When it comes to the moulding of plastic parts, OEMs have had to choose from various technologies that are to a greater or lesser extent based on traditional injection moulding processes. With the trend towards miniaturisation in many industries, some suppliers have attempted to adapt this injection moulding technology to better service the demands for small and precise parts. However, adaptations of macro technologies and processes to the moulding of precision and micro plastic components is not always the best solution.
Now a Spanish company has developed a plastic moulding technology that uses ultrasonics as the polymer melting agent. It is deemed particularly suitable for the production of small and precise plastic parts, which cannot be easily made using traditional injection moulding techniques. The new approach eliminates the need for a screw and barrel, saving energy and minimising waste. At the same time, as ultrasonics induces low viscosity in melted materials, the technique potentially opens up new design freedoms.
Enric Sirera, a director at Barcelona-based Ultrasion, says the Sonorus IG machine will change the way small plastic parts are made. “The ‘go to’ technology for plastic part manufacturers is injection moulding, a tried-and-tested technology. So ingrained in the psyche of the plastics industry is this manufacturing process that when technologies targeting the requirements of micro and precision plastic part manufacturers were needed, micro injection moulding machines were developed, scaling down the size of machines, but in essence using the same process.
“Injection moulding machines, whether for macro or micro applications, work on the same principles. Plastic pellets are placed in a hopper, melted in a screw and barrel surrounded by heating elements, and then injected into the mould under pressure. As such, they require a continual source of energy, and there is significant wastage of material that is melted and not required in production, and there is the age-old necessity to purge machines between cycles. In addition, as the injection pressures in traditional micro injection machines are typically quite high, expensive tooling is required.”
Taking into account these inefficiencies in the injection moulding process, Ultrasion designed a process based around the use of ultrasonics as the melting agent. The technology eliminates the need for a screw and barrel.
“In the Sonorus IG, ultrasonic waves are used to melt plastic granules that are fed directly into the mould, are contacted by an ultrasonic horn, and are melted in milliseconds,” says Sirera.
“Using a dosage 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 which is lowered, and as well as melting the material forces the polymer to flow into the mould cavities. The horn then returns to its original position, and the cycle begins again.”
Sonorus IG: The machine avoids some of the inefficiencies of more traditional techniques
The ultrasound moulding technology is 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% less energy than a traditional micro injection technology.
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 a 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 sprue concept in the Ultrasion technology means that it behaves as an energy director, orientating the waves in the flow direction. So molten material and waves travel together towards the mould cavities, which induces extremely low viscosity (almost as low as water) in the melted plastic.
Sirera says there were no materials that could not be processed using the ultrasonic moulding technology, with successful moulding projects using everything from standard polypropylene to high-density polyethylenes. The machine 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 mould 15mm-long parts with wall thicknesses of 0.075mm, and achievable tolerances are in the region of 0.01mm.
An early use for the machine was on a healthcare project to develop a medical device using coloured polypropylene. This tissue management application required a particularly difficult to manufacture tip. By using the Ultrasion technology, the team managed to produce a tip that was 43mm long, weighing 0.22g, with wall thickness 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, outside diameter of 4.4mm and an internal diameter of 2.9mm. This part – with a membrane overmoulding – was achieved in one operation. This proved impossible to achieve using a conventional micro injection moulding process, says Ultrasion. The alternative ultrasonic moulding process was used to mould the part in one process, and then to glue the membrane in a secondary process.
Finally, ultrasonic moulding was 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 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.
Sirera says that it is difficult to predict the limits of the new technology. “In the case of the ‘tip’ part mentioned 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 thought impossible to make.”
Out of the mould: Parts for an eye surgery tip