Materials processing using microwave technology have numerous advantages compared to traditional materials processing techniques; however, conventional microwave processing technology suffers from variable product quality due to uneven processing or heating. The variable frequency microwave (VFM) technique is an emerging technology that overcomes the inherent difficulties of uneven heating experienced in conventional microwave facilities and was developed for the purpose of processing many of today's advanced materials including polymers, composites and fibre-reinforced materials and is a rapid and controlled approach to uniform heating. By sweeping through a bandwidth of frequencies, the VFM method creates multiple hot spots within the processing cavity that lead to time-averaged uniform heating. The current work focuses on the sweep-rate regime used in VFMs and the effect of varying this parameter. A "recipe" for a non-linear sweep-rate regime is introduced and compared with the current linear sweep regime and conventional fixed-frequency processing in terms of heating uniformity. Theoretical modelling of the linear and non-linear sweep regimes are presented and are experimentally verified by heat-mapping of a multimode cavity with the aid of thermally sensitive material to quantify improvements in the uniformity of heating.