Design-CAD File Part Details and Tooling Part 6

The last time I wrote an article for Design Corner was in November of 2007. That was quite a while ago. As promised in the closing statement of the July/August publication of Design Corner, this issue will address draft, undercuts parting lines and other features that should be included in 3D CAD models for rotational molding. The majority of patterns and molds fabricated today are machined directly from a 3D CAD file provided by the designer. The tool maker typically makes little to no changes to the file with the exception of adding draft and a few features for mold fabrication. Therefore the designer must be extremely careful to include all the proper features to mold a quality part. The remainder of this article will review the importance of draft, undercuts and parting line location in the CAD file.

Unlike injection molding, rotational molding does not require strict adherence to including draft an all surfaces perpendicular to the direction of draw. This is because of the inherent differences between the two processes. Rotationally molded parts can sometimes be molded without draft and even molded with undercuts to a limited extent. This is typically true for exterior surfaces which pull away from the tool walls as the part shrinks during cooling. Polyethylene’s high rate of shrinkage (between 2% and 3%) results in molded surfaces pulling away from exterior mold walls with enough clearance to be demolded without draft. However, internal mold surfaces where the plastic shrinks around the mold do require draft. Examples of such surfaces include any standing feature in the mold or any depression in the molded part. These surfaces should include a minimum draft angle of 1 degree, 2 to 4 degrees draft are preferred. If an etched texture is applied to the mold, additional draft is required. A general rule of adding 1 degree draft for every .025mm of texture depth is recommended for etched mold surfaces.

Although adding draft to a CAD model is generally straight forward for simple rectangular or cylindrical parts, it can get quite complicated for parts with complex 3D surfaces. Most 3D CAD programs do not permit the addition of draft to compound splined surfaces. Therefore one must consider adding draft to the part geometry at the very early stages of the CAD modeling process. A designer must anticipate how the part would be molded and where the parting lines appear at the very early stages of the 3D modeling process. If these considerations are not made early in the CAD modeling development, it could result in a significant amount of design rework, requiring the entire CAD model to be redone.
Undercuts are to be avoided whenever possible; however there are instances when they must be included in a part design. In some cases, undercuts can be included in a part without the need for an extra mold section by taking advantage of the shrinkage rate for polyethylene. This can be easily explained with an example. If a 1000 mm long polyethylene part was rotationally molded, it could shrink 25 to 30 mm. Therefore if one end of such a part included an undercut of 20 mm, the material would shrink far enough away from the mold wall to enable the undercut to clear the protrusion in the vertical wall.

More frequent practical applications of the design technique are used to mold embossed graphics into vertical walls or adding a recess for a snap fit. It is more often advisable to mold undercuts with additional mold sections. Although additional mold sections reduce cycle time and add to mold maintenance, they are sometimes the only method for molding an undercut. When multipart molds are used, the designer should carefully consider parting line location and draft based on how the mold will be opened.

Parting lines are defined by the designer’s choice of draft angles and how he envisions the part to be molded. Since parting lines are the most maintenance prone area of the mold, they do tend to deteriorate with time, resulting in cosmetic detractions. It is therefore important to locate parting lines in areas that are less visible to the viewer. Typical locations are on the underside of the part in the back. A step or recess is sometimes added to disguise the parting line. It is advisable to eliminate radii along parting lines since the tangent of the radius must be closely matched on both halves of the mold to achieve a clean parting line. This is very difficult to achieve in rotational molding and should be avoided.

These design highlights and tips should provide you with a better insight into what considerations to include in your next design project. I look forward to hearing from you with any comments or questions you may have. If you want to contact me, please feel free to contact me at my email address paloian@idsys.com. Your feedback will help me provide content in these editorials which will be of interest to your specific requirements. Until next time, best of luck on your projects.

Michael Paloian, Integrated Design Systems, Inc.