Design-CAD File Part Details and Tooling Part 5

The last time I wrote an article for Design Corner was in November of 2007. That was quite a while ago. For those of you who may have forgotten the theme for the series of design articles, I will try to refresh your memory and continue. The series was based on factors to consider when designing rotationally molded parts in 3D CAD. Previous articles discussed effects of tooling considerations, secondary operations and tolerances on part geometry. This article will continue the series by reviewing CAD techniques and considerations based on the effects of inserts, graphics and other add-ons to a rotationally molded part.

3D CAD files can be considered a virtual model of the production rotationally molded part. This means that the CAD file is based on a particular tooling technology including all the design considerations required for molding such as draft angles, split lines, core pins and threads. Designers typically develop 3D CAD files as finished parts within an assembly which are typically different from files used to cut patterns or molds. The main differences between a production part and a pattern are all the secondary operations and hardware that are added to the production piece.

All cast aluminum rotational molds include a pattern of the part. The pattern is typically derived from the 3D CAD file of the final part. Before a pattern is CNC cut, many modifications must be made by the tool maker to the 3D CAD part geometry file. The first modification is adding shrinkage, which means enlarging the part geometry by a certain percentage according to the aluminum casting process and polymer material to be molded. The 3D CAD file is then further modified by adding surfaces to open areas thus forming one contiguous enclosed form. In addition, any features which have been achieved by secondary machining operations must be modified to include material which will ultimately be removed after the part is molded. Other modifications to the CAD file could include appending additional parts to the base 3D geometry so more than one part is molded from the same tool. This technique of consolidating multiple parts into a single tool is commonly employed when large open ended single walled parts are molded. Individual parts are then separated from the large cluster by cutting and finished routing. 3D part files are further modified by removal of inserts or other molded in hardware that was included within the finished molded piece. The mold maker must understand what inserts will be in the production part and include provisions in the pattern to accept them when the tool is cast.

When rotationally molded parts are removed from a mold, they will be identical to the pattern except for a slightly smaller size, inserts that may have been added and any graphics that may have been included to the part before molding. Typically these parts must undergo a series of secondary operations before a final production part is ready to be shipped. This last series of operations will transform the molded part into its final form.

Designers should understand that it is often desirable to provide tool makers and molders with multiple files of the same part which features at various stages of the production process. This will assure production quality and better control of the end product. Understanding these production steps and including them in multiple CAD files of the same part is sometimes the only way a rotationally molded part can be designed properly.

Although there are many other features and considerations to be accounted for in CAD modeling, understanding these basic concepts will aid you in effectively communicating your design intent to those who will actually manufacture the product. In our next issue of Rototalk, I will discuss draft, undercuts parting lines and other features that should be included in 3D CAD models for rotational molding. Until then, keep in touch and email me Michael Paloian (Integrated Design Systems, Inc.) with any of your comments or questions at paloian@idsys.com

Michael Paloian, Integrated Design Systems, Inc.