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Design 14 MoldMaking Technology —— OCTOBER 2017 By Jeremy Williams This thermos lid molded from polycarbonate is exhibiting stress. ALL STRESSED OUT W e all work long hours and try to meet unreason- able expectations set by someone who has no idea what is required to get the job done. However, we're not the only ones who feel this stress. Plastic also feels stress when it is handled inappropriately. If neither situation is managed properly, the engineer and the plastic part (see Figure 1) will certainly be stressed out. Stress is the load (force) per unit area that tends to deform the body on which it acts. Stress is inherent to injection molding because of flow rates, pressures and temperatures applied to the material during processing. However, excessive stress in a molded part can cause issues with paint or chrome adhesion to the surface. In-molded stress can cause prema- ture functional failures as well, especially if a part must pass chemical testing, physical loading or thermal cycling. A basic understanding of plastic is necessary before tack- ling stress. Close your eyes and visualize it's mid-August at the amusement park, and you're on vacation with your fam- ily. It's a scorching 98°F, and you are standing in line for the seventh ride on the Tilt-A-Whirl. You notice a loose group of people congregating around the main entrance to the line, smiling and conversing with one another. As they inch closer to the single file line, there is less talking, the temperature rises and several individuals start getting feisty because of the lack of space between them. Plastic goes through a similar experience when changing thick- ness, switching directions or squeezing around sharp corners, and these are just a few of the factors that influence overall stress. Part Design The design of a part has a large impact on the overall stress of the plastic. Here are three ways you can decrease stress on your parts through design: 1. Keep wall thickness consistent. This isn't easy given today's complex geometries because as thickness changes occur, orientation, compression and cooling stresses vary dramatically. Orientation stress is the alignment and stretch- ing of the molecular chain during the filling phase. It's like a rubber band. If a polymer chain is stretched too far or too Minimizing inherent molded-in stresses reduces plastic part failures. fast, it can break. Compressive stress occurs during the pack- ing phase when material is added to the cavity to compensate for shrinkage. Cooling stresses happen when the material contacts the cavity wall and the material tries to shrink back to its unoriented, relaxed state. Again, it's like a rubber band. If the molecular chains are stretched and held under constant load, they will eventually break. 2. Eliminate sharp corners. The polymer is already under stress from filling, packing and cooling during the process. A sharp corner is another area where the material may not receive proper treatment. It's highly likely that sharp corners will add stress to plastic, which leads to functionally weak parts. This is because when load is applied, sharp corners become the weakest link in the chain, causing parts to fail in this region. Basically, sharp metal edges in molds will cut a polymer chain as it passes much like a sharp knife cuts skin. Edges with radiuses will not cut the polymer as it flows, in the same way a dull butter knife is unlikely to cut skin. Also, sharp corners can generate shear imbalance within the part's geometry, which further increases the potential for stress. Shear is caused by the injection rate as the polymer chains flow past one another during the filling phase. Shear imbalance is caused by the material changing directions in Images courtesy of RJG Inc.