By LaRoux K. Gillespie, Dr. Eng., FSME, PE, CMfgE, CEng
Society of Manufacturing Engineers
Today’s products require high finishes, burr-free edges, freedom from contamination, and often close tolerances. Electropolishing provides all of those conditions and more in a matter of seconds for many metal parts. It is a process that has been used for more than a hundred years. It is widely known and the science is widely discussed, but its ability to run job shop lots and high-precision high-volume parts in the same equipment makes it a bit unique. You can automate it to any level desired—even to producing 100,000 parts per hour.
Breadth of Electropolishing
Electropolishing units are inexpensive, easy to use and found on benchtops and in laboratories, small machine shops, dedicated electroplating facilities, electronics manufacturing centers, and highly automated specialty shops. The small breadbox-size and larger console units are all manual systems as are many industrial tanks found in electroplating shops. But the ability to automate them is striking.
Many of these systems are semi-automated, which means that the transport of the parts is controlled by the operator, typically via pushbutton or touch screen to move to the next station, i.e., the part is processed automatically, but many elements still involve some manual intervention. In contrast the automated systems can provide completely hands-off automation of product handling, bath composition control and temperature control.
Automated lines reduce labor costs, increase part uniformity, allow digital documentation of process control and part history while eliminating scratching, creasing, or denting.
The material handling aspects of electropolishing involve two distinct issues:
Handling through the electropolishing steps,
Part loading and unloading onto fixtures, reels, or racks so the parts can be inserted into the baths.
Automating the transporting of parts through the baths generally uses one of three approaches: straight-line (also called linear) systems, rotary systems (also called radial), and reel-to-reel systems. While robots have been used for such transportation, they typically are limited to loading and unloading because of the acids and liquids that can find their way into robot tooling and arms.
In the 1950s, manufacturers used continuous overhead conveyor lines very similar to today’s powder coating lines. Parts were hung from a continuous overhead line and moved in and out of each tank as the parts moved at constant rate down the line. Because the part has to smoothly enter and leave each tank, the angle of approach adds considerable length to each tank and to the overall line. Consequently that approach is not typically practiced today.
Straight line systems, as the name implies, involve moving racks of parts horizontally above all the tanks aligned in a straight line for the complete process. Once above the tank a mechanism lowers the rack into the solution.
Straight line systems use one of two different control approaches. In the first approach the transporter stays with the part and immerses it in each process or rinse for the preprogrammed period of time.
The second approach has the transporter able to leave the parts in a process, moving along to do other things and returning at a preprogrammed time in the cycle to remove it.
Because of the time differences you may not be able to run many racks at the same time or you will over polish or over clean because of the time differences.
This approach, common in job shop plating operations, allows the user to skip some tanks to accommodate different chemical steps. In this design not every rack goes into every tank so the number of parts produced per hour will be much lower than the sequential standard straight-line approach. The same system that electropolishes parts might also be used for just cleaning parts by eliminating polishing step from the process sequence. (Some companies will not allow you to mix products or processes on the same line in order to assure that no parts inadvertently run to a different process than others.
When a rack gets to end of the line it (or sometimes just the rack holder) is returned to the beginning of the line for more parts.
The key to high production is to adjust the processing details of each station to exactly match the time required at the most critical station. That is true for all three variations of automation.
Technic Inc. (Cranston, RI) Application Specialist Stu Raifman notes that it typically is not possible to perform passivation in a line that uses radial or sequential transport motion because passivation times are typically 10–20 minutes vs. a minute or so for electropolishing. In contrast hoist transports can be programmed to spend whatever times are necessary in each tank since they do not automatically spend the same time in each tank. Hoist systems can leave a rack in a tank and come back for it at a later time.
For the radial approach the transport mechanism has a fixture holder for each process in the cycle. As the mechanism lifts, indexes and lowers, each fixture sequentially visits every tank. The lift, lower and dwell times are the same for all racks. The number of racks processed is typically higher than for the straight-line system. With 30-sec processing times in solution, you produce a rack of parts roughly every 30 sec whether you have 11 tanks (tanks are often called stations) or 16 (there is a little movement time from one station to the next).
Systems that transport parts by rotating them around a circular path at the end of arms are popular for high production of medical parts. They offer continuous automated production in a relatively small footprint. Electric drives provide the indexing motion while many use pneumatic lift systems to insert and remove the parts from the small tanks. These systems are particularly adaptable to long parts placed one or two to a hanger. While they are not limited to small or long parts, most such systems require only a small tank for each station. While the tanks could have a diameter as small as 6" (152 mm) to accommodate thin and small parts, few equipment builders will honor that size because electropolishing works much more repeatably with a large volume of electrolyte. The larger diameter units can also handle racks of many parts which allow them to meet higher production rates.
These machines vary from about 8 to 36' (2.5–11 m) in diameter, with power supplies from 100 to 7000 A. All of these have programmable time cycles, and almost all of them require that every part be inserted in each tank for the same time. While that need not be a hard and fast limitation, it does provide the highest production rate.
Automated rotary systems, such as Technic’s MP500, use the same approach as many carousel assembly systems. Each part passes on a rotating arm to the next station where the parts are dunked in solution, cleaned, or blown dry for the required time. Every step requires the same time for this approach to work. These systems generally are smaller than the in-line systems since only a single part is typically racked in the clamping device. The tank sizes are smaller than in-line systems, resulting in a very small footprint unit. For example, Technic’s MP500, which produces surgical and other medical equipment, may produce 60 or more parts per hour at a one-at-a-time rate, but several hundred if several parts can be placed on the same hanger. That electropolisher is only 8' in diameter and 10' (3-m) tall. These are ideal for continuous production in a cellular operation because they have very little footprint and can accommodate the production rate of one or several machining centers.
The same system that electropolishes parts might also be used for just cleaning parts by eliminating polishing solution from the tank that normally polishes. You may waste one station, but you still get the same production rate with no additional equipment. (Some companies will not allow you to mix products or processes on the same line in order to assure that no parts inadvertently run to a different process than others).
Automated straight-line transport systems designed for medical components can range from $150,000 to $1,500,000 depending upon what the operation is designed to accomplish, the extent of automation required, and amount of instrumentation. Some users are asking for measurement of specific gravity today and that adds another $15-20,000.
"The key to high production is to adjust
the processing details of each station
to exactly match the time required at
the most critical station. That is true for
all three variations of automation."
While there are not many manufacturers of this type of machine, one manufacturer reportedly has produced up to 10,000 small pieces per shift. The operator runs the machine from a single station, loading and unloading racks as they return after completing the process cycle. Another machine of this type produces 5000 small medical parts per shift. The machine has nine process stations and two 100-A rectifiers. This system has a diameter of about 6' (1.8 m) and an overall height of 10'. All operations are automatically performed by the PLC located in the operator control panel.
Small scale manual systems may cost as little as $25,000.
Reel-to-reel systems have been used traditionally for electroplating electrical connectors, but have now been adapted for electropolishing medical devices. These systems are designed to pull a strip of metal from a reel through the system and return them to a reel. By attaching parts to strips, in a similar fashion to the way a cloth or metal bandoleer is used to hold bullets for machine guns, parts can be processed in a continuous manor. A leader strip at the beginning of the reel of parts is fed to a tensioning device and it is pulled through each bath and then wound on a reel at the end of the electropolishing line. When the end of the strip is reached the system automatically knows that it needs to stop or that another strip is attached using an accumulator. With the exception of the splice, it’s all automatic. This approach eliminates the complexity of dealing with individual piece parts. Production rates on these systems are generally defined as feet per minute rather than parts per hour. A typical rate might be 10–15 fpm with dwell times in each station of 30-90 sec. With parts mounted on 0.5" (12.7-mm) centers that’s 14,400 to 21,600 parts per hour!
Technic’s Raifman notes that automatic pay-out and take-up systems utilize ultrasonic and non-contact sensors to detect the presence of product and to maintain the proper degree of tension. Encoders detect the relative speed of the product in the system. DC power supplies measure the electrical current applied the parts in the electropolishing process. Any significant change in amperage indicates that a smaller surface area is in the tank and that is interpreted as a leader in the tank rather than parts on the strip. While reels of parts may typically be an inch to 3" (76-mm) wide, other sized reels of flat stock or stamped product can be accommodated.High-volume work and precision parts benefit from every part seeing the same operating condition as every other part. Control of voltage and amperage is one requirement. Controlling solution composition is another requirement that is sometime automated. Tachometer-generator feedback speed control is used to maintain synchronized transport speed and eliminate product distortion under all load conditions.
Continuous Conveyor Line Processing
As mentioned earlier, use of continuous conveyor lines is a possible approach to electropolishing automation. Occasionally users rely on such an approach. To overcome the extra length required by an overhead belt approach, systems are built that pass the parts through the wall of each tank into the next tank. Proprietary sealing, collection and pumping systems are used to minimize the solution losses from the process and rinse pass throughs and provide for reuse.
Robot Part Movement
A leading medical device company uses a robotic arm to move parts through electropolishing and cleaning lines, according to MONTIP - Robotics and End of Line Automation (Drangan, Ireland). A long-reach Fanuc M710CL robot with a mechanical gripper accommodates several different racks or fixtures. The cycle details for each type of product vary so a barcode scanner reads the product type from each fixture and adjusts the process accordingly. For this application up to nine different product types can run simultaneously without any instructions from the operator. The cell is completely enclosed and secured by a double trap-key system to ensure safety from robot motions. The result is a fully automatic process capable of running non-stop, 24/7.
The second key element in part handling involves getting parts into or onto the racks, panels or grippers that transporters use. All the common automation approaches work, but firm electrical contact with the part is a requirement that challenges some high-production shapes. Raifman notes that one of their applications uses vibratory feeders to shake small cylinders ¼ diam by ½" (6.3 × 12.7-mm) long automatically into 30 pockets in the rack. The rack closes up and the system indexes to the next rack, all automatically. With a system like this, they were able to electropolish roughly 100,000 parts per hour. There are few applications having such high production. Doing 5000–10,000 parts per hour is common for many small parts, but production runs of 4 million–5 million parts per year can be done with a single line.
Raifman notes that the technology is available to handle 500 lbs (225 kg) of parts or more at a time in an automated line.
Dennis Gardino, president of Jacob Hay Co. (Wheeling, IL), notes that the typical line today is more semi-automated. Most systems have an operator load 40–50 parts into a fixture or rack and then load the rack on the system transporter. The transporter takes that rack and automatically completes the process. One or two operators spend full time loading and unloading fixtures. The programmable hoist system accommodates a variety of processing steps and parts, but it is not a high-production system since there is typically one hoist moving one rack at a time.
Joseph Leonhardt, president of Leonhardt Plating (Cincinnati), notes that they bought their automated straight-line electropolishing system in the late ’90s. They load 40–100 parts per rack off line and then slide the rack in place. They produce 800–1200 parts per hour in their 14-station line. Like most systems theirs was a special design line using modular components. In their case they bought dual electropolish tanks to handle high production but the second electropolish tank sits unused today.
Leonhardt notes that automation of part movement is only one aspect of automation. Close control (automation) of waste water and heat lowers overall costs. His firm won a pollution prevention award for their approach to closed-loop, zero-discharge operation.
Hypodermic needles have been electropolished for several decades as a deburring process, and the smoother surface reduces friction as the needle goes into the body (resulting in less pain). Vascular and biliary stents are also electropolished to remove minute burrs, improve surface finish and clean these medical devices. Over 2 million stents are inserted each year, and electropolishing is one of the processes to manufacture and clean each one. Surgical tubing, stampings, fasteners, investment castings, cutting instruments, sanitary fittings, gears, splines, shafts are other common applications.
Several companies produce automated lines for polishing inside surfaces of stainless steel pipes. Pipes up to 30' (9-m) long can be electropolished in an automated system. In this case the electrolyte is pumped through the pipe. Stationary or moveable cathodes inside distribute the current. One machine polishes 20 tubes simultaneously. Equipment exists for handling tubes as small as 1/8" (3.2-mm) and as large as 6" in diam and for providing finishes of 3–5 microinches (0.0762–0.127 µm) inside. Production rates can be as high as 250' per hour (76 m/hr).
Stainless and other varieties of steel, copper, brass, nickel alloys, aluminum and even titanium are suitable for electropolishing applications. Titanium can now be successfully anodized in automated lines similar to those designed for electropolishing.
Automating Electropolishing is More Than Part Handling
Successful electropolishing depends upon more than just automated parts handling.
Part clamping such that firm electrical contact is maintained is essential for high process repeatability. One of Technic’s solution is to use spring-loaded clam-shell clamping. In the case of the little cylinders cited above, the feeder vibrated the cylinders into holes in a rack which had clam-shell contacts at the bottom. When the rack was full, the system closed the clam shells and indexed the next rack into position. Regardless of what method is used, fully automated systems have to accommodate firm contact throughout the electrical portion of the cycle and in some instances to assure that parts do not fall from racks into the solutions. Maintaining contact cleanliness and configuration throughout months of operation is yet another consideration for automation.
Electrolyte management (concentration, pH, filtration, agitation, etc.), accurate timing, constant solution temperature, and consistent electrolytic action are other key elements to be controlled. The electrolytic lines of force in an electropolishing bath are not uniformly distributed, which in turn means that electropolishing action is not necessarily uniform throughout the tank. Parts processed in different locations in the tank may exhibit different metal removal characteristics. Cathode and rack design optimization are used to mitigate these effects.
Raifman notes that, "Controlling solution dragout is essential to part quality and bath integrity. Where required, unique wiper systems, brushes and air blow-offs are used to minimize solution losses. A chemically resistant wiper system can wick solution off the product, allowing it to be drained back into the process. The resiliency of the wiper material allows it to automatically adjust to changes in thickness of the material being plated without springs or manual adjustments.
"Air knives and brushes are also utilized to enhance the removal of solution. Carefully controlled pressures deliver just the right amount of air to remove solution without drying, staining or passivating the part surfaces. All methods prevent ‘back spray’ and reduce cross-contamination. This careful control of dragout results in more precise process control and reduced rinse water and chemical consumption."
These may seem like simple issues, but attention to these design details prevents a host of problems in an otherwise automated system. ME
This article was first published in the April 2012 edition of Manufacturing Engineering magazine. Click here for PDF.