Reengineering the mold shop

The big drivers reshaping the mold industry include extensive automation of the entire design process and high-speed machining. The automation includes software for designing plates and slides, generating NC toolpaths, and selecting off-theshelf components. And highspeed machining (HSM) has wider ramifications than just cutting tools spinning faster to shorten cycle times. If the two developments work together, they may set in motion dramatic improvements for U.S mold shops.

ON THE DESIGN SIDE
An old design adage says you can have a product faster, cheaper, or better, but only pick two. "Over the last 12 years, faster and cheaper were the real drivers," says Roland Thomas, president of Moldflow Corp., Wayland, Mass. "More recently, the emphasis has been on ‘better'. That emphasis has researchers pushing the boundaries with what can be done on plastic components. In-mold assembly, for instance, is a new focus in the industry." Instead of taking several components and assembling them, they become inserts in the mold. So molding becomes the assembly step.

"Certain automotive relays are made this way," points out Thomas. The development means some conventional design rules for plastic don't apply anymore. Molding around components, for example, doesn't give designers the same control over parts as conventional assembly techniques do. "Thick and thin sections could be next to each other, which designers ordinarily avoid," he adds. "But the reduction in assembly time and cost is so attractive it puts big demands on toolmakers as well as simulation. In fact, every new design or manufacturing change calls for a simulation to show that it will work," says Thomas.

Tying design to simulation seems inevitable considering that even small molds cost upwards of $100,000 and larger ones for auto parts can hit $500,000. Repairing design mistakes are profit killers. With costs like this hanging over manufacturers, it's no surprise software developers have responded by automating many mold-design tasks. For instance, PTC, EDS PLM Solutions, and others have developed mold-design assistants and wizards that turn months of repetitive manual operations into a few interactive hours. Each developer brings its own range of automation to designing molds.

Pro/Engineer Expert Moldbase (www.PTC.com/products/proe/emx/), for example, provides a process for importing part designs and producing digital cores, cavities, and mold bases. Their NC production schemes then take advantage of highspeed machining to cut tool steel without heat treating. PTC engineers have shown how their software with other molding technologies can turn out production tooling in less than 24 hr.

And Mold Wizard from EDS PLM Solutions (www.eds.com) provides a series of process-based wizards, knowledge-embedded models, alternative modeling methods, and collaborative engineering tools to quickly step through the mold design process. The system includes features for handling a wide variety of geometry, designing mold inserts and parting geometry, and libraries for mold bases and standard parts. At least one company credits the system with slashing their design time in half.

Other design approaches include Expert Mold Designer and Expert Mold Manufacturer from Cornerstone Technologies Inc., Windsor, Ont., Canada, (www.cornerstonemold.com). "The software compresses tasks that once took days or hours into minutes," says Paul Coleman, vice president at Cornerstone. "Expert Mold Designer uses Cadkey as its driving system. After supplying a part model, the software steps through a mold-base design process that lets engineers adjust parameters when necessary. The software then produces the rest of the components, such as ejection pins and hole plates. It also routes water lines, and develops 3D models and 2D drawings for components."

Should the user have to modify a plate by adding a screw, for instance, the software knows that it must go into a tapped hole with clearances and tolerances, and places those details in the right locations. The software also captures design intent — the purpose of the hole, for instance. "If a user places a clearance hole, the system applies a machining practice that includes the tolerance used earlier for similar features. But if the hole is to be reamed and held to tighter tolerances, it recognizes that, and plans on more refined machining practices," he says.

Most moldmakers use rulesof-thumb that have been incorporated into the mold-design software, adds Coleman. "But users can edit dimensions to stay true to their own standards."

In general, the software takes the grunt work out of moldmaking. For example, some mold plates call for 200 holes. "Programming that plate manually and missing one hole can add days to a project," says Coleman.

ON THE PRODUCTION SIDE
The mold industry has been fairly static for the last 30 years, then came high-speed machining, according to Charles Farah, a manufacturing engineer and spokesman for PTC, Needham, Mass. "The increase in cutter speed has made it necessary to rethink strategies for machining parts. HSM once meant cutter speeds up to 20,000 rpm, so you could get by with less than the best machining practices because the cutting tool was not working at its limit. But many shops now cut steel at 50,000 rpm, and the speed will continue increasing." Most shops taking advantage of HSM use it as a finishing strategy, says Farah. The big savings, however, come in roughing routines that removes most material.

"Traditionally, roughing toolpaths were developed from part geometry," says Farah. "But that gets into geometry that is bad for the tools. For example, some corners load tools to 100% of their capability, which leads to broken tools and damaged workpieces. High cutter speeds now handle hard machining — machining hardened steel. This avoids postheat treating to harden a tool which often slightly warps it.

PTC's machining plan is to find optimum chip sizes for each material and let cutters work near their limits. Until recently, machinists would break tools because they were driven too hard. And tools burn with too little load. A lot of heat dissipates into the material and the chip, if there is one, is too small. High speeds don't allow the luxury of going gently.

Cutting principles, however, say an ideal chip size gives the greatest rate of material removal. With it, the heat generated is not in the part or the tool — it's in the chip. "Maintain chip size and you can quickly slice through parts regardless of complexity, no matter how many islands or walls," says Farah.

Other NC software features focus on manufacturing modeling, reducing roughing and bench time, says John Callen, a spokesperson for Gibbs and Associates, Moorpark, Calif. "For example, NC software applies shrinkage factors to imported mold models, helps users find parting lines, and provides routines such as plunge roughing to quickly hog out material, and polishing routines to minimize bench time." The parting-line capability, explains Callen, helps find a decent line on the part that separates one side of the mold from the other, an arbitrary feature that's not easily defined. "Some geometry can make parting line geometry extremely difficult," adds Callen.

Plunge roughing works by vertically driving a large-diameter tool into material, one way to minimize roughing cycles. The vertical motion takes advantage of the tool's axial stiffness.

Cusp-height control, another NC feature, minimizes bench time. Cusps are the rows or peaks of material between toolpaths. "Large cusps can be jarring and slow down highspeed machines. Ratcheting down cusp height produces improved surface finishes that need little manual polishing," says Callen.

NC software also minimizes hand work in molds by producing Nurbs output. There are two approaches here. "One takes the polyline toolpath and fits a Nurbs curve to it. The second approach extends the toolpathgeneration algorithm to create toolpaths directly as Nurbs curves. Because the extended algorithms have access to the original surfaces, they ensure the toolpath does not deviate beyond a reasonable tolerance from the original surface, which is lost in the first method," adds Callen.

The Nurbs-based feature cuts bench time other ways. For instance, most NC programs facet surfaces they read in, says Callen. "Most NC software does all its toolpath slicing and dicing off the faceted representation. Look closely at a cut surface and you might actually see artifacts from the faceted representation. This is generally what hand polishing removes." But Nurbs toolpaths generated using extended toolpath algorithms don't have faceting artifacts, so hand polishing is minimal.

Callen sees a higher percentage of models in the future transferred to mold shops in native digital formats or next generation industry-standard formats such as STEP, not just IGES files. It wasn't long ago mold shops predominantly received only drawings or IGES files. Currently, engineering drawings are still used along with CAD files because drawings include critical dimensions, tolerances, and surface finishes.

Another trend is greater reuse of proven processes. "NC software lets users save them. A favorite example is to center drill, drill, and tap threaded holes. Mold shops now store more complex pocketing operations as well, which include toolpaths to semipocket, pocket, semifinish, and finish." The advantage comes when routines are stored in central libraries and made available to several users.

"Tested and proven processes are the shop's family jewels," says Callen. "When standardized and centralized, they become assets that let companies introduce a level of consistency and repeatability that is invaluable. These become the shop's manufacturing best practices."