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Engineering thermoplastics

The Next Generation of Micro Moulding

Author: Enric Sirera, General Manager, Ultrasion

When it comes to 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 industry sectors, some suppliers have attempted to adapt this injection moulding technology in order to better service the demands for small and precise parts. However, as is often the case, adaptations of macro technologies and processes to the moulding of precision and micro plastic parts is not always the best solution. In the area of micro moulding, OEMs now 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 as this article describes, is designed specifically for OEMs of small and precise plastic parts, and therefore addresses the specific problems associated with this sector. With no need for a screw and barrel, 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.

Ultrasonics Versus Traditional Injection Moulding
While in the world of precision and micro plastic part design and manufacture, some of the aspects of the production process require special consideration when compared to the macro world, in essence all OEMs are looking for the most cost-effective and accurate technologies for their specific purposes.

The "go to" technology for many plastic part manufacturers is injection moulding, a tried and tested technology with a pedigree going back decades. 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 basic inefficiencies in the injection moulding process -- inefficiencies that are exacerbated when looking at the particular contingencies of the precision and micro moulder -- Barcelona-based company Ultrasion spent a number of years researching and developing a brand new plastic moulding technology. Encapsulated in the Sonorus IG machine that is now being sold commercially worldwide (see figure 1), Ultrasion designed a process based around the use of ultrasonics as the melting agent, which opens up enormous potential for manufacturers to save on energy, material, and tooling costs, while at the same time providing a technology that is extremely accurate and induces characteristics in the melted polymer that allows product designers to overcome previously assumed limitations and truly innovate.

Figure 1.

Ultrasion decided to work on a technology and machine that catered for the precision and micro moulding sector that had a small footprint, which used as little energy as possible, reduced material wastage, reduced tooling costs, and optimised the properties of melted plastics.

How the Technology Works
Most seasoned plastic manufacturing professionals looking at the Ultrasion machine would immediately notice that there is no screw and barrel. In the Sonorus 1G, 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.

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 or "sonotrode" which is lowered, and as well as melting the material 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% 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% of material processed will be scrapped. Where this material is expensive as is the case of 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
The Ultrasion technology was commercialised towards the end of 2013, and is being sold worldwide into all industrial sectors, with early sales and interest centring on the medical, aerospace, electronic, and military sectors where precision and accuracy are key. Sales are coordinated from the company's Barcelona headquarters, with distributors and representatives being installed globally to cater for demand.

The technology has been designed for ease of use, and requires no more than a short orientation and training session as the machine is installed. There are a few simple adaptations necessary to the tooling for the Sonorus 1G, which are covered in the installation training and in specification documents that the company issues to all customers.

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 0.05 g to 1.5 g.

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 15 mm long parts with wall thicknesses of 0.075 mm, and achievable tolerances are in the region of 0.01 mm.

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 43 mm long, weighing 0.22 g, with wall thicknesses of 0.075 mm, and with an outside diameter of 0.35 mm and an inside diameter of 0.2 mm.

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 0.02 g, with a 0.5 mm wall thickness, and outside diameter of 4.4 mm and an internal diameter of 2.9 mm. Of enormous interest with this part (see figure 2), was that the part -- with a membrane overmoulding  -- was achieved in 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% increase in productivity using the Ultrasion technology.

Figure 2.

Finally, ultrasonic moulding was successfully used in the production of an eye retina surgery tip (
see figure 3) made from raw polypropylene. The final part weighed 0.1 g, had an internal diameter of 0.6 mm with a 0.17 mm wall thickness, and a wall thickness at the tip of 0.1 mm. 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.

Figure 3.

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 0.075 mm thickness along 15 mm with PP, 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 0.003 mm along 3 mm. The customer was astonished as they felt that PP was not supposed to flash at such thicknesses, and this led to the development of parts that it had previously thought impossible to manufacture.

About the Author
Enric Sirera is the General Manager at Ultrasion SL, Barcelona, Spain. He was previously the EMEA Business Manager at Husky Injection Molding Systems, and also Director of Sales Europe (EMEA) at Moldflow Corporation.

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