What is Dry Electropolishing?

How Does Dry Electropolishing Work?

Dry electropolishing is a revolutionary new surface finishing technology for polishing metal parts. Dry electropolishing removes surface roughness from metal parts without affecting the shape or overall dimensions of the part. The resulting surface is bright, smooth and ultra-clean. Dry electropolishing is a non-abrasive, electrochemical surface finishing process. For this reason, it is particularly suitable for polishing and deburring fragile parts and parts with complex geometries. It provides mirror brightness on surfaces for a wide variety of machined, sintered and cast metals, including titanium and nitinol.

What Makes Dry Electropolishing Different?

In dry electropolishing, electrolyte beads are used instead of liquid electrolyte solution. While the traditional electropolishing process uses a liquid electrolyte bath, the dry electropolishing process uses a drum consisting of solid electrolyte beads. The size of these small beads varies between 0.1 mm and 1 mm depending on the application.

How Does Dry Electropolishing Work?

Dry electropolishing works by using electrical current to draw or transport ions from a metal surface into an electrolyte. It operates on principles similar to traditional electropolishing. As in traditional electropolishing, the metal workpiece acts as the positively charged anode in the electrolytic reaction. The drum containing electrolyte beads acts as the negatively charged cathode. Metal workpieces are held in a clamp and then immersed in a drum containing electrolyte particles. The positively charged arm holding the workpieces performs continuous planetary rotation and vertical up-and-down movement to equally expose all areas of the workpieces to the electrolyte beads. Electric current is transmitted from anode to cathode through the electrolyte beads. The electric current causes oxidation of ions on the metal part surfaces and absorption of ions by the electrolyte beads at the physical contact point. The process depends on the electrolyte particles making physical contact only with the peaks and not the valleys. The spherical shape and size of the electrolyte particles ensure that only the burrs and other surface roughness peaks receive electric current, not the valleys. Removal of the protruding portions of the surface structure results in a smoother metal surface.

What are Electrolyte Beads?

Electrolyte beads are a revolutionary alternative to liquid electrolytes. Electrolyte particles are small, porous, sponge-like solid spheres that trap an electrolytic additive inside. Electrolyte particles are safer and less hazardous for workers compared to liquid sulfuric acid and phosphoric acid tanks used in traditional electropolishing. Electrolyte bead sizes range from 0.1 mm to 1 mm. The smaller the bead, the brighter the surfaces become. If mirror brightness is required on your metal part, the smallest electrolyte beads should be used. Smaller beads also provide better performance on parts with complex geometries. The more complex the geometry of the part to be polished, the smaller the sphere. Different metals require different electrolyte particles. Electrolyte beads are currently available in formulations designed for the following metals:

Key Benefits of Dry Electropolishing

1. Safe and Environment Friendly

The electrolyte beads used in dry electropolishing are safer and less hazardous for workers. They do not require special storage like hazardous chemicals. The operator has no direct physical contact with the electrolyte beads. However, if a particle falls into the electrolyte beads, rubber gloves are recommended for the operation, as prolonged skin contact may cause mild irritation. The use of electrolyte particles reduces risk and reduces the burden of safety procedures and regulatory compliance associated with traditional liquid electropolishing chemicals. In addition, the risk of electrolyte particles polluting the environment is much lower compared to the liquid electrolyte of traditional electropolishing. For medical device manufacturers, the dry electropolishing process provides proven biocompatibility. The dry electropolishing process may be considered non-cytotoxic based on a study conducted according to the specifications of the UNE-EN ISO 10993-5:2009, Biological Evaluation of Medical Devices - Part 5: In Vitro Cytotoxicity Tests (ISO 10993-5:2009) standard. Manufacturers must exercise due diligence to confirm that their specific parts are not cytotoxic. However, the process itself meets the standards required for dental and orthopedic implants.

2. Mirror Brightness Appearance

Dry electropolishing achieves brighter part results. Dry electropolishing improves surface roughness (Ra or average roughness) by 10 times. Therefore, if you have a newly machined part with a surface roughness of 80 Ra, you can use dry electropolishing to bring this part to a surface roughness of 8 Ra. In contrast, the best improvement possible with traditional liquid electropolishing is 50% or 40 Ra in this example. This technology represents a major leap forward in surface polishing processes: the difference between 8 Ra and 40 Ra is a quality difference you can see. Most importantly, mirror brightness is achieved in a single step rather than multiple process steps required in mechanical or abrasive surface finishing.

3. Excellent Solutions Where Traditional Electropolishing Falls Short

The dry electropolishing process makes titanium electropolishing and nitinol electropolishing accessible to small manufacturers. For these metals, the electrolyte used in traditional liquid electropolishing is flammable at room temperature, so titanium electropolishing and nitinol electropolishing operations have inherently posed significant hazards. They could hardly be performed without almost cryogenic process temperatures. Dry electropolishing overcomes this barrier with new electrolyte beads specifically designed for titanium and nitinol that do not require special handling. Electrolyte beads for titanium can be used and safely stored at room temperature. Dry electropolishing provides more uniform, homogeneous polishing compared to traditional electropolishing. The electrical flow is more dispersed along the electrolyte beads rather than a straight path through a liquid electrolyte. As a result, consistent quality is achieved across all surfaces of the workpiece.

Dry Electropolishing and Mechanical Polishing

Abrasive polishing, also known as rolling, vibratory finishing or surface finishing, is a polishing process that abrades the surface of a metal part through friction. Examples of mechanical polishing include centrifugal tumbling and vibratory polishing. Dry electropolishing, on the other hand, electrochemically polishes the metal surface through ion transport rather than friction. Unlike mechanical polishing, dry electropolishing preserves edge integrity of the workpiece by selectively removing rough peaks. When larger diameter electrolyte particles are used than the roughness peaks, material removal occurs only at the surface roughness peaks, not in the valleys. Corners remain sharp and do not round. Additionally, dry electropolishing produces no dust or noise.

Electropolishing Internal Dimensions

Polishing the inside of a metal tube is generally difficult. Mechanical polishing struggles to apply sufficient pressure to the inside of the tube. Traditional electropolishing requires precisely positioned cathodes for each metal part. With dry electropolishing, an internal cathode (wire) is placed in the metal tube to allow direct conductivity. The distance of the cathode from the metal surface is less critical than in traditional liquid electropolishing. Dry electropolishing opens the door for stent medical device manufacturers to more easily electropolish stents. Despite the improvements provided by dry electropolishing, limitations remain for electropolishing the internal diameter (ID) of metal tubes. Particularly in small tubes, there may not be sufficient space for electrolyte particles to enter. The geometry of the part may prevent adequate flow of electrolyte particles into the inside of the part.

Dry Electropolishing Process Steps

Since dry electropolishing is a new technology, there is currently no industry standard. The nearest applicable industry standard is ASTM B912. As content, the standard concerns passivation of stainless steels using electropolishing. This standard specifies three stages that can be applied to the dry electropolishing process: 1- Surface Preparation 2- Electropolishing 3- Post-Processing

1. Surface Preparation

At this stage, any liquid oils and greases that could prevent electrical current flow to the metal surface must be cleaned. Parts do not need to be as clean as in traditional electropolishing, but water and liquids on the part surface must be removed.

2. Electropolishing

At this stage, actual electropolishing is performed to smooth, deburr and polish the metal. 1- Metal parts are held in a fixture and the fixture is attached to the dry electropolishing machine. 2- A program is selected from available options based on the material to be polished. For a brighter surface, the process is performed over longer periods. Smaller size electrolyte beads are used. The longer the process time, the better the brightness. Dry electropolishing is not entirely a dry process. The electropolishing machine sprays a small amount of DI (deionized) water on the material during the process. DI water softens the sponge-like electrolyte beads, allowing the electrolyte in the center to contact the metal surface more smoothly. Therefore, in this context, "dry" is a relative term compared to a tank filled with liquid electrolyte. Moisture is an important factor in the dry electropolishing process. Electrical current and moisture are directly proportional. During each cycle, every 2 minutes, a moisture sensor on the fixture measures moisture. Excessive moisture can make material removal too aggressive and result in inconsistent surface finishing. For this reason, the machine adjusts moisture level to optimum. 3- The system is activated. Process time ranges from 5 minutes to 1 hour.

3. Post-Processing

In traditional liquid electropolishing, acid dipping is performed after the process. In the dry electropolishing process, all that is needed to remove residual electrolyte is a simple rinse. Rinsing can be performed with any of the following types:

Common Applications

Dry electropolishing is compatible with a wide variety of metals including stainless steel, cobalt chrome, carbon steel, copper alloys, nickel alloys, carbides, titanium and nitinol. Dry electropolishing is particularly suitable for medical device manufacturers such as orthopedic implants, dental instruments and stents where dimensional tolerances require high-precision polishing. Common applications of dry electropolishing include dental equipment, surgical instruments, orthopedic implants. Furthermore, this technology produces very successful results for hip, knee and ankle joint implants requiring polishing, skull, jaw and bone plates, titanium bone screws and nitinol stents. Dry electropolishing is also very suitable for the aerospace and space industry where smooth surfaces are critical to performance. Dry electropolishing can achieve a surface roughness average (Ra) of 3.5 micro-inches or 0.09 micrometers. The process provides mirror surfaces with predictable costs and repeatable industrial processes.  

Tips for Best Results with Dry Electropolishing

First, the material must be known. The composition of the metal is very important in determining which electrolyte beads will be used. Additionally, the manufacturing process of the metal part must be known. A laser-sintered, additively manufactured part tends to be rougher than a cast part and therefore requires slightly higher voltage. Surface roughness must be known. Not only the expected surface roughness (Ra) value of the finished part, but also the surface roughness of the part before processing must be known. These factors affect the cycle time, voltage and electrolyte bead size required for optimal results. Geometry must be recognized. The size, shape and complexity of the metal workpiece affect the flow of electrolyte beads and electricity from the part. Fixturing and rack configuration are very important in electropolishing. Determining optimal parameters for a new part type may require some experimentation. Titanium runs hotter. For titanium, electrolyte beads are consumed at approximately twice the rate of other metals due to higher heat. For other metals, each electrolyte particle has a life of 100 hours. The average life of electrolyte particles for titanium is approximately 50 hours. Increased heat causes electrolyte particles to dry out faster.   Alparslan Kuru Training and Business Development Specialist Rösler Kromaş