TWI Knowledge Summary

Hot bar and impulse welding of plastics

Summary

Hot bar and impulse welding, sometimes referred to as heat sealing, are techniques for joining thermoplastic films typically less than 0.5mm thick. A heated metal tool applies pressure to the two films, melting the plastic at the joint and creating a weld.

Scope

The equipment consists of one or two electrically heated bars, one of which is hinged for the insertion and removal of the films. The film is placed on the base plate, the upper bar is brought down and pressure is applied either mechanically by the operator, or pneumatically. After the required weld time the bar is lifted to release the joined film. The bars are often PTFE coated to prevent molten polymer from adhering to them.

Diagram of impulse welding

Hot bar welding is a rapid process. Typical weld times for a thermoplastic sheet of 100µm in thickness, are in the range from 1-3 seconds. A reduction in welding time can be achieved by the use of two heated bars, one on either side of the films to be joined. The quality of the weld produced is dependent on the welding parameters (bar temperature, weld pressure, weld time and cool time) and these should be optimised for each application. The necessity for the heat to conduct through the film to the joint, imposes a restriction on the thickness of material that may be welded. A practical limit of approximately 1mm exists.

Impulse welding is an advanced form of hot bar welding in which both the heating and cooling regimes are controlled whilst the joint is still under pressure.

The welding is effected by a pulse of electrical current which is passed through nickel-chromium resistance wires or ribbons located in either top and/or bottom jaws. These ribbons transfer heat at a very high temperature for a short period of time. The films melt at the interface and co-solidify during the cooling cycle whilst in the jaws to form a weld.

Materials

Films used for heat sealing applications are manufactured in two different ways:

• Mono layers - a single layer of material often with an adhesive (lower melting point) coating. Different coatings used include amorphous PET (used for high temperatures), EVA (used for microwave products) and PE.
• Laminates - multi-layer films formed from two or more different substrate materials. The sealant layer is dictated by the material, the type of seal needed ( e.g. weld or peel) and cost.

Polyethylene is widely used for heat sealed bags, sachets and shrink wraps. PP, PVC, PS, PA, aluminium foil and board materials are also used extensively.

Applications

Hot bar and impulse welding are most widely used in the packaging industry. The food sector in particular extensively uses the processes for the lidding of various containers such as pots and trays, as well as formation of pouches and sachets.

Many medical devices and products are also heat-sealed in packages of thermoplastic and plastic coated paper e.g. syringes, intravenous bags, Tablet blister packs.

Further information

Additional content on hot bar and impulse welding of plastics can be found in 'A review of joining processes for packaging with plastics' (May 1999)

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