Martin Ganz, Micro Systems UK, explains why PEEK offers success for the manufacture of medical parts and the rise behind its popularity.
Close to the Bone
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.
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 reaches quickly 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. The 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 successes 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 demonstrate 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 control process temperature of 400°C and injection pressure up to 3500Bar, 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, it is evident 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 for this is the efficiency of energy, particular when running this device in the clean room, additional electric heater creating less dust and are safer for the machine operator.
Also due to the fact that medical grade PEEK price can rise up to over £2000 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 than 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 saves 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 depending 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 dimension below 0.2 mm are critical in PEEK and must be examined in practical test.
The complete detachment of ligaments, tendons or other soft tissues from their associated bones within the body is relatively commonplace injuries, 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.
For many years Micro Systems UK has processed PEEK for different applications from thin wall up to 5mm wall thickness, from part weight of 0.001gramme to over 1g.
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!