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Mold Components 32 MoldMaking Technology April 2015 ing as it does. If this compression is not prevented or at least greatly minimized, the molding surfaces may not be in the proper position at the end of the shot. Gaps, variation in part geometry and flash will result. Fighting for an Infinite Processing Window So, if flex and bending were not enough to challenge us, we now have to consider steel compression in our quest to maximize our molding process windows. In his October 2014 Plastics Technology article "Flash-Free Molding." Randy Kerkstra addresses the role of the tool in achieving a broad process window. He says, "My main focus is to have a robust process window, and this typically relies heavily on the tool- ing. I've been successful in focusing on the tool to address root causes to help achieve a broad processing window. While I agree that processing is the number-one tool for eliminat- ing molding problems, every time you process around a root cause, your overall window becomes smaller." An "infinite" processing window relative to machine capa- bility is preferred. A mold with zero dimensional change (zero movement) and thus zero part change up to maximum machine injection pressure would have an infinite processing window. Pressures and flow could be changed to fix other problems without worrying about the effects of pressure in the tool. Often when a mold is in production, it must withstand more pressure than intended. Ideally, the mold builder would create a perfectly stiff mold (no flex or compression) that would have an infinite processing window. However, the mold builder is challenged with manufacturing very rigid molds, from non-ideal materials, that fit in the smallest pos- sible space (smallest press size) and still produce perfect parts. Unfortunately, mold stiffness is a challenge and often results in a tradeoff between mass and size. To combat bend- ing and flex, more steel is added to the mold to increase stiffness and reduce movement under load, but this is coun- terproductive to making molds smaller. In other cases, com- pression is expected, so a mold core may be made longer than necessary (steel safe) and then cut back after tool sampling. There are numerous other methods for accommodating bending and flex, but they require process tweaking, which also entails adjustments to the core and limits to the process- ing window. To avoid this, there is an option not often con- sidered: preload. FIGURE 2 Before Injection: Preload force (PF) generates a load on the stops and stop force (SF) is created. SF=PF During Injection: Injection force takes up and reduces some of the stop force. Zero Movement If preload force (PF) is > injection force (IF), then stop force (SF) always is > 0. Core is always touching stops, and there is no movement. Fixed-lock slide preloaded cannot move. It cannot compress if injection force is less than preload force. PRELOAD: CORE FACE STAYS IN POSITION Fixed Stop SF SF IF USING PRELOAD FORCE DESIGN Fixed Stop This preloaded side-action core shows load sharing.