Stamp forming of hydroxyapatite filled ethylene vinyl acetate co-polymers: Process optimization using a right angle V-mould

Hydroxyapatite (HAP) filled ethylene vinyl acetate co-polymer (EVA) composites are developed in an attempt to formulate a surgeon friendly material for renovating impaired skull contours. A cost effective technique for obtaining these composites in the clinically significant forms would indeed be a landmark accomplishment. Stamp forming is one of such processes where the cost as well as the performance of the product strikes the right balance. This study was carried out prior to the stamp forming process optimization of the composites into three-dimensional (3-D) contours, essential for applications like cranioplasty. This paper discusses the V-bending results for HAP filled EVA and an attempt to identify a processing window for real manufacturing situations (3-D forming) is made. The processing conditions, such as the stamping temperature, time, and stamping rate, required to give high-quality right angle bends, have been established. The quality of stamped forms is also examined in terms of shape conformance and variation in wall thickness. It has been found that the stamping temperature and velocity were the key factors, which determined the quality of the stamped part. Too high temperatures as well as too high stamping rates lead to severe thinning and degradation of the formed parts. On the other hand, when the temperatures and the stamping rates are too low, the composites do not conform to the mould contour.