When to go Direct Digital Manufacturing

Sept. 25, 2008
DDM has its own set of rules.

Who, What, Where
Authored by Joe Hiemenz Stratasys Inc. Minneapolis, Minn.

Edited by Leslie Gordon [email protected]

Resources Stratasys Inc., stratasys.com
, redeyerpm.com
FDM primer — tinyurl.com/6btt4z
Cobalt-chromium alloys — tinyurl.com/6d25u9
Rapid mold making — tinyurl.com/6c8oa5
Tips on rapid prototyping — tinyurl.com/6lhhoc

You are probably already familiar with direct digital manufacturing (DDM). This is the term for the technology that uses additive fabrication machines to make prototypes and functional parts directly from digital CAD data. DDM might be a good bet when one or more of the following factors come into play.

First, the part should have a relatively low production volume. In conventional manufacturing, production volume means quantity. In contrast, with DDM, production volume is literally a volume, expressed in the total cubic in. produced per year. To illustrate, DDM might be practical for 100 parts the size of a basketball, 800 parts the size of a baseball, and 2,400 parts the size of a golf ball.

Calculate production volume by multiplying the annual quantity by the part’s physical volume. For example, 100 parts with a volume of 2 cubic in. would have a production volume of 200 in3. When making calculations, do so for each product revision. A changing design might boost or lower annual production.

Unfortunately, there is no universal sweet spot for cubic in./yr. So each company must find its own breakeven point. Note, though, that additive systems continue to improve in speed. This means more and more parts are potential candidates for DDM.

Relatively high design complexity. Although DDM can produce such simple objects as, for instance, cylindrical parts like handles and knobs, there are much greater costs and time advantages when parts have complex shapes. As an example, consider a two-part housing, the back side of which has hundreds of features. While in the past, some rapid methods could not handle features such as overhangs, this is no longer the case. DDM can handle any geometry, no matter how complex.

In fact, parts impractical or impossible to make with traditional methods are feasible with DDM because conventional design-for-manufacturability rules no longer apply. In effect, DDM provides geometry for “free” — no time or cost penalties apply when manufacturing complex components. DDM also eliminates long lead times, because there are no tools to make.

Probability of change. With traditional manufacturing, design changes are expensive and time consuming. So one goal becomes minimizing changes to maximize productivity and profit. In contrast DDM gives users the freedom to redesign at will.

That’s because the manufacturing of a revised design is simply a matter of modifying the CAD data, exporting a new STL file, and running the DDM machine. There is no additional cost for rework or retooling and no interruption in production schedules. Many manufacturers are also using DDM as a “bridge” to high-volume, traditional production. Companies can produce in low volumes until conventional tooling such as a high-production mold is built. Companies in the medical and dental fields were early adopters of DDM because it lends itself well to custom products.

High start-up investment. It takes a substantial investment of labor, time, and money to create traditional fixtures, molds, and machinery. For example, a single injection mold can cost $75,000 or more and take eight to 16 weeks to make. In contrast, products made with DDM incur no tooling costs and there is no waiting for the first production parts. This minimizes start-up investment for a new product. In turn, the company has a better cash flow and profitability. What’s more, a lower initial investment on one product frees a company to potentially introduce others.

Fewer materials to choose from. Compared with traditional manufacturing, DDM has few materials from which to select and even fewer varieties of any one material. Thus, mechanical, electrical, and thermal properties might not exactly match those of a specified resin or alloy. A successful DDM project must consider the material properties required and possibly revise the design. For example, this might necessitate adding gussets to stiffen a thin wall so the part mechanically performs as required.


Finished parts are easy to separate from a build sheet.


As participants in a marine competition, students at Flower Mound High School in Texas used RedEye, a business unity of Stratasys, to build the frame for their entry, an unmanned underwater vehicle. The frame is ABS Green.


A technician unloads a tray of parts that were built at the same time in the FDM machine.

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