Low Cost, High Volume Orders and Electrochemical Machining
When manufacturing small parts in high volumes, engineers have a variety of manufacturing options to consider, each with different capabilities, strengths, and weaknesses. One way to compare these techniques is by part costs.
Although the above graphic is an oversimplification of the costs of these techniques, on average this trend holds true when manufacturing higher part volumes (e.g. >10,000/yr). Voxel has already discussed how EDM and CNC milling processes compare to ECM/PECM; however, the medical, automotive, and electronics industries require high volumes of small parts -- for <$20 per part. Metal injection molding and progressive stamping are popular techniques to accomplish this, so in the following paragraphs we will compare and contrast these techniques with electrochemical machining processes. This side-by-side view will show how PECM can be a viable alternative to MIM and stamping, in certain cases.
When parts are formed through stamping, a flat sheet of metal goes into a press where a die forms and/or cuts the metal into the desired part. In a progressive stamping operation, multiple dies of different shape press the metal in sequence to create a 3D shape. This process can be very fast and inexpensive for small components – producing parts in seconds for <$1/part.
Notwithstanding, the contact nature of stamping does result in some limitations. The dies used in stamping are physically deforming the materials and must be made from hardened materials that can withstand the contact forces, both immediate and accumulating over time. If part materials which are harder or abrasive are used, they will wear out the die quickly, leading to a drift in the part geometry or frequent die replacement. Thick materials cannot be easily processed due to the very large forces required. Therefore, thinner, softer and more ductile materials are generally required to enable the necessary material deformations and bend radii without fracturing the metal. And even with these concessions in the materials used for the parts and dies, burrs are still formed at the part edges where it has been separated.
PECM – A Non-Contact Option
In some ways, PECM could be considered a non-contact electrochemical stamping operation. PECM utilizes one (or maybe a few) electrodes and repeatedly forms the shape, microns at a time, with each cycle of the process, instead of “hitting” the workpiece with a progressive set of dies.
Although both pulsed electrochemical machining (PECM) -- also known as precision electrochemical machining-- and stamping require a set of dies to be manufactured (and the design and iteration of these dies can take some time), the wear on these dies is one of the major contrasts between these processes. Due to the non-contact nature of electrochemical processing, the electrodes don’t wear significantly over time and can incorporate small or thin-walled features that would not survive a physical stamping process.
In short, electrochemical forming takes longer than stamping, but it can have some significant advantages in workpiece materials, aspect ratios, and design freedoms.
Metal Injection Molding (MIM)
In MIM, a finely powdered metal is mixed with a polymer binder material. This creates feedstock which can be heated to melt the polymer binder so that it can be injected into a mold in the shape of the final part. Once the part cools, a “green” part remains, which is ~20% polymer by volume. Subsequent binder removal and sintering steps remove all the polymer elements and apply enough heat to sinter the remaining metal, causing a corresponding shrinkage of 20%. The result is a part with high feature counts and sufficient material properties for many applications. By forming parts in multi-cavity molds and sintering in large batches, the MIM process can achieve high throughputs leading to prices <$10/part. Similar to stamping and PECM, a complex mold must be designed, manufactured, and iterated to achieve the desired performance and throughput. Unlike stamping, this mold can be used for 100,000s of parts without replacement, allowing the vendor to amortize those mold costs. This is similar to the electrodes required for PECM, which require an up-front investment, but the initial costs can be written off over the duration of a production job.
The required injection process and shrinkage during sintering also lead to some specific challenges. First, MIM requires wall sizes that are typically >0.5mm thickness and is limited in a due to the challenges of filling a viscous binder material into thin-walled areas. Second, the shrinkage due to the sintering process can cause the part to crack or fail if poorly designed, creating limits on total part sizes, maximum wall thickness, transitions from thick-to-thin region.
Molds for MIM also need a way to permit ejection – this means the inclusion of draft angles, parting lings, and gates/sprues. All of these features, which are required in the mold, are then copied into the part and sometimes need a secondary processing operation to remove from the part surface.
PECM – A Viable Alternative
Although the average PECM part may be more expensive than a MIM’d part (or certainly a stamped one), it can achieve features, tolerances, and part sizes that are out of reach for MIM or stamping while still providing affordability in volume production. For example, PECM has demonstrated 20:1 aspect ratios, can be used on parts of virtually any size or weight, and creates surfaces free of any parting lines or blemishes.
That said, another option to consider is a hybrid approach whereby PECM could be a secondary operation to a MIM or stamped part. As long as a proper datum structure can be maintained or relocated between the stamping/MIM and PECM operation, the most economic way to achieve a critical design may be to use multiple operations.
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Stamping, MIM and PECM all require complex dies and can produce multiple parts per cycle. The custom tools required for each of these processes can be costly and time consuming to both make and change, so a well-thought out design is critical and any tool ought to be reviewed in the early stages of manufacturing, including when using PECM. However, ECM can accommodate any part size and weight, and is compatible with more complex geometries and higher aspect ratios, allowing for many more options for design.
If you would like help determining if PECM would be a good supplemental or alternative manufacturing process for your particular application, please contact us.
This article is part of our ongoing “PECM vs. Competing Processes” series that compares PECM to other, popular machining processes. Find other articles in the series below:
CNC Milling vs. ECM
EDM vs. ECM
Electropolishing vs. ECM