Acetabular cups for "dual mobility" use are generally made from cobalt chrome or titanium. They are produced through a milling process followed by a fine grinding or fine turning operation to smooth the initial surface roughness. After this machining operation, the Ra value of the raw components usually ranges between 1.0 and 0.6 µm. However, before the cups can be implanted into the body, their surface must be significantly smoother. The maximum surface roughness must not exceed an Ra value of 0.05 µm. This ensures optimal functionality of the acetabular cup, sufficient coverage with lubricants produced by the body, and prevents unwanted friction between the PE insert and the metal cup.

For this finishing task, the special challenge is not achieving the required surface roughness values, but rather the critical position of the surface area to be treated on the inside of a half-sphere. This represents a so-called "cup shape" that normally prevents the processing media from exerting any significant pressure. If the workpieces were to tumble freely in the processing bowl of the rotary vibrator, the internal space of the workpieces would be filled with media. However, effective grinding would only take place on the external surface. There would be insufficient "rubbing" between the media and the workpieces on the internal surface areas.

In this particular case, Rösler was able to utilize its decades-long experience in surface treatment, especially its profound knowledge of mass finishing and its in-house engineering and manufacturing capabilities. "The acetabular cups are finished in a rotary vibrator without an inner dome. These machines were initially developed for the aerospace industry, where high-value, complex components must be finished gently while achieving high metal removal rates at the same time. Compared to traditional rotary vibrators, the processing bowl does not contain a vibratory motor in the inner dome, but instead has two motors on the outside of the bowl. This provides space in the center of the bowl for a clamping device with a unique inlay, in which the acetabular cups can be fixtured," explains Michael Striebe, Global Sales and Process Expert at Rösler.

Clamping the workpieces allows the processing media to exert ample pressure on the inner workpiece surface, making the grinding and polishing process highly effective. For the finishing process, a plastic grinding media specially developed for medical technology applications and a dry polishing media are utilized. With the extremely fine structure of the grinding media, very low surface roughness values of under Ra = 0.05 µm are achieved during the pre-grinding operation. The highly homogeneous and fine structure of the grinding media creates the perfect surface prior to the final polishing step. This further reduces the surface roughness value to 0.02 µm or lower.

The cycle times depend on the base material from which the acetabular cups are made and the initial condition of the raw workpieces.

Robotic surface grinding and polishing of acetabular cups may be fast, but the programming of robot movements, process controls, and wear compensation for the grinding and polishing tools for every single workpiece type makes the process very expensive. With robotic polishing operations, only a single workpiece can be picked up and clamped. In addition, each workpiece size requires its own precisely defined robotic program. Moreover, users must cope with considerable wear of the equipment, such as polishing tools, brushes, or other special tools. Due to tool wear, the metal removal rate changes constantly. This must be considered when planning a robotic finishing process. Because it always performs the same movements, the robot itself cannot compensate for these wear rates. The same applies if the final polishing step is performed manually. Manual polishing is also fast but prohibitively expensive. Fluctuating finishing quality poses another risk in manual finishing because the operator cannot always maintain the same operating parameters. Finally, it is becoming increasingly difficult to find experienced personnel for such grinding and polishing operations.

In contrast, once the workpieces are clamped in the processing bowl, all that is required for a mass finishing operation in a center-less rotary vibrator is setting the processing time. The rest is handled by the machine. The process requires no operator and no additional automation tools. Only suitable grinding and polishing media, water, and electrical power are used. Of particular importance is the fact that wear of the grinding media does not affect the finishing results whatsoever. The only requirement is that the media level in the machine must be kept constant.

"All this results in more consistent, repeatable, and overall higher finishing quality with a more homogeneous structure," explains Michael Striebe, who has been supporting customers in the medical engineering field for the last 10 years in his role as Global Sales and Process Expert at Rösler. "My experience clearly indicates that the surface finishes generated with mass finishing have a finer structure and higher quality. We consistently achieve surface roughness readings that are lower than the specified values. Depending on the initial surface roughness of the raw workpieces, surface readings of Ra = 0.01 µm are possible without jeopardizing the dimensional tolerances of the workpieces. This extraordinary quality proves the efficiency and effectiveness of mass finishing technology."

Surface finishing operations for medical components, especially orthopedic implants, demand not only high cost-efficiency but also compliance with the specified surface roughness values and dimensional tolerances of the workpieces. In addition to achieving absolutely consistent, high-quality finishing results, the entire process must be completely replicable in terms of design, stability, and repeatability. It must also be fully documented. Comprehensive workpiece and process monitoring is an essential prerequisite for manufacturing processes that are subject to validation and audits.

All these demands can be met with the latest version of the Rösler digitization software Smart Solutions Comfort (RSS). The digital software automatically monitors process water quality, automatically takes water samples at defined time intervals, documents the measured values, and corrects deviations from preset values through targeted dosing of chemical additives. This ensures stable process water quality. Furthermore, it significantly supports process repeatability and thus guarantees consistently high finishing quality on the processed workpieces. Whenever the system identifies additional required actions, it informs the operator and issues precise, documentable action proposals. Any adjustments to the process are also documented and completely traceable.

With the development of this efficient grinding and polishing process in a center-less rotary vibrator, Rösler offers an economical alternative to robotic and traditional manual finishing operations. The high manpower costs required for manual polishing can be reduced by 70–80%, because the only remaining manual operation is placing the workpieces in the rotary vibrator. Excellent surface quality, absolutely repeatable results, and the option for digital process monitoring and control with Rösler Smart Solutions software (RSS) qualify this process as an excellent solution for the precise surface finishing of acetabular cups.

"When we developed the new process, we carefully studied the ‘pain points’ of the systems available on the market and adapted our solution accordingly," explains Michael Striebe. "This was only possible by combining our competence in equipment manufacturing, process development, and media and compound production, all done in-house. The future of acetabular cup finishing lies in the center-less rotary vibrator."

For Information: 

Rösler Oberflächentechnik GmbH 

Vorstadt 1, 96190 Untermerzbach, Germany

Michael Striebe

Tel. +49.9533.924-0

E-mail: m.striebe@rosler.com

www.rosler.com