Magnesium Castings Solve Design Issues of Video Projector Parts

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The lightweight castings, a conversion from plastic injection molding, provide thermal and dimensional stability, as well as substantial strength, stiffness, and durability.

As Sales Manager for Prototype Casting Inc., of Denver, Colorado, Brett Peak recently had a special reason to smile: Earlier this year, Prototype Casting was named the 2002 winner of the American Foundry Society's Casting of the Year Award (Industry News). The company earned the award for a set of magnesium castings that it produced for the portable LP 130 LCD digital business projector from InFocus (Wisonville, Ore.), a long-time customer providing portable projection equipment for road warriors. The projector, which is used with laptop computers, has only a 6.7-inch x 8.6-inch footprint and weighs less than three pounds. It projects both still and full-motion video.

The AZ91D magnesium castings for the LP 130 were produced using a rubber/plaster mold (RPM) process. In keeping with road warriors' demands for lightweight portable devices, the projector castings weigh only 0.22 lb. and 0.05 lb., respectively, and have a common wall thickness of only 1.2 mm. The entire prototype creation process took only 20 days from when CAD files were transferred to Prototype's foundry, through to post-machining.

Conceptualization

According to Greg Sample, senior tooling engineer at InFocus, and Brian Heintz, the OEM's principal engineer of product design, the conceptualizing of new projects at InFocus often begins simply, in artboard or foamcore. Those materials help them decide where various elements might go, after which manufacturing and service people go through mock builds and disassemblies to input their thoughts on the overall concept.

In addition to SLA and SLS, InFocus uses a Fused Deposition Modeler (FDM) to create models. Using ABS as the modeling material source, FDM digitizes parts and lays down the part around support material that later dissolves in a vat of water. "Those parts are good for building up products to understand interferences," Sample says. "We can also use them for early airflow measurements, though without lamps.

"Prototype Casting can deliver magnesium prototypes through the RPM process in two to three weeks. They were able to gravity-pour liquid magnesium into plaster molds produced from urethane masters. The urethane masters, in turn, were made from silicone 'tools' created with SLA patterns generated from our CAD software.

"We add wall-section thickness in some cases to ensure that the part is manufacturable," Sample continues. "We know the Prototype Casting processes, so we try to accommodate magnesium flow into the casting where we can. We work closely with them, and the speed with which they create prototypes shows it really works."

The prototype castings thus created are not production parts, but they work: the entire model can be analyzed from myriad perspectives, the lamp can be lit, and images can actually be projected on a wall—all within three to four weeks, sometimes less.

No "Typical" Parts

Prototype Casting turns around complex requirements quickly for clients such as Black & Decker, John Deere, H-P/Compaq, Milwaukee Tools, Bose, Raytheon, and Parker-Hannifin. There is no "typical" part, Brett Peak tells JST, but the firm often makes thin-walled housings for electrical and other applications, boxes for electrical components, bevels for monitors, and a lot of very light, thin-walled parts using magnesium. Products range from small items, such as the projector castings for InFocus, to larger items like automotive steering columns and gear boxes.

Why magnesium for the LP 130? "The lamp in the LP 130 is incredibly hot," Peak says, "which creates special challenges to any parts that would be used close to it. InFocus needed a durable, strong, rigid material that could take the heat and that could also be readily mass-produced. Their parts also had to be very light, because of InFocus's concentration on the needs of road warriors. Its customers don't want to travel with anything heavy. Strength was equally important."

Automotive steering columns aren't exactly thin, lightweight components, and since Prototype Casting makes many of them, you can't really characterize Prototype as a specialist in casting lightweight magnesium parts. But Prototype is well known for doing precisely that.

"We're one of few companies that does rapid prototyping and casting of magnesium," Peak explains, which is one reason InFocus contacted them for the LP 130. Moreover, InFocus had been working with Prototype Casting since the mid-1990s.

Magnesium parts don't necessarily move from drawing board to point of assembly in a straight line. "In this case," Peak says, "InFocus showed us their design and said, 'This is what we're looking to do in prototype, and this is what it will look like in production.' We discussed the process and we discussed capabilities, and, ultimately, we suggested a few tweaks to the part process that would enable us to quickly produce a part that they could use to create projectors for testing."

Prototype Casting's prototype development process begins just after a customer or prospect e-mails the firm a 3D "STL" stereolithography file. After company review and some customer phone contacts, the STL is used to create an SLA, which is then used as the pattern for the casting process. Prototype Casting creates silicon negatives of the pattern, and from the negatives, it makes urethane positives. It then pours plaster into the urethane positive to make plaster mold halves.

A Difficult Material to Handle

Handling magnesium requires some exceptional competencies. "Magnesium is dangerous to work with," Peak understates, "so a lot of companies won't handle it. It only tolerates a narrow temperature spectrum. At low-end temperatures, it can freeze. At high-end temperatures, magnesium can burn. You have to get it hot enough so it's molten and can 'fill in' the part without freezing off, but if you get it too hot you could wind up with a fire and an explosion. Most magnesium parts have very thin walls, and the metal just wants to freeze while the molten material is going into the mold."

Prototype Casting has developed proprietary procedures to prevent freezing and explosions. On top of its ability to work with magnesium safely and productively, the company is able to turn around prototypes quickly, a crucial advantage for customers who are seeking fast, competitive time-to-market. From the time Prototype receives a 3D STL file, not much more than seven to 21 days usually pass until the company ships a prototype magnesium part. Usually, prototypes are delivered in under two weeks.

Deep Inside the LP 130

InFocus's Heintz says that three external LP 130 parts—the top, bottom, and rear bezel—are thixomolded magnesium. "Thixomolding is very similar to injection molding," he explains. "It uses pelletized magnesium in machines which are very similar to those used for plastic injection molding. The magnesium chips are heated to a semi-solid state and then injected into the mold, where they go through a thixotropic transformation due to injection pressure and friction. This mechanism causes the semi-solid chips to become liquid and allows for molding temperatures 100 degrees Celsius less than conventional injection molding. In general, thixomolded magnesium has greater density, better surface finish, lower tolerances, and thinner wall capabilities than can be realized through typical die-casting techniques."

Typical wall sections measure 0.050 inch, or 1.2 mm thick. "We wanted to conduct heat away from the internal subassemblies and dissipate that heat through convection off of the case," Heintz continues. "By doing that, we could minimize the airflow that would otherwise be required. In a plastic enclosure, the heat would not have dissipated as efficiently as it does in thixomolded magnesium." A 120 watt high-pressure mercury lamp is inside the projector.

Two of the parts Prototype Casting created for the LP 130 were for its cover plates, or skins. The other structure was the thixomolded interface casting, which both couples and aligns the lamp assembly to what InFocus calls its "optical engine." The interface casting also internally mounts two fans and some circuit boards, making it a critical component in terms of dimensional accuracy, strength, durability, and heat resistance—not to mention the quality of the projected images. Wall thicknesses vary from 0.030 inch to 0.050 inch. The interface casting accurately "guides" the lamp module and indexes it to the rest of the optical path. "It's a non-descript looking, L-shaped part with a tube hanging off of it, but it's important," Heintz says.

According to Greg Sample, thixomolding wasn't the only casting alternative that InFocus considered, although its designs were optimized for the thixomolding process. "The goal was near net shape, that is, to get as many features molded to the correct size as possible just through the molding process," he explains. "The plan was to keep manufacturing costs low by reducing the number of features that had to be secondarily machined."

Hot-chamber die casting is a more accurate process than cold-chamber, Sample says, but thixomolding has "a leg up" over hot chamber. "The parts we get through thixomolding are clearly very accurate," Sample says. "Surface finish is better, density is better, and there is virtually no issue with porosity when you thixomold magnesium."

Heat resistance was one reason why magnesium was selected for the parts, but other metals could have handled heat virtually as well. Magnesium was chosen also because of its light weight and its strength at that weight.

Some of the features of the LP 130 could not be created with two mold halves. Prototype Casting had to eliminate cores in the LP 130 mold to make the parts more dimensionally stable. Some areas of the mold had to be filled in during the prototyping process, then post-machined.

"We had some challenges filling the part because the walls are so thin," Peak recounts. "We worked with InFocus and elected to add runners to help the metal flow throughout the entire part, then we post-machined to remove those features."

Slick Surface Sheen is Significant

The surface appearance of the LP 130 was another consideration in the selection of magnesium as a molding medium. Sample, Peak, and Heintz all agree that magnesium has a high-quality feel to it, and imparts a feeling that the device is a substantial piece of hardware. "Plastic just wouldn't have the same effect," Heintz says.

Peak says that industry advances in pattern making, SLA patterns, SLS patterns, and other rapid prototyping models have enabled Prototype to respond quickly to customer demands. "Our process includes advances in silicon and urethane and a pretty slick production process which doesn't waste time," says Peak.

"We do not gear our shop to run at 100% capacity all the time," Peak adds. "We gear it so we can turn around projects very quickly. We have full machining capability here, we have three four-axis machining centers, we have our own foundry, we have our own toolmaking—everything is in house."

According to Sample, the firm has had numerous dimensional inspection reports performed on as-cast parts to understand the accuracy of various features and how those correspond to the different tool features. "Effectively, we have mapped out by feature type and tool type exactly what the accuracy level is on these parts," he says. "We were demanding tolerances on this project over and above what we had required before. In the end, we all understand better just how high our process capabilities have risen to achieve these results."

Because of its size and performance, the LP 130 is not InFocus's lowest-cost portable projector. Nonetheless, in a somewhat laggard market for technology peripherals, Heintz says the product continues to sell very well.

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