In addition to a line of cobalt-chromium alloy surgical implants, one of the top manufactures of implants recently introduced artificial knee joints cast in titanium alloys – leading it to select new casting finishing functionality. For the final surface treatment of the casting prior to implantation, the manufacturer adopted the Rösler RMSF 4/800 “multi-surf” finishing system.
Compared to the “drag finishing” process typically used in such an application – in which workpieces are mounted on fixtures and "dragged" through a stationary bed holding a finishing media – the new method completes the task in a cycle time that is four-times faster, according to Rösler Oberflächentechnik GmbH. Depending on the initial surface roughness readings of the raw components, the grinding operation with specially developed plastic processing media produces surface roughness readings of less than Ra = 0.1 µm in cycle times of about 20 minutes.
In addition, the critical “box” surface area of the implant piece frequently requires a manual grinding operation, and that can be fully automatically ground and polished. This improvement was possible because of Rösler’s novel surf-finishing system, which it calls “the most intensive mass finishing technology.”
Surf-finishing was developed for treating high-value workpieces with complex shapes that require the targeted finishing of precisely defined surface areas. Similar to drag finishing, the knee joint implants are mounted to a rotary spindle equipped with a servo motor.
The actual treatment of the workpieces is done in a rotating bowl filled with grinding media, or more specifically, polishing media. Individually adjustable tilting and rotary motions guarantee highly precise and entirely repeatable surface treatment of the various implant components on specifically defined surface areas.
The process parameters, such as cycle time, rotary speed, and direction, are stored in the equipment PLC as part of workpiece-specific processing programs. Pneumatic clamping of the workpiece fixtures and a loading window, as part of the standard machine version, allow easy automation of the loading operation with an industrial robot.
Saving space, resources, and energy
Intensive treatment in multi-surf finishers offers additional benefits too, including that the process stage that precedes grinding with ceramic grinding media (necessary for drag finishing) is not needed. This reduces the number of machines required for surface grinding and polishing to two multi-surf finishers rather than three drag finishers. This saves capital equipment costs, raw materials, and process energy requirements, and it requires less floor space for the surface finishing operation.
Another benefit is that the grinding times on CNC machines can be shortened by transferring some of the finishing work to the Rösler multi-surf finisher.
Visualization and digitalization
To maximize process safety and stability, Rösler augmented the standard equipment controls with a process visualization module. This allows the machine operators to visually monitor the individual workpiece finishing stages at any moment during the process.
Cleaning and recycling of the process water from the wet surface smoothing operation is handled by a fully automatic, Z 1000 centrifuge. The control panel of the centrifuge is presents an “Advanced” digital, process water-management module from Rösler Smart Solutions. This interactive software continuously monitors, records, and evaluates up to 14 process parameters, such as the compound concentration, the pH value, the microbiological load, etc. As soon as one or several of these parameters exceeds the pre-defined range, those discrepancies are displayed.
In addition, the operators obtain easy-to-understand recommendations for immediate corrective action, so that workers with no particular knowledge or training can quickly adjust the process water parameters to the pre-defined values, to ensure consistently high product quality. Continuous recording of the parameters over time will indicate when the process water quality has declined to a point that it must be refreshed or replaced. This results in substantial water and compound savings and guarantees that the workpiece surface quality remains high.
Finally, the continuous recording of the process water parameters can be used to document process quality and stability for quality audits and documentation purposes.