Direct wire-tension measurement based bowing correction in hot wire cutting of polystyrene

The accuracy of parts produced by hot wire cutting of expanded polystyrene (EPS) is severely affected by bowing of the wire. The degree of bowing depends on the current supplied and the feed rate of the hot wire. Bowing occurs when the current is insufficient to melt the foam ahead of the wire and the mechanical interaction between the foam and the wire increases. This causes an increase in the tension in the wire and this is directly measured using a load cell arrangement. Based on this direct wire tension measurement a real time closed-loop feedback mechanism is implemented which regulates the current to maintain a constant wire tension and eliminates the bowing of the hot wire during the cutting process. For a particular feed rate the feedback mechanism precisely fine tunes the current supplied to the wire such that there is no mechanical drag force between the foam and the wire. The sensitivity of the feedback mechanism is high and responds to very minute contact between the foam and the wire and nominally regulates the current variation in the hot wire, hence, maintaining a constant kerf width throughout the cut. This mechanism allows to significantly improve the accuracy of the EPS part even in complex 3D cutting paths.