Don’t Print the Drawing — Print the Part-Drawing — Print the Part

Oct. 9, 1997
The need for quick output of concept designs has spawned a new type of rapid-prototype machine that works handin- hand with CAD workstations in office settings.

Say good-bye to the days of trying to visualize a solid 3D part from 2D drawings. There’s no need to make critical design decisions based on vague images because desktop RP machines can produce physical models right in design offices — and they do it fast.

Desktop RP machines, called 3D printers, are no bigger than office copiers and run just as clean and quiet. Design teams can use RP models to exchange ideas and make changes early when they are cheapest. When changes are made to the CAD model, 3D printers spit out a new part, letting clients and designers get their hands on it and get a better idea of what the final product will look like. Holding a part also allows improving ergonomics to levels unattainable with 2D drawings.

What’s more, 3D prints only cost a few dollars each. So when a customer or design team can’t pick a best choice for a particular design, they can print a variety of choices and make a better decision after examining and discussing each one.

The need for speed
Three-dimensional prints are simple prototypes used in early product development for communication and basic design testing. The machines that build them work with CAD models created by design engineers. Before computer-aided design, prototypes were built using NC tooling or by a craftsman, a process that took weeks or months and cost thousands of dollars. Rapid-prototyping machines, in contrast, build precise parts in a few days at most by bonding layers of material to each other.

Although high-detail parts from conventional RP machines can be used throughout the design cycle, the parts are often impractical in early product development stages for several reasons:
• The machines cost between $100,000 and $400,000, a difficult sum to justify in concept modeling.
• Each part can cost well over $1,000 which limits the number of design iterations.
• High precision parts usually are not necessary during early development.
• Since precision RP is intended as a final step before production, machines are installed in the manufacturing shop, making them inconvenient for the design engineer.

While precision modelers have these limitations, they still quicken the pace in machine shops. To keep up with the pace, design engineers need their own machines nearby to produce models early in the design process. They need a part they can get their hands on and present to customers for analysis and design verification. Three-dimensional printers meet this need by building simple parts for concept modeling. The machines cost from $40,000 to $65,000, and produce parts for as little as $10.

How to use 3D prints
Costs of design changes increase substantially with each step of product development. In addition, changing a functional prototype has the disadvantages of high cost and increased delays, from weeks to months, in getting the product to market.

Low cost parts and fast generation, on the other hand, make 3D printers the best solution for early concept modeling. Changes early in the design process mean less backtracking, which saves time and money. After changing a simple CAD model, its 3D print can be made the same day.

While 2D drawings are also easily revised, 3D prints are a welcome alternative to complicated mechanical drawings. 3D prints are not only easier to understand but also have a wider range of applications. Uses include: visualization and discussion, form and fit testing, and occasionally models for casting and hard-tooling production.

Visualization and discussion are vital to product development and arguably most important during early concept modeling. For example, an eye-catching part can be the strongest selling point for prospective clients who enjoy handling it and picturing its use to determine whether it will meet their needs.

Aside from touchy-feely functions, 3D prints have other uses. For instance, more rigorous form testing occurs during early concept modeling and involves examination of a part to determine if it looks and feels right, and matches what was originally visualized.

When several parts are to be assembled, fit tests determine whether each part is geometrically correct. Some 3D prints stand up to fit tests better than others. But precision RP models are usually the best choice to test fit and are the only choice for function testing.

While accuracy is critical for a fully functional prototype in final testing, 3D prints need not be highly accurate because of their short life span. “The effective life of any concept model is 5 minutes,” according to Scott Hill, product manager for Integrated Systems Technologies Inc., Willoughby Hills, Ohio, a reseller for 3D Systems, Valencia, Calif. “When a model is shown at a presentation it is either accepted and immediately replaced by a functional prototype, or rejected and immediately replaced by a revised concept model.”

Three-dimensional prints have additional uses. For example, how many times is there only one option for the final part? More often there are several design possibilities and somebody has the tough task of choosing the best one before the project begins.

Three-dimensional printers can shorten the decision cycles. Engineers can start with a preliminary CAD model and create a variety of designs from the original. Three-dimensional printers can build each design, letting customers take a look at a model of every design choice, talking over the strengths and weaknesses of each design and making a more educated choice. “One manufacturer came to us with a half dozen different designs for sunglasses and wasn’t sure which would be best,” says Kou-Rey Chu, director of manufacturing technology for Phoenix Analysis & Design Technologies, Gilbert, Ariz. “Each design was only a 2 or 3-hr print. By the next day we had all six pairs printed.”

Some 3D prints can also be used for casting patterns. While most 3D printer manufacturers recommend precision models, Sanders Prototype Inc., Wilton, N.H., says their parts are sufficiently precise for casting patterns. The precision of Model Maker II parts comes from combining the additive build process of 3D print jets with a subtractive milling operation.

How the machines work
CAD solid-model data are output in an .STL file, the most common input format for 3D printers. An .STL file approximates a CAD-designed object using an array of adjoining triangular facets. Software reads the .STL file and divides it into tiny “slices,” each representing a cross section of the part to be built. Slice thickness varies from 0.0005 to 0.013 in. depending on the machine.

The material deposition process is similar to that of an inkjet printer. An individual jet or bank of jets moving in the X-Y plane deposits material on a Z-axis platform. Highly accurate jets rapidly apply material one drop at a time, each bonding to previously laid droplets, eventually forming one slice of the model. When a layer completes, the platform lowers and the jet deposits the next layer. Slight differences in layer dimensions, however, create stairstepping, an undesirable result of attempting to duplicate freeform shapes with discrete layers. Although thinner layers reduce stairstepping, thick-layered printed parts are still useful for concept modeling. The five-axis positioning system of the Personal Modeler, from BPM Technology Inc., Greenville, S.C., adds two rotational degrees of freedom to the three translational ones found on other 3D printers, which is said to minimize stairstepping.

Unconnected islands and thin overhanging material are common during the build process and can move or sag before they cure. This requires temporary support structures to hold the part together until completion. Support structures also elevate the part from the build platform to allow easy part removal and prevent the part itself from bonding to the platform.

Postprocessing may be necessary on some RP prints either for design integrity or aesthetics. For example, support structures leave small nubs that may require sanding. Some parts need painting to make them more appealing for presentations. Other parts are cleaned and treated to make them stronger. For instance, Z Corp., Somerville, Mass., builds models from a paper powder and water-based binder. Completed models are initially fragile but can be strengthened by dipping them into molten wax, or a twopart epoxy for maximum durability. “Postprocessing adds very little time to the build,” says Marina Hatsopoulos, president of Z Corp. “Parts need only be dipped for about 10 min and the bath can be filled with multiple parts and left unattended.”

Build envelopes, of about an 8½-in. cube limit 3D print sizes. Occasionally larger parts can be built in segments and assembled after completion, but a better solution may be to scale the part to fit the envelope. Similarly, the build envelope can be filled by magnifying the size of small parts. “When you have a small part and want to see its detail, just scale it up 300 or 400%,” says Sharon Christopherson, marketing communications specialist for Stratasys Inc., Eden Prairie, Minn. Another way to fill the envelope and maximize production time is to “pack” the machine and build several small parts at the same time.

Build materials for each 3D printer exhibit a range of qualities that help select a particular printer. While all parts are durable enough to handle for presentations and form testing, the more rugged ones allow worry-free shipping, fit, and even limited function testing. Wax parts require a little more care when handled, but can be used for castings. On the other hand, some materials withstand light machining. “Motorola uses an Actua 3D printer with Thermojet 45 build material at their industrial design center,” says Mervin Rudgley, product manager at 3D Systems. “It lets them drill holes in 3D prints and add lead to simulate the weight of production parts.”

© 2010 Penton Media, Inc.

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