Frequently optimised injection units and screw diameters are therefore required for micro injection moulding. The aim of optimisation is to reduce dwell time and retain an acceptable screw displacement in order to be able to precisely control the injection process.
A possible solution is the further miniaturisation of the proven three-zone plasticising screw, with realistic screw diameters currently in the region of 15 mm. However, the sizes of granulate currently available (minimum diameter 2 mm), prevent any further reduction in screw diameter. The reason for this is the required depth of the screw channel in the inlet zone (minimum 3 mm). The core diameter of the screw would be reduced to such an extent that the torques generated during plasticising could no longer be tolerated.
However, the opposite approach is possible. Grains of granulate could be reduced to create micro granulates with grain diameters of less than 1.5 mm. ARBURG has developed a modified screw for the injection unit 30 with a diameter of 12 mm specifically for processing micro granulate. A reduced screw channel depth in the inlet zone ensures the required mechanical stability of the screw.
Experiments carried out in the ARBURG research centre with this 12 mm screw and granulates with grain diameters between 0.5 and 1.5 mm have shown that the dwell time, which is often critical in micro injection moulding, can be almost halved. The results obtained also demonstrate significantly better reproducibility.