Small wonder
How can micro parts be produced by a small footprint machine that is designed to use as little energy as possible, optimise the properties of melted plastic, reduce material wastage, and reduce contingent tooling costs? Enric Sirera reveals innovation in micro moulding for medical OEMs
Abstract
Over the course of the last few decades, the technology of plastic injection moulding, and its more recent development into the area of ‘micro’ injection moulding has seen little in the way of radical re-interpretation and evolution. Recently, a new and innovative approach to micro moulding has been introduced and commercialised that reinvents the entire process. This opens up significant advantages for medical device OEMs in terms of the costs of manufacture and the possibilities that now exist in terms of product design due to the characteristics of the process. With an ability to reduce melted polymer to the viscosity of water, ultrasonic micro moulding heralds a new dawn for the design and manufacture of micro plastic parts.
Summary
Across industry in general, but in the medical industry in particular, the drive in terms of product and component design is towards miniaturisation. In the medical sector, beyond the usual industrial desire for use of less material and weight savings, miniaturisation is driven by the desire on behalf of medical product manufacturers to produce products that allow for ease of implantation or that facilitate surgical procedures with minimal invasiveness.
Of course, there is a trade off between miniaturisation and the actual function of a medical product. Smaller and smaller is not always better and there are limits as to the extent to which medical products can be reduced in size and still produce optimal patient outcomes. A few argue that there is a danger in some instances that the oft quoted design mantra that “form should follow function” can become lost as medical product designers continually strive for miniaturisation. But in general terms, medical designers are forever pushing the boundaries of increased precision and reduced size in balance with part functionality and user ergonomics.
There are particular areas where continued requirements exist for efficacious parts that are as small as possible, for example in various in vivo diagnostic applications such as embedded sensors, in the development of intravascular ultrasound catheters, and for numerous micro-invasive technologies treating a variety of chronic medical conditions.
Whether product designers are focused on such cutting edge developments, or are striving for miniaturisation in other areas of medical product development, very often there is a limit on product design parameters from the nature of (and availability of) cost-effective manufacturing alternatives.
In the area of micro plastic part manufacture, until recently, these parameters have been dictated by the nature and capabilities inherent in micro ‘injection’mmoulding. Today, however, a new and innovative technology for the manufacture of micro plastic parts is making some waves in various industry sectors, and is forcing product designers to re-evaluate the limits on what is now possible.
Moulding plastic parts
The history of plastic moulding is a long one, actually stretching back to the 1870s. Early machines were manual, and it was not until the 1930s that electric machines began to be developed. The injection moulding machines that many people relate to today, with screws and barrels have been used commercially since the 1950s, with fully electric machines coming into existence in the early 1980s.
For much of its history, plastic injection moulding has evolved through variations on a theme. The basic underlying process of plastic pellets being placed in a hopper, melted in a hot runner, and then injected into the mould has remained the same.
Indeed, when looking at developments in the area of moulding ‘micro’ plastic parts, what can be seen is a migration of ‘macro’ injection moulding technologies to cater for the manufacture of micro parts, and therein lies the potential for huge inefficiencies in terms of energy usage, material wastage, and unnecessary and expensive tooling.
For the first time in the history of plastic moulding, a new approach has been commercialised that uses ultrasound as the agent of material melting, and which has been developed specifically for the manufacture of micro parts. The technology approaches the manufacture of micro plastic parts from a completely different perspective. How can micro parts be produced by a small footprint machine that is designed to use as little energy as possible, optimise the properties of melted plastic, reduce material wastage, and reduce contingent tooling costs? From this perspective, an innovative technology and solution to micro plastic part manufacture was developed and is now offering numerous hitherto impossible opportunities to medical product designers.
Ultrasonic Micro Moulding
The company behind the development of the ultrasonic micro moulding technology is Barcelona-based Ultrasion, and the machine now being sold worldwide is the Sonorus 1G.
What is most obvious when looking at the technology is that there is no barrel and screw. In this technology, ultrasonic waves are used to melt plastic granules that are fed direct to the mould, and are melted in milliseconds once contacted by an ultrasonic horn. The technology was developed through years of research and development at the industrial research group ASCAMM in Barcelona, which attempted to marry an expertise and understanding of the versatile nature of ultrasonics in various industrial applications to the particular draw backs and issues that were confronted when attempting to mould plastic at a micro level.
The machine has 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 polymer at room temperature. The material is then contacted by an ultrasonic horn or ‘sonotrode’ which is lowered, and as well as melting the polymer forces the material to flow into the mould cavities. The sonotrode then returns to its original position, and the cycle begins again.
Advantages for medical part design and manufacture
In essence, the Sonorus 1G is a technology solution for plastic micro part production that has been designed for this purpose and which has sought to address many of the problems associated with traditional micro ‘injection’ moulding.
Very often, micro injection moulding suffers from the fact that the technology requires high moulding pressures, which means that the technology solution is oversized for the manufacture of micro plastic parts. Also, the micro ‘injection’ moulding process — using as it does electrical heater bands — is extremely energy inefficient, (running continuously at 1.7 kW/h) and such a heating process and the speed of material injection required leads to significant raw material degradation.
Add this to the inherent complexity and cost of traditional micro injection moulding machines and the various necessary peripherals, and the process can be costly and restrictive in terms of micro part manufacture.
Ultrasion’s ultrasound micro moulding technology is outstandingly precise, uses no heaters, and the process means there is no material residence time, and no material degradation. Also, as energy is only imparted for a split second when the ultrasonic horn contacts the raw polymer to induce melt, the technology uses up to 90% less energy than traditional micro plastic moulding.
Perhaps of paramount importance, however, especially for medical part manufacture where raw materials are often disproportionately expensive, is the reduction in material waste.
Material wastage using traditional injection moulding technologies can be a huge problem when making micro parts, with in some instances 99% of material processed being scrapped. In the ultrasound micro moulding process, only the material required is dosed, and so runner and sprue wastage is minimised.
Also, the ultrasonic moulding technology requires low moulding pressures and is highly replicable, much of this being due to a specific characteristic of the process that opens up countless possibilities for medical product OEMs as they attempt to overcome obstacles in the manufacture of micro plastic parts.
Opening up product innovation
This particular characteristic is that materials melted via ultrasonics exhibit significantly reduced viscosity, allowing the production of thinner walled, flatter, and longer parts than has ever been achievable before.
It is not just the use of ultrasonics that reduces fluidity. The new sprue concept in the Ultrasion technology means that it behaves as an energy director as well as part of the ejection system. The energy director orientates the waves in the flow direction, therefore the molten material and waves travel together toward the cavities, reducing the viscosity of the polymer.
It’s all about applying a high intensity mechanical vibration that transmits energy directly into the polymer molecular structure resulting in a very fast and efficient melting ‘inside-out’ rather than ‘outside-in’ which is typically how melting occurs in injection moulding via electric heaters.
In real terms, this particular characteristic of the ultrasonic moulding process means that 15 mm long parts with wall thicknesses of 0.075 mm are easily attainable. Achievable tolerances are as tight as 0.01 mm.
The Sonorus 1G accommodates shot weights from 0.05 g to 1.5 g, and is being used across the world by various OEMs in the medical and other industry sectors. The proprietary nature of many of the applications of the technology are such that precise case studies are not possible, but it is possible to give some details of recent OEM projects and the materials used and dimensions achieved.
The first was 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, 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.
Finally, ultrasonic moulding was successfully used in the production of an eye retina surgery tip (see figure 4) 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 mouding.
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.
Conclusion
For any new technological development to enhance the existing manufacturing environment, it must of itself contain significant advantages, and also open up design and manufacturing possibilities previously unattainable.
Ultrasion’s new ultrasonic micro moulding process is one such technology that is beginning to redefine the parameters that were deemed to be in place when looking to manufacture micro medical plastic parts. This technology represents a new and innovative approach to micro plastic part manufacture, and as an alternative to traditional micro-injection moulding technologies exhibits significant advantages in terms of power consumption, reduced material wastage, and reduced tooling costs.
Perhaps of most interest, however, is that the nature of the process, and especially the reduced viscosity characteristics that ultrasonics can achieve as the melting agent, opens up the possibility of part design and part characteristics that have hitherto been unattainable. It is here that the interest that Ultrasion is attracting from OEMs across industry is focussed, most especially in the areas of medical devices and microfluidics.