1.4.3.2 Injection Pressure Program Control The injection pressure control is usually divided into one injection pressure, two injection pressures (holding pressure) or more than three injection pressures. The proper timing of pressure switching is very important to prevent the pressure in the mold from being too high and to prevent overflow or shortage of material. The specific volume of a molded product depends on the pressure and temperature of the melt at the time the gate is closed during the pressure holding period. If the pressure and temperature are the same each time the pressure is switched from the holding pressure to the product cooling phase, the specific volume of the product will not change. At a constant molding temperature, the most important parameter that determines the size of a product is the holding pressure. The most important variables that affect the dimensional tolerance of a product are holding pressure and temperature. For example, after the mold is filled, the holding pressure is reduced immediately. When the surface layer is formed with a certain thickness, the holding pressure rises again. This allows the use of low clamping force to form thick-walled large products and eliminate pits and flashes.
The holding pressure and speed are usually 50%~65% of the maximum pressure and speed when the plastic fills the cavity, that is, the holding pressure is about 0.6~0.8MPa lower than the injection pressure. Since the holding pressure is lower than the injection pressure, the load of the oil pump is low during a considerable holding time, the service life of the solid oil pump is prolonged, and the power consumption of the oil pump motor is also reduced.
The three-stage pressure injection can not only make the parts fill smoothly, but also does not appear weld line, depression, flash and warpage. For the molding of thin-walled parts, multiple small parts, and long-flow large-scale parts, even molded parts with unequal cavity configurations and molded parts that are not tight have advantages.
1.4.3.3 The program control of the filling amount of plastics injected into the cavity adopts a certain amount of pre-adjustment, so that near the end of the injection stroke, a small amount of melt (buffer amount) remains at the end of the screw, according to the filling condition in the mold. Further application of the injection pressure (secondary or three injection pressures) with a little melt. In this way, the product can be prevented from sinking or adjusting the shrinkage of the product.
1.4.3.4 Program control of screw back pressure and rotation speed High back pressure can make the melt material obtain strong shear, and low rotation speed can also make the plastic get a longer plasticization time in the barrel. As a result, the control of the back pressure and the rotational speed at the same time has been used more often. For example: in the full stroke of screw measurement, the first high speed, low back pressure, then switch to lower speed, higher back pressure, then switch to high back pressure, low speed, and finally plasticize at low back pressure and low speed. In this way, the pressure of the melt at the front of the screw is mostly released, which reduces the rotational inertia of the screw, thereby improving the accuracy of the screw metering. Excessive back pressure tends to increase the discoloration of the colorant; the mechanical wear of the barrel screw of the pre-molding mechanism increases; the pre-molding cycle lengthens, and the production efficiency decreases; the nozzle is prone to runny, and the amount of recycled material increases; even if self-locking is used The nozzle will also cause fatigue failure if the back pressure is higher than the designed spring blocking pressure. Therefore, the back pressure must be adjusted properly.
With the advancement of technology, it has become possible to incorporate small computers into the injection molding machine control system and use computers to control the injection molding process. Nippon Steel Works N-PACS (Microcomputer Control System) can perform four feedback controls (pressure adjustment, mold press adjustment, automatic metering adjustment, resin temperature adjustment) and four process controls (injection speed program control, pressure maintenance) Inspection, screw speed program control, back pressure program control).
1.4.4 Preparations before Injection Molding Preparations before molding may include many topics. Such as: material processing performance test (measurement of plastic fluidity, moisture content, etc.); dyeing and granulation of raw materials before processing; preheating and drying of pellets; cleaning and preheating of inserts; Wait.
1.4.4.1 Pretreatment of raw materials Based on the characteristics of the plastic and the conditions of the materials, the appearance and process properties of the raw materials should generally be examined before forming. If the plastic used is powder, such as polyvinyl chloride, it should be compounded and dry blended; if the product has coloring requirements, it can be added with an appropriate amount of colorant or color masterbatch; the supplied pellets often contain different levels of moisture , fluxes and other volatile low molecular weight materials, especially those with a hygroscopic tendency, always exceed the allowable limits of processing. Therefore, it must be dried before processing, and determine the moisture content. Water-sensitive polycarbonates that are sensitive to water at high temperatures require a moisture content of 0.2% or less, or even 0.03% to 0.05%, so they are often dried in a vacuum oven. The dried plastics must be properly sealed and stored to prevent the plastics from absorbing moisture from the air and lose the drying effect. For this purpose, a drying chamber hopper can be used to continuously provide dry hot materials for the injection molding machine, which simplifies operations, maintains cleanliness, and improves quality. It is advantageous to increase the injection rate. The loading capacity of the drying hopper is generally 2.5 times the amount of material used per hour for the injection molding machine.
1.4.4.2 Preheating of Inserts Injection molded products require metal inserts embedded in the product for assembly and strength requirements. In the injection molding, when the cold metal insert and the hot plastic melt cooled in the cavity are cooled together, due to the significant difference in the shrinkage ratio between the metal and the plastic, a large internal stress is often generated around the insert (especially in the case of polyphenylene). Rigid polymers such as ethylene are more pronounced. The presence of this internal stress causes cracks around the insert, resulting in greatly reduced use performance of the product. This can be done by using metals (aluminum, steel, etc.) with large coefficients of thermal expansion as inserts and preheating inserts (especially large metal inserts). At the same time, large thick walls and other measures are arranged around the inserts when designing the product.
1.4.4.3 Barrel Cleaning The injection molding machine barrel needs to be cleaned or disassembled before the newly purchased injection molding machine is used or when it is necessary to change the product, change the raw materials, change the color, or find that there is decomposition in the plastic. .
Washing barrels are generally heated barrel cleaning method. Cleaning materials generally use plastic materials (or plastic recyclates). For heat-sensitive plastics, such as polyvinyl chloride stocks, low-density polyethylene, polystyrene, etc. can be used for transition refueling, and then replace the transition cleaning material with the new material processed.
1.4.4.4 Selection of Mold Release Agents Mold release agents are substances that make plastics easy to release. Zinc stearate is suitable for general plastics other than polyamides; liquid paraffins are good for polyamides; silicone oils are expensive, use less troublesome, and less useful.
The use of mold release agents should be controlled in appropriate amounts, with minimal or no use. Excessive spraying will affect the appearance of the product and adversely affect the color decoration of the product.
1.5 Precautions for the Operation of Injection Molding Machines Developing a good operating habit for injection molding machines will greatly improve the life of the machine and the safety of production.
1.5.1 Before starting up: (1) Check whether there is water and oil entering the electrical control box. If the electrical appliance is damp, do not turn it on. The maintenance personnel should blow the electrical parts dry before starting up. (2) Check that the supply voltage is in compliance, generally should not exceed ±15%. (3) Check the emergency stop switch, whether the front and rear safety door switch is normal. Verify that the motor and oil pump are in the same direction of rotation. (4) Check whether the cooling pipes are unblocked and pass cooling water to the oil cooler and the cooling water jacket at the end of the barrel. (5) Check if there is any lubricant (fat) in each active part and add enough lubricant. (6) Turn on the electric heat and warm the various sections of the barrel. When the temperature of each section reaches the required temperature, it is kept for a period of time to stabilize the temperature of the machine. The holding time varies according to the requirements of different equipment and plastic materials. (7) Add enough plastic in the hopper. According to the requirements of injection molding different plastics, some raw materials are best dried. (8) To cover the heat shield on the barrel, this can save energy, and can extend the life of the electric heating coil and contactor.
1.5.2 During operation: (1) Do not arbitrarily cancel the function of the security door for the sake of convenience. (2) Observe the temperature of the pressure oil. The oil temperature should not exceed the specified range. The ideal working temperature of hydraulic oil should be kept between 45~50°C, and it is generally suitable in the range of 35~60°C. (3) Pay attention to adjust each stroke limit switch to avoid the impact of the machine during the action.
1.5.3 At the end of the work: (1) Before shutting down, the plastic in the barrel should be cleaned to prevent oxidation of leftovers or long-term thermal decomposition. (2) The mold should be opened so that the toggle mechanism is locked for a long time. (3) The workshop must be equipped with lifting equipment. Care should be taken when installing or removing bulky components such as molds to ensure production safety.
1.6 The causes of defects in injection molding products and their handling methods may be caused by poor handling of raw materials during the injection molding process, unreasonable product or mold design, lack of proper operating conditions for the operator, or mechanical reasons. The product produces defects such as imperfections, depressions, flashes, bubbles, cracks, warpage, and dimensional changes.
The evaluation of plastic products mainly includes three aspects. The first is the appearance quality, including the integrity, color, and gloss; the second is the accuracy between the size and the relative position; the third is the mechanical and chemical properties corresponding to the purpose. Electrical properties. These quality requirements, in turn, vary depending on the use of the product.
The production practice proves that the defects of the products mainly lie in the design, manufacturing precision and wear degree of the molds. In fact, technicians in plastic processing plants often suffer from the difficulty of using technical means to make up for the problems caused by mold defects and have little effect.
The adjustment of the process in the production process is a necessary way to improve the quality and yield of the product. Because the injection cycle itself is very short, if the process conditions are not well mastered, waste products will continue to flow. When adjusting the craft, it is best to change only one condition at a time and observe several times at a time. If pressure, temperature, and time are unified and adjusted, it is easy to cause confusion and misunderstanding. If problems arise, you do not know what the reason is. The measures and means for adjusting the process are various. For example, there are more than a dozen possible solutions to solve the problem of unsatisfactory products. To solve the problem, one or two major solutions must be selected to solve the problem. In addition, we should also pay attention to the dialectical relationship in the solution. For example: The product has sunken, sometimes to raise the material temperature, sometimes to reduce the material temperature; sometimes to increase the amount of material, sometimes to reduce the amount of material. It is necessary to admit the feasibility of solving the problem with reverse measures.
The holding pressure and speed are usually 50%~65% of the maximum pressure and speed when the plastic fills the cavity, that is, the holding pressure is about 0.6~0.8MPa lower than the injection pressure. Since the holding pressure is lower than the injection pressure, the load of the oil pump is low during a considerable holding time, the service life of the solid oil pump is prolonged, and the power consumption of the oil pump motor is also reduced.
The three-stage pressure injection can not only make the parts fill smoothly, but also does not appear weld line, depression, flash and warpage. For the molding of thin-walled parts, multiple small parts, and long-flow large-scale parts, even molded parts with unequal cavity configurations and molded parts that are not tight have advantages.
1.4.3.3 The program control of the filling amount of plastics injected into the cavity adopts a certain amount of pre-adjustment, so that near the end of the injection stroke, a small amount of melt (buffer amount) remains at the end of the screw, according to the filling condition in the mold. Further application of the injection pressure (secondary or three injection pressures) with a little melt. In this way, the product can be prevented from sinking or adjusting the shrinkage of the product.
1.4.3.4 Program control of screw back pressure and rotation speed High back pressure can make the melt material obtain strong shear, and low rotation speed can also make the plastic get a longer plasticization time in the barrel. As a result, the control of the back pressure and the rotational speed at the same time has been used more often. For example: in the full stroke of screw measurement, the first high speed, low back pressure, then switch to lower speed, higher back pressure, then switch to high back pressure, low speed, and finally plasticize at low back pressure and low speed. In this way, the pressure of the melt at the front of the screw is mostly released, which reduces the rotational inertia of the screw, thereby improving the accuracy of the screw metering. Excessive back pressure tends to increase the discoloration of the colorant; the mechanical wear of the barrel screw of the pre-molding mechanism increases; the pre-molding cycle lengthens, and the production efficiency decreases; the nozzle is prone to runny, and the amount of recycled material increases; even if self-locking is used The nozzle will also cause fatigue failure if the back pressure is higher than the designed spring blocking pressure. Therefore, the back pressure must be adjusted properly.
With the advancement of technology, it has become possible to incorporate small computers into the injection molding machine control system and use computers to control the injection molding process. Nippon Steel Works N-PACS (Microcomputer Control System) can perform four feedback controls (pressure adjustment, mold press adjustment, automatic metering adjustment, resin temperature adjustment) and four process controls (injection speed program control, pressure maintenance) Inspection, screw speed program control, back pressure program control).
1.4.4 Preparations before Injection Molding Preparations before molding may include many topics. Such as: material processing performance test (measurement of plastic fluidity, moisture content, etc.); dyeing and granulation of raw materials before processing; preheating and drying of pellets; cleaning and preheating of inserts; Wait.
1.4.4.1 Pretreatment of raw materials Based on the characteristics of the plastic and the conditions of the materials, the appearance and process properties of the raw materials should generally be examined before forming. If the plastic used is powder, such as polyvinyl chloride, it should be compounded and dry blended; if the product has coloring requirements, it can be added with an appropriate amount of colorant or color masterbatch; the supplied pellets often contain different levels of moisture , fluxes and other volatile low molecular weight materials, especially those with a hygroscopic tendency, always exceed the allowable limits of processing. Therefore, it must be dried before processing, and determine the moisture content. Water-sensitive polycarbonates that are sensitive to water at high temperatures require a moisture content of 0.2% or less, or even 0.03% to 0.05%, so they are often dried in a vacuum oven. The dried plastics must be properly sealed and stored to prevent the plastics from absorbing moisture from the air and lose the drying effect. For this purpose, a drying chamber hopper can be used to continuously provide dry hot materials for the injection molding machine, which simplifies operations, maintains cleanliness, and improves quality. It is advantageous to increase the injection rate. The loading capacity of the drying hopper is generally 2.5 times the amount of material used per hour for the injection molding machine.
1.4.4.2 Preheating of Inserts Injection molded products require metal inserts embedded in the product for assembly and strength requirements. In the injection molding, when the cold metal insert and the hot plastic melt cooled in the cavity are cooled together, due to the significant difference in the shrinkage ratio between the metal and the plastic, a large internal stress is often generated around the insert (especially in the case of polyphenylene). Rigid polymers such as ethylene are more pronounced. The presence of this internal stress causes cracks around the insert, resulting in greatly reduced use performance of the product. This can be done by using metals (aluminum, steel, etc.) with large coefficients of thermal expansion as inserts and preheating inserts (especially large metal inserts). At the same time, large thick walls and other measures are arranged around the inserts when designing the product.
1.4.4.3 Barrel Cleaning The injection molding machine barrel needs to be cleaned or disassembled before the newly purchased injection molding machine is used or when it is necessary to change the product, change the raw materials, change the color, or find that there is decomposition in the plastic. .
Washing barrels are generally heated barrel cleaning method. Cleaning materials generally use plastic materials (or plastic recyclates). For heat-sensitive plastics, such as polyvinyl chloride stocks, low-density polyethylene, polystyrene, etc. can be used for transition refueling, and then replace the transition cleaning material with the new material processed.
1.4.4.4 Selection of Mold Release Agents Mold release agents are substances that make plastics easy to release. Zinc stearate is suitable for general plastics other than polyamides; liquid paraffins are good for polyamides; silicone oils are expensive, use less troublesome, and less useful.
The use of mold release agents should be controlled in appropriate amounts, with minimal or no use. Excessive spraying will affect the appearance of the product and adversely affect the color decoration of the product.
1.5 Precautions for the Operation of Injection Molding Machines Developing a good operating habit for injection molding machines will greatly improve the life of the machine and the safety of production.
1.5.1 Before starting up: (1) Check whether there is water and oil entering the electrical control box. If the electrical appliance is damp, do not turn it on. The maintenance personnel should blow the electrical parts dry before starting up. (2) Check that the supply voltage is in compliance, generally should not exceed ±15%. (3) Check the emergency stop switch, whether the front and rear safety door switch is normal. Verify that the motor and oil pump are in the same direction of rotation. (4) Check whether the cooling pipes are unblocked and pass cooling water to the oil cooler and the cooling water jacket at the end of the barrel. (5) Check if there is any lubricant (fat) in each active part and add enough lubricant. (6) Turn on the electric heat and warm the various sections of the barrel. When the temperature of each section reaches the required temperature, it is kept for a period of time to stabilize the temperature of the machine. The holding time varies according to the requirements of different equipment and plastic materials. (7) Add enough plastic in the hopper. According to the requirements of injection molding different plastics, some raw materials are best dried. (8) To cover the heat shield on the barrel, this can save energy, and can extend the life of the electric heating coil and contactor.
1.5.2 During operation: (1) Do not arbitrarily cancel the function of the security door for the sake of convenience. (2) Observe the temperature of the pressure oil. The oil temperature should not exceed the specified range. The ideal working temperature of hydraulic oil should be kept between 45~50°C, and it is generally suitable in the range of 35~60°C. (3) Pay attention to adjust each stroke limit switch to avoid the impact of the machine during the action.
1.5.3 At the end of the work: (1) Before shutting down, the plastic in the barrel should be cleaned to prevent oxidation of leftovers or long-term thermal decomposition. (2) The mold should be opened so that the toggle mechanism is locked for a long time. (3) The workshop must be equipped with lifting equipment. Care should be taken when installing or removing bulky components such as molds to ensure production safety.
1.6 The causes of defects in injection molding products and their handling methods may be caused by poor handling of raw materials during the injection molding process, unreasonable product or mold design, lack of proper operating conditions for the operator, or mechanical reasons. The product produces defects such as imperfections, depressions, flashes, bubbles, cracks, warpage, and dimensional changes.
The evaluation of plastic products mainly includes three aspects. The first is the appearance quality, including the integrity, color, and gloss; the second is the accuracy between the size and the relative position; the third is the mechanical and chemical properties corresponding to the purpose. Electrical properties. These quality requirements, in turn, vary depending on the use of the product.
The production practice proves that the defects of the products mainly lie in the design, manufacturing precision and wear degree of the molds. In fact, technicians in plastic processing plants often suffer from the difficulty of using technical means to make up for the problems caused by mold defects and have little effect.
The adjustment of the process in the production process is a necessary way to improve the quality and yield of the product. Because the injection cycle itself is very short, if the process conditions are not well mastered, waste products will continue to flow. When adjusting the craft, it is best to change only one condition at a time and observe several times at a time. If pressure, temperature, and time are unified and adjusted, it is easy to cause confusion and misunderstanding. If problems arise, you do not know what the reason is. The measures and means for adjusting the process are various. For example, there are more than a dozen possible solutions to solve the problem of unsatisfactory products. To solve the problem, one or two major solutions must be selected to solve the problem. In addition, we should also pay attention to the dialectical relationship in the solution. For example: The product has sunken, sometimes to raise the material temperature, sometimes to reduce the material temperature; sometimes to increase the amount of material, sometimes to reduce the amount of material. It is necessary to admit the feasibility of solving the problem with reverse measures.
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