A key complexity involved in micro injection moulding is the fact that the quantities of materials involved are miniscule. It is only possible to manufacture products exactly in series with shot weights which are often just a few milligrams — corresponding to the weight of a single grain of granulate — if both the dwell [resistance] time of the melts in the injection unit and the level of material shearing can be kept correspondingly low.

Moreover, the prepared melts must be perfectly homogeneous, both thermally and mechanically. It must be possible to regulate the cylinder temperature precisely and the plasticising process must be easily reproducible. However, it is also important that the entire range of “normal” plastics can be used, not just specially prepared materials like micro-granulates.

Micro Injection Module: Requirements Met Arburg, one of the leading global manufacturers of injection moulding machines, has developed a new micro injection module which helps to minimise these complexities. It combines a small injection screw with a second screw for melting the material. Unlike other alternatives on the market — which use a combination of screw plasticising and piston injection — the two screws “share” the preparation, dosing and injection of the material.

A servo-electrically driven screw pre-plasticising section, which is installed at 45 degrees to the horizontal injection unit, ensures standard granulates are prepared under ideal conditions. In terms of screw channel depths, the plasticising screw is similar in design to a conventional three-zone screw. Once prepared, the molten material is then transported from the pre-plasticising stage to the injection unit. The injection screw is used purely for transport purposes. It has a diameter of only 8 mm, is fitted with a non-return valve and operates according to the screw/piston principle. This permits the smallest shot weights to be achieved with great precision and the required travel distance. At the same time, the perfect interplay of screw pre-plasticising and injection ensures excellent processing that is gentle on the plastic. The melt is continuously fed from the material inlet to the tip of the injection screw. This ensures that the First-In-First-Out principle is fully observed.

A homogeneously prepared, newly dosed melt is always available for every shot. This keeps the material dwell times correspondingly short, preventing thermal damage to the material. The result is high processing quality. An additional advantage of the two-screw system is that it is highly leak-proof.

Arburg has also succeeded in fully maintaining the modularity of its components with this new development. The micro-injection module is designed specifically for use on electric Allrounder A machines with a size 70 injection unit. Its enclosed construction means that it can be changed rapidly and then used on various machines, like any other Arburg cylinder module. This also means that the range of the machine’s applications is not limited to micro-injection moulding. Consequently, other larger cylinder modules can also be used on this machine when a product change is required.