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In micro-injection moulding, the size of the production is increasing as we move forward in time and now often runs into millions. Scientists at the Kunststoff-Zentrum (Plastics Centre) in Leipzig/Germany (KUZ), together with Hasco, developed a technology for economical production on conventional injection moulding machines.

The demand for injection moulded micro-components with a size of a few cubic millimetres and a shot weight of only a few milligrams is growing constantly, especially in medical, automotive industry and consumer electronics. Here, numbers of units in the triple-digit million range are no longer uncommon. In principle, these requirements can be met in two ways. Apart from having a large number of dedicated micro-injection moulding machines with a comparatively small number of cavities, correspondingly high quantities can be achieved by using high-cavity mould solutions with standard injection moulding machines — although this necessitates a certain amount of extra work. While this offers higher process reliability and less probability of failure, it is considerably more expensive in terms of investment and space requirements. The second variant thus offers considerable economic advantages, but brings with it other process-related challenges.

The experts at the Kunststoff-Zentrum in Leipzig (KUZ) have been working on micro-injection moulding since the late 1990s [1]. Dr. Gábor Jüttner, team leader in microplastics technology, provides an insight into the technological principle: “One of the first projects was the development of our Formica Plast micro-injection moulding machine with a two-stage piston injection unit.” In this process, the granules are first melted in the pre-plasticising cylinder and conveyed by the pre-plasticising piston into the injection cylinder. From there, a servo-electrically driven micro-piston with a diameter of a few millimetres pushes a correspondingly small amount of melt into the cavity with a high level of precision. However, since this technology is designed for gentle and precise processing of very small melt volumes of approx. 4 to 400 mm3, upscaling quickly reaches its limits.

“When it became apparent about three or four years ago that a production run of a few hundred thousand pieces was no longer sufficient for an increasing number of applications, we decided to tackle the task of high-cavity precision manufacturing for micro-moulded parts,” explained Steffen Jacob, project manager at the KUZ.

As part of the publicly funded project Scale-Mi [2], the above-mentioned servo-electric micro-piston technology has been expanded over the past two years to accommodate a larger number of cavities. With this approach, the melt is fed from the plasticising unit of a conventional screw injection moulding machine into a hot runner manifold, where it is split up, for example, over four injection modules. On each of these modules, a micro-piston injection unit actively injects the melt into a mould area that each has, for example, four cavities. In this example, 16-cavity production can thus be achieved. In this way, the advantages of screw/piston plasticising for providing a melt quantity for higher volumes are combined with the advantages of the injection dynamics and the precision of small piston injection units.

Since each of the injection modules, within certain limits, can be individually controlled or adjusted in terms of shot volume, injection speed, etc., different moulded parts can also be produced. Thus, the realisation of family moulds is possible without the usual disadvantages.

However, this combination of the special micro-piston technology with a hot runner poses a major challenge for the production of the melt-carrying runners.

Here, Hasco’s additively manufactured Streamrunner not only enables considerable space savings, but also, through its optimised geometry, it is possible to keep the amount of melt held in reserve as small as possible, minimising the residence time of the melt.

The additive manufacturing technology (selective laser sintering — SLS — of steel powder) of the hot runner offers maximum design freedom and can be individually developed for each specific task and rheologically optimised on the basis of filling simulations.

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“The free three-dimensional design of the melt channels makes it possible to optimally balance the melt distribution and to completely avoid sharp edges and areas with poor flow, i.e. dead corners,” says Tobias Kröber Technical Sales Engineer Hasco hot runner. “Thanks to the flexible tubular heating elements used, we also achieve optimum temperature distribution, which also helps to ensure a melt flow that is gentle on the material.” The surfaces of the flow channels, which are still quite rough due to the 3D sintering process, are polished by pushing, with pressure, a special fine but still sufficiently abrasive polishing compound through the flow channels. In this way, surface finishes of Rz 2-3 can be achieved.

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In a joint development with Hasco, the KUZ scientists have now integrated a micro-injection piston with a diameter of just 3 mm into the melt channel of the Streamrunner close to the cavity for each injection module. The modular design of the Streamrunner® enables the realisation of several such injection modules with excellent synchronisation of the melt distribution at the same time.

“We have been working with Hasco on development projects for a long time,” says Steffen Jacob, “and now the Streamrunner came at just the right time for us, not only because of the considerable space-saving possibilities. In the context of this development project, we once again very much appreciated Hasco's willingness to try something new.”

For example, the bore-holes for the micro pistons were prefabricated by Hasco during the 3D printing process and then finished later by eroding and grinding at the KUZ with the corresponding fine fitting. “Here, a particular challenge was to place the prefabricated bore-holes and the runners as precisely as possible in the same plane,” added Tobias Kröber.

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Several demonstration mouldings were designed to demonstrate the functionality and to highlight the advantages of the system, with the aim of increasing the number of units produced in micro-injection moulding. A small flow spiral with a volume of 58 mm3 and a diameter of 17.4 mm was created to investigate the balancing of the manifold. The balancing is evaluated via the achieved flow path length and the weight of the moulded part. A small clamp in a 4-cavity design, each of 6 mm3 with four active injection drives and thus 16 cavities was used to highlight the advantages of the concept.

A further advantage of the flow-optimised design of the polished runners is that it enables a rapid colour change. The test with the clamp moulding showed that a change of moulding compound or colour can be made by flushing the melt distributor twice by spraying through with the injection unit.

Presented for the first time at K 2019, Hasco also demonstrated the possibilities of the Streamrunner for multi-component production at this year's World Plastics Fair in Düsseldorf. The free three-dimensional design of the runners opens up completely new possibilities here. Different plastic components or colours can be distributed in a very confined space and the channels can be intertwined. This enables product designers to overcome previous limitations in the design of moulded plastic parts and to exploit new design options.

Finally, a look at the economics. The KUZ scientists drew up a cost comparison for an example of a demonstration moulded part with a volume of around 10 mm3 and a production volume of 30 million units. On the one hand, 14 micro-injection moulding machines with 4-cavity moulds require total costs (investment and manufacturing costs) of just under EUR 7 million in the first year. In contrast, production on three standard machines with 16-cavity moulds and the additional equipment developed by the KUZ, the scale microinjector, costs just under 1.7 million euros.

The additively manufactured Streamrunner from Hasco allows greater design freedom for single- and multi-component injection moulding. With state-of-the-art manufacturing technologies, it thus opens up new possibilities for mould makers and injection moulders. The Streamrunner is a 3D printed hot runner manifold using the laser sintering process with the highest degree of freedom in design. The flow channels can be optimally designed rheologically so that, for example, sharp edges and areas with poor flow are completely avoided.

The combination with the scale microinjector technology from the KUZ enables the production of high volumes in micro-injection moulding on conventional injection moulding machines. The next steps to be taken by the Leipzig-based company are, among other things, to expand the field of application of the new mould and process engineering, and the long-term testing of corresponding systems.

[1] Jüttner, G.: Plastifiziereinheiten für kleinste Schussgewichte. Kunststoffe 94 (2004)1, S. 53–55

[2] Jacob, S.: Abschlussbericht zum Forschungsvorhaben Scale-Mi 49VF190007, BMWK INNO-COM

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