Drag finishing systems deliver targeted surface finishes on femur components

The exterior surface of the femur component requires a homogeneous, very smooth and polished finish.

Situation

The mission of medical technology company Smith+Nephew is to replace defective body parts to improve quality of life for the patients. Its new plant in Penang, Malaysia, part of the company’s Orthopedic Reconstruction division, produces primarily endoprosthetic knee and hip implants.

To function properly and last for a long time in the human body, orthopedic implants require precisely defined finishes on different surface areas. For instance, on knee implants made from cobalt chrome alloys, 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 must have a very smooth, polished surface finish to prevent friction causing premature wear. Therefore, some targeted outer surface areas of the femur components must undergo an extremely demanding finishing operation with consistent results.

Resolution

For this task, the company chose Rösler R 6/1000 SF drag finishers. These mass finishing systems were developed for the precise and targeted surface finishing of high-value and delicate workpieces with complex shapes. All process parameters, precisely adapted to the stringent finishing requirements, are stored in the PLC panel to guarantee repeatable, consistent results.

To date, the Penang plant is equipped with two manufacturing cells. Each cell contains three drag finishers for pregrinding with ceramic media, prepolishing 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 for running the dry polishing process. Four of the six drag finishers also are suitable for pregrinding and prepolishing.

The drag finishers comprise a 1,000-mm processing bowl and a carousel with six rotary spindles. They are equipped with workpiece fixtures to which the femur components are mounted. These fixtures clamp the workpiece in a position that allows the finishing of precisely defined surface areas. A fixture redesign now accommodates three femur components on one fixture without jeopardizing the all-around treatment and finishing quality.

“We are now able to process 18 femur components in one single batch compared to only 12 we processed in the past,” said Jürgen Preiser, senior manufacturing engineer. “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 maintains the specified depth of various workpiece types. Separate, independently controlled carousel and spindle drives allow the optimal adaptation of the processing intensity to the various femur types.

The media level is monitored visually, as well as with special sensors. A message is displayed in the control panel if operator action is required. In addition, once per week, a media classification system screens and discharges undersized media from the operating mix to maintain finishing consistency.

For cleaning and recycling of the process water from the wet pregrinding and prepolishing operations in the two manufacturing cells, the company purchased two fully automatic Z1000 centrifuges. This allows operation of two separate process water cycles for finishing operations: one for ceramic media and one for plastic media.

The control panel of each centrifuge is equipped with the digital process water management system from Rösler Smart Solutions. This interactive software allows the monitoring, data collection, and evaluation of up to 14 individually selectable process parameters. These include 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.

If parameters drift out of the predefined range, the system displays an error message and issues recommendations. Recording of the parameters over time helps identify long-term trends in the process water so that necessary changes can be scheduled with minimal disruption to the manufacturing operations.