Another concern related to wall thickness is referred to as “read-through”, which is a cosmetic phenomenon that occurs on the outside surface of the part directly over the thin wall. The visual effect of read-through is a distinct change in gloss level. This particular change in gloss level can be more distinct at the transition point but carries through across the entire area of the thin wall. Read-through is caused by temperature changes in the material at the flow front and a localized change in cavity pressure. There are several contributors that may drive a design to require thin areas. One example is an interlock feature where there is a reduced wall section at the parting line to accommodate a mating interlock feature. As a general rule, when thinning is required it’s best to avoid reducing wall thickness by more than 20%. Molding resin, color, and surface finish are all things to consider when wall thinning is required. Whenever possible it is also best to avoid sharp transitions from thick to thin. Adding a fillet, or chamfer, or transitioning gradually will help hide the read-through.
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Be on the lookout as we discuss the importance of proper rib and boss design in part 3 of this series: Boss and Rib Design.