High power short wave infra-red emitters from Heraeus Noblelight Ltd, of Neston, are being used in an adapted rig at The Welding Institute, in Cambridge, to demonstrate and prove the benefits of infra-red non-contact welding of plastics over competitive techniques.
Heraeus Noblelight recently launched a range of infra-red emitters, specially developed for applications in the plastics industry. These can be supplied with different emission spectra to match the absorption properties of the material to be heated and in various shapes and sizes to meet specific heating applications and geometries.
The rig at the TWI is currently investigating infra-red butt welding of plastic pipe but it can be adapted to suit a range of products and plastic materials. Specifically, it consists of two banks of six short wave 1.5kW emitters clamped and spring-loaded on either side of a movable platten to give a total installed power of 18kW. (This equates to less than 2kW for a hot plate system, as the power is required for only one tenth of the operating cycle.) The movable platten is advanced to its working position between the butt ends of the pipe to be welded and the power is switched on. The platten is then retracted and the power switched off, while the two butt ends of the pipe, whose facing circumferences are now softened, are brought together under pressure to effect the weld.
It has been found that weld times of around 5 seconds can be achieved and that the technique is probably applicable to most polymeric materials in use. It offers a number of significant advantages over competitive techniques such as hot plate, vibration and friction welding and plastics gluing. Being a non-contact method, it suffers none of the disadvantages of hot plate methods, such as plastic adhesion to the heating surface, which necessitates frequent tool cleaning. It is also capable of handling large surface area products, as it is a simple operation to add more emitters to a heating bank. In addition infra-red heaters can be custom-built to match the geometry of the product to be welded.
The newly developed, high power short wave emitter is also proving more efficient and effective than infra-red emitters previously considered for welding applications. Its high power density, developed at a lower operating temperature, means that it can transfer energy much more efficiently than halogen emitters, while its lower mass filament makes it much more responsive than ceramic emitters. Energy efficiency is also another advantage it enjoys over other techniques, as power is used only during the short times that the system is working. (With hot plate systems, the power must be switched on continuously throughout a working shift to ensure that the plates are at the right temperature. With the high power infra-red system, the installed power is drawn, on average, for only one tenth of the operating cycle.)
The TWI infra-red welding rig is available for proving trials on a variety of plastics and product geometries.
Apart from high power short wave emitters, specifically developed for plastics welding, Heraeus also offers a range of infra-red emitters, which can be matched with virtually any plastic formulation and plastics processing technique. These include medium wave emitters to heat the surface of a product for adhesion processes and drying and to heat thin materials and foils; and short wave emitters to provide uniform, volumetric heating.