11.17.06
Electropolishing Can Improve Medical Part Performance
A decades-old technology has been updated to achieve new and profound results for the orthopedic industry.
David Z. Pokvitis
The term electropolishing still conjures up the idea of bright and shiny. From a strictly technical perspective, the removal of surface metal using chemicals and electricity has long been used to brighten many metals, especially stainless steel. Even sectors such as the food processing industry have long used electropolishing to make food contact products look and behave better.
The photos at top, before electropolishing, and below, after electropolishing, show the tip of a drill used in orthopedic surgery. Manufactured from 440A stainless and heat treated, this expensive and critical part is machined to tight tolerances. |
Electropolishing is a controlled metal-removal process similar in theory to electroplating. Parts are immersed into a precisely controlled chemical bath, and when electrical current is applied to the metal parts, metal ions are dissolved from the surface. Electroplating uses similar techniques to apply metal ions to parts.
Under higher magnification (top photo) the opposing machining marks are clearly visible on the various surfaces, moving metal to a fine and fragile burr on the lead surface. After electropolishing (bottom photo), enough metal was removed to remove most machining marks together with the burr and yet preserve the highly engineered drill surfaces. This was accomplished without change to the hardness of the drill. |
There are six key reasons electropolishing is specified on orthopedic products:
1. Deburring. Electropolishing removes surface metal and works especially well on burrs left from stamping, grinding or machining. Since the process is non-mechanical, it dissolves burrs from all surfaces simultaneously. The more complex or multifaceted a part, the more
2. Microfinishing. As surface metal is removed, grinding, tooling and other marks are reduced, leaving the part much smoother both visually and measurably by profilometer. This drive for smoother parts can result in tools that operate with less applied force, smoother insertion for cannulas or biopsy needles, more consistent drug delivery devices and surgical tools that operate in ever-smaller environments. In general, Ra surfaces are improved by 50%. For example, a 16Ra finish is improved to a finer 8Ra, again on all surfaces simultaneously. Nearly all orthopedic products, especially implantable parts, benefit.
3. Corrosion resistance. Stainless will corrode, especially once contaminated by fabrication. Electropolishing removes surface metal and, with it, imbedded contamination that encourages corrosion. Decades of corrosion testing have proven that electropolished surfaces will resist corrosion 30 times more than commonly specified chemical passivation. All implantable parts and multiuse surgical tools are excellent examples, especially those that call for passivation in an attempt to prevent damaging corrosion. With increasingly stringent sterilization techniques subjecting parts to corrosion, many engineers are looking for treatments that will exceed the limited benefits of passivation.
5. Decontamination. Most metalworking processes leave a metal surface porous and spongy, a condition called an “amorphous” or “white layer” of smeared metal. Surfaces that are machined, ground or lapped often leave an amorphous layer that can be observed under 100X-300X magnification. This imbedded contamination also has significant air space that is perfect for trapping bacteria or water/chemicals. Research
This surgical cutting blade (Top photo) is fabricated from 420 stainless steel and heat treated. The middle photo (400X magnification) shows how it looked before electropolishing. The cutting teeth were ground, and the cutting surfaces held residual burrs and other fine material. The objective was to remove enough metal to remove loose metal fragments but not to reduce cutting effectiveness. After electropolishing (bottom photo, 400X magnification), the surfaces were cleaned of all fragments. All teeth were done on all angles from root to tooth in a single process step. |
6. EDM/Laser Recast Removal. EDM and laser fabrication have made great strides over the past few years. An increasing number of complex, fragile and tiny parts is cut and shaped by these methods. EDM especially is a preferred fabrication method for products in design for FDA approval, as slight design changes can be accommodated with minimal tooling charges. Unfortunate byproducts of these methods are re-cast or molten metal and contaminates that adhere to the part. As the wire (EDM) or laser beam cuts metal, some of it is vaporized, while a fair portion re-attaches to the metal part. In most cases, this recast metal is extremely brittle and difficult to remove by conventional mechanical finishing operations. Electropolishing dissolves recast on nearly any metal alloy, removing the molten metal, carbon and other contamination. As described earlier, complex curves, parallel and certain interior surfaces are treated simultaneously without damage to the base part.
A Beneficial Process
Thanks to process improvements and exhaustive surface analysis, more people are beginning to see the full range of benefits from controlled metal removal via electropolishing. Restoration of a metal surface almost always is more beneficial than coating a defect, especially when the process is non-mechanical and applicable on fragile and complex parts. In volumes large and small, the process has proven itself as a valuable part of orthopedic product manufacturing.