Paul Dvorak, Senior Editor
It's not hard for tooling costs to devour 40% of a development budget. And once constructed, molds can make only one product. And when production finishes, molds usually collect dust on warehouse shelves for years. A few recent ideas, however, promise to turn these traditions on their head.
For instance, a laminate mold can chop up to 10% off traditional tooling lead times. And when put into production, its conformal and flood-cooling channels help shave even more time off manufacturing cycles.
Another technique aimed at forming large aerospace and marine parts eliminates a lot of roughing. It also cuts up to 90% off the time usually needed for lowtemperature molds. When this tool is no longer needed, its surface can be adjusted and cut again for new parts.
MOLDS LAYER BY LAYER
Engineers at Fast4m, Troy, Mich., borrowed an idea from rapid prototyping to make laminate molds. RP equipment builds parts from many thin layers of plastic. So why not build injection molds layer by layer out of sheet stock?
The technique, called laminate tooling, starts with a CAD model of a part. Software builds a mold base around the part and then slices this assembly into many layers, each the thickness of selected sheet stock. A high-speed laser cuts and punches details of each slice of the mold into a steel or aluminum sheet. The layers are then pressed together, bonded to achieve about 94% of the tensile and shear strength of P-20 steel, and finished machined. This last task puts a fine finish on the part surfaces and machines ejectors and other mold features. Class-A surfaces may require plating the laminate tool. "However, some users say as-finished laminate tooling part surfaces turn out acceptable parts," says Fast4m Vice President Rob Esling.
Laminate molds are made of coldrolled steel, 300 and 400 stainless steels, and 6061 T-6 aluminum. Steels work best for high-temperature and pressure-injection molds while aluminum is better for low-temperature and pressure applications. But stainlesssteel tools have advantages over aluminum versions. For instance, good venting and thin-wall construction let stainless-steel tools perform similarly to aluminum tools but stainless does not corrode, patterns are more accurate due to less thermal expansion and contraction, and it far outlasts aluminum.
Laminate molds also let designers place cooling lines where they are most useful. Heat conducts from the molten plastic to the mold and is then removed by coolant flowing through a network of internal channels. Thermal analysis of the mold in the design stage accurately predicts the location of hot spots. Cooling lines can then conform to the geometry of the part. These conformal cooling lines remove four to five times the heat of traditionally gun-drilled lines.
Another innovation, flood cooling, uses large-surface channels that create turbulence in the flow to carry away more heat than laminar flow in smoothwall lines. With either method, the goal is to maximize heat transfer from the part. A well-cooled mold lets parts cool uniformly and faster, thus minimizing internal stresses and trimming production times. Another benefit is uniform surface temperatures across the core and cavity of the mold.
"More efficient cooling lets manufacturers shave 30 to 50% off a 60-sec injectionmolding cycle," says Esling. "Large parts are usually made in singlecavity molds. So in a machine capable of 700-ton clamping force, that equates to saving about $0.43/part. If the job calls for 100,000 parts/year, that comes to $43,000 annually," says Esling.
Laminate tooling can also build large molds. However, current equipment restricts molds of about 4 to 6-ft3. Generally, laminate mold construction costs less because it's made from sheet stock, a relatively inexpensive material that is readily available. "Costs for a laminate mold may be 8 to 12% less than the same mold from a traditional shop. And it may take about 10% less time," says Esling.
SUBTRACTIVE PIN TOOLING
Surface Generation in the U.K. developed an approach called Subtractive Pin Tooling (SPT) to build molds that use a grid of rectangular "pins" mounted on threaded rods. Their height can be adjusted to form a rough net-shape surface. The pins are clamped and held in place by a bolster while the working mold surface is roughed and finished with traditional milling equipment. Molds made this way are useful in about a dozen operations including composite manufacturing, superplastic forming, vacuum forming, and pattern making.
"SPT shrinks lead times and costs associated with large and short-run component manufacturing by creating the front face of the tool, as opposed to an entire solid mold insert," says Jim Gray of Jim D. Gray & Assoc. Inc., the North American distributor for SPT in Richardson, Tex. "Pins can be of plastic, metal, ceramic, and even wood. Graphite could be used to make a large electrode and we've even proposed pins of Inconel for a high-temperature application."
According to the company, SPT tools slash cost and lead times by up to 90%, and it cuts time to market by 35% for large components. And ROIs are projected in six to 18 months.
"This approach allows rapid design iterations by adding material, removing it, or both. Over 90% of the mold can be reused in future projects. We suggest saving the model, not its mold. And the system can economically produce one part," says Gray.
"It's imperative that the first mold be made as quickly and cheaply as possible," adds Gray. "Tooling is only an asset while its being used. So for low-volume work, it is essential to have a reusable tool. When the surface on the tool is no longer useful, pin heights can be adjusted and recut for new parts." Gray adds that SPT also lets users verify assemblies, manufacture one-off's, and produce several prototypes from the same mold to assess competing designs.
SPT performance is governed by how closely the pins can produce the near-net shape of the required geometry. In most cases, composite SPT tools are more stable than conventional steel tools.
For ductile materials, machining parameters can be set to blend away the pin-to-pin joint, usually less than 50 m. And when necessary, a temporary bond along the pin boundary may be used to create a "single" surface.
MODEL TO CASTING IN 10 DAYS
Fast tooling also comes from skillfully handling a rapid-prototyping machine to build with low-melt materials. These can be used to make patterns for lost-wax casting.
For instance, when engineers at Tecumseh Products Research Laboratory, New Holstein, Wis., spotted design changes in a two-cylinder engine they were developing, they would modify a CAD file and sent it off to the rapid-prototyping facility at the company's compressor division. "The cylinders measured about 12 X 14 X 16 in.," says Manufacturing Engineer David Wadsworth at Tecumseh Compressor Co., in Dundee, Mich. "The SLS rapidprototyping equipment from 3D Systems built the engine cylinder in wax overnight." After forming, the wax pattern was coated in several layers of ceramic, which was baked and sent to a foundry. Finish machining followed. But, notes Wadsworth, total time from receipt of the CAD file to new functional part took only 10 days.
Prototype parts from production plastics
Getting working prototype parts in days let a vacuumcleaner motor maker turn out working models of a new design in a week. What's more, engineers at Ametek Inc., Paoli, Pa., received injection-molded parts in Rynite, a material strong enough to survive the heat and pressure generated in performance tests.
Before building the parts, Protomold Inc., Maple Plain, Minn., let the motor maker "analyze part models for elements that might affect lead time," says Ametek Vice President of Engineering James Shawcross. "Changing designs online instead of describing alterations over the phone proved a huge advantage in getting a functional model in front of clients within one week." Protomold created 100 production-quality parts for tests by the engineering and manufacturing departments, and clients.
Most traditional rapid-prototyping methods, however, deliver only one part per run and use materials that deform in rigorous performance tests. To solve such problems, functional testing was often put on hold until production molds could be developed, typically a 16-week wait.
Moldmaking in a Rush
Rush or "Rapid use of shop hours" is a lean-manufacturing technique that shortens the time required to make molds. It was devised by Ron Pleasant, president of Pleasant Precision Inc., Kenton, Ohio, to let moldmakers work with key suppliers, share 3D design data, reduce the number of changes, and trim errors.
Pleasant uses Pro/E Wildfire to create molds based on customer designs. Then he uses his own Modular Mold System for standardizing mold components, and high-speed milling machines.
"The associative features in Pro/E Wildfire lets us design molds even when its details are incomplete. It lets us describe mold cavities without knowing the shape of the block itself. This lets us move ahead electronically, while changes and fixes are made along the way, confident that all elements will be included in the final design."
At a trade-show demonstration, Pleasant and his team set up a system that showed how a mold could be completed in just one day. Injection molds typically take eight to 12 weeks. Each morning of the show, the team designed a new injection mold and generated the corresponding NC toolpath with Pro/E, then machined and assembled the production injection mold from tool steel. At the end of each day, a newly finished mold was installed on a 190-ton injection press. Over the course of the week, the team designed and built five different molds.
Production plastic ready for RP machines
PC-ABS, a blend of polycarbonate and ABS that combines the strength of PC with the flexibility of ABS, is ready for several FDM (fused-deposition modeling) rapid-prototyping machines. Stratasys Inc., Minneapolis, says that although PC-ABS blends are widely used to manufacture parts, a blend has not been available for rapid prototyping and production.
The company says PC-ABS has excellent thermal and mechanical properties. It is stronger than ABS, and feature detail is similar to that of Stratasys ABS modeling material.
The new material works in several Stratasysmachines, including the FDM Titan, Vantage S, and Vantage SE. Layer thickness can be set for 0.005 or 0.010 in. A 0.007-in. layer is planned for future releases. Users must upgrade to the most current software to use the material.
The problems with traditional tooling
Large tools made the traditional way usually start with large blocks of wrought material that take weeks to procure. Sheet stock used in a laminate tool designed by Fast4m, on the other hand, is readily available.
Water lines are another problem. They are usually the last consideration in tool design because other factors, such as ejector locations, are considered more important. So cooling channels are placed wherever they fit, often not in the best locations. In addition, the usually straight-drilled passages rarely bring coolant to hot spots. Straight-line cooling tends to produce laminar flows through the tool that transfer heat less efficiently than turbulent flows.
And lastly, manufacturers impatient with the performance of a traditional mold may just dial down the cooling cycle. But parts that cool unevenly have more internal stress and can warp.
3D Systems, (661) 295-5600, 3dsystems.com
Fast4m Tooling, (248) 457-9611, www.fast4m.com
Pleasant Precision Inc., (419) 675-0556, ppiteam.com
Protomold Inc., (763) 479-3680, protomold.com
Stratasys Inc., (952) 937-3000, stratasys.com
Surface Generation, (972) 699-9976, surface-generation.com
Tecumseh Research, (517) 423-0342, tecumsehrp.com