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Arburg main
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The injection screw has a diameter of just eight millimetres and is purely a conveyor screw.
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The micro injection module from Arburg combines an 8 mm injection screw with a second screw for melting the material.
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System solution for micro components: the production cell for micro clamping frames includes an electric Allrounder 270 A with a micro injection module.
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At various trade fairs, the micro injection module demonstrates its performance capabilities in the production of micro clamping frames with a moulded part weight of just 0.0036 g.
In recent years, the increasing miniaturisation of assemblies and components has caused the injection moulding of miniature and micro components to grow in importance in many industrial applications. The requirements on the injection side in the manufacture of miniature and micro parts have been clearly defined for some time. The problem was simply this: how could these requirements be achieved and implemented technically? The extent to which conventional screw injection units can be further adapted to achieve even smaller shot weights is severely limited and piston injection is also no perfect solution. However, Arburg has developed an efficient solution to enable minute shot weights to be achieved in future using the tried and tested screw injection technique: the micro injection module combines an 8-mm injection screw with a second screw for melting the material.
Reminder: what requirements of the processors does micro-injection moulding have to meet? Precision manufacture of items with shot weights that can correspond to just a few grains of granulate. These can only be achieved if the dwell time of the melt in the injection unit and the shearing of the material can be kept sufficiently 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.
Part weight and shot weight may differ considerably
Many micro-injection moulding applications can be implemented using 15-mm screws. One reason for this is that the shot weight and part weight may differ considerably. For example, with a part weight of a few milligrams, the total shot weight may be several hundred milligrams, because the complexity of the moulds, with regard to demoulding and temperature control of the sprue, demands a compromise.
In the case of extremely low shot weights of less than one gram, 15-mm screws approach their limits in terms of dwell time and reproducibility. In some cases, the dwell time of the material in the injection unit becomes very long, bringing the risk of thermal damage to the plastic. In addition, the low injection volume means that the movement of the screw is extremely small, almost reaching the limit of the machine's positioning accuracy.
Solutions: Reduced screw diameter or piston injection unit
One solution is the miniaturisation of the tried and tested three-zone screw. By using a 12-mm screw, the dwell time can be reduced and the dosage volume maintained, due to the smaller diameter. At the same time, the mechanical stability of the screw is guaranteed through reduced screw channel depth in the inlet zone. However, micro-granulate must then be used. Its grains have a diameter of 1 mm and weigh approx. 0.01 g. Consequently, the shot weight consists of several grains of granulate, guaranteeing extremely constant product properties. However, the disadvantage is that only a limited range of materials is available due to the low purchase quantities and hence low number of raw material suppliers. Furthermore, dosing precision greatly depends on the quality of the micro-granulate.
Another solution commonly found on the market is the use of a piston injection unit with screw pre-plasticising instead of a screw injection unit. The principal advantages of a piston injection unit are that standard granulates can be used and the piston diameter can be a great deal smaller than that of a plasticising screw. As a result, much smaller shot weights are possible.
Further challenges are the complex design and problems with system leakage. Whereas with a screw, the leakage flow is caught by the adjoining screw threads of the metering zone, for example, in the piston this melt visibly escapes from the cylinder behind the piston.
Finally, a decisive disadvantage of the piston injection unit is that in contrast to a screw system, the material is not transported forward evenly and continuously. As the first-in, first-out principle is therefore no longer assured, individual particles may suffer thermal damage, possibly leading to degenerated material in the moulded part.
Micro-injection module meets all requirements
The micro-injection module from Arburg is a perfect solution that unites all advantages in one system: it combines an 8-mm injection screw with a second screw for melting the material. Two screws therefore share the tasks of preparing, dosing and injecting the material. Firstly, a servo-electrically driven screw pre-plasticising section, which is installed at 45° to the horizontal injection screw, ensures that standard granulates are prepared under ideal conditions. In terms of screw channel depths, the plasticising screw used is similar in design to a conventional three-zone screw.
The molten material is then conveyed from the pre-plasticising stage to the injection screw. This screw is used purely for conveying the material. It has a diameter of only eight millimetres, 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 with the necessary travel distances. At the same time, the perfect interaction between the screw pre-plasticising section and screw injection ensures excellent, gentle processing of the plastic. The melt is continuously fed forwards from the material inlet to the tip of the injection screw. This ensures optimal adherence to the first-in, first-out principle.
Constant injection conditions through dosage control
A prerequisite for constantly high part quality is a homogeneous melt feed. To achieve this, the pressure at the transfer point between the pre-plasticising screw and the injection screw is monitored and regulated. For this purpose, an appropriate nominal pressure and maximum permitted screw circumferential speed (limitation) for the pre-plasticising screw are entered via the Selogica machine control system. If the actual pressure deviates from the nominal pressure, this deviation is compensated by adjusting the rotational speed within specified limits.
In this way, a homogeneously-prepared, newly dosed melt is 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 design is the leak-tightness of the system.
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 injection moulding machines with 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. At the same time, the application range of the machine is not restricted to micro-injection moulding. Consequently, other, larger cylinder modules can also be used on this machine when a product change is required.
Production of micro clamping frames
The micro-injection module was presented at the Technology Days 2011 on an electric Allrounder 270 A with a clamping force of 350 kN and size 70 injection unit. The exhibit was equipped with an 2-cavity mould and produced micro clamping frames from POM in a cycle time of 15 s. Its moulded part weight was a mere 0,0036 g.
The micro clamping frames were handled by a horizontally operating Multilift H robotic system, which was equipped with a special gripper system based on the application of pneumatic force and pneumatic drives. Due to the parts’ intricate structure and small size, removal was performed individually via vacuum. In order to ensure precision removal, the removal gripper of the robotic system was centred on the mould. Two vacuum circuits for the gripper and discharge functions, as well as two further circuits, were required to ensure reliable pick-up of these micro components.
True pioneering technology
With its micro-injection module, Arburg meets all the requirements for the production of miniature and micro components. It is particularly important, however, that the entire range of plastics can be used in production without suffering losses in terms of precision or part quality. Materials that have already been successfully processed with the micro-injection module include ABS, LCP, PA6, PA 6.6, PBT, PP, PEEK and POM. Compelling advantages of this technology are the homogeneous preparation of the melt with short dwell times of the plastics used, a highly reliable process and adherence to the first-in, first-out principle. These can be achieved through the combined use of two full-value screw systems for pre-plasticising and injection.