Chapter 6 Quantitative Production Equipment and mold maintenance 6. 1 Injection Molding Machine 6.1.1 Basic Principles of Injection Molding Machine The injection and molding is achieved through the injection molding machine and molds. Though in various types, the injection molding machine, no matter which type it is, comprises only two basic functions: 1)to heat plastics and make them fuse, 2)to apply high pressure on the fused plastics, make them eject and fill the mold cavity E 1. SP108A injection molding machine, 2. drier and hopper, 3. water tower; 4. water pump 5. control machine for rapidly heating and cooling injection molding According to the mode of plastication, the injection molding machine falls into the following two 1)Plunger Injection Molding Machine The plunger injection molding machine pushes forward the fused plastics in the charging barrel
Chapter 6 Quantitative Production Equipment and Mold Maintenance 6.1 Injection Molding Machine 6.1.1 Basic Principles of Injection Molding Machine The injection and molding is achieved through the injection molding machine and molds. Though in various types, the injection molding machine, no matter which type it is, comprises only two basic functions: 1) to heat plastics and make them fuse; 2) to apply high pressure on the fused plastics, make them eject and fill the mold cavity. According to the mode of plastication, the injection molding machine falls into the following two types： 1) Plunger Injection Molding Machine The plunger injection molding machine pushes forward the fused plastics in the charging barrel Fig.6-1: highlight and no-welding-line injection molding system 1. SP108A injection molding machine; 2. drier and hopper; 3. water tower; 4. water pump; 5. control machine for rapidly heating and cooling injection molding 3 4 1 5 2
hrough the to-and-fro movement of plunger therein, and the spreader further injects them into the mold cavity through the nozzle. The plastication of plastics in the charging barrel mainly depends on the heat provided by the outer heater of charging barrel. The poor thermal conductivity of plastics results in the unbalanced plastication to the inner and outer layer of materials in the charging barrel and therefore, the plunger injection molding machine is not suitable for the molding of plastic parts with low fluidity. strong heat sensitivity and overlarge amount of injection 2)Screw Injection Molding Machine The screw injection molding machine differentiates from the plunger injection molding machine mainly in that the horizontally-pushed plunger in the charging barrel is replaced by a rotary screw which undertakes all the fusion, plastication as well as injection of the materials in the charging barrel. The screw can not only rotate in the barrel but also can move forward and backward, and thereby is able to achieve such functions as sending materials, compacting, plasticating and transmitting pressure Currently, the screw injection molding machine yields the highest productivity and is also the most widely applied. As indicated in Fig. 6-1 is a complete set of super-luminous and traceless injection molding system, wherein Yanhing SP108A injection molding machine is one of the screw type. The subsequent discussion shall be based on the screw injection molding machine The course of injection and molding of plastics is a cycling process, wherein the completion of one cycle represents the completion of one injection cycling period. Each period mainly comprises quantitative feeding, fusion and plastication, pressurization and injection, mold filling and cooling as well as mold opening and taking out the parts etc. The next cycle follows after the plastic parts have been taken out and the molds have been closed. The completion of each injection cycling period represents the completion of a working cycle for the injection unit and mold clamping unit of the injection molding machine, namely, each moving part of the two units has moved once according to the scheduled The cycling process is indicated in Fig. 6-2 Plastication Raw materials 塑料原料一预塑 主座后退 注座前进 Injection unit moving backward Injection unit moving forward 具预热 计量 Preheating mold 插嵌件一闭校一 清理模具← 顶出塑 Opening mold Clear-up mold Pressure maintaining Fig6-2: cycling process of plastic injection molding Three stages shall be needed for the completion of injection and molding: plastication, injection and mold 1)Plastication Ejecting parts The raw materials of plastics fall into the charging barrel from the hopper and are sent forward by the rotating screw. While in the charging barrel, the plastics, on the one hand, receive heating and warming from outside of the barrel, and on the other hand, are compacted due to the gradual reduction of the volume of spiral flute. Meanwhile, strong stirring and cutting shall be applied to the plastics by the rotation of screw in the charging barrel, resulting in the intense friction between plastics and between plastics and the barrel as well as screw, and producing a lot of heat which leads to the gradual plastication and fusion of plastics while being pushed forward. The screw while rotating also moves
through the to-and-fro movement of plunger therein, and the spreader further injects them into the mold cavity through the nozzle. The plastication of plastics in the charging barrel mainly depends on the heat provided by the outer heater of charging barrel. The poor thermal conductivity of plastics results in the unbalanced plastication to the inner and outer layer of materials in the charging barrel and therefore, the plunger injection molding machine is not suitable for the molding of plastic parts with low fluidity, strong heat sensitivity and overlarge amount of injection. 2) Screw Injection Molding Machine The screw injection molding machine differentiates from the plunger injection molding machine mainly in that the horizontally-pushed plunger in the charging barrel is replaced by a rotary screw which undertakes all the fusion, plastication as well as injection of the materials in the charging barrel. The screw can not only rotate in the barrel but also can move forward and backward, and thereby is able to achieve such functions as sending materials, compacting, plasticating and transmitting pressure. Currently, the screw injection molding machine yields the highest productivity and is also the most widely applied. As indicated in Fig.6-1 is a complete set of super-luminous and traceless injection molding system, wherein Yanhing SP108A injection molding machine is one of the screw type. The subsequent discussion shall be based on the screw injection molding machine. The course of injection and molding of plastics is a cycling process, wherein the completion of one cycle represents the completion of one injection cycling period. Each period mainly comprises: quantitative feeding, fusion and plastication, pressurization and injection, mold filling and cooling as well as mold opening and taking out the parts etc. The next cycle follows after the plastic parts have been taken out and the molds have been closed. The completion of each injection cycling period represents the completion of a working cycle for the injection unit and mold clamping unit of the injection molding machine, namely, each moving part of the two units has moved once according to the scheduled sequence. The cycling process is indicated in Fig.6-2. Fig.6-2: cycling process of plastic injection molding Three stages shall be needed for the completion of injection and molding: plastication, injection and molding. 1) Plastication The raw materials of plastics fall into the charging barrel from the hopper and are sent forward by the rotating screw. While in the charging barrel, the plastics, on the one hand, receive heating and warming from outside of the barrel, and on the other hand, are compacted due to the gradual reduction of the volume of spiral flute. Meanwhile, strong stirring and cutting shall be applied to the plastics by the rotation of screw in the charging barrel, resulting in the intense friction between plastics and between plastics and the barrel as well as screw, and producing a lot of heat which leads to the gradual plastication and fusion of plastics while being pushed forward. The screw while rotating also moves Raw materials Injection unit moving forward Preheating mold Insert Plastication Injection unit moving backward Measurement Closing mold Parts Clear-up mold Opening mold Injection Pressure maintaining Ejecting parts
backward due to the pressure from the fusion of plastics by the head of screw, so that the plasticated fusant can be stored at the top of the charging barrel for use during injection. The backward stroke of screw is determined by the quantity of materials injected as required by each molding of plastic parts 2)Injection When the screw stops rotating and moves backward, the mold clamping part begins to work to close the mold and the injection unit subsequently moves forward to make the nozzle and mold spruing gate joint with each other; the injection oil tank plunger drives the screw to move forward rapidly, and the injection pressure and speed as required by the molding technics inject the fused plastics into mold cavity. To avoid refluence of the materials injected into the mold and to supplement the small amount of materials needed due to the contraction caused by the cooling and molding of parts, the screw,upon completion of injection, shall still keep certain pressure on the fused materials, i.e. pressure maintaining until the materials at the gate have cooled and molded 3)Molding The fused materials in the mold cavity restore from viscous and fluid state to glassy state after being cooled. After complete cooling and molding, the plastic parts in the mold cavity are ejected by the ejector unit during mold opening, thereby the whole process of molding of the injected parts being Barrel Rotating screw Hydraulic motor Single check valve Closing mold 油压马达注q Injection cylinder Injection nozzle Heate Screw going ahead Pressure maintaining 冷却(可型化测量) Cooling(Plastication measure) 螺杆转 Screw backing off Demolding Screw rotating Fig 6-3: injection molding As indicated in Fig. 6-3, centering on the screw, three procedures shall be needed to complete an injection cycling period 1)The injection tank plunger, under the scheduled pressure, pushes the screw forward to inject the 2)The injection tank plunger, under the scheduled pressure, further pushes the screw forward to supplement the materials and to maintain pressure 3)The oil pressure motor drives the screw to rotate and to supplement materials to be plasticated
backward due to the pressure from the fusion of plastics by the head of screw, so that the plasticated fusant can be stored at the top of the charging barrel for use during injection. The backward stroke of screw is determined by the quantity of materials injected as required by each molding of plastic parts. 2) Injection When the screw stops rotating and moves backward, the mold clamping part begins to work to close the mold and the injection unit subsequently moves forward to make the nozzle and mold spruing gate joint with each other; the injection oil tank plunger drives the screw to move forward rapidly, and the injection pressure and speed as required by the molding technics inject the fused plastics into mold cavity. To avoid refluence of the materials injected into the mold and to supplement the small amount of materials needed due to the contraction caused by the cooling and molding of parts, the screw, upon completion of injection, shall still keep certain pressure on the fused materials, i.e. pressure maintaining, until the materials at the gate have cooled and molded. 3) Molding The fused materials in the mold cavity restore from viscous and fluid state to glassy state after being cooled. After complete cooling and molding, the plastic parts in the mold cavity are ejected by the ejector unit during mold opening, thereby the whole process of molding of the injected parts being completed. Fig.6-3: injection molding machine operation process As indicated in Fig.6-3, centering on the screw, three procedures shall be needed to complete an injection cycling period: 1) The injection tank plunger, under the scheduled pressure, pushes the screw forward to inject the plasticated fused materials; 2) The injection tank plunger, under the scheduled pressure, further pushes the screw forward to supplement the materials and to maintain pressure; 3) The oil pressure motor drives the screw to rotate and to supplement materials to be plasticated; Rotating screw Hopper Hydraulic motor Injection cylinder Heater Barrel Single check valve Injection nozzle Injection Pressure maintaining Cooling(Plastication measure) Screw backing off Screw rotating Screw going ahead Demolding Closing mold
the screw overcomes the scheduled pressure on the injection tank plunger, moves backward until the work has been completed, and prepares for he next injection; on the other hand, after the cooling and molding of plastics in the mold cavity, the ejection shall be made 6.1.2 Structural Composition of Injection Molding Machine A universal injection molding machine mainly comprises such three parts as the injection system the molding clamping system and the hydraulic control system 1. Injection System The function of injection system is to plasticate a certain amount of plastics uniformly into fused state within certain time, and inject them into the mold cavity with sufficient pressure and speed. Above all, the system is able to maintain pressure and supplement materials for the fused materials in the mold cavity after injection. As indicated in Fig. 6-4, the most extensively-applied system is the to-and-fro screw injection device which plasticate the materials through the rotation of a screw that injects the plasticated fused materials into the mold cavity. It mainly consists of the plastication parts(including a nozzle, a charging barrel and a screw), a hopper (including a drying device), a screw rotation device, an injection device, an injection seat as well as a movable oil tank for the injection seat 18 1. nozzle; 2. front adapter: 3. screw head assembly: 4. heater band; 5.barrel 6. injection screw; 7. hopper; 8.bearing: 9. hydraulic motor: 10. oid tank for injection seat 11. cylinder rod for injection seat: 12. oil tank stopper for injection: 13 injection oil tank: 14. injection unit base block: 15. nozzle center adjusting bolt: 16. driving housing: 17. driving housing guide rod: 18 guide rod bracket Fig 6-4: injection unit structure Its working process: With the continuous rotation of screw, granular materials keep falling into the spiral flute of the screw in the charging barrel from the hopper and are pushed to the front of the charging barrel by the rotating screw, during which the materials are gradually plasticated and fused under the heating by the outer heater of charging barrel as well as the cutting and mixing by the screw
the screw overcomes the scheduled pressure on the injection tank plunger, moves backward until the work has been completed, and prepares for he next injection; on the other hand, after the cooling and molding of plastics in the mold cavity, the ejection shall be made. 6.1.2 Structural Composition of Injection Molding Machine A universal injection molding machine mainly comprises such three parts as the injection system, the molding clamping system and the hydraulic control system. 1. Injection System The function of injection system is to plasticate a certain amount of plastics uniformly into fused state within certain time, and inject them into the mold cavity with sufficient pressure and speed. Above all, the system is able to maintain pressure and supplement materials for the fused materials in the mold cavity after injection. As indicated in Fig.6-4, the most extensively-applied system is the to-and-fro screw injection device which plasticates the materials through the rotation of a screw that injects the plasticated fused materials into the mold cavity. It mainly consists of the plastication parts (including a nozzle, a charging barrel and a screw), a hopper (including a drying device), a screw rotation device, an injection device, an injection seat as well as a movable oil tank for the injection seat. 1.nozzle；2. front adapter；3.screw head assembly；4.heater band；5.barrel； 6.injection screw；7.hopper；8.bearing；9. hydraulic motor；10.oid tank for injection seat； 11. cylinder rod for injection seat；12. oil tank stopper for injection；13. injection oil tank；14. injection unit base block； 15.nozzle center adjusting bolt；16.driving housing；17. driving housing guide rod；18. guide rod bracket Fig.6-4: injection unit structure Its working process: With the continuous rotation of screw, granular materials keep falling into the spiral flute of the screw in the charging barrel from the hopper and are pushed to the front of the charging barrel by the rotating screw, during which the materials are gradually plasticated and fused under the heating by the outer heater of charging barrel as well as the cutting and mixing by the screw
and then are pushed between top of the screw and the nozzle. With the increase of fused materials at the top of charging barrel, the anti-resistance on the forward-moving materials also grows. When the resistance exceeds that returned by the injection oil tank plunger, and when the stored materials reach the quality required by the parts, the measurement control unit begins working and the screw stops rotating jection. The mold at this moment is closed, and the screw pushed by the injection plunger injects the fused materials into mold cavity with a certain pressure and a high speed. To avoid refluence and to supplement the materials contracted due to cooling, a certain period for pressure maintenance shall be needed Morphing screw( used by extrusion molding machine L1=340 (1)漸变形蝶杆(挤出机用 S=65 L1=330 2)突变形螺杆(注塑机用)一 Mutation screw ectIo on molding machine) LyLength of measurement section: S--Pitch: D--Screw diameter: e-Width of screw rib: hr-Depth of feeding Section hy-Depth of plastication section e-Screw rib width R,. RyFillet radius of the root screw rib Fig 6-5 Geometric shape and dimension of injection molding machine's screw Table 6-1: section lengths of injection screw/mm eeding Section Plastification Section Measurement Section Mutation lype 65~70 (1~15)D 0~25 D is screw diameter General Type 45~50 20~30 The screw is an important part of injection unit as indicated in Fig.6-5. Its major parameters are as follow Diameter and Stroke. The injection molding machine's maximum quantity of injection is determined by the diameter and maximum stroke of the screw. The diameter proportionates to the stroke, with a ratio of 3-5. If the ratio is too large and the stroke too long, the working length of screw shall be shortened, thereby influencing the plastication quality of material Should the ratio be too small and the stroke too short, to ensure the quantity of injection the diameter of screw must be increased and the diameter of injection oil tank shall also be increased accordingly to increase power consumption o Screw Lift Angle Rib Width and Radial Clearance Generally screw lift angle g=17°40。 Screw rib width e=(0.08-0.12)D(D is screw diameter). For materials with poor viscosity he rib width should be taken as large as possible since if it is too small, overflow shall easily occur yet when the width is too large, power consumption shall be increased, which may cause over-heatin
and then are pushed between top of the screw and the nozzle. With the increase of fused materials at the top of charging barrel, the anti-resistance on the forward-moving materials also grows. When the resistance exceeds that returned by the injection oil tank plunger, and when the stored materials reach the quality required by the parts, the measurement control unit begins working and the screw stops rotating to prepare for injection. The mold at this moment is closed, and the screw pushed by the injection plunger injects the fused materials into mold cavity with a certain pressure and a high speed. To avoid refluence and to supplement the materials contracted due to cooling, a certain period for pressure maintenance shall be needed. L—Length of screw L1—Length of feeding section L2—Length of plastication section； L3—Length of measurement section；S—Pitch；D—Screw diameter；e—Width of screw rib；h1—Depth of feeding Section h2—Depth of plastication section e—Screw rib width R1、R2—Fillet radius of the root screw rib Fig.6-5 Geometric shape and dimension of injection molding machine’s screw Table 6-1: section lengths of injection screw /mm Types of Screw Feeding Section Plastification Section Measurement Section Notes Morphing Type 30~50 50 20~35 Mutation Type 65~70 （1~1.5）D 20~25 General Type 45~50 20~30 20~30 D is screw diameter The screw is an important part of injection unit as indicated in Fig.6-5. Its major parameters are as follows: z Diameter and Stroke. The injection molding machine’s maximum quantity of injection is determined by the diameter and maximum stroke of the screw. The diameter proportionates to the stroke, with a ratio of 3~5. If the ratio is too large and the stroke too long, the working length of screw shall be shortened, thereby influencing the plastication quality of materials. Should the ratio be too small and the stroke too short, to ensure the quantity of injection the diameter of screw must be increased and the diameter of injection oil tank shall also be increased accordingly to increase power consumption. z Screw Lift Angle, Rib Width and Radial Clearance. Generally screw lift angleϕ = 17°40'。 Screw rib width e = (0.08 ~ 0.12)D (D is screw diameter). For materials with poor viscosity, the rib width should be taken as large as possible since if it is too small, overflow shall easily occur, yet when the width is too large, power consumption shall be increased, which may cause over-heating. Morphing screw ( used by extrusion molding machine ) Mutation screw (used by injection molding machine)