Heat distribution and thermal analysis of joining thermoplastic composites using microwave energy

Industrial microwave technology for processing polymers and polymer-based composites is currently in a state of considerable flux. In this study, a microwave facility consisting of a 0.8 kW variable control power generator operating at 2.45 GHz is used to join random glass or carbon fibres reinforced thermoplastic composites. The 2.45 GHz microwave was selected for the study because the magnetron at this frequency is readily avalable at low cost. The input power can be varied in four steps. A cycle time of 30%, 50%, 80% and 100% of full power can be selected. The heat absorbed and heat flow in the sample materials are studied. The temperatures at different points of the samples are also measured using infrared thermometer. The effect of power input and cycle time on the temperature distribution in the test piece is detailed together with the underlying principles of sample material interactions with electromagnetic field. The material used for the research is 33% by weight of random glass fibre reinforced polystyrene [PS/GF (33%)]. Araldite has a high loss tangent and can absorb microwave energy and convert it into heat much better than the thermoplastic, polystyrene and the glass fibre. The samples were therefore lapped joined using Araldite as primer. The Araldite consists of 100% liquid epoxy and 8% amine. Computer software package (MATLAB 6) is utilised to analyse the heat distribution in the samples and its relation to the lap shear bond strength of the joints obtained. It is found that sufficient combination of power level and duration of exposure to microwave irradiation is required for obtaining proper bonding with good strength. It is also discovered that proper bonding will only form when the rise in temperature is significant, ie the sample temperature should be above 30 C, at which the Araldite will cure quickly and properly. Based on these findings, the lap shear bond strength of the test pieces after microwave joining can be predicted by reading at the temperatures reached by the samples without carrying destructive testing.