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Mold Components 34 MoldMaking Technology April 2015 into position on an injection mold. A core can be preloaded against a stop by applying a large force to the core and then capturing that force (or locking it in place) during the injec- tion cycle. Rather than reacting to the force of the plastic and trying to resist it like a large finger-tight bolt, a preloading approach compresses the core, thus preventing movement, much like torque to a bolt. Although the physics may seem a bit strange, the stop on the tool does resist the initial loading of the core with an equal and opposite force. As the plastic presses on the core, that load simply reduces the load on the stops without any movement. The stop loading and the plastic pressure always add up to a force equal to the preload force. The core does not move and the desired part shape is maintained regardless of the plastic pressure (see Figure 2). While any method for applying a preload force to a core would work, the most common is using a hydraulic cylin- LEARN MORE Understanding the effects of injection on the core, slide and associated components is critical to selecting the best side-action me thods for a given application. This frst of two articles will discuss the basic physics underlying all side-actions. shor t.moldmakingtechnology.com/sideaction der (or similar actuation means) and maintaining that force throughout the injection cycle by maintaining, capturing or locking it in place. Three methods include: Hydraulic Maintaining Cylinder (hydraulics only): Applying a large force with hydraulics only can work, but it requires the hydraulic pressure to be maintained throughout the injection cycle, which is often not possible. It also often demands larger cylinders and a large hydraulic supply volume, both of which take up space and gallons per minute (gpm), which is not always available from the machine hydraulics to maintain cycle speed. Cylinder and Heel Block (capture method): Another option is using a hydraulic cylinder to set and preload the core, and then capturing that force mechanically with a block of steel placed behind the core. This does not solve the large hydraulic volume requirement for preload and requires very accurate placement of the steel block after preload. It overcomes the need to maintain pressure during injection, and with correct timing on smaller cores, this method can be very effective in providing preload without much sacrifice in cycle speed. Preloading and Locking Cylinder (integrated method): A fully integrated approach is to use a force-intensifying (large force in a small size) and preloading locking cylinder. Such a system applies very large preloads in a small package to cre- ate a very rigid coupling between the core and the stop. This results in zero core movement during injection, and when This side-action core is preloaded and locked toward core stops. FIGURE 3 CORE SET CORE PRELOADED PRELOAD COMPRESSION INJECTION ZERO MOVEMENT - FULLY COMPENSATED T h i s s i d e - a c t i o n c o r e i s p r e l o a d e d a n d l o c k e d t o w a r d c o r e s t o p s . Z E R O M O V E M E N T - F U L L Y C FORCE