Implants: Selective processing of femur parts with high process reliability and productivity

Medical implants must have different surface properties for different areas. One manufacturer installed special vibratory finishing equipment for selective surface finishing of femur parts for high process reliability.

For optimal patient care, the different areas of implants must have different surface properties. At Smith + Nephew's new high-tech plant in Malaysia, two production cells, each with three R 6/1000 SF drag finishing systems, are currently in use for the selective surface treatment of femoral parts. For the preparation of the process water, the equipment of the two automatic Z1000 centrifuges with the digital process water management system "Advanced" from Rösler Smart Solutions ensures increased process reliability. The continuous monitoring and regular preparation of the grinding media also contributes to this.

Smith + Nephew is one of the leading medical technology companies dedicated to restoring people's bodies and confidence using technologies that take the limits out of life. To this end, around 18,000 employees in over 100 countries work every day to improve the excellence of the product portfolio in the three global business areas of orthopedics, advanced wound management, and sports medicine and ENT through the development and application of innovative technologies. This is also visible in the new high-tech plant in Penang, Malaysia, which will primarily support the company's orthopedics business by manufacturing products for knee and hip arthroplasty.

"There are precise specifications for the various implants as to what surface finish they must have," explains Jürgen Preiser, Senior Manufacturing Engineer at Smith + Nephew. He is supervising the set-up of the surface machining cells for knee implants made of cobalt-chromium alloys at the new plant in Malaysia. For example, defined roughness must be achieved on the inner surfaces of femoral components for optimum integration, while homogeneous, very smooth and highly polished surfaces are required on the outside to prevent abrasion during movements. This necessitates a partial but extremely high-quality and reproducible machining of the outer surfaces of these femur parts.

For this demanding task, the company decided to use the Rösler R 6/1000 SF drag finishing systems. These are special vibratory finishing systems that were developed for the precise, selective individual machining of complexly shaped, high-quality and damage-sensitive workpieces. The femur parts must not touch each other during the grinding or polishing process. Exactly repeatable process parameters, controlled via PLC and adaptable to the respective requirement profile, ensure reproducible results. "Since we use identical machines and processes in our US plant in Memphis for this application, corresponding experience and know-how are available. This makes approval easier and faster. Another aspect that spoke in favor of Rösler as a partner is the company's global presence with a representative office in Singapore. In addition, Rösler develops and produces both the equipment and the process media, so that everything is optimally coordinated and security of supply is guaranteed," says Jürgen Preiser, explaining the reasons for the decision.

The plant on the Malaysian peninsula is currently equipped with two production cells. Three drag finishers are integrated into each cell for the process steps of pre-grinding with ceramic abrasives, fine grinding with plastic abrasives and dry polishing with a vegetable medium. In order not to have to interrupt the process chain during maintenance work, the dry polishing process can be carried out in all cells, and in four of the six drag finishing cells, both the dry process and the wet process can be carried out.

The machines have a 1,000 mm diameter work bowl and a carrier carousel for six work spindles. They are equipped with part-specific workpiece fixtures in which the femur parts can be positioned so that machining only takes place in the defined areas. Optimization of the fixtures means that three parts can now be clamped instead of the previous two femur parts, while still ensuring high-quality machining on all sides. "This allows us to machine 18 femur parts simultaneously. This corresponds to a throughput increase of almost 50 percent and is a considerable increase," notes Jürgen Preiser enthusiastically.

For the grinding or polishing process, the rotating carrier carousel immerses the workpieces clamped to the spindles, which also rotate, into the stationary grinding wheel mass. A standard immersion depth control ensures that the part-specific immersion depth is precisely maintained. The machining intensity can be optimally adapted to the various femur parts by means of the separate and independently adjustable carousel and spindle drives. The higher load due to the additional loading of the spindles, each with an additional femur part, naturally also requires that the entire system technology, including the drives, is prepared for the higher performance. This is the case with Rösler systems as standard, which is why all other machining parameters, including the plunging depth of the workpieces, are maintained and the shortest cycle times can be achieved.

Important process parameters when it comes to ensuring machining reproducibility are the filling volume and the composition of the grinding media in the work bowl. The filling volume in the work bowl is monitored both visually and with sensors, and the operator receives an indication when action is required. "To ensure machining quality, we also chose the grinding wheel preparation system. In this system, which is equipped with a screening station, the ceramic and plastic grinding wheels are cleaned once a week and all those below a defined size are sorted out," describes Jürgen Preiser.

Smith + Nephew has invested in two fully automatic Z1000 centrifuges for cleaning the process fluids from the wet processing steps of pre-grinding and fine grinding in the two manufacturing cells. "This enables us to treat the process water separately for ceramic and plastic grinding media, which helps avoid problems caused by foaming, for example," reports Jürgen Preiser. In addition, the digital process water management system from Rösler Smart Solutions is integrated into the controls of the centrifuges. This innovative, interactive monitoring solution for process water purification and treatment enables the monitoring, recording and evaluation of up to 14, individually selectable process parameters, including compound concentration, pH value and microbiological load. The target and tolerance ranges of the relevant parameters are stored in the software and the results measured in each case are entered. If there are deviations in one or more parameters, this is advertised and personnel automatically receive easy-to-understand and directly implementable recommendations for action to bring the process water back within the specified range and thus ensure product quality.

"This digital solution not only gives us better control over process water treatment, it also simplifies the training of employees who previously had no experience with this topic," explains Jürgen Preiser. In addition, the automatic storage of values allows changes in the process water to be detected over a longer period of time, so that a necessary change can be planned into the operating sequence accordingly. Savings in water and compound consumption can also be realized in this way. The seamless recording of data is also helpful as proof of process quality and stability for quality audits and for documentation purposes. "We are currently in the validation phase with the first two production cells, and the start of series production is planned for spring 2023. In parallel, we are currently planning two further manufacturing cells with Rösler for identical products and processes," concludes Jürgen Preiser. (OM-07/23)