2-D cutting (laser cutting, waterjet cutting, CNC plasma table, 2-D wire EDM, the electrolytic method described in Electrolytic 2-D cutting and related electrolytic digital fabrication processes, etc.) is a highly efficient way to make things, especially at large scales and when you have high precision. It occurred to me that by combining it with electroforming and anodic dissolution (ECM) it can be substantially more powerful.
If you want to 3-D print a 100-mm prolate spheroid that is 50 mm in its minor diameters, you need to 3-D print 131 milliliters of volume. With a typical 30% infill, this works out to 39 milliliters of actual plastic for the interior. If you use an 0.8-millimeter line width to print 3 perimeters around each layer, you have about 2.4 mm of thickness on the shell, which is 38 of those 131 milliliters, leaving only 93 mℓ for the infill, using 28 mℓ of plastic, for a total of 66 mℓ. At a typical layer height of 0.2 mm this is 138 meters of extrusion; at a typical 50 mm/s, this is 46 minutes of printing.
XXX recalculate that, it’s calculated with a shell thickness of 2.4 mm which is wrong on the bottom and top
By contrast, if you instead print out the same object by sheet lamination (“laminated object manufacturing”), you only need to cut the perimeters. If you were to use the same 200-μm layer height, you would only need to make 78.5 meters of cuts, which at the same movement speed of 50 mm/s would only take 26 minutes. The resulting object is normally fully dense, which is an advantage in some contexts, since it makes it stronger and stiffer. In cases where it is not, if the inside contour of the object (where infill would have been placed) is not strictly defined, you can often hollow it out by nesting multiple layers one inside the other, avoiding the need for extra cutting time.
Often the movement speed is not the same; many 2-D cutting processes can run at much higher speeds than additive processes can usually manage.
This advantage increases for larger objects and decreases for smaller ones; for a meter-scale object instead of a factor of 1.8 it’s a factor of 18. Larger objects tend to benefit more from being fully dense, because they need proportionally more cross-sectional area to support their own weight, or their own mass under the same accelerations.
By electrodepositing a thin layer of metal on a metal object made by metal sheet lamination as described above, we can get several important advantages:
The layers are connected together by the deposited metal. Although it won’t penetrate the layers, under some circumstances it can have sufficient adhesion to them to form a solid object. This depends crucially on their surface condition, which is more controllable in this situation (sheets freshly cut out, produced by a controlled process) than under some other circumstances.
The alternative to connecting the lasers is to run slots or holes through many layers and put a sliding fastener through them; such fastener-based or sliding-joint construction can also be fixed “permanently” by such electrodeposition. (If the glue metal being deposited can be anodically dissolved at a more moderate voltage than the base metal, the glue metal can be selectively removed later by electrolysis, permitting disassembly.)
The electrodeposited metal can smooth out layer lines which could otherwise interfere with appearances, fluid flow, optical performance, smooth sliding, human comfort, etc.
The electrodeposited metal (or composite) can be a material that can’t be processed by the original sheet-cutting process, or with more difficulty. For example, copper and nickel cannot be cut with oxy-acetylene torches, but you can very rapidly cut out a form from cheap mild steel on a CNC oxy cutting table, then electrodeposit them on its surface. This is especially true of nanolaminates, whose properties can be tuned to the application.
If the originally deposited sheets can be anodically dissolved at a more moderate voltage than the newly electrodeposited metal, contrary to the anodic ungluing process described above, they can be selectively removed after the electroforming process is complete.
Electrodeposition suffers from a positive feedback process of dendrite growth, in which protuberances on the surface are exposed to greater electric fields; as a secondary, much weaker, effect, they physically obstruct ion transport (“mass transport control”) to nearby parts of the surface. Consequently small protuberances grow into larger protuberances, potentially bridging all the way to the cathode while most of the material is only thinly plated. This is exacerbated by the anisotropic nature of crystal growth; as I understand it, many “brighteners” used in electroplating work by introducing grain boundaries to prevent the creation of large crystals. Others work by reducing ionic flow so that deposition rate is limited by ionic concentration rather than electric field.
Electrolytic cutting or electropolishing suffers no such effect (on the workpiece); instead of causing small irregularities in the surface to grow faster, it causes them to shrink faster, making the feedback negative. This permits the usage of much larger currents and correspondingly larger material removal rate.
So you can get faster free-form fabrication by alternating electroless plating with anodic removal of the unwanted part of the deposited layer, for example using a movable array of separately controlled cathode electrodes, each removing a controlled amount of material in a particular area of the workpiece. By limiting the degree to which the workpiece is dipped into the electroless plating bath, you can prevent material from depositing elsewhere on the workpiece than the current layer, enabling a layer-by-layer printing process. With electroless codeposition you can even print in a metal-matrix composite, and with the right choice of baths you can switch between two or more different baths to produce a nanolaminated material.
This procedure, as described, requires moving the workpiece back and forth between an electroless plating bath and a second bath where it is electrolytically cut by cathodes in precise positions relative to it (which may be moved in the process). You probably cannot use a single bath because the electroless plating solution will probably all plate out on your cathodes; you might be able to find a cathode material that doesn’t do this, but it may be difficult. An alternative using a single bath is to alternate between selective electrodeposition (at higher current densities than are normal for electroplating) and selective electropolishing (to smooth out any irregularities in the surface that are unwanted). This permits much finer layers and eliminates the dead time and material loss between the two baths.
Unfortunately, both the negative feedback in electropolishing and the positive feedback in electrodeposition increase with the resistivity of the electrolyte.