Advancing the art of moldmaking

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.

Manufacturing engineers with Tecumseh Compressor Co., in Dundee, Mich., say they made about 20 variations of the small-engine cylinder in cast metal, each in less than two weeks.

The flywheel-fan for a compressor is made of DuraForm, a rugged plastic that works on RP machines from 3D Systems. Tecumseh's Wadsworth made the part that was later bolted to a compressor for physical testing.

The rectangular pins in a Surface Generation machine are being adjusted for height. Afterwards, a clamp holds the pins tightly together for roughing and surface finishing, often on the same machine. This Subtractive Pin Tooling reduces lead times and costs for large molds by up to 90%.

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.

Protomold design specialists and ProtoQuote, the company's Webbased quoting and design-analysis system, let Ametek engineers receive working pilot parts in five days for customer demonstrations and design approvals.

The motor developed by Ametek (right) relocates the carbon brushes and air-handling diffuser. The armature support and carbon-brush system hang upside down instead of topside up.

MAKE CONTACT:
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