Design-CAD File Part Details and Tooling Part 3

This issue of Design Corner will continue with Part 3 of CAD modeling techniques and details to be considered for rotationally molded parts. In my prior column I discussed the importance of deciding on the type of mold construction at the beginning of the project and its affect on partdesign. Now I will try to outline some of the techniques and design considerations that should be included during the CAD development of a rotationally molded part. Although rotationally molded parts are molded hollow, it is unnecessary in most cases to shell (core out the geometry) a 3D CAD file. Almost all of the parts we design are modeled as a solid 3D part without any internal shelling. This saves us lots of time due to the difficulties that are sometimes encountered shelling a complex 3D part. Since rotationally molded part molds are based on the exterior surface of the part geometry, a shelled model is of no use to the pattern maker. A solid 3D CAD model is all that is required to construct a rotational mold. However, one has to be careful to not to ignore the basic design guidelines specific to the rotational molding process. For example the 5:1 rule for minimum separation of two parallel walls of a desired wall thickness must not be overlooked. If the internal walls of a rotationally molded part are critical in an overall assembly, the part may have to be shelled. Other factors that must be considered are inserts and kiss-offs which are sometimes difficult to visualize if a part is not shelled. As a general rule, you are not required to shell a rotationally molded part unless it is required by you as the designer to better understand an overall assembly.

Another design consideration in CAD modeling is the final part design versus the "as molded part design". Unlike injection molding, rotationally molded parts are typically subjected to secondary machining or cutting operations. In some cases a single molded part is cut into multiple individual parts which can be assembled to various unrelated subassemblies. This introduces complications into the CAD modeling technique since the pattern geometry will cut from a fusion of all the parts that are derived from it. The arrangement of these parts representing the pattern should be discussed with the molder and tool maker before they are assembled. CAD techniques for creating such a pattern can be accomplished by either assembling individual parts into the desired pattern or creating a master model and separating it into the individual parts. Since the designs are usually derived throughout the development process, it would probably be more likely to assemble the final parts to each other. If two parts are related to each other such as a lid and a tank, it is relatively easy to construct the part as it would be molded in one piece and then separate them in the CAD modeling into the individual pieces.

A basic understanding of the molding process and secondary operations is essential in designing a rotationally molded part since the CAD model of the part may be required to portray the part at various stages of production. For example, it is not unusual to provide a CAD model of the part at one level to the mold maker. Another CAD model of the same part may be provided to the molder to illustrate the location of molded in inserts. Athird CAD model of the same part may illustrate trimming operations, where a shelled geometry is required. A fourth CAD model may be required to illustrate assembly of other plastic or metal parts to the base piece.

Hopefully this third installment of a series of articles pertaining to CAD modeling techniques has been informative. In our next issue of Rototalk, I will continue to elaborate on these, as well as other design considerations during the development of 3D CAD models for rotational molding. Until then, keep in touch and email me with any of your comments or questions at paloian@idsys.com.

Michael Paloian, Integrated Design Systems, Inc.