You could find a long line of mold builders who have encountered it ... computer-aided part design geometry that's not mold-ready when it shows up at the mold shop. The solid model, often described as the Bible, arrives with no additional detailed dimensions. The mold designer is left without the critical information to begin the design.
More often than not, draft (the angles to allow the ejection of the part) is not designed in the original CAD database. When it is, it may be incorrect due to parting-line jumps or mold actions that are pulling from separate planes. Part tolerances are another detail not conveyed with the solid model, and tolerances can affect how the tool will be designed.
This puts the mold-building company in a precarious position. Should the mold designer play God and revise the Bible? Better that than build a mold with unmoldable conditions. So the mold designer asks his customer about possible revisions, and then edits the solid model.
All the while, the ``I need it in six weeks'' clock is ticking and the mold builder is making no-charge edits. And when it's finished, who is responsible if the molded part that quality control is inspecting differs from the original flow-analyzed solid model?
Sure, it would be nice to have a mold designer involved in the part design. But because of the scarcity of mold designers, and the rush to forward information to the mold builder that is the exception rather than the rule.
Pressure is building for CAD software vendors to cooperate on a common-language protocol. Any progress in this area will assist mold designers, but it still leaves them with the issues of moldable geometry and tolerancing practices. These two issues combine to be a big problem in the plastics tooling industry, and will only get worse as the use of solid modeling increases and delivery schedules shrink.
The only solution is with standards. And it should occur with the cooperation of all appropriate mold-building trade organizations.
First, a group such as the American Mold Builders Association is positioned perfectly to develop a standard policy for what the mold builder should expect from his customer when a solid model is used. And if it is not provided as a moldable part, the policy should state how the solid model will be revised, along with who is responsible for the editing, and how the impact on delivery will be handled.
Second, an organization such as the Society of the Plastics Industry Inc. can assist in developing guidelines for communicating tolerances of the solid model.
Also, the more than 3,000 members of the Society of Plastics Engineers' Mold Making & Mold Design Division can be surveyed for their input and assistance.
We've seen industry cooperate for mold-surface-finish guidelines, terms of payment for mold builders, mold classification and other issues. In process right now is an SPI committee to develop guidelines for mold safety considerations to be included in the mold design. A standard for the handling of solid models is another issue that needs to be addressed.
If your firm has developed written policies for editing geometry for moldability, or has developed guidelines for communicating tolerances, please consider sharing that information to assist in developing an industry standard.
Glenn Starkey, president of Progressive Components, is past chairman of the SPE Mold Making & Mold Design Division, and currently is on the SPI Mold Safety Committee. To provide input on part geometry standards, contact Glenn at tel. (800) 269-6653 ext. 235, fax (800) 462-6653, or e-mail [email protected] comps.com.