By Jan H. Schut
Conformal cooling channels in injection molds have been built by additive manufacturing, or 3-D printing, developmentally since the late 1990s using successive thin layers of metal. Unlike conventional straight drilled cooling lines, conformal cooling channels curve around deep shapes in a mold, equidistant from the mold surface, which is the source of heat. They can only be built by additive manufacturing and are designed with cooling line requirements secondary only to part and parting line requirements, unlike traditional drilled cooling lines which are located where space is available late in the design process.
Conformal cooling, however, was limited by the capabilities of the early metal powder welding process, which used metal powders coated with polymer binder, then evaporated out the binder leaving porous metal, and infiltrated the porous space with bronze. Since about 2006, direct additive manufacturing of pure metal powders became possible with the integration of fiber lasers into metal printing vs. the original CO2 lasers. 3D Systems Inc., Rock Hill, NC (www.3dsystems.com), also added a compaction step to make a dense metal powder bed before laser melting thinner layers—20 microns vs. about 100 microns before.
All that makes “100% metal mold inserts possible with reasonably smooth surfaces, providing a rock solid tool for mold makers,” explains Scott Young, engineering manager at Bastech Inc., Dayton, OH (www.bastech.com), a service bureau for additive manufacturing since 1994 and a reseller for 3D Systems. Bastech has built molds using additive metal manufacturing since 2000 and using pure metal additive manufacturing since 2015.
GOING BEYOND ROUND CHANNELS
Conformal channels themselves, however, haven’t changed much in two decades of development. Primary cooling channels can transition to small capillary channels and back to primary trunk lines again to get cooling closer to the mold surface or to cool small mold details. Capillary channels can be closer together than large channels. (The cross section of the supply channel has to be equal to or greater than the sum of the cross sections of the capillaries, and the cross section of the return line has to be equal to or less than the sum of the cross sections of the capillaries.) But conformal cooling channels themselves are still typically round like drilled water lines.
Graphics for advanced state-of-the-art conformal cooling from 3D Systems; EOS GmbH in Kraillingen, Germany (www.eos.info); and Renishaw PLC in Gloucestershire, U.K. (www.renishaw.com), three suppliers of metal-powder-based additive manufacturing systems, invariably show only round channels. But round may not be the best channel shape, according to a recent study by Bastech, presented at the AMUG 2016 conference (www.additivemanufacturingusersgroup.com) in April in St. Louis, MO. Bastech compared cooling efficiency of round, square, diamond, tear drop, triangular tear drop, and triangular channels, based on surface cooling area calculated from channel perimeter. So for channels with the same length and volume, the channel with the longest perimeter will have the most surface area and cool best.
Using 3D System’s Cimatron 13 software, Bastech compared a round channel with a 1.374 inch perimeter to the other shapes and found that a tear drop has a 1.454 inch perimeter; square has a 1.474 inch (rotating the square into a diamond shape has the same perimeter but is structurally stronger in a mold); triangular tear drop has a 1.574 inch; and triangular has a 1.587 inch. So a triangular channel cools 16% more efficiently than a round one. “More important, triangular channels have a long flat surface that can be oriented to parallel the mold surface,” Bastech’s Young points out.
THE ‘X’ FACTOR
The most complex conformal channels Bastech has built so far were done this year for a mold core for an in-house promotional product, a 5-inch-tall cold drink sleeve. The 8-inch high mold core, also presented at AMUG 2016, is cooled by two inter-twined helical flow channels with X-shaped cross sections (X’s have even more surface area than triangles). The supply side channel splits to start one helix at the base of the core and the other helix and the top, creating counter flow, shorter channels, and more even cooling. The helical channels then reconnect into the return line to exit the core.
Recent patent literature also mentions additive metal manufacturing of alternative cooling channel shapes. Star-shaped cooling channels are described by Siemens AG (U.S. Pat. # 8922072) and “triangular, rectangular, square, semi-circular, and ellipsoidal” channels by General Electric Co. (U.S. Pat. Applic. # 20140202163), both to cool heavy machinery, not injection molds. 3D Systems discussed the concept of star-shaped cooling channels at an electronics trade show in January this year, but without showing simulations.
Star-shaped channels, however, would be difficult to build by additive metal manufacturing “because unsupported structures shouldn’t overhang by more than 45 degrees,” notes Maximilian Boulter, manager for additive manufacturing at Renishaw’s LBC Engineering service bureau in Pliezhausen, Germany (www.renishaw.com), “though the angle is dependent on a few things like material, size, and angle toward the recoating wiper.” The X shape is also difficult to build. “The X pattern would have been doomed in a single helix, but a double helix gave us enough pitch to create the shape,” Bastech’s Young explains.
Cleaning and maintenance, however, are issues with non-round channels, notes Mads Jespersen, a partner at FlowHow ApS, Sydiylland, Denmark (www.flowhow.dk), a consultant on conformal cooling. Jespersen calculated how much steel was needed for a mold core not to collapse with wear over time and filled all the open space with cooling water. “That made some weird but very effective cooling channel shapes,” he says.
He then simulated the flow rate of cooling water using Moldex 3D software from CoreTech System Co. Ltd., Chupei City, Taiwan (www.moldex3d.com), which includes computational flow dynamics (CFD). CFD showed potential areas in the cooling channels with no flow. The risk with ‘no flow’ areas is that “these irregular channels, while effective, would also be sensitive to corrosion and deposits and impossible to clean without use of chemicals and the risk of damaging small features in the cooling channels,” Jespersen says. He concludes that “conventional round channels may be the best solution to minimize the cost of cleaning, spare parts, and break downs.”
Instead of increasing the surface area of the channel with alternative shapes that may be hard to clean later, cooling can also be improved by turbulent flow. FlowHow’s Jespersen simulated and built unusual “fish net” structures with additive manufacturing in combination with round channels to increase turbulent flow where more cooling is needed.
Like FlowHow, Bastech also used additive manufacturing to reduce the mass of metal in molds, but it wasn’t to increase cooling water volume. It was done because less metal in the mold means faster mold start up and shorter additive manufacturing time. “We only need 0.25 inches of tool steel for mold walls, plus an inch or so for the cooling channels. Everything outside of that we don’t need,” Bastech’s Young explains. For the drink sleeve core instead of a solid metal mold, Bastech built a structure of trusses and supports, leaving diamond shapes of metal out and removing roughly 25% of the metal. The drink sleeve core took only 38 hours to build vs. 42 hours for CNC programming and machining to build the same core out of solid tool steel with conventional spiral baffles.
An earlier important study on conformal cooling was done internally by Lego Group, Billund, Denmark (www.lego.com), in 2010. Lego used Moldex3D software and MSC Nastran finite element analysis software from MSC Software Corp., Santa Ana, CA (www.mscsoftware.com), to simulate and build three different cores for Lego blocks: one out of standard tool steel, one with an Ampco bronze cooling insert, and one with conformal cooling. The study found that cycle time went down to 13.1 seconds for cores with the bronze insert and conformal cooling vs. 23.2 seconds for standard tool steel, but conformal cooling made better parts with less warpage than bronze inserts. Temperature differences between actual mold inserts and simulation were reportedly within 5%. Lego is now believed to have built more conformally cooled production molds than any other company in the world.