Material Curing with Induction Heating

Curing involves any process where heat is used to catalyze or initiate chemical and molecular level structural changes in a polymeric materials such as epoxies, phenolics, polyesters and silicones. These materials are applied in many ways to various products for bonding, protective coating, sealing, insulation and other uses.

Air curing and hot fan curing are often used for curing small production runs, despite inconsistent results. Large production runs often are cured in batches in large ovens which must be run continuously. Induction heating provides a much better solution for adhesive curing.

Modern induction heating can solve many of these problems. Heating with induction provides reliable, repeatable, non-contact and energy-efficient heat in a minimal amount of time, so that the curing process can be completed with minimal energy and time. Improved temperature ramping cycles can be achieved with computer control of the solid state power supply. To eliminate extra steps for loading and unloading ovens, induction heat stations can be incorporated into a production line. Finally, induction heating can be performed in extremely clean environments, vacuum conditions or special atmospheres, allowing for unique curing solutions.

Although induction heating is normally used with metals or other conductive materials, plastics and other non-conductive materials can often be heated very effectively by using a conductive metal susceptor to transfer the heat. Typical RF power supplies for curing applications range from 1 to 5 kW, depending on the parts and application requirements.

Induction heating is widely used for adhesive bonding of metal parts at temperatures of approximately 200°F. Adhesive bonding relies on the localized heating of a metallic substrate to achieve accelerated melting/curing of the adhesive. The adhesive bond in this process is cured from the inside out, an obviously desirable condition. All trapped gasses are removed from the adhesive.

Induction heating of metal parts to adhesive curing temperatures is utilized in a many automotive processes, such as the use of thermosetting adhesives to produce clutch plates, brake shoes and auto bumper components. Shafts are typically bonded to the squirrel cage rotors in the manufacture of small motors. In copying machines, plastic components are adhesively bonded to aluminum rotors; a thermoplastic glue is used to hold foam rollers on metal shafts. Once the rollers wear out, the shaft is heated and the foam replaced.

These components have been dried by having their centre portions heated to a temperature that is above the temperature that would normally destroy the sealing compound around the edge.

This high temperature slowly "soaks" from the centre to the edge over a period of time giving the compound a longer at-temperature heat profile.

A special work coil design is used that allows the preferential heating of components on a simple continuous motion conveyor system i.e. not indexing. This means that the production rate can run into thousands of parts per minute.

This photograph shows steel wire that has been formed into "U" shapes for bra under wires. The ends of the "U" have been heated and coated with Estrabond powder. By controlling the heating and dipping process, we have developed a production system that allows control over the final shape of the end protection coating.

The use of Estrabond as an end protection is far superior to the usual resin dip process which often fails if the garment washing instructions are ignored and the article is put in a tumble dryer.

In the left illustration, an induction coil is being used to heat the tube prior to the tube passing in to a powder coating spray booth on the left. The heat generated in the tube cures the surface coating (induction heater not shown).

This photograph on the right shows an induction heating coil on a former under construction at our works. This coil is for a pipe surface curing application.

The pipe to be coated is in the order of 2 metres (6ft) in diameter.

Filter assemblies are produced by curing a thermosetting material between a paper assembly and a steel or aluminum cover. As shown below, dual pancake induction coils are used in a static setup to provide heat to the top and the bottom plate. A dynamic setup utilizes long pancake coils for the heating. Localized temperatures of 150-175°C associated with such bonds avoid damage to the components and provide a high strength bond upon cooling. Heat cycle times are in the order of 5-10 seconds.