Job Knowledge 55
The purpose of this article is to give an overview of the variety of techniques available to industry for the thermal joining of thermoplastics.
The techniques used can be divided into three distinct groups based on the method used to introduce heat to the weld. These are:
- by mechanical movement,
- by an external heat source
- from electromagnetism
Welding techniques where heat is generated by mechanical movement
In linear vibration welding the parts to be joined are brought into contact under pressure before being rubbed together in a linear reciprocating motion. The resulting friction melts the material at the interface after which the vibration stops; the parts are then aligned and held together until the weld solidifies.
Most thermoplastic materials can be welded using this technique, which is used extensively in the automotive industry for joining components such as two-part bumpers, fuel tanks, manifolds, rear light clusters and inner door panels.
In spin welding the joint areas are always circular and the motion is rotational. The technique has been exploited for applications as diverse as the manufacture of polyethylene floats, aerosol bottles, transmission shafts and PVC pipes and fittings.
Ultrasonic welding involves the use of high frequency mechanical energy to soften or melt the thermoplastic at the joint line. Parts to be joined are held together under pressure and then subjected to ultrasonic vibrations, usually at a frequency of 20 or 40kHz. Ultrasonic welding is a fast process, with weld times typically less than a second, and can be easily automated. It is a popular choice for assembling components in the automotive, medical, electronic and packaging markets.
Welding techniques using an external heat source
Hot plate welding is possibly the simplest plastic joining technique, used for various applications ranging from small automotive fluid reservoir vessels to pipelines in excess of 1000mm in diameter.
The technique involves heating the ends of the parts to be joined against an electrically heated platen until they are sufficiently molten. The heater plate is then removed and the parts pressed together. A cooling cycle follows, allowing the weld to develop strength.
Hot bar and impulse
This technique is mainly used for joining thermoplastic films with a thickness of less than 0.5mm. It works on the principle that if two films are pressed against a heated metal bar, they will soften and allow a joint to be made between them. Weld times are rapid, around two seconds for 100µm film.
The principle of impulse welding is the same. Here the heat comes from a brief burst of electrical energy through a nickel chromium wire triggered as the films are pressed together. This method is used in packaging for the rapid sealing of polyethylene bags.
In hot gas welding of thermoplastics, the parts to be joined, typically sheet sections up to 30mm in thickness, are prepared in a V-butt or T-butt configuration before a stream of hot gas is directed towards the joint area. This causes melting of the joint area and also of a consumable filler rod of the same polymer type as the parts being joined. The weld is formed from the fusing together of the joint with the filler material.
The main advantage of hot gas welding is that the equipment is easily portable. However, the process is slow and weld quality depends greatly on the skill of the operator. Training and Certification of operators is recommended to achieve high standards.
Extrusion welding is similar to hot gas welding, sharing some of its characteristic advantages and disadvantages. Molten thermoplastic filler material is fed into the joint preparation from the barrel of a mini hand-held extruder based on an electric drill. The molten material emerges from a PTFE shoe shaped to match the profile being welded. At the leading edge of the shoe a stream of hot gas is used to pre heat the substrate prior to the molten material being deposited, ensuring sufficient heat is available to form a weld.
The process is used typically for assembly of large fabrications such as chemical storage vessels, with wall thicknesses up to 50mm.
Welding techniques which directly use electromagnetism.
This involves trapping an electrically conducting implant between the two parts to be joined before applying a high electric current to cause resistive heating. As the implant heats, the surrounding thermoplastic material softens and melts. Application of pressure ensures the molten surfaces fuse together to form a weld.
A widely used application of resistive implant welding is the electrofusion technique for joining thermoplastic pipes using specially designed socket couplers containing an integral electrical heating coil.
Induction is similar to resistive implant welding as an implant is generally needed at the joint line. However, in this process a work coil connected to a high frequency power supply is placed close to the joint. As high frequency electric current passes through the work coil, a dynamic magnetic field is generated whose flux interacts with the implant. Eddy currents are induced in the implant, heating it and the surrounding joint area.
High frequency (dielectric)
High frequency (dielectric or radio frequency) welding relies on the ability of the plastic being joined to generate heat in a rapidly alternating electric field. Hence the technique is generally restricted to PVC, EVA and polyurethanes.
During the process, the parts to be joined are subjected to a high frequency electric field applied between two metal bars. The dynamic electric field causes molecular vibration in the plastic. Some of the resulting oscillatory motion is converted into thermal energy, causing the material to heat.
Products manufactured by high frequency welding include stationery wallets, inflatables, tarpaulins and blood bags.
During infrared welding the parts to be joined are brought into very close proximity with an electrically heated platen. The technique is similar to hot plate welding although no actual physical contact is made with the heat source. After sufficient time has elapsed the parts become molten and can be forced together to form a weld.
Infrared welding is generally faster than hot plate welding with typical welding times being reduced by around 50%. The fact that heating is achieved without physical contact eliminates the possibility of contamination entering the weld from the surface of the hot plate. The technique is used for joining thermoplastic pipes.
The laser welding technique uses a focused beam of intense radiation, usually in the infrared area of the electromagnetic spectrum, to melt the plastic in the joint region. The type of laser used and the absorption characteristics of the plastic determine the extent of welding possible.
Clearweld® transmission welding, recently patented by TWI, uses a colourless infrared absorbing medium at the joint interface of two transmissive plastics. Thus two optically clear plastics may be laser welded with an almost invisible joint.
Laser welding has the advantage of being a quick, clean, non-contact process which generates minimum flash and distortion.
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