A methodology for the determination of foamed polymer contraction rates as a result of cryogenic CNC machining

The ability to produce products, suited to a particular individual is becoming more prevalent in today’s society. This requires more efficient and rapid methods for manufacturing of bespoke products. One such method being currently developed is the cryogenic CNC machining of soft materials for producing personalised shoe insoles and outsoles. A major element of cryogenic CNC machining is the freezing of the soft polymers, which subsequently contract. This paper describes a method for predicting and compensating for the effect of cryogenic material contraction for the commonly used shoe midsole foamed polymer, ethylene vinyl acetate (EVA). Using the linear coefficient of thermal expansion a scaling factor for EVA has been developed to enable it to be accurately cryogenically CNC machined. This factor is then applied to the X, Y and Z scaling within the Delcam CAM software to shrink the model. The process is tested for a series of EVA cube test pieces and the results provide a scaling factor, which shows that the linear scaled dimension are within 1% of the measured contracted X, Y and Z dimensions. The scaling factor is subsequently used on an example low density EVA orthotic insole, which is cryogenically machined and then compared dimensionally with the original CAD model. It was found that using the cryogenic contraction factor the cryogenically machined insole had a dimensional X, Y, Z error of less than 1% when compared and analysed with the original CAD model.