Leading US-based extrusion machinery supplier American Kuhne (AK) has successfully extruded PEEK tubing using a vertically upward extrusion line. There is a patent pending on the technology. In a series of experiments, the company has found that by passing tubing through an oven placed above the die head, it can be annealed and the outer diameter and ovality can be controlled precisely. The outer diameter can be kept to a tolerance of ±0.0007” (0.18 mm) while the ovality to less than 0.001” (0.25 mm). The process of annealing gives PEEK the time to crystallise uniformly as it cools, giving optimum physical properties.
AK’s discovery of the vertically upward extrusion and inline annealing process followed unsuccessful attempts at extruding PEEK horizontally and vertically downwards. The horizontal attempt, performed with a 24” (61 cm) gap between the die head and the quench bath, resulted in a semi-crystalline tube but with inconsistency in outer diameter (OD) and ovality. Horizontal extrusion resulted in a non-uniform level of crystallinity between the top and the bottom of the tube. The vertically downward attempt resulted in a “snake-like” product with bends and kinks in the tube, due to the PEEK not having enough melt strength to hold the weight of the extrudate.