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Mold Components 30 MoldMaking Technology April 2015 In Search of the Infinite Processing Window By Mark Scanlan Figures courtesy of Mark Scanlan. E mbarrassment is a great teacher. In early 1995, after earning two engineering degrees, I ended up in the molding industry, selling common, spring-activated locking cylinders to mold builders. The key words here are spring and lock. I was having a tough time achieving customer satisfaction because, although the cylinders' force was suf- ficient for holding loads on the side cores, customers often commented that the cores were flashing, and some claimed the cylinders were unlocking and backing up. Of course, I knew that the cylinders were not unlocking and the locks were not backing up, but then what was hap- pening? It seemed to all concerned that the customers must be doing something wrong or that something was moving somewhere in the system. After many visits to custom- ers, application reviews and plenty of frustration, I knew something else must be causing the cores to flash, but I still couldn't figure out what. Side-action preload can yield zero core movement during injection and help molders achieve a robust molding process. I finally sent a cylinder to a test lab and had it loaded to failure. The test results surprised me. After reviewing the data, I realized that I had simply failed to consider the phys- ics associated with the steel molds and to use my training to question my own assumptions about the extent of steel compressibility. Steel is more compressible that we might imagine (see Figure 1). Steel is Very Compressible in Molding Steel is so compressible that even typical injection forces can negatively impact performance of steel molds. We know steel can bend and stretch because we see this happening when we work with bolts, but we never really think about it compress- ing. We know it does, but we still sometimes assume that the parts we are working with are too large to compress or that compression is insignificant. After all, when do we ever really crush something to failure? A block of steel always seems to just hold its own no matter what. And that is the challenge— the steel often holds, but in molding, holding is rarely enough. Typical injection pressures of 10,000 to 20,000 psi can cause steel to not always maintain form or position. The steel may still hold the applied loads, but it might be compress- For a static core with constant major area (A), the FLEA formula can be used to calculate the defection (D) for a given injection force (F) and core length (L). Material modulus (E) is assumed at 30,000,000 lbs/ in 2 . F = pressure (P) x core exposed area (C) D = (F x L) / (E x A) For a core with the exposed area equal to the core body cross-sectional area, the FLEA formula simplifes to D = (P x L) / E, and a chart can be produced for defection as a function of pressure and length. FIGURE 1 Constant Core Deflection vs. Core Length and Pressure