Best of 2018: Strategies for orthopedic component processing

Five major suppliers claim approximately 85% of the highly competitive orthopedic device and implant market, with more than 200 other companies vying for the remainder. In light of such intense competition, recent developments allow medical manufacturers to continually seek ways to machine components faster and more cost efficiently. Some of these innovations and strategies include dry coolant and 3D printing, along with advanced cutting tools and high-speed milling.

Orthopedic components are typically machined from bar stock, castings, or forgings, then ground and polished. For hip and knee implants, the most common workpiece material is titanium, with the use of cobalt-chrome alloy increasing. A typical cobalt chrome alloy is similar to CoCr₂₈Mo₆, and the titanium alloy Ti₆Al₄V is most common.

Keep it cool

Because the materials used in orthopedic implants typically generate excessive heat when machined, coolant use is required. However, using traditional coolants is often prohibited or limited to prevent part contamination, where time-consuming and expensive post-machining cleaning processes are needed. In addition, coolant poses environmental issues for employee health, safety, and disposal. An alternative coolant technology uses supercritical carbon dioxide (scCO₂) dry-cutting technology. The scCO₂ acts as a vehicle to deliver dry and enhanced lubrication to a cutting zone.

Developed by Fusion Coolant Systems, the process can machine parts without oils, emulsions, or synthetics. When carbon dioxide is pressurized above 74 bar (1,070psi) and 31°C, it becomes a supercritical fluid, scCO₂. In this state, it fills a container like a gas, but with a density similar to a liquid. When delivered to the cutting zone, scCO₂ expands to form dry ice, though it does not create a cryogenic substance like liquid nitrogen. The result is an effective coolant that often outperforms existing systems that incorporate high-pressure water/oil, minimum-quantity lubrication (MQL), liquid CO₂, or liquid nitrogen.

3D printing

Another nontraditional manufacturing technology being applied more often in orthopedic device production is 3D printing. The process uses titanium and cobalt-chromium alloy powders to produce complex, near-net-shape parts. In the medical industry, selective laser melting (SLM) melts the powders to build components layer by layer. The process allows medical manufacturers to generate special part contours and dimensions custom- tailored to individual patients. The process also produces consistent micro-pore surfaces that expedite bonding between the part and living bone.

For finish machining, 3D-printed parts maintain most of their metals’ machining characteristics. However, some parts may need post-printing treatments to relieve uneven stresses generated during processing. In addition, for post-machining, fixturing can sometimes be a challenge due to the parts’ near-net shapes and complex contours.

Surface finish

The metal alloy components of orthopedic implants, such as knees and hips, must possess excellent surface finishes to minimize wear of plastic parts and permit the joint to function for its projected lifetime of 20 years or more. In a knee replacement, the femoral component and tibial tray must be perfectly smooth to protect the plastic bearing insert against wear.

Accordingly, the manufacture of orthopedic components typically requires grinding operations follow the milling process to achieve sufficiently smooth surfaces. However, grinding is time consuming and impacts overall manufacturing efficiency and flexibility. Equally critical, grinding generates high temperatures and stress in components, leading to dimensional errors that can affect the product’s strength and performance.

Grinding can be supported – or in some cases replaced – with the application of advanced cutting tools and high-speed milling strategies. The goal of milling is to achieve a burr-free outside profile and a superior surface finish that delivers the exact required surface quality, integrity, and dimensional accuracy. If post-treatment is required, such as polishing, the time for that task can be minimized because of the defined surface roughness and structure achieved in milling. With high-speed milling, tooling should meet the parallel goals of long, reliable life with maximum productivity.

Strategies, results

In a representative application, a cast cobalt-chrome femoral component was finished using a ballnose end mill on a 5-axis milling machine. High-speed copy milling strategies and high-performance end mills eliminated a grinding operation, reducing part cycle time 50% to 11 minutes per part compared with the prior method. The change from grinding to milling of the condyle surface reduced scrap parts. The solid-carbide end mills made from a high-grade carbide micro-grain substrate offered superior toughness compared to standard carbide grades. In addition, a patented high-abrasion-resistant coating maximized tool life. The tools were engineered to provide high metal removal rates and smooth cutting action for superior finishes and minimized polishing time.

The complex contours of orthopedic components often require specific sequences of specialized tools. The tibial tray, for example, can require up to seven separate machining operations including roughing, tray base roughing, tray base finishing, chamfer milling, T-slot undercut machining, wall finishing/chamfering, and undercut deburring. The challenge is to achieve superior surface finishes with minimal manual intervention as well as reliable tool performance with the best combination of productivity, cost, and quality.

Traditionally, performing these multiple operations dictated separate special tools to produce each required contour, dimension, and surface finish. Special tools require design and development time and expense, and due to their low production volume, may have extended lead times and availability constraints. A new approach involves developing standardized tools with flexibility that enables them to be used in a variety of similar orthopedic parts.

Global demographic and economic trends strongly indicate that demand for sophisticated orthopedic components will grow. At the same time, consumer desires and the determination of medical parts manufacturers to differentiate themselves from competitors are promoting development of personalized orthopedic components. Surprisingly, variable part specificity can be achieved with relatively new innovations and strategies that include dry coolant and 3D printing, along with advanced cutting and high-speed milling tools that are less specialized, more flexible, and more cost-efficient than the custom ones previously applied.

Seco Tools LLC
www.secotools.com