TWI Knowledge Summary

Infra red welding of plastics

The process

Infrared welding is a relative newcomer to the field of commercially developed welding techniques for thermoplastics. The possibility of using this heat source for processing polymers has been known for many years and there have been applications in the area of preheating of polymers prior to compression moulding or vacuum forming since the 1960s.

For the welding of thermoplastics, two different approaches to infrared welding have emerged, both based around the principle of hot plate welding.

Non-contact hot plate

This system uses an electrically heated metal plate, which in some cases is coated with a ceramic. The hot plate is heated to a temperature between 310°C and 510°C, depending on the thermoplastic to be welded, and the size of the welding machine. When the parts to be welded are brought in close proximity to the hot plate, (typically 0.2mm) but without touching, they heat up due to radiation and convection and subsequently soften and melt. The plate then withdraws and the parts are forced together to form a weld.

IR lamp welding

In this system, the standard heater plate is replaced with two banks of short wave infrared emitters clamped and spring-loaded on either side of a movable platen. Powers can be very much greater compared with conventional hot plate welding but weld times are significantly shorter. This technique is also capable of handling large surface area products, as it is a simple operation to add more emitters to the heating bank.

The newly developed, high power short wave infrared emitter is also proving more efficient and effective than infrared emitters previously considered for welding applications. Its high power density, developed at a lower operating temperature, means that it can transfer energy more efficiently than halogen emitters, while its lower mass filament makes it more responsive than ceramic emitters.

Advantages

Infra red 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 from the hot plate entering the weld.

One further advantage claimed by the infrared welding process is the ability to weld low modulus materials such as thermoplastic elastomers. This is because there is little or no shear of the weld piece during heating.

Typical applications

Infra red welding has been used for the joining of plastics pipes, since the early 1990s. Other potential applications include welding of uPVC window frames, biomedical products where high production rates of welded seals are often required in flexible materials, and in the fabrication of laminates or in the manufacture of parts from thermoplastic composite materials, where it is able to achieve high energy densities over large areas.

References

[1] Taylor N S, Klaiber T and Wermelinger J, 'Assessment of PP and PVDF joints made by a new infrared jointing system'. SPE ANTEC 1993, pp 2647-2656.

Further information

Additional information about infra red welding can be found in the items detailed below:

Butt and socket fusion welding of plastic pipe - a guide to best practice

Thermal welding of polymers (Technology Briefing 667, Jan.1999) available to TWI Industrial Members only

Tomorrow's solutions today - new plastics joining methods (Bulletin, July 1997) available to TWI Industrial Members only

You can use the Weldasearch literature database to supplement what you find in JoinIT.

For specific enquiries please contact the Polymer Group at TWI (e-mail: pol@twi.co.uk).