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Filling process analysis in injection molding

The requirements for injection molded products gradually move towards high-precision, ultra-micro or ultra-thin, ultra-thin and ultra-thin and special environmental requirements, and the appearance and functional requirements of injection-molded products are increasingly demanding and refined. Standard injection molding The share of traditional 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 being subdivided and specialized. This article analyzes the most important plastic filling mechanism in injection molding and hopes to help injection molding engineers develop injection molding processes.

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:

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This is an injection molding process for a thin-walled product: the injection pressure is 2750 kgf/cm2, the injection time is 3 seconds, the 5-step speed is 15, 45, 85, 190, 20 mm/s, and the speed is switched by position. In the domestic injection molding machine, the injection pressure is also controlled in many stages as well as the injection speed. Each stage speed corresponds to a certain pressure. This is because the domestic injection molding machines are generally open-loop control, and many times the set speed conversion does not reach the machine. Execution, so that each segment is given pressure control individually, allowing the machine to execute every segment of speed.

In fact, many practitioners are not very familiar with the relationship between injection speed and injection pressure, thinking that pressure and speed are two concepts. In actual fact, the real situation can be understood as this: a theoretically completely flawless injection molding machine can use its 100% pressure to do injection work, and injection molding engineers can absolutely not make any adjustments in injection pressure, even in injection molding. The machine operator panel does not need to list this parameter, as long as the injection speed control is accurate. Injection pressure is a necessary condition for injection speed. The ability to accurately perform the injection speed is the mission of injection pressure. When injection pressure is not available, the injection speed cannot be set regardless of the setting. On the other hand, from the perspective of the injection molding machine, the "injection molding" action is the oil valve to the oil cylinder, pushing the cylinder piston forward to promote the forward movement of the injection molding machine screw to achieve the purpose of injection molding. As the injection speed increases, more hydraulic oil is required per unit time to enter the cylinder and push the screw forward to achieve the goal.

Injection speed conversion

Injection molding machine in the process of injection molding, the screw forward to push the plastic melt to the mold to complete the injection process, the size of the injection speed changes in order 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 melt fracture, to avoid the silver lines or material flow pattern;

5, trapped air;

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 produced during the injection process are completely different. The molten material maintains the same distance and speed in the mold. The mold temperature is high and the mold temperature is low. The injection volume is completely different: the same injection speed, low mold temperature, rapid cooling of the melt, and thicker solidified layer, the injection volume will become smaller; The temperature is high, the solidified layer is thin, and the injection volume will increase. This makes the actual situation of the injection molding machine after the execution of the injection molding machine must be tested in order to truly grasp.

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We know that the production of injection speed is to inject oil into the injection molding machine and push the screw forward to complete the action. If we set a certain injection speed to 50 mm/s (most injection molding machines show the injection speed as a percentage, the principle is the same), how long does it take for the injection molding machine to go from 0 mm/s to 50 mm/s? How much time and distance does it take for the injection molding machine to change from a speed of 50 mm/s to a speed of 0 mm/s? This is a very important parameter in the injection molding machine design. The slow response is equivalent to that the injection speed is not implemented properly. Speed is meaningless, especially on high-speed injection molding machines. The speed of response directly determines whether injection molding can produce qualified products.

The injection molding machine's execution speed command response is fast enough. As shown above, the 190 mm/s can be perfectly executed within 20 milliseconds. After that, the command speed after the 190 mm/s becomes 20 mm/s. Many injection molding machines cannot perform 20 mm at all. /s. Because the inertia caused by the 190 mm/s command operation 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 parameter is useless. Can not control. To make this 20 mm/s instruction executable, it is necessary to wait until the inertia generated by the 190 mm/s instruction is completed. This is similar to the principle of braking when the car is moving at high speed. To stop the car in a preparatory position, it is necessary to start braking beforehand.

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 perform the 190 mm/s instruction perfectly. In fact, most injection molding machines do not have a closed-loop control system at all, and many injection molding machines designed by injection molding engineers cannot execute the injection molding machines. Therefore, precision injection molding products cannot produce precise-sized products.

When an injection molding engineer designs an injection molding process, the injection speed is dependent on sufficient injection pressure in an injection molding machine with a sufficiently fast response time. A 50 mm/s speed parameter, when executed, the actual pressure displayed by the injection molding machine has reached the pressure set by the injection molding engineer. This speed is mostly not achieved. The actual injection speed in the injection molding process is the same under this injection pressure. The highest speed achieved, not the engineer's design speed. The position where this speed is reached is the position where the inertia speed of the injection molding machine screw position is lower than the speed of the latter stage. The above 190 mm/s was originally performed to the 12 mm position, but due to the 190 mm/s speed inertia, the rear 20 mm/s is not implemented at 12 mm, and the closed-loop control system may make the screw speed below 20 mm at 11.5 mm. s, the injection molding machine began to perform 20 mm/s speeds from there. Without closed-loop control, the injection molding machine could directly overshoot 11 mm, and the speed of 20 mm/s could not be fully implemented.

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 instructions are executed must be controlled by the process. Such a process may be stable.

Injection molding process pressure switching point selection

Injection molding from filling to holding pressure switching, known as V-P switching, is speed-to-pressure switching. Generally, V-P switching is performed when the product filling volume reaches 95-98%. Switching too early leads to a lack of glue in the product, and switching too late results in a product with a sharp edge and high internal stress. There are several ways to switch:

1. Time switch: Set an injection time. Once the time is up, hold the pressure immediately. This method is generally used for high-speed high-pressure injection molding, the screw position is difficult to accurately stop, the products are very thin, requires a very short time (2 seconds) can material into the mold cavity;

2. Pressure switching: pressure switching includes cavity pressure switching, nozzle pressure switching, and system pressure switching. The cavity pressure switching and nozzle pressure switching must be installed in the corresponding position to realize the pressure sensor, truly high precision. Pressure switching is cavity pressure switching, the disadvantage is higher cost, each set of molds must make a set of sensors;

3, position switch: the screw position as the injection V-P switching basis, which is the most common method of injection molding production, low cost and more accurate, but compared with the cavity pressure switching, the accuracy is not high enough.

At the actual injection molding production site, V-P switching is often not rigorous, as shown in the following figure:

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The parameter setting is the position switching mode, but the setting of the rotary hold pressure position is 0 mm. When the injection molding machine is executing the process, the instruction can only be executed until one shot or two shots are fired. The injection time is reached 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 execute the 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.