Today more and more manufacturers of medical products are using PEEK (polyetheretherketone) for bone surgery, but also in areas of traumatology and orthopaedics. Why PEEK? Because this material is biocompatible, inert to body fluids and can be processed relatively easy with high precision micro moulding for individual implants or surgical instruments.

PEEK anchor to re-attach soft tissue to bone

PEEK anchor to re-attach soft tissue to bone. The complete detachment of ligaments, tendons or other soft tissues from their associated bones within the body is a relatively commonplace injury, particularly among athletes. Such injuries are generally the result of excessive stresses being placed on these tissues. For example, tissue detachment may occur as the result of an accident such as a fall, over-exertion during a work-related activity, during the course of an athletic event, or activities. For a complete detachment, however, surgery may be needed to re-attach the soft tissue to its associated bone or bones. Numerous devices made out of PEEK material are currently available to re-attach soft tissue to bone. Examples of such currently- available devices include screws, staples, suture anchors and tacks.

PEEK micro molded parts used for minimally invasive surgery produced by microsystems

100,000 Instrument housings moulded in PEEK with 0.3 mm wall thickness (weight 0.014 gramme) used for minimally invasive surgery, or keyhole surgery, is a modern surgical technique in which operations are performed far from their location through small incisions of about 0.5-1.5 cm somewhere in the body. There are a number of advantages to the patient with keyhole surgery versus the more common, open procedure. Pain and haemorrhaging are reduced due to smaller incisions and recovery times are shorter.

Size comparison, original PEEK granulates with moulded PEEK parts

Size comparison of original PEEK granulates (left) with 0,018 grmmme weight and moulded PEEK part (right) with 0,006 grammme weight used for an implantable device.

For medical products, PEEK provides a great benefit: it saves weight, allows more design freedom and a greater functional integration, it also scores with X-ray transparency and elasticity, which corresponds approximately to bone. At the same time it is a lower-cost alternative to Titan, the classic implant material, especially when using medical instruments or endoscopes, where the good electrical insulation properties of PEEK come into play.

PEEK parts are often produced by standard injection moulding, but this process quickly reaches physical boundaries for high precisions medical parts. This technology can often not achieve the recommended part precision, especially not the necessary long- term quality stability and besides always has a great material loss by needing oversized sprue. To improve this situation Micro Systems UK Ltd Austria Operation is using micro moulding machines to process PEEK materials. The focus of our production starts with a design study through to mass production of high precision micro plastic part. The key of success is using only latest and suitable production equipment, optimising all influencing parameter by DOE and stabilising environment production condition. Micro Systems UK operates a quality assurance management system with accreditation to ISO 9001 and ISO 13485.

Medical devices that are intended for long-term contact with body tissues must meet particularly high quality requirements for registration in Europe or in the US. The manufacturers have to demonstrate, on the one hand, that the raw materials are suitable for the respective area of application, and, on the other hand, how they ensure consistent quality. For example, in further processing using injection moulding, different cooling speeds influence the material properties of PEEK. Also, the duration and temperature of the heat treatment has a direct influence on the crystallinity of the PEEK polymers and thus their mechanical properties. That is, on the one hand, that the material properties can be specifically controlled, on the other hand also, that any variation in the production process change the quality of the parts. So it is evidently important to stabilising every relevant process conditions to get over a long production time of only high quality parts.

Besides, the injection moulding machine which controls the process temperature of 400°C and injection pressure up to 3500 Bar, is one key feature of a stable injection moulding process. Due to the fact that PEEK needs a cavity surface temperature of up to 220°C, is it important to take into account,
in the design phase, a uniform tempering of the tool. The heat distribution in the tool has to be calculated and optimised before manufacturing like mould flow analyses, which are used to calculated the filling characteristic of the moulded parts. For micro moulds we use electric elements to heat the tool instead of oil tempering device; the reason to use electric heaters is the efficiency of energy, particularly when running this device in the cleanroom, additional electric heater creating less dust and are safer for the machine operator.

Also due to the fact that medical grade PEEK price can raise up to over £2,000 per kilo, it is evident that the sprue weight is as small as possible in relation to the micro part.

On standard injection moulding machines there is a physical limitation reducing the shot weight due to the reason of the cold material slug on the machine nozzle or the hot runner tip.

Optimised micro moulding machines allowed us much smaller sprues in the design thn using standard moulding machines. The micro moulding processes guarantee injection of thermal homogenies material without a cold material slug so 0,1 gram shot weight or below is possible. With this feature the weight ratio sprue to part is much smaller and save material and costs. Of course only in combination with optimised screw geometry to ensure that the resistance time of the plastic in the barrel is still below the specification of the material supplier. In general micro moulding is physically equal to conventional moulding, only faster with a higher dynamic and more accurate.
Micro parts out of PEEK with wall thickness below 150 μm are achievable depend on flow length and PEEK type. Regarding the size of injection point, we recommend always using as big as possible to achieve a stable process, but often on micro parts there is not enough space on the part to design an optimal gate. Gate point dimensions below 0,2 mm are critical in PEEK and must be examined in practical test.

For many years Micro Systems UK has processed PEEK for different application from thin wall up to 5 mm wall thickness, from part weight of 0,001 gramme to over 1 gramme.

By using the latest micro moulding technology gives a significant improvement of part quality and confirmed this with simultaneous cost reduction due to faster cycle time and smaller sprue. The improved long-term stability of the production process allowed additionally reducing operator time. It is relatively straightforward to process PEEK as long as you know exactly your process window and equipment. Strict operating procedures are recommended for heating up and shutting down the machine cell enabling to reach a proper and stable productions process. Monitoring and checks are carried out regularly throughout the entire process, ensuring that the micro injection mould parts are all of the same high quality before shipping to the customer. I personally look forward to the next PEEK project.

Micro System UK Ltd is a leading micro mould manufacturer, with over 50 toolmakers working in England and Microsystems Singapore. Since Micro Systems UK was founded in 2003, it has continuously grown and is now a key player in the high precision mould production industry.

Micro Systems Austrian Branch is the subsidiary of the renowned English toolmaker Micro Systems UK Ltd. We are a certified subcontractor, ISO 9001 and ISO 13485, of high precision micro parts, our service starts from parts design to fully automated mass production, especially in the medical field. Micro systems produced in Austria under cleanroom conditions are fully automatic micro parts from bioresorbable to high temperature materials like PEEK. Technical challenges and innovative solutions are our daily work. Our vision as a supplier of micro plastic parts is to establish a long-term reliable partnership with our customers.

Article by Martin Ganz
Micro Systems (UK) Limited, Austria Operations

Published in CMM Magazine – commercial micro manufacturing international Vol 9 No. 4