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Smaller equals more complicated
Micro-injection moulding places high demands on development and components/Arburg launches new micro-injection module onto the market
As a reminder, what are the requirements stipulated by the processor that need be met for microinjection moulding? Manufacturing products exactly in series with shot weights that are often just a few milligrams, corresponding to the weight of a single grain of granulate, is only possible if the dwell time of the melts in the injection unit as well as 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 and not just the specially prepared materials like microgranulates.
Microinjection module: requirements met
One problem with minute shot weights of less than one gram is, in some cases, the very long material dwell time in the infection unit. Furthermore, the displacement distance of the screw is extremely short because of the small injection volume. An effective solution is the new microinjection module from Arburg. Unlike other alternatives on the market which use a combination of screw plasticising and piston injection, this operates using two screws which "share" the preparation, dosing and injection of the material.
Firstly, a servoelectrically driven screw preplasticising 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 used is similar in design to a conventional threezone screw. The molten material is then transported from the preplasticising stage to the injection unit. The injection screw is used purely for transport purposes. It has a diameter of only 8 millimetres, is fitted with a nonreturn 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 preplasticising 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.
Constant injection conditions through dosage control
A prerequisite for continuously high part quality, is a homogenous melt feed. In order to achieve this, the pressure at the transfer point between the preplasticising screw and the injection screw is monitored and regulated. For this purpose, appropriate nominal pressure and maximum permissible screw circumferential speed (limitation) values for the preplasticising screw are entered via the Selogica control system. If the actual pressure deviates from the nominal pressure, this deviation is compensated by adjusting the rotational speed within specified limits.
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 design with two screws is the leaktightness of the system.
Arburg has also succeeded in fully maintaining the modularity of its components with this new development. The microinjection 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 microinjection moulding. Consequently, other larger cylinder modules can also be used on this machine when a product change is required.
True pioneering technology
With its new microinjection module, Arburg has fulfilled all the crucial requirements for manufacturing miniature and micro components. It is particularly important, however, that the entire range of plastics can be used in production without forfeiting anything in terms of precision or part quality. Compelling advantages of this technology are the homogeneous preparation of the melt with short dwell times of the plastics used, process reliability as well as working by the firstin firstout principle. These can be achieved through the "combined" use of two fullvalue screw systems for preplasticising and injection.
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