What to Check in a Mold-Design Review

A mold design can be like a soup. The designer has ensured that the movements that the part geometry requires are designed (the meat), the gates are properly placed to fill the part (the vegetables) and that the mold fits in the press (the broth). But, has the designer checked that the cavity and core blocks have the proper number of screw holes to retain them in the base, or whether there are any? Having a proper mold-design review and checklist can and will save time to market by ensuring that the designer has even accounted for the small potatoes, or the design details. Oakley has developed an extensive mold-design checklist and following are some of the top items that can be applied to all tool designs. Mold designers check that:

1. The mold fits in the desired press. (The designer checks all water lines, hot sprues or heater rods and wires, injection locations and mold height minimum and maximum to clear all press tie or safety bars.)
2. The gate sizes and locations are correct. (They should be 50 percent to 80 percent of the part thickness at the gate location.)
3. The runners are properly sized. (Working from the part, the runner adjacent to the part should be no smaller than the maximum part-wall thickness and each branch should be sized up by multiplying 1.259 to the previous branch.)
4. Venting has been addressed. (The designer starts with a ring vent on the Z puller, which is the first cold slug and initial point to remove unwanted out-gassing. It is self-cleaning with every ejection cycle with a natural dynamic vent.)
5. Cooling lines, heater rods and oil lines are adequate and appropriate for the plastic material.
6. The part remains on the proper side of the mold for ejection. (The designer reviews the part draft and slide delays to help.)
7. Mold components are poka-yoked (mistake-proofed), or they are keyed for proper assembly and timing (or both).
8. The leader pins are taller than any other mold components above the parting line to keep the insert interface safe from misalignment.
9. Parting-line side or taper locks are in place to protect shut offs (if necessary), or inserts or blocks are designed with interlocking features to protect delicate interlocking features.
10. The mold components are made from proper materials to accommodate plastic part requirements, and the component Rockwell hardness ratings differ by more than two points to ensure that interfacing components do not gall. (Some considerations include the high polish that is required for the plastic part, corrosive out-gassing from plastic processing and heating and cooling requirements that can limit which steels to use.)
11. The slides are held effectively in the open position. (The designer checks whether a heal lock can be used as a kick-back feature to place the slide back into the open position if it is moved or whether springs are required.)
12. The slide or the lifter travel (or both) is enough to clear when the part is ejected.