Part Details Review
Even though you desire to keep uniform wall sections in your part, frequently functional requirements force you to use varying wall thicknesses. Variable wall thickness leads to a variety of shrinkage and flow related problems, and produce undesirable residual stresses and warpage in the part. As a designer, your aim is to design the part in the best possible way within the constraints imposed by functional requirements. When working with thick and thin sections in a part, place the gate in the thick section ensuring the flow from thick to thin. If the flow goes from thin to thick, the thin section will freeze before the thick section has a chance to be properly packed. The under packed thick section could form internal voids and sink marks to relieve shrinkage stresses. When injection and holding pressures are increased to pack out the thick section, the thin section gets over packed and becomes highly stressed. Flow from thick to thin generally offers the best solution with the least number of problems. However, in certain situations, especially when thin section is surrounded by thick section, flow from thick to thin could cause hesitation and air entrapment problems. This is discussed in more detail under the heading -"Flow Hesitation". The direction of melt flow in the mold also influences the direction of molecular orientation in the molded part. Almost all injection molded parts have some degree of "frozen-in" molecular orientation. During injection, the molecular chains on the part surface tend to align in the direction of material flow. For maximum strength, design parts so that the molecular orientation is aligned such that primary mechanical loads would bend the polymer chains over their length instead of across the aligned polymer chains. This is particularly important in fiber-filled materials and in flexible features such as living hinges. |