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Cost Factors Affecting Productivity P2

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Required Shot Size of the Mold Shot size is an important characteristic of the molding machine, which affects the molding cycle. But first, what is the difference between “shot size” and “rated shot size”? “Rated shot size” is the amount (in grams or ounces) of polystyrene (PS) that the injection system (ram screw or two-stage) can inject at every cycle, and is indicated in all machine specifications. “Shot size” depends on the mass (W) of the molded product (in grams). | 70 Copyrighted Materials Copyright 2006 Hanser Publishers Retrieved from www.knovel.com Figure 3.21 Runner system for a 32-cavity 2-plate mold for caps Previous Page 3 Cost Factors Affecting Productivity 3.7.6.2 Required Shot Size of the Mold Shot size is an important characteristic of the molding machine which affects the molding cycle. But first what is the difference between shot size and rated shot size Rated shot size is the amount in grams or ounces of polystyrene PS that the injection system ram screw or two-stage can inject at every cycle and is indicated in all machine specifications. Shot size depends on the mass W of the molded product in grams . There are several common methods to determine the mass Weigh a sample or a handmade model . If the sample or model is of the same material as the desired product this is the mass of the product. Otherwise divide by the specific weight of the sample model and multiply with the specific weight of the desired plastic. Establish the volume by completely immersing the sample or model in a graduated container filled partly with water. The difference in filling levels gives the volume. Calculate the volume from the drawing dimensions this can be very cumbersome and time consuming Multiplying the established volume with the specific weight of the desired plastic gives the estimated mass weight of the product see Appendix for charts of average specific densities weights for various plastics . At this point it should be determined which runner system would be most suitable for the planned mold. If a cold runner system is selected the mass of the cold runner must be added to the shot size. And finally the number N of cavities should be determined. All these calculations will have to be repeated several times with different assumptions before settling on the final selections for the planned mold. For 2- and 3-Plate Molds Only Shot weight SW N W plus the mass of the cold runner The runner size mass is a very important. The .