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Rapid 3-D Printing Process Offers Flexibility of Over-molding


ANAHEIM, Calif.--The ability to quickly create--without tooling--complex parts and models that incorporate more than one material is a rare advantage in the world of manufacturing. But GROWit (now Forecast 3D), an Anaheim, Calif., startup specializing in leading-edge rapid prototyping and 3-D printing services, is offering this capability through a process that combines the flexibility of over-molding with the speed of rapid prototyping. GROWit became the first service bureau on the West Coast to purchase a Connex 500TM 3-D Printing System from Objet Geometries, Ltd., earlier this year, empowering its customers to save time and costs on projects that traditionally required double-injection or over-molding, or the gluing of separate parts made of different materials. The company is said to be one of only three service bureausvin the United States that is currently using the new machine.

Based on Objet's PolyJet MatrixTM technology, the Connex 500 is the first system capable of printing--in a single build cycle--parts and assemblies made of multiple model materials with different mechanical or physical properties. It can create Digital MaterialsTM on the fly by mixing and matching, during the build process, composite materials that have pre-set combinations of mechanical properties.

"This opens up the options for over-molded products," David Gurrola, co-founder and president of GROWit, told Design-2-Part Magazine in a recent phone interview. "In most standard RP processes, you would first create two masters and then an RTV mold (a room temperature vulcanized, rubberized mold used in place of injection mold tooling, adequate for 100 to 200 parts). You would have to do a few steps to either mold the rubber onto the hard plastic, or glue it onto the hard plastic. If you use the glue, it is susceptible to coming off. With the Connex machine, it actually fuses the two materials together tightly."

The PolyJet materials come in five basic colors: white, black, gray, blue, and translucent amber. By mixing these materials, the machine offers a variety of different shades that can be used to distinguish different components on the same part. GrowIt also uses a 3-D printing machine produced by ZCorp, the Spectrum 510TM, which makes parts in a single plaster-based material, but produces 24-bit full color parts.

The PolyJet process uses a photo-sensitive polymer, just as a stereolithography (SLA) machine would use, but it cures the polymer using UV lights, rather than a UV laser. The PolyJet machine uses printing heads--similar to an inkjet printer--that spray droplets of material down onto a platform that will hold the final part. The printing matrix is 600 x 600 dpi. When processing is initiated, droplets are smoothed down on the surface with a roller that planes each layer of material, which guarantees a smooth, flat surface.

"One of the benefits from Objet machines is they can grow the part in 16-micron layers, which is about one half of a thousandth of an inch," says Gurrola. "Most other RP technologies, like SLA, FDM, or SLS, would have you build in about five thousandths of an inch. So it's about ten times more accurate than the industry standard in rapid prototyping, meaning PolyJet processes can achieve tighter tolerances."

An SLA machine employs a resin trough--a big tub of resin--and it has a platform that lowers the part into the trough of material. A laser then cures the resin after each layer is added. On a PolyJet machine, the UV lights do the curing.

"We think this Connex 500TM is the latest and greatest RP process," Gurrola insists. "It has revolutionized the way rapid prototypes and other products are produced--say, for example, a pistol grip for a power tool that may have over-molding on it, or the door handles on a car door. We can do these products much faster and in a more cost-effective way now. This revolutionizes over-molding for prototyping and low-volume production parts."

If the user is just trying to simulate the look and feel of a product, the Connex 500™ machine can simulate different shore values indicating the stiffness of materials. "Typically, we can hit anywhere between 27 and 100 Shore scale A, primarily for softer materials," Gurrola affirms.

One of the materials that GROWit uses is DurusWhiteTM FullCure®430, recently released by Objet Geometries for use with the Connex500. DurusWhite simulates the toughness, strength, and flexibility of polypropylene; it can be used in hinged mechanisms because its mechanical properties simulate polypropylene's resistance to fatigue and enable snap-fit functionality over time. By permitting more flexible parts, such as living hinges, the material encourages a new way of thinking for assemblies. It can be folded over onto itself, replacing metal pin joint hinges with polymer material. Gurrola says that the new material won't break after being folded two or three times.

Gurrola sees a strong interaction between rapid prototyping and rapid manufacturing. "In the case of investment casting, for example, rather than creating a steel or aluminum mold for the master wax pattern, we can produce the wax pattern on our RP machines," he says. "You no longer have to create a mold, pour wax in it, break the mold out, and then do the casting. The RP part becomes the mold. We've done quite a few investment casting molds using aluminum, or Chromoly 4130. We can do some fairly large, intricate parts using a direct wax material that's in one of the machines."

GROWit can also integrate PolyJet printing with the fused deposition modeling (FDM) process, which uses ABS plastic, a very robust polymer. After producing rubberized parts from the PolyJet machines, the company uses epoxy to fasten them to parts made on an FDM machine for a much more durable application. Many of the UV cured resins, according to Gurrola, aren't preferred for volume production because they are unstable in some environments. Some customers use the ABS parts in actual production applications because of their strength.

GrowIt also performs a great deal of work for inventors who have a design that they want to produce. For these inventors, who usually don't have $20,000 to spend on mold tooling, rapid prototyping eliminates the high cost of metal tooling. But GrowIt can help them design the part, and then produce a prototype that can be used for market research. They can then go into limited production using RTV molds. The per-part price will be more than injection molding, Gurrola adds, but the total cost is still much less. It might cost $200 to $300 for an RTV mold, rather than thousands.

"Companies that are willing to step out on a limb and try new things have found that the PolyJet printing process completely revolutionizes the way prototype parts are produced," says Gurrola.

For more information, visit Forecast 3D

PolyJet Matrix, DurusWhite, and Connex500 are trademarks, and FullCure is a registered trademark, of Objet Geometries Ltd.

FDM is a registered trademark of Stratasys, Inc.

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