Whether it is hydraulic or electric injection molding machine, all the movements in the injection process will produce pressure. Only by properly controlling the required pressure can we produce finished products with reasonable quality. The pressure control and metering system is on the hydraulic injection molding machine, and all movements are performed by the oil circuit responsible for the following operations:
1. Screw rotation in the plasticizing stage.
2. Slide material way (injection nozzle is close to the injection sleeve).
3. Axial movement of the injection screw during injection and holding.
4. Close the base material on the ejector rod until the elbow rod is fully extended or the piston clamping stroke is completed.
5. Start the jacking of the assembly ejector to eject the components.
On all electric presses, all movements are performed by a brush less synchronous motor equipped with a permanent magnet. Through the ball bearing screw which has been used in machine tool industry, the rotary motion is transformed into linear motion. The efficiency of the whole process depends in part on the plasticizing process, in which the screw plays a key role.
The screw must ensure that the material is melted and homogenized. This process can be adjusted by means of back pressure to avoid overheating. The mixing element shall not produce too high flow rate, otherwise, polymer degradation will be caused. Each polymer has a different Zui flow rate. If the limit is exceeded, the molecule will stretch and the main chain of the polymer will break. However, the emphasis is still on controlling the forward axial movement of the screw during injection and pressure maintaining.
The subsequent cooling process, including internal stress, tolerance and warpage, is very important to ensure product quality. All of this is determined by the quality of the mold, especially when optimizing the cooling channel and ensuring effective closed-loop temperature regulation. The system is completely independent and does not interfere with mechanical regulation. Die movement such as die closing and ejection must be accurate and efficient. The velocity distribution curve is usually used to ensure the moving parts are close to each other accurately.
The contact holding force can be adjusted. Therefore, it can be concluded that without considering the same energy consumption, mechanical reliability and additional conditions (such as die quality), the product quality mainly depends on the system of controlling the forward movement stage of the screw. In the hydraulic injection molding machine, this regulation is realized by detecting the oil pressure. Specifically, the oil pressure activates a set of valves through the control panel, and the fluid acts through the manipulator, and is regulated and released.
Injection speed control includes open-loop control, semi closed-loop control and closed-loop control. The open loop system relies on a common proportional valve. The proportional tension is applied to the required proportion of the fluid so that the fluid produces pressure in the syringe barrel and the injection screw moves at a certain forward speed. The closed-loop proportional valve is used in the half closed-loop system. The loop is closed at the position where the closing port is located, and the closing port controls the flow proportion of oil through the movement in the valve. The closed-loop system is closed at the translational speed of the screw.
The speed sensor (usually potentiometer type) is used in the closed-loop system to detect the tension drop at a fixed time. The oil from the proportional valve can be adjusted to compensate for the speed deviation. Closed loop control relies on special electronic components integrated with the machine. The closed-loop pressure control can ensure that the pressure is uniform in the injection and holding stages, and that the back pressure is uniform in each cycle.
By comparing the detected pressure value with the example valve, the deviation compensation is made according to the set pressure value. In general, hydraulic pressure can be monitored, but detecting melt pressure in the nozzle or cavity is another effective method. A more reliable solution is to manage by reading the injection nozzle or cavity pressure readings compared with the example valve. Temperature detection is added on the basis of pressure detection, which is particularly beneficial to process management.
It is also helpful to predict the actual weight and size of the molded parts according to the set pressure and temperature conditions. In fact, by changing the holding pressure value, more materials can be introduced into the mold cavity to reduce the part shrinkage and meet the design tolerance (including preset injection shrinkage). Near the melting condition, the semi crystalline polymer shows a great change in specific volume. In this regard, overcharge will not hinder the ejection of components.
Oil pressure equipment and discharge and pressure regulation
Centrifugal pumps produce an average hydraulic pressure of 140 bar, which is particularly suitable for injection molding. At other stages of the cycle, the requirements are significantly lower, except for specific situations (such as pet injection drawing one-step injection molding machine) that require rapid plasticization.
In order to reduce energy consumption, variable displacement pump and pressure storage cylinder can be used in the peak discharge period. The fixed displacement pump moves the same amount of oil in each rotation, so the selection of oil pump depends on the amount of oil to be moved in a specific time. The speed of three-phase motor is generally 1440 rpm, and it is usually required to install two pumps. Only in the plasticizing process (power up to 100%), the utilization rate of the oil pump can reach the Zui limit. In the process of standstill, machinery does not need energy consumption, even if it is needed, it is also power loss.
All injection molding machines adopt proportional servo valves with different quality levels. Two or more proportional valves are installed on the injection press to accurately control the following aspects:
Opening speed (two levels), closing speed (two levels), closing safety, injection (3-10 levels), feeding (3-5 levels), suction and ejector (two levels).
Mold opening pressure, mold closing pressure, mold safety, mechanical fixture (barrel or elbow bar), injection (once in mold filling stage, 3-10 times in subsequent stages), suction and back pressure (3-5 levels). Screw rotation speed (level 3-5).
The speed of sliding seat approaching (the mechanical injection nozzle is close to the injection liner on the fixed half mold