Replacing polishing with roller burnishing - A success story from medical technology
Can roller burnishing replace expensive and unproductive processes such as polishing? This question can easily be answered with "it depends". We will explain exactly where the limits lie and what requirements can be achieved using a customer example from the medical sector.
Hip implants place high demands on production
In medical technology, the demands placed on the products are extremely high. We all want to be and remain fit and mobile. We want to be able to move without pain and remain active even at an advanced age. Implants or artificial joints therefore play a major role when it comes to fulfilling these wishes. Unfortunately, the reality is that in many cases our joints are already worn out and we need a replacement. Current figures show that the number of operations for implants has risen by 7% in 2023. In Germany alone, 220,000 hip joints are implanted every year. On average, we assume that one in four people will need an artificial joint in the course of their lives.
Unfortunately, not all operations are successful and, regrettably, such a joint may not last forever. The approximate lifespan of an artificial joint is around 10 years. If the quality is not good, the lifespan is reduced and premature revision occurs, which always entails the risk of another operation. In short, we all have a considerable interest in ensuring that the production of a hip implant, for example, meets the highest standards and that the quality of the joint is absolutely perfect.
One requirement placed on the joint is low friction between ball and socket. Both surfaces must be extremely smooth to prevent chipping due to highly stressed surface areas. To achieve this goal, the balls, which are usually made of CoCr steel or titanium, are polished in the final processing step. For the doctor, the shine of the surface is a decisive criterion that speaks for or against the use of the implant.
Polishing as a finishing process
The final polishing process is carried out in several stages. The surface roughness is reduced step by step with increasingly finer polishing tools. In terms of productivity, the low material removal is offset by the required roughness of usually Ra < 0.02 µm. In order to achieve a mirror-like surface, polishing tools with extremely small grain sizes must be used. The grit size is reduced in each polishing stage.
In order to increase productivity, our customer questioned this process chain and looked for alternatives to the polishing process.
Roller burnishing reduces the polishing effort considerably
The obvious idea and therefore also the goal for this project was to completely replace the polishing process with a roller burnishing process. By using a hydrostatic roller burnishing tool of type HG6-9, the roughness value could be reliably reduced to an Rz value of less than Rz < 0.6 µm. A rolling speed of vw = 150 m/min and a rolling feed rate of fw = 0.1 mm were used.
In roller burnishing, the roughness is reduced through a forming process by pressing a roller or ball onto the surface. This means that the rolling body generates such a high contact pressure through the roller burnishing force that the material of the roughness peaks begins to flow. This forces the material from the peaks into the valleys, which lowers the peaks and raises the valleys. In this case, a rolling force of Fw = 750 N was used. In other words, a force is used that is manageable for conventional processing machines.
By using the roller burnishing process described above, polishing could not be completely replaced, but it could be significantly reduced. It was possible to eliminate some of the polishing steps and thus achieve an overall cost reduction of -27%.
