Drying plastics with microwaves The quality of plastic products depends partly on the correct moisture level in the polymer after processing. Research by the European Polydry project has provided an innovative microwave-based prototype system that can be used on a range of plastics. Industrial Technologies © Thinkstock Drying polymers minimises or eliminates problems caused by too much or too little moisture in a plastic material during its processing. The extent to which moisture affects the quality of a product depends on the plastic being used and the intended use of the product. Uniform drying helps to prevent defects like environmental stress cracking and assists in the moulding or extrusion process. The EU-funded project Polydry aimed to develop a new low-cost, real-time microwave-based drying system. Apart from the obvious advantages for plastics quality, the number of rejected products would be reduced thereby reducing the amount of waste material to be granulated. Dealing with waste is fraught with hazards for operators such as contact with cutters, fire and explosion. Polydry scientists investigated interactions between a range of polymers and microwave drying and also compared the results with conventional thermal drying. Microwavability of thermoplastic polymers depends mainly on the dielectric constant (linked with insulating properties). Research therefore concentrated on the chemical structure of the polymers relating to dielectric characteristics as well as transport of water properties. To measure moisture content, the Polydry project developed new sensing equipment. Appropriate software for moisture measurement and drying was designed according to a requirement analysis and high level design of the V-cycle model. The Polydry team carried out extensive trials and successfully dried different raw materials in a lab-based microwave system. Design ideas for a 'pilot' microwave drying system produced a 3D prototype model incorporating an inner and an outer drum while software determined the critical positioning of the magnetrons. An intelligent control module provides further cost reduction and efficiency of the system. Enabling closed loop control of the continuous moisture measurement process, there is integration with downstream processes. The results of this innovation are far-reaching. Not only does the polymer drying system mean savings in drying time and less rejected products and waste, but overall there will be more competitively priced plastic products in the EU for competition in the global market.
