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November 28, 1999

4 Min Read
Solid model interoperability revisited

In addition to supplying a vast range of plastics, DuPont Engineering Polymers offers a design consultancy service to its larger customers, assessing manufacturing feasibility and end-product characteristics. Importing 3-D solid models from many different CAD/CAM systems used to be a major issue for the engineers who provide this service, requiring them to rebuild models that didn’t translate accurately into their CAD system. They were hit with the same data interoperability problems that molders and moldmakers face when receiving files from a diverse customer base.

Not only did incompatibility waste time, it also tended to reduce accuracy as files were rebuilt. The solution? DuPont finds that a class of software intended to repair IGES translations of 3-D solids has actually eliminated the need to rebuild solid models.

Analysis Tools
“Companies like Ford are pushing design responsibility further and further down the supply chain,” explains Paul Abraham, engineering manager at DuPont Ltd. (U.K.), “often providing just a broad specification. It is then up to the suppliers to come up with the detailed design, which places more burden on us as we are called upon to make sure the chosen designs are going to work.” Many of DuPont’s customers are Tier One and Tier Two suppliers to major automotive OEMs, an environment where defects in finished components can mean losing a lucrative contract, so designers can’t afford errors.

Paul and his team have an established set of tools for performing engineering analysis of designs. Principally, these are Ansys for general structural finite-element analysis, and Moldflow for moldfilling simulation. For these tools to be effective, though, fully defined 3-D representations of the proposed designs are a prerequisite. This used to cause a bottleneck.

“Three years ago our main communication medium was still the engineering drawing,” explains Abraham. “We would manually translate drawings into 3-D models in our own CAD system, or sometimes directly in Ansys, and move on to meshing and solving from there.”

Since that time, there has been an explosion in desktop solid modeling and an associated increase in the inherent complexity of computer-aided designs. Abraham explains that designers are now unwilling, or unable, to translate their efforts into engineering drawings. As a result, DuPont’s primary way to receive design intent has become the 3-D solid file.

The 3-D Predicament
Placing such emphasis on 3-D CAD models quickly uncovered some severe limitations between the communicating file formats. Even with a relatively modest number of modeling kernels, there is still a bewildering range of proprietary formats in use across the solid modeling market, each of which has its own peculiarities. Even standard file formats such as IGES cause problems, because they are handled so differently during import and export by each CAD, CAM, and CAE tool.

According to Abraham, plugging away with IGES at DuPont required significant rework of 3-D data to isolate and correct errors and inconsistencies following the IGES import. Ansys does have its own IGES translator, but because of the wide range of possible sources of native data, each with its own standards for tolerances, it was extremely rare for a solid model to come in complete and correct the first time, and not uncommon for it to fail altogether. All too often it would be back to the solid modeler, remodeling components from scratch. And without the reassurance of engineering drawings to fall back on, this hardly represented progress.

The Fix is In
Abraham began using CADfix (from ITI), a software package with tools for assessing geometric and topological imperfections in solid models and making corrections to cure them. Much of the time this can be performed automatically, but most users tend to step through the corrections interactively.

An initial analysis graphically highlights surfaces that are incorrectly connected and need further attention, and merges any duplicated points, edges, surfaces, and faces. Users can step through the detected imperfections and choose a method for curing them, typically closing a loop or fusing edges that lie within a specified tolerance. The end result is a fully formed, closed solid model that can be taken to downstream applications—in DuPont’s case, Ansys FEA and Moldflow packages.

“We are working with a truer representation of the proposed design and we can therefore perform more accurate analyses,” he says. Time savings, though, is another benefit. Abraham estimates that, on average, the previous method involved as much as three times more work for each component. “This greatly increases our analysis capacity and therefore boosts the service we can offer our customers. Analysis is completed more quickly, we can perform more design iterations, and the end products are better engineered as a result.”

Contact information
ITI
Milford, OH
Carl Izurieta
Phone: (800) 783-9199
Fax: (513) 576-3994
E-mail: [email protected]
Web: www.cadfix.com

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