April 2012 / Volume 64 / Issue 4|
By Clare Goldsberry
| Sizing up the state-of-the-art of micro-moldmaking.
Manufacturing molds used to produce plastic parts with features that can be too small to be seen with the naked eye or measured with standard devices is challenging. That’s why moldmakers that perform this work are specialists in the nuances of designing and building micromolds.
According to Ryan Katen, general manager of Micro Mold Co. Inc., Erie, Pa., machining mesomolds with micro features as well as micromolds is not only challenging, it’s tedious. “It requires a high level of attention and concentration on the part of the moldmaker,” he said.
Courtesy of Matrix Tooling
Recognizing the trend toward microparts and the need to accommodate moldmakers, specialty machine tool builders have raised the bar in equipment for machining micromold cores and cavities. Demand for these machine tools has increased as the market for injection-molded microparts grows, pushing demand for the molds.
While alternative processes can be used to make micromolds for producing plastic parts, standard machining methods still predominate. Machine tool builders such as Makino Inc. and Datron Dynamics Inc. have improved machine tool technology to meet the needs of micromanufacturers, including moldmakers. For example, Makino’s V22 vertical machining center can maintain accuracies under 2µm.
“The reason [micro-moldmakers] choose our technology is because the machines are designed from the ground up for high-speed machining with the small tools required to produce small, intricate molds,” said Stephen Carter, marketing manager for Datron Dynamics Inc., Milford, N.H. Datron recently launched the C5 for milling small parts, including micromolds. With its simultaneous 5-axis capability, the machine can mill micro molds from steel, aluminum, titanium and plastics. The C5 can impart fine surface finishes.
Mark Kinder, president of Plastic Design Corp. (PDC), Scottsdale, Ariz., which designs and builds micromolds for parts smaller than a standard resin pellet, noted that Makino’s V22 provides excellent surface finish and resolution. “Makino’s new machine, the IQ300, has even better resolution from our experience, but we cannot discern the improvement with our current metrology capabilities,” Kinder said. “We’re looking at some new metrology equipment to allow us to measure this. It’s the continual leapfrogging of technology. We use a Nikon VMR video measurement system with a through-lens laser. To work with finer detail, we are looking at the Alicona InfiniteFocus, an optical 3-D micro coordinate system.”Devil in the Details
Matrix Tooling Inc., Wood Dale, Ill., specializes in building and running micromolds but finds that even with a lot of experience the challenges remain daunting. When working with CNC machine tools and EDMs to create microscopic core and cavity details, measurement is the biggest challenge. One key to Matrix’s success, according to Mike Martin, shop manager, is trusting the machines used to make micromolds.
Micro Mold’s Katen agrees. “You have to rely on the accuracy of the CNC equipment to hit the dimensions, and you have to trust those numbers,” he said.
“As the machine technology gets better, the challenges aren’t as tough,” Martin said. “It helps to have machine tool centers that can handle the smaller details. In some areas, yes, it’s the same type of work as in larger molds, but in larger molds you can get away with milling and machining the detail work [on the machining center]. With micromolds, you must use sinker and wire EDMs for details, and you need more electrodes because they are so small and details are so fine that smaller electrodes are adversely affected more than large electrodes. Since they erode faster, we need more electrodes to get the fine details. It also helps to get a smaller wire. Our standard wire is 0.010" in diameter and we go down to 0.004". That allows us to create smaller details, such as corners.”
Courtesy of Datron Dynamics
Katen added that while some micromold manufacturers use coordinate measuring machines for measuring, Micro Mold uses manual devices such as micrometers, gage blocks and ball indicators, and finds those “extremely accurate.” He related the story of a customer who searched for 6 months to identify a different system that Micro Mold could use to measure its cores, cavities and detail dimensions, but came up empty.
Katen said measuring the electrodes also verifies dimensional part accuracy. “It makes it tough for moldmakers because they’re burning features into parts they can’t measure,” he explained. “We believe [the mold] will be dimensionally correct based on the electrode, but no [measurement] technology exists that ensures you’ve manufactured the mold correctly. Some features are blind, particularly in deep-draw areas, and you can’t physically measure the blind pockets, so we’ll measure the electrode instead. We’re confident we have the needed accuracy.”
Rob Cooney, manufacturing manager for Plastikos Inc., the micromolding division that grew out of Micro Mold, noted that verifying the steel mold by measuring the plastic part produced from it is another way to measure dimensional accuracy. “We can get close measuring the steel, but we’ll do verification in the plastics,” he said.
However, even verifying the mold by measuring the plastic parts can be a challenge. Brent Borgerson, process engineer for Matrix, noted that in cases in which the actual molded part isn’t much more than a dot at the end of the sprue, the company uses Optical Gaging Products microscopes and vision systems, a Vision Engineering video microscope and a CGI cross-sectional scanner for first-article inspection.
Measuring mold components and details and the molded plastic parts requires a lot of creativity on the part of the quality team, according to Gary Johansson, vice president of quality and regulatory at Matrix. “Any sort of tactile touch when measuring these parts is almost impossible,” he said. “It’s like killing a fly with a sledgehammer. We use high-resolution instruments to get into these parts. Fixturing these parts is another challenge, however. We have to find a way to hold them so we can see them. Typically, when the engineers design these parts, they don’t make the dimensions easy to get to.”
When it comes to measuring the details, Johansson explained that some mold conditions can be inspected, but “some geometries are formed only when the mold is together (closed) as a unit. We can measure one side and then the other side and add the two. However, at the end of the day, we’re selling the plastic part, not the mold, so it’s the part that has to be in spec.”
Interestingly, there are times when the mold can’t be made to the original print and still make a good part. Johansson said: “With scientific molding, we can get a good process, but we have to make the steel mold technically incorrect to make a part to spec. It’s rare but it does happen. We’ll confirm steel when we can, but, when we can’t, we use the part to confirm the steel.”
Some non-traditional machining processes are being used in micro-moldmaking. For example, lasing is being used more, but the capital expense appears to be a big drawback. The benefit of using a laser is that it can machine complex and precise cores and cavities, primarily because a precision laser can cut features within nanometers. Newer laser machines also come with 5-axis cutting capability to accommodate a range of angles and shapes.
PDC’s Kinder explained that operators are more familiar with milling and EDM-ing the features than using lasers. However, he recommends a hybrid approach. “Laser is catching on,” he said. “But because your base material is a mold steel, you can still hard mill that, putting in all the larger features with dimensions of 25µm to 100µm. Where you have details in the 2µm to 5µm range, you do those with a laser.”Alternatives Slow to Catch On
While several alternative technologies for micro-moldmaking have been proposed, few have had commercial success, according to the sources interviewed for this article. For example, it was thought that LIGA, a German acronym for lithography, electroplating and polymer replication, might work well in manufacturing micromold details.
Courtesy of Datron Dynamics
However, “LIGA doesn’t commercialize well” for the moldmaking industry, according to PDC’s Kinder. “It doesn’t work well with microfluidic parts because you have core pins and ejector pins in the mold that get in the way,” he said. “We were interested in it at one time, but the state-of-the-art was not as far along as everyone claimed. The LIGA process works well, but it was difficult to integrate it into the other moldmaking processes. The parts looked good in the lab but the technology turned out not to be good for mass production.”
LIGA has other issues as well in micro-moldmaking, the biggest one being the nickel electroplating process. “In high-temperature molding—250° F and above—you have to deal with the thermal expansion of the nickel,” said Kinder.
Another alternative process might prove more successful than LIGA, according to some sources. Direct metal laser sintering, an additive manufacturing (AM) process developed by Electro Optical Systems (EOS) GmbH, Germany, has been used to make several types of parts.
In DMLS, a laser melts powdered metals to create very thin layers that are then built up to create a part. Because of the composition of the materials used in DMLS, which include powdered titanium, aerospace and medical parts can be created, and the process can also be used when micro-moldmaking.
While there are a few moldmakers in the U.S. that see the value of the process, few—if any—are using DMLS to build cores and cavities. John Tenbusch, president of Linear Mold & Engineering, Livonia, Mich., began using DMLS several years ago to create conformal cooling channels in molds, but does not use the process for other applications.
However, EOS has developed what it calls micro laser sintering (MLS) for making small, complex parts. The company is working on creating cores and cavities for micromolds. “For example, we are working on prototype molds driven by the need to deliver injection-molded plastic parts very quickly to mobile phone manufacturers, which need to get their products on the market extremely fast,” said Joachim Goebner, project manager for EOS. “There’s a good chance that using this method can reduce their time to market by 20 to 50 percent.”
Courtesy of Matrix Tooling
Goebner also noted that building micromolds conventionally using milling and EDMing is time-consuming because only one step can be performed at a time. “We have some strategies that can save some steps, and we see the potential to do some steps concurrently, thus saving time as well with MLS,” he said.
Materials used so far with the MLS process include single-phase molybdenum and 316-L stainless. These materials have additive layer thicknesses of 2µm and 4µm, respectively. The thinnest wall section produced thus far using MLS is 21µm wide × 0.5mm tall. Wall thicknesses of 60µm can be produced reliably, Goebener noted. The deepest features created thus using the process far have been 150µm in diameter and 6mm deep (a realized aspect ratio of 40:1).
One strategy may be to combine conventional moldmaking processes with MLS, according to Goebner. “The idea would be to create the larger features via milling or EDMing, and produce the smallest features via MLS. To get micro definition with MLS, the focus diameter of the laser beam is 30µm and the layer thickness is 5µm.”
Courtesy of EOS
PDC’s Kinder has studied the use of AM in micro-moldmaking, but has concerns as to whether or not it can achieve finer surface finishes than standard machining. “When using MLS, you have to consider the size of the grain and the size of the laser burst and what is the resolution,” he said. “With all of your additive processes, it’s a function of the bonding method that determines how smooth a surface you get.
“For good resolution, you need submicron powders and highly focused lasers with high energy bursts,” Kinder continued. “Some machining centers offer 0.2µm resolution. With a machine tool—if you can create the geometry in the cores and cavities—you can produce better resolution than just about anything out there.”
So far, creating micromolds using the MLS process “looks promising,” Goebner added. “But,” he cautioned, “we don’t have the final results. We’re just in the middle of this development. We believe the trend to make things smaller will continue for some years, so we’ll need smaller solutions.” CTEAbout the Author: Clare Goldsberry is a freelance writer for the plastics injection-molding and moldmaking industries. Contact her at (602) 996-6499 or firstname.lastname@example.org.
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