and can be lower if vice versa. The injection speed should be properly selected, since if the speed is too low, the parts can easily get cold joint and the contour of the parts with complicated shapes cannot be ensured; if the speed is too fast, plenty of friction heat may be produced, resulting in the decomposition and color change of the materials, and meanwhile the gas in the mold cavity may not be discharged freely and air bubble will possibly occur in the molded parts. After the injection, the pressure should be maintained to ensure that the fused materials fill the mold cavity and provide supplement to the contracted materials caused by cooling The ejector unit should be provided with sufficient ejection force to ensure smooth ejection of the parts. For ejector unit with a plurality of pivots, the ejection force on each pivot should be uniform and the protruding length of the ejector pin should be the same. Meanwhile, the moving speed of the ejector pin should be steady and adjustable CLAMPING CYLINDER EJECTOR CYL CARRIAGE CYLINDER INJECTION CYLINDER b 中中,。,中,路 SV3 OPERAT vi sv2 Sv3 Sv4 SV7 Sv9 SV 区b Fig 6-8: hydraulic schematic diagram 6.1.3 Basic Parameters of Injection Molding machine The basic parameters of injection molding machine, as basis for the design, manufacture, purchase s well as use of the machine, include such three aspects as injection, mold clamping and comprehensive 1. Theoretical Quantity of Injection The quantity of injection reflects to a certain extent the processing ability of the injection molding machine and indicates the maximum weight of the plastic parts that can be produced. It is therefore usually used as a parameter representing the specification of the injection molding machine. Two rendering methods are generally available thereof: the theoretical injection volume and the injection
and can be lower if vice versa. The injection speed should be properly selected, since if the speed is too low, the parts can easily get cold joint and the contour of the parts with complicated shapes cannot be ensured; if the speed is too fast, plenty of friction heat may be produced, resulting in the decomposition and color change of the materials, and meanwhile the gas in the mold cavity may not be discharged freely and air bubble will possibly occur in the molded parts. After the injection, the pressure should be maintained to ensure that the fused materials fill the mold cavity and provide supplement to the contracted materials caused by cooling. The ejector unit should be provided with sufficient ejection force to ensure smooth ejection of the parts. For ejector unit with a plurality of pivots, the ejection force on each pivot should be uniform and the protruding length of the ejector pin should be the same. Meanwhile, the moving speed of the ejector pin should be steady and adjustable. Fig.6-8: hydraulic schematic diagram 6.1.3 Basic Parameters of Injection Molding Machine The basic parameters of injection molding machine, as basis for the design, manufacture, purchase as well as use of the machine, include such three aspects as injection, mold clamping and comprehensive performance. 1. Theoretical Quantity of Injection The quantity of injection reflects to a certain extent the processing ability of the injection molding machine and indicates the maximum weight of the plastic parts that can be produced. It is therefore usually used as a parameter representing the specification of the injection molding machine. Two rendering methods are generally available thereof: the theoretical injection volume and the injection
D)Theoretical Injection Volume When the injection molding machine injects to the air, the volume of fused materials injected during the maximum injection stroke made by the screw or plunger is represented by cm. The formula for the theoretical injection volume Vi is VL=+D'S Wherein: VL -Theoretical injection voluM D-Diameter of screw(or plunger ), cm S-Maximum injection stroke of screw(or plunger),cm During the injection, on the one hand, density of the fused materials will change with temperature and pressure, on the other hand, small amount of refluence will occur under the pressure, ar pressure maintenance, the materials must be supplemented due to the contraction resulted from cooling therefore, the actual quantity of injection is less than the theoretical quantity of injection, which needs to corrected with injection coefficient VS =aVl Wherein: Vs-Actual quantity of injection, cm a-Injection coefficient The injection coefficient is related with many factors, such as structure and parameters of screw, injection pressure and speed, back pressure, structure of mold, shape of molded parts as well as nature of plastics etc. It is usually selected between 0.7-0.9 2)Theoretical Weight of Injection When the injection molding machine injects to the air, the maximum weight of polystyrene(PS) materials injected during the maximum injection stroke made by the screw or plunger is represented with gram(g). If the materials of the plastic parts differentiate from Ps, conversion for the quantity of injection should be made per the following formula W=MX Wherein: p-Density of the plastics, g /cm M-Quantity of Injection rendered by PS plastics, g For example, the density of PP is 0.909/cm, the standard parameter for the quantity of injection of an injection molding machine is 288g, and hence, the quantity of injection during the production of PP parts with this machine is 2880.9/1.05=2468g According to the actual production experience, total weight of plastic parts(total sum composed by the weight of parts and that of gate system) should be preferably controlled within 85% of the quantity of injection. For noncrystalline plastics, large value can be taken, whereas for plastics with high viscosity. smaller value should be taker 2. Injection Pressure During the injection of fused materials, the screw (or plunger) must apply pressure large enough onto the materials to overcome the resistance when the materials flow through the nozzle, the spruing channel and the mold cavity. The pressure thereby applied is called the injection pressure. The formula whereof is as follow
weight. l) Theoretical Injection Volume When the injection molding machine injects to the air, the volume of fused materials injected during the maximum injection stroke made by the screw or plunger is represented by cm3 . The formula for the theoretical injection volume VL is: VL D S 2 4 π = Wherein: VL ―Theoretical injection volume, cm3 ; D ― Diameter of screw (or plunger), cm; S ― Maximum injection stroke of screw (or plunger), cm. During the injection, on the one hand, density of the fused materials will change with temperature and pressure; on the other hand, small amount of refluence will occur under the pressure, and during pressure maintenance, the materials must be supplemented due to the contraction resulted from cooling; therefore, the actual quantity of injection is less than the theoretical quantity of injection, which needs to be corrected with injection coefficient. VS = αVL Wherein: VS ― Actual quantity of injection, cm3 ; α ― Injection coefficient. The injection coefficient is related with many factors, such as structure and parameters of screw, injection pressure and speed, back pressure, structure of mold, shape of molded parts as well as nature of plastics etc. It is usually selected between 0.7-0.9. 2) Theoretical Weight of Injection When the injection molding machine injects to the air, the maximum weight of polystyrene (PS) materials injected during the maximum injection stroke made by the screw or plunger is represented with gram (g). If the materials of the plastic parts differentiate from PS, conversion for the quantity of injection should be made per the following formula: 1.05 ρ W = M × Wherein: ρ ―Density of the plastics, g / cm3 ; M ―Quantity of Injection rendered by PS plastics, g. For example, the density of PP is 0.909/cm3 , the standard parameter for the quantity of injection of an injection molding machine is 288g, and hence, the quantity of injection during the production of PP parts with this machine is 288× 0.9 /1.05 = 246.8g . According to the actual production experience, total weight of plastic parts (total sum composed by the weight of parts and that of gate system) should be preferably controlled within 85％ of the quantity of injection. For noncrystalline plastics, large value can be taken, whereas for plastics with high viscosity, smaller value should be taken. 2. Injection Pressure During the injection of fused materials, the screw (or plunger) must apply pressure large enough onto the materials to overcome the resistance when the materials flow through the nozzle, the spruing channel and the mold cavity. The pressure thereby applied is called the injection pressure. The formula whereof is as follows: