All-encompassing finishing of implants improves quality and lowers costs
Significant factors for a comfortable life with a knee implant are its shape and the final finish of its surface. By combining their knowhow, Haas Schleifmaschinen GmbH and Rösler Oberflächentechnik GmbH are now able to offer a comprehensive system for the manufacture and surface finishing of medical implants – from the raw casting all the way to the finished high-tech implant. The result is a clearly defined and repeatable manufacturing process without any interface problems that results in higher quality products at lower costs.
Medical device and component engineering is a growing industry all over the world. A significant portion of this growth can be attributed to endoprosthetics, especially to knee implants. The growth in this sector is projected to amount to 10 – 12% per year. Reasons for this increased demand for knee implants are a higher life expectation as well as increased wear and sickness of body joints due to extreme sports and increasing obesity. Suppliers are reacting to this trend by expanding their manufacturing capacities for implants made from cobalt-chrome, titanium, special alloys and ceramic. In line with increasing their capacities the manufacturers are also striving for further quality improvements and lowering their costs. To assist in this endeavor the leading companies in the field of grinding and polishing of implants, Haas Schleifmaschinen GmbH and Rösler Oberflächentechnik GmbH have joined forces in a technical cooperation in the field of medical implants. The equipment and manufacturing engineering knowhow of the grinding specialist Haas has been setting the standard for shaping parts with different geometries all over the world. For the subsequent final fine grinding and polishing the drag finishing technology and process consumables developed by Rösler has become the preferred finishing technology by many of the major manufacturers.
Fine tuned process technology
The two partners are helping manufacturers by co-operating with the customer early and are exchanging their technical ideas during the planning phase of a project. In doing this the customer is assured of the optimum process technology by defining the parameters of the various interdependent grinding steps, like for example the surface smoothness that can be achieved by geometric grinding in an economically feasible manner. This guarantees that the implants can not only be manufactured consistently in the specified geometrical shape but also at the lowest possible costs.
Another benefit, especially in the case of automated manufacturing systems, is the consideration of the various interface points between the steps of geometric grinding, parts transfer and drag finishing. Other aspects of this cooperation are the joint R&D activities for further optimizing the surface finishing processes. This gives the customer complete choice of a part spending longer on the grinding machine, at the same time achieving shorter drag finishing times or shorter cycle times on the grinding machine with the extra finishing work being done in the drag finishing machines.
It’s an approach that clearly works as more than 1,000,000 femorals are processed by Haas Multigrinders and Rösler SF drag finishing machines every year.
High precision geometric grinding
For creating the geometric shape of the implants, grinding machines of the type Multigrind from Haas are utilized which are now available with an integrated belt grinding system. The Multigrind system allows the simultaneous processing of two implants. After clamping the part, aligning it and removal of gates and risers, the functional surfaces, condyles and patellas, are treated first. Subsequently, the outer contours and – depending on the part – the “box” are milled. In a further step, with an integrated belt grinding unit, the transitional surface areas can be levelled without having to re-clamp the part. In this case the grinding belt is controlled by the same geometrical data as the grinding wheel. The fact that the complete grinding process takes place by clamping the part only once results in saving time and also contributes to a significantly improved quality. After geometric grinding the implants show readings of Ra = 0.40 µm – Ra = 0.20 µm on all critical surface areas. The time required for the geometric grinding process varies between 12 and 30 minutes (depending on the part) for two implants.
Drag finishing produces optimum surface finishes
The femural parts receive their final surface finish in the newly developed compact drag finishing systems from Rösler that allow the processing of up to 28 parts in one batch without the parts ever touching each other. Based on tried and tested, proven Rösler drag finishing technology these machines have become the standard for femoral finishing worldwide. After having been mounted onto special work piece fixtures the implants are immersed into a bowl filled with a grinding media and „dragged“ through the media in a circular manner by a drive system that is equipped with two independently controlled drive motors. As with all Rösler processes dedicated grinding media and compounds guarantee an excellent surface finish with an absolutely even metal removal all over the required surfaces without negatively affecting the geometry of the parts. These unique „consumables“– specially developed and produced by Rösler Germany - generally must undergo rigorous testing before being approved and certified by the implant manufacturers and various medical agencies. The metal removal takes place evenly around the complete parts surface and leaves no secondary burs whatsoever. An additional benefit is the edge-breaking (radiusing) of the outer contours of the femur which facilitates its subsequent attachment to the human tissue. One major prerequisite for this excellent finish is an efficient undersize media classification system that ensures a stable and homogeneous mix of grinding media in the work bowl. And, of course, the process taking place with the constant addition of water and compound, simplifies the subsequent cleaning and washing steps. The fine grinding process which produces a surface reading of Ra = 0.03 µm is followed by a high gloss polishing step taking place in a mass of organic dry polishing media. This step produces an extremely smooth and shiny surface on the implants with surface readings of Ra = 0.02 µm.
For parts with surface readings of Ra =0.8 µm up to Ra = 1.0 µm after the geometric grinding, that is where a manufacturer has decided on a short grinding machine production strategy a three stage drag finishing process will be required. In this case the fine grinding step is preceded by a pre-grinding process with ceramic grinding media. This additional step allows a manufacturer to produce more components per working day on their Haas equipment without additional investment.
However the parts come-off the grinding machine the pre-defined and constant process parameters of the drag finishing technology guarantee absolutely consistent and high quality surface finishes, irrespective of parts geometry and quantity. At the same time, this technology is ecologically very advantageous: The process liquids – a mix of water, compound and abrasive fines – can be cleaned and recycled in Rösler centrifuges, and the resulting sludge, consisting mainly of media fines, can be disposed of on a sanitary landfill.
Benefits that also apply to other industries
The optimal fine tuning between the geometrical grinding process and the drag finishing technology is not only advantageous for the treatment of implants but also for any parts that require high dimensional precision and excellent surface finishes. This integrated approach not only improves the overall parts quality, at the same time it results in reduced manufacturing times and lower costs, benefits sought by many of our customers. Examples of applications outside of the medical implants are turbine blades for aerospace applications, the manufacture of all kinds of gears and pinions, precision work piece fixtures as well as drill bits and milling tools.
The drag finishing technology is the most intensive type of mass finishing. The grinding media utilized for this process are literally “mini precision tools” for the fine finishing of very complex, delicate and high value parts and are specially developed for specific applications. Rösler, the world’s largest manufacturer of media and compounds for mass finishing applications, offers a range of over 6,000 types of media with different shapes, sizes and compositions.