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Targeted Surface Finishing, with Process Stability, Productivity

July 11, 2023
To function properly – and to last a lifetime – orthopedic parts require precisely defined finishes on different surface areas. Smith+Nephew has two new work cells to achieve that.

Orthopedic implants require precisely defined finishes on different surface areas, in order to function properly and to last. For the targeted surface finishing of femur components in its new plant in Malaysia, Smith+Nephew is using two manufacturing cells, each with three Rösler R 6/1000 SF drag finishers. Two fully automatic Z1000 centrifuges, equipped with the “Advanced” digital process water-management system from Rösler Smart Solutions, ensure that the process water is reliably and efficiently cleaned and recycled.

More process stability is provided by continuous monitoring and classification of the grinding and pre-polishing media.

Implants specialist Smith+Nephew employs around 18,000 people in more than 100 countries, including a new plant in Penang, Malaysia, where it produces mainly endoprosthetic knee and hip implants.

“For each of our implants we have specifications that precisely define the required quality of their surface finish,” Jürgen Preiser, senior manufacturing engineer, explained. At Penang plant he is coordinating the start-up for the surface-finishing manufacturing cells for cobalt-chrome alloy knee implants. For example, the inner surface areas of femur components must have a precisely defined surface roughness to promote optimal integration into the bone.

On the other hand, the outer surface areas of the implants must have a very smooth, polished surface finish to prevent friction, which causes premature wear. So, some targeted outer surface areas of the femur components must undergo a demanding finishing operation with absolutely consistent results.

Optimal surface finishes

For this task, the customer chose the Rösler R 6/1000 SF drag finishers. These are special mass finishing systems developed for precise and targeted surface finishing of high-value and delicate workpieces with complex shapes.

It’s extremely important that the femur components do not touch each other during surface grinding and polishing. All process parameters, precisely adapted to the stringent finishing requirements, are stored in the PLC control panel. This guarantees absolutely repeatable and consistent results.

“Since our plant in Memphis, Tenn., is using identical finishing processes and machines for the same application, we have plenty of experience and know-how,” Preiser explained. This facilitated and expedited the approval for the respective equipment and finishing processes significantly.

Another point in favor of Rösler was its global presence with, for example, a distributor in Singapore. And, Rösler develops and produces all its products in-house, so its able to adapt equipment and consumables and guarantee stable supplies.

So far, the Malaysian plant has two manufacturing cells. Each cell contains three drag finishers for the process stages pre-grinding with ceramic media, pre-polishing with plastic media and dry polishing with a specially prepared organic polishing medium. To prevent any disruptions of the surface finishing operation, all drag finishers can be used to run the dry polishing process. Furthermore, four of the six drag finishers are suitable for the pre-grinding and pre-polishing as well as the dry polishing operation.

Higher workpiece throughput

The drag finishers consist of a processing bowl with a diameter of 1,000 mm and a carousel with six rotary spindles. These are equipped with specially designed workpiece fixtures, to which the femur components are mounted. The design of the fixtures allows clamping of the workpieces in a position that allows the finishing of precisely defined surface areas. A redesign of the fixtures now allows to increase the number of femur components clamped to one fixture from two to three without jeopardizing the all-around treatment and finishing quality. Jürgen Preiser comments on this design change enthusiastically: “We are now able to process 18 femur components in one single batch compared to only 12 we processed in the past. This increases our throughput by around 50 %, a truly remarkable boost of our productivity.”

During the start phase of the grinding and polishing process, the rotating carousel is lowered. This causes the workpieces attached to the rotating spindles to be immersed into the media contained in the stationary processing bowl. The built-in, immersion depth control system ensures that the specified immersion depth for the various workpiece types is precisely maintained. Separate, independently controlled carousel and spindle drives allow optimal adaptation of the processing intensity to the various femur types.

The Rösler drag finishers, including the drives for the carousel and spindles, are designed to be very sturdy. Therefore, no design change was necessary to cope with the higher operating load caused by clamping three workpieces to a spindle instead of two. Achieving all processing parameters, including the immersion depth of the workpieces, was no problem whatsoever.

When it comes to achieving consistently high finished quality, it is important to maintain the optimal media fill level in the processing bowl – and the optimal media operating mix is important, too. The media level is monitored visually as well as with special sensors. A message is displayed in the control panel, whenever an action by the operator is needed.

“In addition, to maintain a consistently high finishing quality, we installed a media classification system, consisting primarily of a vibratory screening unit. Once per week we pass the ceramic and plastic media through the screening unit, where all undersized media are discharged from the operating mix,” according to Preiser.

Digital process-water management

For cleaning and recycling of the process water from the “wet” pre-grinding and pre-polishing operations in the two manufacturing cells, Smith+Nephew purchased two fully automatic Z1000 centrifuges.

“The dual centrifuge concept allows us to operate two separate process water cycles, one for the finishing operations with ceramic and one for the operations with plastic media,” he explained. “Maintaining two separate water cycles helps us prevent all kinds of problems, for example, foaming issues.”

The control panel of each centrifuge is equipped with the Rösler Smart Solutions digital process water management system – an interactive software for monitoring, data collection, and evaluation of up to 14, individually selectable, process parameters. These include the compound concentration, pH value, and microbiological contamination. The parameter target values and their respective tolerance range are stored in the software, and the actual values are entered for comparison purposes. Whenever one or several parameters are drifting out of the pre-defined range, the system displays an error message and issues easy-to-understand recommendations allowing a quick return to the specified tolerance range. This ensures a consistent high quality of the process water.

“With the digital management system we not only control the process water cleaning and recycling operation a lot better,” Preiser detailed. “In addition, it helps us with the training of those employees who know very little or nothing about how to handle the process water.”

Moreover, the recording of the parameters over a longer time period helps identify long-term trends in the process water so that necessary process water changes can be precisely scheduled with minimal disruption of the manufacturing operations. The digital process-water management system also significantly reduces the consumption of compound and fresh water. And finally, carefully recording of all relevant water parameters represents an excellent tool for documenting the process quality and stability for quality audits and documentation purposes.