welding Institute.
Hot gas welding of thermoplastics is similar to the gas welding technique used for metals; as the gas used is often air, it is also known as hot air welding. Developed in the 1930s, it is the oldest welding technique for plastics, and today is in widespread use in many demanding applications. This article describes the process and some of its more important uses.
Hot gas welding
The process involves melting the weld area by a stream of hot gas from a hand torch. Filler material is usually fed into the prepared joint and the hot gas heats both filler rod and parent material, see Fig.1. The filler rod is not melted in the gas stream, but it is sufficiently softened to allow it to fuse with the parent material as it is forced under light pressure into the joint. The filler rod can be circular in section, typically up to 3mm diameter, but recently profiled rods have been introduced as they allow multi-run welds and fillet welds to be made more easily, see Fig.2.
Fig.1. Principle of hot gas welding of plastics
Fig.2. Sections of typical profiled welding rods. Dimensions in mm
Hot gas welding equipment is simple and cheap. The torch consists of a nozzle and heater unit to heat the gas and to direct it to the workpiece. Today the gas is normally heated by an electrical resistance heater, although in the past a gas flame was used to heat the welding gas indirectly. Almost any convenient gas can be used as long as it is clean and dry. Oxygen is claimed to produce the strongest joints, whereas those made with CO 2 are weak. Very often compressed air is adequate and this can be supplied locally by a blower unit, see Fig.3. Some plastics, notably polyethylene and acetal, oxidise easily so an inert gas such as nitrogen is used. The shape of the gas nozzle can be chosen to suit the weld preparation: standard nozzles for tacking or manual welding; or 'speed welding' nozzles with an opening through which the filler rod is fed. Speed welding is easier and faster than fully manual welding, but joint properties are relatively poor. Nozzle designs can vary depending on the material to be welded.
Fig.3. Equipment for hot gas welding incorporating a blower unit for compressed air supply
The only parameters which can be set on the equipment are the gas flow rate and temperature. Typically flow rates are 15-60 litre/min, and temperatures 200-300°C. The technique usually employed in manual welding is first to heat the tip of the filler rod, which is then pressed into the root of the weld. Welding then proceeds by heating both the joint and the filler at a uniform rate using an oscillating motion of the torch. Overheating the weld results in charring, blistering or foaming.
Underheating results in the filler and parent material becoming insufficiently soft, so that notches, cavities and bad adhesion occur. For good joints both parent material and filler rod must be clean and free from grease. Mechanical scraping is probably most reliable, but chemical cleaning with petroleum spirit is also widely used. Cleaning fluids that attack plastics, like acetone, are not suitable.
Hot gas welding is used mostly for butt and fillet welding, but also for lap welding of thin sheet. In lap and fillet welding no joint preparation is needed. In fillet welding the filler is fed directly into the 90° angle of the fillet, and for lap welding no filler is used. For butt welding, the edges of the parent material are bevelled by scraping, filing, turning or milling to give an included angle of 60-70° depending on thickness, and a root gap of 0.5-1 mm, see Fig.4.
Fig.4. Recommended joint designs for hot gas butt welds [4]
Applications
Hot gas welding is the only plastics welding process that is manual and fully portable. It is therefore often the only suitable technique, for example for large complex fabrications or for repair operations. However, it is a relatively slow process so it is not suitable for high volume mass production.
There are three main areas of application: rigid materials, flexible materials, and repair. About 80% of hot gas welding is performed on three materials: polyvinylchloride (PVC), polyethylene and polypropylene. Other materials that are often hot gas welded are polycarbonate, ABS, polymethylmethacrylate and fluorinated polymers.
Rigid materials up to about 50mm thick can be welded, although thicknesses less than 15mm are the most common. Typical applications are industrial plant, where plastics are used for corrosion resistance, for example fume extraction hoods, scrubbers, piping and ductwork. Similar uses include chemical process and storage vessels, and pipework, Fig.5. Hot gas welding is widely used to fabricate tanks for pickling and plating lines. On the continent the pipework for district heating systems is often welded using hot gas welding.
Fig.5. Hot gas welding of polypropylene chemical plant pipework using the speed welding technique and an integral blower welding torch Courtesy Welwyn Tool Co
Flexible materials, usually plasticised PVC, up to several millimetres thick can be lap welded. Typical applications include roof coverings, liners for tanks, refuse tips, swimming pools, fire ponds and so on ( Fig.6), truck tarpaulins, flooring ( Fig.7), conveyor belts, and cable coverings.
Fig.6. Hot gas welding of flexible PVC refuse tip lining Courtesy Welwyn Tool Co
Fig.7. Hot gas welding of floor covering using the speed welding technique
The portability and flexibility of the hot gas welding process make it ideal for repair operations. Repairs on welded plastics fabrications are common, but of growing importance is the use of the process to repair cars and other vehicles which now contain large numbers of plastics components. Automotive components that may require repair include bumpers and body panels, heating and air conditioning units, spoilers, batteries and oil and water reservoirs. For example, a small crack (100mm) in a bumper can be repaired in less than 10min by first making a weld preparation along the crack with a rotary burr, hot gas welding with matching filler, and then sanding off the excess material. After painting, the repair should be strong and cosmetically acceptable.
A good weld can only be achieved using a filler rod of the same composition as the parent material. This can pose a problem for repair welding as the type of parent material may not be known. The very large number of different plastics makes identification difficult, although with experience many of the commoner materials can be identified by burning a small piece with a match, observing the flow of the material, the flame, smoke and smell. Manufacturers often supply information on the materials used in cars, for example a 1986 Ford Sierra has polyamide (nylon) hub caps, filler caps, wheel trim and door mirrors, an ABS grille, and polycarbonate bumpers. Batteries, heaters and air conditioning units are often polypropylene; water and oil reservoirs are usually polyethylene; and lamp housings and instrument panels are often ABS. A useful trend is that manufacturers are beginning to mark the type of plastic on car parts. This is principally to allow the plastics to be recycled, but will help identification prior to repair as well. On the other hand, it can be very difficult to identify the material of an old piece of plant for which no data can be found.
Weld properties and quality
In spite of its long history and widespread use, it is generally agreed that the hot gas welding process suffers from a lack of standardisation and widely accepted procedures for fabricators to follow. Similarly there is very little technical literature on hot gas welding. References
[1-7] are those most commonly used for guidance. Some of these are old, although still very useful. Reference
[3] is the most authoritative, but Ref
[4] gives the most useful guidance, particularly on techniques for specific materials,
Fig.8.
Fig.8. Relationship between weld strength and angle of filler rod for hot gas welds in UPVC [4]
It is believed that most thermoplastics are weldable by hot gas welding, provided that care is taken to optimise the welding conditions. Short term weld strengths, as determined by tensile testing, can be as good as the parent material. However, long term strengths, as determined by creep tests, are more relevant to service performance, and Diedrich and Gaube, [8] in tests on polyethylene and polypropylene, have shown that the long term strengths of hot gas welds made under optimum conditions are considerably less than parent material values. Creep rupture tests were carried out at elevated temperature to shorten the test period, as is normal practice. The 'long term weld strength factor' was quoted, this being the ratio of the stress values at which equal life was obtained for the welded seam and for the parent material. Long term weld strength factors for three run welds in 4mm sheet were 0.6 and 0.65 for polyethylene and polypropylene respectively. In 20mm sheet welded with 15 runs the long term weld strength factors for both materials were only 0.35. These values were much poorer than with some other plastics welding processes: for example heated tool butt fusion welds gave long term weld strength factors better than 0.9 in the same tests. The author is unaware of any investigation that has explained the relatively poor long term performance of hot gas welds, so it is not clear whether it is an inherent feature of the process or not.
Good design will, of course, make allowances for the long term strength of welds. However, as it is a manual process, the quality of hot gas welds is entirely dependent on the skill of the operator. In critical applications it is essential that the highest quality welds are made and therefore, that the operators are well trained and understand what is required. In some countries there are recognised training and certification schemes for operators, but this is not currently the case in the UK. Unfortunately, there are no reliable or recognised non-destructive test techniques available at present, that can be used with hot gas welds to detect anything other than gross defects. One NDT technique which is used, particularly on chemical plant, is the spark test. This uses a high voltage probe to detect leak paths through the weld, but finer defects cannot be found. An elegant solution that has been adopted for roofing membranes and flexible liners, is to make two seams in parallel at a lap joint. The space between the seams can then be inflated so that any loss of pressure will indicate the presence of a leak path.
Extrusion welding
Extrusion welding is a welding technique for plastics that is related to hot gas welding and should therefore be mentioned here. It is not widely used in the UK, but has found favour on the continent for large fabrications because for thick material it is faster than hot gas welding. The equipment is heavy and bulky, so it is not as adaptable as hot gas welding, and although hand welding units are available, it is most often operated as an automatic or semi-automatic process, Fig.9.
Fig.9. Hand held extrusion welding equipment
In extrusion welding, instead of a solid rod, molten filler material is extruded into the joint. The parent material is softened by a hot gas jet or sometimes by radiant heaters. Long term weld strength factors are in the range 0.5-0.6 of the parent material value, [8] and are less affected by material thickness than are hot gas welds. Thus for thick sheets, say >10mm, extrusion welds are stronger than hot gas welds. Wolters and Venema [9] have pointed out, however, that extrusion welding equipment must be set up very carefully to avoid the presence in the root of the weld of notches, which can seriously reduce both short and long term mechanical properties.
Concluding remarks
Hot gas welding is a well established and simple process suited to the fabrication of complex structures and to repair operations. However, it probably deserves more attention in terms of procedure development, training and quality assurance.
References
| N° | Author | Title |
|
| 1 | | 'Recommended practices for joining plastics piping.' IIW Doc. XVI-322-78-E, 1978. | Return to text |
| 2 | BS 4994: 1973: | 'Specification for vessels and tanks in reinforced plastics.' | |
| 3 | | 'Welding of thermoplastics: principles.' DIN 16960, February 1974. | Return to text |
| 4 | | 'Thermal and chemical welding of plastics materials.' Engineering Equipment Users' Association Handbook No.35, London, 1976. | |
| 5 | Kaminsky S J and Williams J A: | 'Handbook for welding and fabricating thermoplastic materials.' Kamweld Products Co, Norwood, Mass., 1964. | |
| 6 | Thomas D W: | 'Making better plastic welds.' Laramy Products Co., Cohasset, Mass., 1962. | |
| 7 | Institute of Welding, Joint committee on the welding of thermoplastics: | 'Data on the welding of thermoplastics.' The Institute of Welding, London, 1966. | |
| 8 | Diedrich G and Gaube E: | 'Creep rupture strength and long term weld strength factors for welded high density polyethylene and polypropylene pipes and sheets.' IIW Doc. XVI-223-73-E, 1973. | |
| 9 | Wolters M and Venema B: | 'Mechanical properties of welds in polyethylene pipe systems.' Proc Conf 'Plastics Pipes V.' University of York, September 1982, Plastics and Rubber Institute, London, 1982. | Return to text |
