Design-CAD File Part Details and Tooling Part 2

In my last Design Corner editorial I discussed the evolution of the design process from drafting tables to CAD. I also mentioned the added responsibility designers have undertaken as technology has progressed and development cycles have decreased. In this issue of Rototalk, I would like to limit my discussion to tooling selection and its affects on part design.

Most designers develop their ideas with 3D CAD solid modeling software which is typically emailed or transferred to a mold maker for machining the molds. Most tool makers construct the molds directly from the part geometry as defined in the CAD files. It should be noted that the hand-off of files from the designer to the mold maker always requires some verbal communication, especially for rotational molding. This is due to the many possible interpretations of the design by a tool maker as well as the various tooling methods. An essential early decision for a designing a rotationally molded part is selecting the type of mold which can be either machined aluminum, cast aluminum or fabricated sheet metal. The type of mold will have a major impact on the design. As an example, a machined mold will yield parts with the tightest tolerances and parting line match. Machined molds are also the most expensive. However, machine molds are very cost competitive with cast molds if parts are relatively shallow, such as a pallet or auto panel. Complex surfaces can be accurately reproduced directly from CAD files by CNC machining into an aluminum block. Machined molds become less cost effective as part geometries become complex, very large and deep. Improved tolerances are realized because of the elimination of the cumulative tolerance buildup of patterns, casting aluminum and secondary hand work associated with cast aluminum tools. Therefore designers can design parts with tighter clearances, better parting lines and improved surface matches when designs are based on a cast aluminum tool. Cast tools are the most commonly specified molds for rotational molding. They provide the best balance of cost, design freedom and range of part size. However, unlike machined molds, casttools vary significantly in quality from one mold makerto the next. Quality cast molds can be as much as fourtimes the cost of a poor quality mold. These wide variations in quality and cost are based on the manually intensive complexities associated with cast molds. An identical part design could be successfully molded in a quality tool and be a dismal failure in a poor quality mold. Mold quality will have an affect on draft angles, surface finish, tolerances, wall thickness variation, kiss offs and numerous other design parameters.

If a part is designed with planaror cylindrical surfaces, it would be well suited to a sheet metal mold. Most water tanks, simple pallets and otherfunctional products are molded in sheet metal molds. As a general rule, whatever shape can be achieved with a piece of paper can be achieved with a sheet metal mold. Compound curves and complex shapes are not well suited to sheet metal molds. In addition, parts molded in sheet metal molds typically exhibit the widest tolerance range due to the inherent construction process. Some tool makers will use 3D CAD files to construct the sheet metal molds. However, in most cases the CAD are used as a guide to cut the sheet metal and weld it into the final desired shape. It should be noted that complex parts can be attained with sheet metal molds, but geometries must be limited to basic geometric forms of rectangular, cylindrical, trapezoidal and conical forms.

Hopefully these highlights have provided you with the basic differences in tooling options and their general effects on part design. In our next issue of Rototalk, I will go into a bit more detail about specific considerations in 3D CAD modeling for each of the tooling options. Examples will include inserts, engraved logos, threads and textures. 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.