The Soup to Nuts of Dies and Molds

To describe the die and mold industry as broad or far-reaching is like calling a Swiss Army knife a fancy screwdriver. Progressive stamping dies, press brake tooling, blow molds, die-casting molds, and forging dies—these are just a few of the machined products that a person might learn to construct when earning their Tool and Die certificate at the local vocational-technical institution (which more young people should pursue), a place that helps keep the wheels of civilization turning.

Despite this article's title, it's far from a comprehensive dive into toolmaking. That would require many hundreds of pages and input from dozens of industry experts. It will, however, provide some insight to the machining technology behind the complex, three-dimensional shapes found in plastic injection molds as well as some guidance on the software needed to design, simulate, and optimize these sophisticated assemblies.

In addition, much of what's described here applies equally to other toolmaking applications, such as stamping dies used to form automotive body panels or the molds for the thermoformed packaging that kept your takeout dinner warm on the drive home last night. So do the cutting tools, CNC machinery, and CAM software used to produce these highly accurate, often free-form surfaces, all of which are just as necessary to produce the components found in industries such as aerospace and medical. The question then becomes: What's so special about die and mold work?

Sean Shafer, segment manager for Makino Inc.'s die and mold market, has a few ideas. The Mason, Ohio, machine builder is well known for its presence in this industry, and Shafer—along with product marketing manager Dave Ward—has been with the company longer than many readers have had their driver's licenses. The industry veterans listed several key differentiators between this and other types of machining, starting with complexity.

"There's been a lot written lately about Tesla and the Idra Group's Giga Press, but that's just the tip of the iceberg," Shafer said. "The auto industry overall has seen the value of combining multiple structural components into a single die casting. It reduces installation time, cuts weight, and adds structural rigidity to the vehicle."

Yet, as Ward pointed out, producing these parts requires larger, deeper molds than in the past, work for which traditional three-axis machining centers are less productive. "You need the ability to tip the spindle, so as to achieve the most effective attack angle and reach these deep features without having to stick the tool out overly far. This approach not only improves tool life and surface finish, but significantly increases metal removal rates."

They’re talking about five-axis CNC machining centers, technology that's changing how shops of all kinds make parts. Makino recently expanded its offering in this area with two machines, the V100S and the D2, both "built from the ground up specifically for large mold and die components." The company also continues to offer automation in the form of integrated robotics and pallet-changing systems. Laughing, Ward noted: "We visit a lot of shops and there are three main trends right now: five-axis, five-axis, and automation."

Machining on a three-axis mill means the cutter is always oriented perpendicular to the workpiece, Ward pointed out. This is fine for drilling, face milling, and other common machining operations, but with mold cavities, which are typically finish machined using ball-nose end mills, the tool tip often does much of the cutting. Unfortunately, surface speeds approach zero the closer you get to this "null point," greatly reducing metal-removal rates.

"Now, if you can tip that end mill at an angle as with a five-axis machine, you’re cutting with the periphery of the ball," Shafer said. "You might get six effective flutes instead of one. This generates a much better shearing action, and since you can stub up the tool, you eliminate vibration and can feed far more aggressively. It's benefits like these that are making five-axis machining centers the preferred machine tool in many die and mold shops. They’re the only way to go, no matter what size work you’re doing."

Ironically, five-axis is beginning to reduce the need for ball-nose units. Ward and Shafer agreed that, when not discussing "five-axis, five-axis, and automation" with their customers, they’re answering questions about circle segment cutting, aka barrel milling. So is Takuro "Tak" Sato, the technical applications manager for the Die and Mold Team at Mitsubishi Materials USA (MMUS), Costa Mesa, Calif, a group that represents the company's Moldino brand of cutting tools.

Sato listed a host of indexable and solid-carbide conical barrel cutters, the GP1LB, GF2T, GF3L, and GS4TN among them. Suggested applications include stamping dies, die casting and plastic injection molds, blisks, impellers, and turbine blades, molds used in tire production, and so-called bathtub molds.

All are part of the company's Gallea series of circle segment tools and, like all such cutters, have flutes that contain one or more precision-ground arc segments much larger than that of a comparably sized ball end mill. This allows greater step downs on mold cavity walls, and, in the case of "lens" type barrel cutters, bigger stepovers on cavity floors. The result is faster finishing—up to 70 percent in some cases—and smoother surface finishes than what's possible with ball-nose cutters.

Given the wide variety of barrel-mill radii, shapes, sizes, and manufacturers, selecting one can be a confusing process. Sato explained that much of the variation comes from the diverse range of applications for this groundbreaking machining technology. As noted earlier, aerospace manufacturers use them to cut turbine blades and impellers, while those making orthopedic implants routinely barrel-mill complex bone shapes.

However, practically any free-form surface currently being machined with a ball-nose end mill is likely a candidate for conical barrel milling. The trick is finding the right tool and then generating the correct toolpath. "It's far from one size fits all," Sato cautioned. "For instance, we made our radii smaller than some of our competitors to address the needs of moldmakers. A smaller radius reduces tool pressure and the chance of chatter, so is a better choice for the hard materials used in this industry."

Which tool is most suitable for the application is ultimately up to the programmer, he added, a statement with which Jesse Trinque agrees. A sales and applications engineer for Tolland, Conn.-based CNC Software LLC, makers of Mastercam, he has a great deal to say about circle segment cutting, starting with the fact that it's not for everyone.

When it comes to die mold, barrel cutters are most effective on surfaces without a huge change in curvature relative to their size, he said. "So the bigger the surface and smaller the curvature, the better the application set you’ll have. In other words, they’ll work great on a mold for a Pringle chip, less so on one with ripples."

That's especially true if the Pringle is actually a supersized car door. Trinque pointed out that these large, gently curved surfaces tend to consume the most machining time, giving circle segment cutters a distinct advantage. And yet, they’re not for every customer, he warned. As Sato suggested earlier, programming them requires the correct tool, machine, and CAM software, and no small amount of machining knowledge.

"It's important that you understand your machine and how it moves, and really just having a solid grasp of five-axis programming techniques before taking that step," Trinque added. "You don't want to run before you learn to walk. Circle-segment tools definitely have their place but they can also lead people down the wrong path very quickly, leaving them frustrated and unlikely to give it a second chance."

Learning to walk also applies to the machinery, regardless of the cutting tool being used. When asked about "understanding your machine" and five-axis techniques, Trinque noted that a CNC program might look perfectly fine on the screen but behave differently on the various styles of five-axis machining centers. Depending on the manufacturer, these include head-head, table-head, and table-table constructions, each of which has distinct kinematics that influence performance and programming approach.

"Whatever you’re machining, it's critical that you know the equipment's strengths and weaknesses, and steer away from the latter as much as possible," Trinque asserted.

Any CNC programmer knows that Mastercam does much more than generate circle-segment toolpaths. So do Autodesk Inc.'s Fusion 360 and PowerMill products, both of which have strong followings in the mold and die segment (and others).

But as Hanno Van Raalte is quick to point out, success in this industry goes well beyond the use of high-quality machine tools, cutters, and CAD/CAM systems. It also increasingly depends on a digital twin. "More and more, being able to respond quickly to late changes is becoming critically important, and a digital twin of the plastic design, the mold, and even the molding process provides a huge advantage," he said.

Van Raalte is the product manager for Moldflow, a software tool the San Francisco, Calif.-based Autodesk describes as "plastic-injection and compression-mold simulation for design and manufacturing." With it, he noted, toolmakers can simulate the molding process from art to part, and beyond.

"Let's say you’ve designed a housing for a new line of vacuum cleaners. What's to say it's moldable? Are the walls the right thickness? Are there features that will interfere with part ejection, or lead to warpage? Moldflow allows users to analyze for these and many other design rules and make smart decisions up front, long before the first toolpath is generated or chip made."

Autodesk simulation business manager Mark Hennebicque noted that moldability is only the starting point. By constructing a digital twin early in the process, moldmakers can perform FEA simulations of material flow and thermal characteristics, then use this information to optimize tool design. They can then install the virtual mold into a twin of the molding equipment, and determine the best settings for injection pressures and material temperature. Doing so reduces cycle time and therefore cost, while helping to eliminate common problems such as flash, sink, and dimensional variability.

This last issue is a common pain point throughout the industry, said Hennebicque. "Moldmakers are always looking to make accuracy and cost improvements, but since they have to make certain assumptions during the design process, this can be challenging. At the same time, the production floor or, in some cases, a subcontractor, might make decisions after the fact that should have been accounted for earlier on."

Regardless, simulation with a digital twin provides an iterative, collaborative approach that encompasses the entire design and manufacturing process. It also captures all of the data associated with these processes—if there's a crash six months later and the shop needs to rework a mold cavity or produce a new insert, there's no need to reinvent the wheel. "Everything they need to reproduce the original tool is contained within the twin," he concluded.

Van Raalte agreed. "In many industries, moldmaking included, the digital twin is changing the way companies do business. It allows them to make better products. Confidence increases, risk goes down, and everyone can do more with the time they have. Because of that, the twin is quickly becoming a must-have for many manufacturers."

David Hill of Hexagon Manufacturing Intelligence offers much the same advice regarding digital twins. The director of the company's Commercial Operations in Canada, he’ll tell you that North Kingstown, R.I.-based Hexagon offers competing design and engineering tools, and encourages those in the moldmaking and other industries to leverage their immense capabilities. But for this article, Hill discussed another critical component of mold and die manufacturing: metrology.

"I concur wholeheartedly with the statements made earlier regarding recent advancements in product and CAM simulation and molding analysis, but from a metrology perspective, one of the most important technologies today is the ability to measure where you manufacture," he said.

Clearly, in-machine probing and scanning eliminates regular trips to the inspection room, thereby improving overall equipment effectiveness levels and reducing the possibility of a scrapped part due to machining errors. These benefits are significant for all manufacturing processes, but when the workpiece weighs hundreds or perhaps thousands of pounds as with many molds and forming dies, validating it in the machine can be the difference between profit and loss.

Spindle-mounted touch probes are nothing new. Even laser and structured light scanning have become increasingly common on the production floor.

What's less common is the use of these technologies to quickly gather millions of data points and then use that data to validate the simulations produced in software like that offered by Hexagon and others. This allows toolmakers and engineers to compare in-process dimensional values to the CAD file, analyze spring back, verify main datums to form with best fits, and troubleshoot areas of concern, eliminating expensive recuts and non-conformance.

But what about accuracy? Any machinist worth her salt knows that critical measurements should never be performed on a CNC machining center, no matter whose logo is plastered to the sheetmetal. Right?

Maybe so, but as Hill pointed out, today's high-end machine tools are plenty precise for many measurement functions, never mind the fact that portable laser and structured light systems (such as the company's HP-L-10.10, AS1 laser scanner and Structured Light Systems) can approach 5-micron accuracy levels.

"Granted, in-machine and portable metrology will never truly replace the CMM [coordinate-measuring machine], but many in this industry are finding that these technologies give them a competitive advantage," said Hill. "It lends itself quite nicely to reverse engineering, component inspection, fine-tuning of the machining process, and many other uses, all of which can be performed quickly and easily without moving the workpiece. Time's critical in manufacturing, and while I hate to use a cliché, these solutions are game changing."

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