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Introducing a next generation machine for micro moulding

08 March 2015

When it comes to the moulding of small plastic parts, OEMs have had to choose from various technologies that are based on traditional injection moulding processes. Enric Sirera describes a new ultrasonic process that he claims is more economical and energy efficient.

The Sonorus 1G can accommodate shot weights from 0.05g to 2.0g
The Sonorus 1G can accommodate shot weights from 0.05g to 2.0g

Today, OEMs have a new and innovative technology to assess as they strive for a cost-effective, accurate, and efficient manufacturing technology. This new micro moulding process is based on the use of ultrasonics as the agent of polymer melting, and is designed specifically for manufacturers of small and precise plastic parts.

With no need for a screw, barrel, and heater bands, the technology is extremely energy efficient and minimises waste. At the same time, as ultrasonics induces extremely low viscosity in melted materials, product designers can now innovate as they have never been able to before. Barcelona-based company Ultrasion spent a number of years researching and developing this new plastic moulding technology, now embodied in its Sonorus IG machine.

Using a dosing system that delivers the correct quantity of standard pellets for every shot, the production cycle begins with the mould already closed and dosed with raw material at room temperature. The material is then contacted by an ultrasonic horn or 'sonotrode'; and as well as melting the material, it forces the polymer to flow into the mould cavities. The sonotrode then returns to its original position, and the cycle begins again.

The ultrasound moulding technology is extremely precise, uses no heaters, and the process means that there is no material residence time, and no material degradation. In addition, as the energy needed in the process is only at the point when the ultrasonic horn contacts the raw material to induce melt, it uses upwards of 90 percent less energy than a traditional micro injection technology.

Material wastage, a problem in all sizes of injection moulding machines, is a massive issue in precision and micro moulding applications, where in some instances upwards of 99 percent of material processed will be scrapped. Where this material is expensive, as is the case with some critical medical mouldings, this becomes an even bigger problem. In the Ultrasion process, only the material required is dosed, and so runner and sprue wastage is all but eliminated.

The nature of the ultrasonic moulding process is such that material melt characteristics are very different from those produced in injection moulding machines. The application of high intensity mechanical vibration that transmits energy directly into the polymer molecular structure results in an extremely fast and efficient melting process 'inside out' rather than 'outside in', which is how melting occurs in injection moulding via the electric heater bands.

In addition, the new sprue concept in the Ultrasion technology means that it behaves as an energy director, orientating the waves in the flow direction meaning that molten material and waves travel together towards the mould cavities, which induces extremely low viscosity (almost as low as water) in the melted plastic.

Application results
There are no materials that cannot be processed using the ultrasonic moulding technology, with successful moulding projects using everything from standard polypropylene to high density polyethylenes. The Sonorus 1G machine — which has been designed specifically for precision and micro applications — can accommodate shot weights from 0.05g to 2.0g.

In all materials, the reduced viscosity allows for the attainment of especially long parts or parts with extremely thin walls. The machine can easily mould 15mm long parts with wall thicknesses of 0.075mm, and achievable tolerances are in the region of 0.01mm.

The results achieved by some OEMs using the Ultrasion technology show the versatility of the machine and the precision achievable. One was for a healthcare project for a medical device using coloured polypropylene. This tissue management application required a particularly difficult-to-manufacture tip. By using the Ultrasion technology, this OEM managed to produce a tip that was 43mm long, weighing 0.22g, with wall thicknesses of 0.075mm, and with an outside diameter of 0.35mm and an inside diameter of 0.2mm.

In another application for the manufacture of a cap with a filter for an ear protection device made from raw polyamide 12 (PA12), the ultrasonic moulding process successfully manufactured a part weighing 0.02g, with a 0.5mm wall thickness, and outside diameter of 4.4mm and an internal diameter of 2.9mm.

Of particular note, the part — with a membrane over-moulding — was achieved in just one operation. This proved impossible to achieve using a conventional micro injection moulding process - the alternative to Ultrasion’s ultrasonic moulding process being to mould the part using one process, and then to glue the membrane in a secondary process. The manufacturer reported a 300 percent increase in productivity using the Ultrasion technology.

Ultrasonic moulding was also successfully used in the production of an eye retina surgery tip made from raw polypropylene. The final part weighed 0.1g, had an internal diameter of 0.6mm with a 0.17mm wall thickness, and a wall thickness at the tip of 0.1mm. The tool for this application used two extremely small core pins sitting head to head, which would have broken using the high pressures of conventional micro injection moulding.

While these achievements are in themselves impressive, the bottom line is that Ultrasion do not know what the limits are. In the case of the 'tip' part mentioned above with 0.075mm thickness along 15mm with polypropylene, when working on this project, Ultrasion generated flashes at the top of the tip due to a mould misalignment.

The company has been unable to measure such flashes precisely, but they are definitely at least as thin as 0.003mm along 3mm. The customer was astonished as they felt that polypropylene was not supposed to flash at such thicknesses, and this led to the development of parts that it had previously thought impossible to manufacture.

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