Developments in Micromoulding
Dr Paul Reay describes some of the micro-injection moulding work underway at the University of Bradford’s Polymer Micro and Nano Technology Centre.
Recent and emerging advances in micro- and nanotechnology are realising devices with a huge range of applications. This demand has led to the emergence of micro-injection moulding (micromoulding) as an optimum technique for micro component manufacture that offers all the benefits of conventional injection moulding such as high production capacity at low marginal cost.
The Research and Knowledge Transfer Centre for Polymer Micro and Nano Technology (Polymer MNT) is a leading facility within the Polymer Interdisciplinary Research Centre (Polymer IRC) at the University of Bradford. Under the direction of Dr Ben Whiteside, the Centre was created in May 2009 as a focal point to allow access to equipment and the expertise required for companies of all sizes to develop a successful micromoulding or nano scale surface feature moulding process.
Since its establishment, the Centre has been approached by numerous companies seeking expertise to enable them to get a product to market. It is currently undertaking research and development in application areas such as dental components, surgical devices, micro implants, micro optics, energy harvesting devices and anticounterfeit technologies. The nature of the work can range from a raw concept for a device that needs a full process specification to get it into the marketplace, to an existing product that requires refinement or redesign to reduce cost or increase functionality. Polymer MNT offers assistance throughout the evolution of the manufacturing process from materials formulation through to processing stages, robotic handling/automated inspection systems, and Internet-based data management and control systems to provide full traceability at each stage.
The Centre’s equipment includes sensors and user-friendly software to support manufacturing data and
trials. The hardware, designed for micromoulding and nano moulding activities, includes three Wittmann Battenfeld Microsystem 50 micromoulding machines (www.wittmann-group.com
), a Wittmann Battenfeld MicroPower 15 micromoulding machine, two Fanuc 2000i Roboshot injection moulding machines
), a Rondol High Force 5 injection moulding machine (www.rondol.com
), and an Polymer IRC–Rondol designed machine with in-line compounding for twin screw compounding and micromoulding.
The combination of equipment gives Polymer MNT the flexibility needed to make a variety of sizes from a range of materials such as engineering thermoplastics, nano composites, and metal and ceramic powders. For each of these processes, bespoke systems have been developed for monitoring the behaviour of the raw material and the equipment dynamics in production processes. Ongoing work is focused on developing applications for novel sensor technologies such as ultrasound and optical methods, and data acquisition systems for full traceability in the product manufacturing environment. Techniques used for assessing the shape and mechanical properties of micro moulded components and surface features include atomic force microscopy, optical profilometry, nanoindention, nanoDMA (for dynamic mechanical property measurements) and scanning electron microscopy.
The size of the components typically means that inspection is impossible with the naked eye and automated machine vision techniques are crucial. High-resolution 2-D and traversing single camera systems are used for the 3-D assessment of micro scale products and surface details during the manufacturing process. Robotic handling systems make processes reliable and more easily transported to the next stages for quality assessment and packaging. Class 7 certified cleanroom manufacturing environments are available to ensure process and products are free from contamination.
Polymer MNT is a partner within the Nanofactory (www.nanofactory.org.uk
) consortium, which is supported by £2,511,835 investment from the European Regional Development Fund. Nanofactory helps accelerate innovation and create commercial opportunities in micro and nano technologies by leveraging the world-class expertise offered by six research intensive universities in the Yorkshire and Humber region.
Development of a dental micro component
An example of a current programme of work between Polymer MNT and a local company is the development of a novel root canal treatment. The Centre was approached by DRFP Ltd (www.smart-seal.co.uk
) to assist with the manufacture of a long, thin component of 150-µm diameter tip size and a 2.5ş taper that forms the core of the root canal filling.
The obturating point has a 150-µm diameter tip and a 2.5ş taper
DRFP needed the Centre’s help to mould the part using a heavily filled ceramic powder composite. The Centre assisted with material specification and compounding and then designed the moulding process. It is now building an automated product inspection and feedback system that involves a camera-based system with custom optics and illumination for inspection of the tip dimension that feeds back to the handling system. The product has provided a step change in treatment performance, and the Centre is now assisting with scale-up of the existing process. This involves optimisation of fail-safes to allow lights out unsupervised operation to achieve outputs in the region of one million components per year.
Root canal treatment (endodontics) is needed when the pulp of the tooth has become infected through decay or injury. An estimate is that in excess of 100 million root canal treatments are performed worldwide per annum. Obturation points are tapered cones, traditionally made from gutta-percha (GP; trans-1, 4-polyisoprene) that are forced (or by using heat to improve flow) into the prepared and cleaned canal together with sealant. If heat is used to improve flow, on cooling the GP can shrink to create cavities that allow the potential for microbial growth. Up to 30% of initial treatments have been reported to fail, usually due to re-infection because of poor cleaning, inadequate sealing, incomplete filling or material deterioration, and hence require revision or removal of the tooth. Generally, the procedure is difficult, fiddly and time consuming, which persuades many general practitioners to refer cases to endodontic specialists rather than undertake the treatment themselves.
DRFP was established to exploit hydrophilic polymer technology in the medical sector. The first product, smartpoint, targeted at providing an easier and quicker means to obturate root canals, was launched in 2007, and an improved version, propoint was launched in 2010. Both propoint and smartpoint comprise a radio-opaque core with a hydrophilic polymer coating that expands laterally as it absorbs water from the tooth, thereby creating a tight seal that prevents re-infection. The hydrophilic coating is proprietary DRFP technology based on materials from the contact lens industry. The radio-opaque core was developed with Polymer MNT and necessitated the twin characteristics of providing the appropriate handling characteristics (stiffness to insert with the flexibility to accommodate curvature in canals) and the ability to ensure that the coating expands radially rather than axially.
The design specification posed a number of challenges in material compounding and conditioning, tool manufacture and micromoulding. First, a multi-component polymer matrix and micro powder filler material needed to be accurately blended to provide good consistency and the correct degree of radiopacity. The use of such heavily filled hygroscopic materials then required careful material preparation and handling prior to, and during, the micromoulding process. To achieve the required feature sizes and tolerances of 150 µm ±5 µm at the tip diameter, specialised manufacturing techniques were needed to produce the cavity forms for the micromoulding process. Through Polymer MNT’s network, Microsystems UK (www.microsystems.uk.com
) was selected as the provider of the cavities based on its reputation for supplying high precision tooling for injection moulding and its micromoulding activities using advanced manufacturing techniques such as micro machining and micro electrodischarge machining.
The product form required the accurate high pressure and speed injection rates offered by a Microsystem 50 micromoulding machine to ensure that the high aspect ratio cavity was adequately filled. The machine was also able to accurately dose material in a highly repeatable manner to minimise variation of product properties throughout a production batch. Critically, the required narrow optimised processing window was achieved through the expertise of the staff at Polymer MNT and
the bespoke equipment at the Centre.
This dental micro component has a radio-opaque core with a hydrophilic polymer coating
A clinical trial of three years duration with annual
check up for patients began in late 2007 using smartpoint. Informal feedback based on cases that have been monitored for more than three years has indicated the clinical results are good with good healing and no complications. A user panel of approximately 20 dentists has been set up to provide feedback on DRFP’s products. To date, there have been more than 1,200 cases documented with 14 failures (retreatment required). This failure rate of approximately 1% compares favourably with the figures for standard treatments that are reported to be up to 30%. Informal feedback from various sources (customers, user panel member and clinical trial) indicates that patients report low levels of post-operative pain, which was attributed to a less "aggressive” procedure (no force is required to compact the obturation point) and the good healing attributes of the product.
More than 100,000 points have been manufactured and there has been an increasing demand for the device from the dental community in mainland Europe and the UK. DRFP won recognition for propoint at the Medical Design Excellence 2011 awards. The collaborative work is now concentrating on process scale up. Critical to this goal will be an increase in core output utilising multi-cavity tooling with improved process repeatability and better product quality with tighter tolerances.
Dr Paul Reay is Business Development Manager, Polymer Engineering, School of Engineering, Design and Technology
University of Bradford, Richmond Road Bradford BD7 1DP, UK
tel. +44 (0)1274 233 744
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