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Hot Runners 28 MoldMaking Technology —— AUGUST 2016 An internal mold-venting valve can be placed inside a mold cavity, which allows gas to escape from a venting hole that automatically closes under the pressure of the injected plastic. FIGURE 5 In addition to at runner side, another option for exter- nal mold venting is at the last filling point, whereby, while the melt flows forward in the cavity, air and gases flow out through the valve. Thanks to the low air counter pressure in the cavity, this allows injection parameters to be managed in a larger window, optimizing injection time, part aesthetics and mechanical characteristics. Although external venting generates a small plastic protru- sion on the part that must later be removed, a tunnel gate will allow for automatic cutting of this protrusion during part ejection (see Figures 3 and 4). An internal mold-venting valve can be placed inside the cavity of a mold and allows gas to escape from a venting hole that automatically closes under the pres- sure of the injected plastic. This type of valve consists of an outer sleeve, a slid- ing shaft and a central pin that serves to close off the venting hole. (see Figure 5). During filling, a spring of the appropri- ate length, wire diameter and preload pushes the sliding shaft forward in open position. While in this open position, air and gases can flow through the central hole and along the wide internal vent- ing channels. When the injected plastic meets the valve, it applies pressure on the shaft surface, compressing the spring and closing the valve. While in closed position, the central pin seals off the venting hole, ensuring the plastic does not fill the valve. The proper working of FIGURE 3 FIGURE 4 With external dynamic mold-venting at the last filling point, a tunnel gate can allow for automatic cutting of any plastic protrusion during part ejection. A properly designed external venting channel at the last filling point can allow maximum performance. the valve is guaranteed by accurate sizing of the venting hole, the safety zone and spring preload. These parameters allow the valve to close before the melt fills the wide venting chan- nels (see Figure 6). The design of this type of valve requires the sliding shaft to stick into the cavity while the valve is in open position. This guarantees open venting channels. The central shaft moves up and down for a defined stroke to suit plastic part wall thick- ness. The shorter the stroke, the longer the plastic protrusion