The requirements for injection molded products gradually move toward high precision, ultra-micro or ultra-large, ultra-thin and ultra-thin, and special environments require parts, etc. The appearance and functional requirements of injection molded products are also increasingly demanding and refined. Standard injection molding The share of conventional production models such as machines and universal injection molding machines continues to shrink, and the proportion of customized injection molding machines is also increasing. At the same time, the skill requirements of injection molding practitioners are increasingly subdivided and specialized. This article is the most important in injection molding. The plastic filling mechanism is analyzed, and it is hoped that injection molding engineers can help injection molding process.
Speed and pressure during plastic filling
The injection molding machine sends the melted plastic melt from the front end of the injection molding machine to the mold cavity, which is a filling process in the injection molding process. In the injection molding machine operation panel, the instructions given to the injection molding machine are as follows:
This is an injection molding process for thin-walled products: Injection pressure 2750 kgf/cm2, injection time 3 seconds, 5 speeds 15, 45, 85, 190, 20 mm/s, switching speeds by position. Injection molding machine above, injection pressure and injection speed is also divided into many sections of control, each section of the speed corresponds to a section of pressure, this is because domestic injection molding machine is generally open-loop control, many times the set speed conversion can not be performed by the machine, Therefore, pressure control is given separately for each stage of speed, which allows the machine to perform each stage of speed.
In fact, many practitioners are not very familiar with the relationship between injection speed and injection pressure, so that pressure and speed are two concepts. In fact, the real situation can be understood as this: A theoretically complete and flawless injection molding machine , Can use its 100% pressure for injection molding work, injection molding engineers can not do any injection pressure adjustment, even on the injection molding machine operation panel does not need to list this parameter, as long as the injection speed control accuracy is correct. Injection pressure is a necessary condition for the injection speed. It is the mission of injection pressure to make the injection speed accurate and accurate. When the injection pressure is not available, the injection speed cannot be performed regardless of the setting. On the other hand, the injection molding machine is considered. , 'Injection' This action is the oil valve to the oil cylinder, push the cylinder piston forward to promote the forward movement of the injection molding machine screw to achieve the purpose of injection. When the injection speed increases, more hydraulic oil per unit time will need to enter the cylinder Push the screw forward to reach the goal.
In fact, in the injection molding process, the amount of plastic filled on the same road segment at a low speed injection molding and high speed injection is quite different during a certain distance of the movement of the injection screw. For example, the injection speed is 10 mm/s. The possible injection volume is 10 g; the injection speed is 60 mm/s, and the injection volume is 35 g. This is the viscosity of the plastic after melting, the temperature of the mold during injection, the structure of the injection runner, the temperature of the plastic during injection and There is a close relationship between the screw of the injection molding machine and the filling gap of the barrel. The injection speed is small, and the melting material loses more in the barrel. The energy loss through the nozzle, the flow path, the mold inlet, and the wall thickness of the product is large, so the injection volume Also correspondingly smaller.
Injection speed conversion
During the injection molding process of the injection molding machine, the screw pushes the plastic melt forward to the mold to complete the injection process. The size change of the injection speed is to meet the following requirements:
1, faster injection of molten material into the mold to prevent the problem of insufficient filling;
2. The intensity and appearance adjustment of the strands at the stream fusion point;
3, to prevent gas lines at the glue point;
4, the product thin wall to prevent the melt broken, to avoid the generation of silver lines or material flow pattern;
5, trapped;
6, mold sub-surface or some weak position Pi Feng.
Changes in the injection speed also mean changes in the injection volume of the melt at the same distance, which makes the injection process more complicated. Even if the temperature of the mold during the injection is different, the results from the injection molding process are completely different. The melt is in the mold. Keep the same distance and speed, mold temperature is high and mold temperature is low, the injection volume is completely different: the same injection speed, low mold temperature, rapid melt cooling, solidified layer thickness, injection volume will become smaller; mold temperature is high, solidified layer Thin, the injection volume will become larger. This makes the injection molding machine instructions executed, the real situation of the product injection molding must be tested in order to truly grasp.
We know that the injection speed is generated by injection molding on the injection molding machine to push the screw forward. If we set a certain injection speed to 50 mm/s (most injection molding machines show the injection speed as a percentage, the principle Same time) How much time and distance does it take for an injection molding machine from 0 mm/s to 50 mm/s, and how much time is needed when the injection molding machine goes to a position from 50 mm/s to 0 mm/s? The distance? This is a very important parameter in the injection molding machine design. The slow response is equivalent to the injection speed not being implemented. This speed is meaningless, especially in high-speed injection molding machines. The speed of response directly determines whether the injection can be produced. qualified products.
The injection molding machine's execution speed command response is fast enough. As shown above, the 190 mm/s can be performed perfectly within 20 milliseconds. Then a command speed after 190 mm/s becomes 20 mm/s. Many injection molding machines cannot perform 20 mm at all. /s. Because the inertia generated by the operation of the 190 mm/s instruction directly covers the 20 mm/s command, the 20 mm/s command is not executed at all, that is, the injection molding engineer feels that the parameters are not adjusted. In order to make the 20 mm/s instruction executable, it is necessary to wait until the inertia generated by the 190 mm/s instruction is completed before it is completed. This is the same as the principle of braking when the car is moving forward at high speed. Stopping the car in a preparatory position requires starting the brakes before it is useful.
In the injection molding machine design, the closed-loop control system is designed to allow the 190 mm/s instruction to perform better, but it is not possible for the closed-loop control system to perfectly execute the 190 mm/s instruction. In fact, most injection molding machines have no closed loop at all. In the control system, many injection molding machines designed by injection molding engineers can't execute the injection molding machine. Therefore, precision injection molding cannot produce products with precise dimensions.
When the injection molding engineer designs the injection molding process, the injection speed is achieved in an injection molding machine with sufficiently fast response time depending on the injection pressure. A 50 mm/s speed parameter, the actual pressure displayed by the injection molding machine during execution is already reached. Injection molding engineers set the pressure, which is mostly not implemented. The actual injection speed in the injection molding process is the highest speed that can be achieved under the injection pressure, not the engineer's designed speed. It is the position where the injection machine's screw position inertia speed is lower than the later speed. The above 190 mm/s was originally performed to the 12 mm position, but because of the 190 mm/s speed inertia, the rear 20 mm/s is not executed at 12 mm. , Closed-loop control system may make the screw speed below 20 mm/s at 11.5 mm, where the injection molding machine begins to perform 20 mm/s. Without closed-loop control, the injection molding machine may directly punch 11 mm, and 20 mm /s This speed is completely unimplemented.
In order to make the injection molding process consistent with the instructions, when engineers design the injection molding process, the speed must be guaranteed by the injection pressure. The position where the instruction is executed must be controlled by the process. Such a process may be stable.
Injection molding process pressure switching point selection
Injection molding changes from filling mold to holding pressure, which is called VP switching. It is speed-to-pressure switching. Generally, VP switching is performed when the product filling amount reaches 95-98%. Switching leads to product lack of glue prematurely, switching too late leads to product Fu Feng, internal stress is large. There are several ways to switch:
1. Time switch: Set an injection time, once the time is up, immediately maintain the pressure. This method is generally used for high speed and high pressure injection, the screw position is difficult to accurately stop, the products are very thin, very short time (2 seconds) Inside) can project material into the mold cavity;
2, pressure switching: Pressure switching cavity pressure switch, nozzle pressure switching and system pressure switching mode, mold cavity pressure switching and nozzle pressure switching must be installed in the corresponding position pressure sensor can be achieved truly high precision Pressure switching is cavity pressure switching, the disadvantage is higher cost, each set of molds must be made a set of sensors;
3, position switch: The screw position is used as the basis for injection V-P switching. This is also the most common method for injection molding production at present. The cost is low and accurate, but the accuracy is not high enough compared with cavity pressure switching.
At the actual injection molding production site, V-P switching is often not rigorous, as shown in the following figure:
The setting of this parameter is the position switching method, but the setting of the reel-pressing position is 0 mm. When the injection molding machine is executing the process, the instruction can only be executed for one shot or two shots. The injection time is up and the process is no longer carried out. It is just a VP switch completed by time switching (some injection molding machine programs will no longer alarm). In the case of strict product requirements, the non-strict injection molding process must be unstable and the product qualification rate is not high .