As a mold designer involved in creating tooling quotes for over 13 years, I have encountered a number of errors in design decisions, which, if they were considered during the quoting stage, would have saved both time and money down the road for all parties involved.

For example, here are four common areas where mistakes are often made that impact quote accuracy:
  1. Draft analysis. There also needs to be a balance between the depth of a rib and the width (created from draft) where it meets the attached surface. If the rib gets too thick, then the potential of sink becomes an issue. Can the product accept an ejector flat if it is too thin for an ejector?
  2. Corner radii. Product designers will often not include radii at the bottom of a rib. When the plastic material is flowing across an edge, it prefers to move around an edge with a radius rather than a sharp corner, reducing stress in the part. The larger, the better, as long as the radius does not affect fit and function along with increasing the wall thickness that will create sink in the molded material. Most moldmakers will want a .005-inch/.01-inch radius minimum where a part fillet would be.
  3. Parting line and bypass shutoffs. When a stepped parting line is required, the product designer needs to consider the impact on draft, part removal (via a slide or lifter instead of a bypass shutoff) and surface finish (a witness line not meeting marketing requirements). If a bypass shutoff is used, then the mismatch of opposing drafts needs to be considered. Will this be an issue with the part function or appearance?
  4. Press selection. This is very critical, yet so open to interpretation. There are standard (rule of thumb) numbers for the tons per square inch needed to hold the parting line closed during the injection cycle depending on the type of material. Wall thickness needs to also be considered. Once the material and tonnage are identified, multiplythe part’s footprint (square inches) to identify the required molding press tonnage. This calculation gives a baseline to determine the estimated clamping force required and the minimum press size.

OEMs, custom molders and mold builders must address these areas before the quoting stage, otherwise costs will increase and deliveries can be delayed.