Part Details Review

Ribs usually project from the main wall parallel to the direction of mold opening and are formed by blind elongated holes in the mold steel. To facilitate part ejection from the mold, ribs are generally designed with at least 1/2° draft on each side. The only benefit of using draft for the ribs is reduction in the ejection force. Draft causes thinning of the rib at the other end which becomes difficult to fill. For this reason, more than 1° draft should be avoided in Makrolon polycarbonate. Apec resins often require 1° to 2° of draft for easy ejection. See Figure 1.

While the draft helps in reducing the ejection force, it also makes it difficult to use ejector pins for assisting ejection. Small ejector pins are fragile and sometimes push into the rib during ejection. Blade ejectors are effective but are expensive to make and maintain.

The most frequently used solution, which is practical as well as economical, is the use of ejector pin pads. See Figure 2. The pads effectively increase the area of contact of the ejector pins at the ribs and reduce the compressive stress at the pin that can cause the pin indentation on the rib during ejection. Ejector pad diameters are usually made equal to, or slightly less than, the rib base thickness.

The base of the rib that meets the base wall should have radius at least 0.125 times the base wall thickness to reduce stress concentration. Too large a radius, while effective in reducing stress concentration, contributes to forming a thick section that promotes sink marks, increases molded-in stresses, and can alter the mold filling pattern. For this reason, fillet radii at the base are generally kept less than 0.250 times the base wall thickness.

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