Integrated Design

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.

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