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October 12, 1998

4 Min Read
Designing molds via PC, economically

ArticleImage1563.jpg

ArticleImage2563.jpgMold set for a food bowl (top) was rendered in transparent mode using AutoCAD. Using CSG Editor, the A-side of the mold for a smaller bowl was produced (right) by replacing the bowl profile and resizing and removing other features.

Although we did not undertake a definitive survey, it seems that nearly every molding and moldmaking facility in the U.S. contains at least one seat of AutoCAD. Robert Rush, president of Rush Mold Design (Kokomo, IN), tells IMM that his company puts its software to good use by combining it with a solid-modeling add-on. "Imagine designing molds for up to 80 percent less than generic AutoCAD or more expensive workstation-based CAD/CAM systems," Rush says. "That's what we're able to do."

Rush apprenticed as a mold designer at GE in 1965, and watched the growth of 3-D solids and wireframe during his career in several tool shops. When he began his own design business, he noticed more customers were producing part designs in solids, and he decided to make the switch. "High-end workstations are perhaps the simplest way to design molds in three dimensions," he contends, "but at $60,000 to $100,000, they generally go beyond the budget of all but the largest mold shops. Training periods are relatively long as well, and it's difficult for smaller shops to spare employees for extended periods." So Rush made the decision to go with PC mold design.

After trying AutoCAD's newest version, release 13, he found he was coming up short. "I still could not edit the primitive shapes that make up a solid model. Autodesk's Mechanical Desktop with parametric modeling can convert solids shapes, but all of the primitives are lost in the conversion."

Rush tried an add-on product compatible with AutoCAD called CSG Editor from Context CAD (Sausalito, CA). It lets users edit solids in the same way they were created. For example, Rush can import 3-D wireframe geometry from his customer into AutoCAD, build a solid model of the part, then produce the mold geometry by subtracting the part from the mold block. "Results are virtually identical to models produced by high-end CAD/CAM systems and can be accomplished in far less time," he adds.

'High-end workstations are perhaps the simplest way to design in three dimensions, but they generally go beyond the budget of all but the largest mold shops.'

Here's a typical scenario using the editware: Designers start by importing wireframe customer geometry into AutoCAD, then use it as a template to build the solid model of the part. (Rush uses release 13 tools to create the initial model so that it can be used to check shape and dimensions.) But rather than continuing with AutoCAD, users then switch to CSG Editor to modify geometry. Increasing block thickness, for example, would normally require creation of a new solid to take up the additional volume. Instead, users select the primitive shape and change its dimensions. Commands for the add-on match AutoCAD commands in style and syntax, so users learn its operation quickly. Changes to primitives can include length, width, height, radius, deletion, move, copy, path, taper angle, fillets, chamfers, and cutting planes. And the program operates through a single command or by dragging.

Those who design multicavity or complex molds will appreciate another advantage of using this program, according to Rush. "When an array is created, changing any member of the array automatically updates all of the other members," he says. A recent project contained arrayed parts with seven columns and five rows of features, each designed to hold other parts of the assembly. While the mold was being designed, the product feature changed several times. "Without CSG Editor, I would have had to edit all 35 instances of the feature each time a change occurred," he notes. "Instead, I can use a single edit."

When asked to tally his time savings with the new software, Rush estimates he saves 35 percent of the time he'd normally spend designing molds with either AutoCAD alone or a high-end workstation system. And at $295, the CSG Editor is priced within the reach of many moldmaking concerns.

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