How to control the molding pressure during the custom plastic injection molding process?
Whether it is a hydraulic or electric injection molding machine, all movement during the injection process creates pressure. Appropriate control of the required pressure can produce a finished product of reasonable quality. Pressure regulation and metering system On hydraulic injection molding machines, all movements are performed by the oil circuit responsible for the following operations:
1． Screw rotation in the plasticizing stage.
2． slide seat channel (note nozzle close to the nozzle bushing).
3．Axis movement of the shot screw during injection and holding.
4． Close the substrate to the plunger until the toggle is fully extended or the piston clamping stroke is complete.
5. Start the top of the assembly ejector to eject the components.
On a full voltage machine, all motion is performed by a brushless synchronous motor with permanent magnets. The rotary motion is converted to linear motion by the ball bearing screw that has been used in the machine tool industry. The efficiency of the entire process depends in part on the plasticization process, where the screw plays a key role.
The screw must ensure that the material is melted and homogenized. This process can be adjusted with back pressure to avoid overheating. The mixing element does not produce excessive flow rates which would otherwise cause degradation of the polymer. Each polymer has a different maximum flow rate, and if it exceeds this limit, the molecules will stretch and the polymer backbone breaks. However, the focus remains on controlling the forward axial movement of the screw during injection and holding.
Subsequent cooling processes, including intrinsic stress, tolerances, and warpage, are important to ensure product quality. This is all determined by the quality of the mold, especially when optimizing the cooling channels and ensuring effective closed-loop temperature regulation. The system is completely independent and does not interfere with mechanical adjustments. Mold movements such as closed mold and ejection must be accurate and efficient. A velocity profile is usually used to ensure that the moving parts are in close proximity.
Contact maintenance can be adjusted. Therefore, it can be concluded that the product quality is mainly determined by the system that controls the forward movement phase of the screw without considering the energy consumption and mechanical reliability, and the same additional conditions (such as mold quality). On hydraulic injection molding machines, this adjustment is achieved by detecting oil pressure. Specifically, the oil pressure activates a set of valves through the control plate, and the fluid acts through the manipulator and is regulated and released.
Injection speed control includes options such as open loop control, semi-closed loop control, and closed loop control. The open loop system relies on a shared proportional valve. The proportional tension is applied to the desired proportion of fluid so that the fluid creates pressure in the injection barrel, allowing the injection screw to move at a certain forward speed. The semi-closed loop system uses a closed loop proportional valve. The loop is closed at the position where the closed port is located, and the closed port controls the flow ratio of the oil by movement within the valve. The closed loop system closes at the screw translation speed.
A speed sensor (usually a potentiometer type) is used in the closed loop system to periodically detect the drop in tension. The oil flowing out of the proportional valve can be adjusted to compensate for the speed deviation that occurs. Closed loop control relies on dedicated electronics integrated into the machine. Closed-loop pressure control ensures uniform pressure during the injection and hold-up phases and ensures uniform back pressure in each cycle.
The proportional valve is adjusted by the detected pressure value, and the deviation compensation is performed 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 the proportional valve by reading the nozzle or cavity pressure readings. Increasing temperature detection based on pressure detection is particularly beneficial for process management.
Knowing the actual pressure the material can withstand also helps predict the actual weight and size of the molded part based on set pressure and temperature conditions. In fact, by changing the holding pressure value, more material can be introduced into the cavity to reduce component shrinkage, in line with design tolerances (including preset injection shrinkage). Semi-crystalline polymers show great specific volume changes near melting conditions. In this regard, overfilling does not prevent the component from ejecting.
Hydraulic equipment and discharge volume and pressure regulation
The centrifugal pump produces an average hydraulic pressure of up to 140 bar, which is particularly suitable for injection molding. At other stages of the cycle, the requirements are significantly lower, except for the specific case where rapid plasticization is required (eg, PET injection-blow one-step injection molding machines).
In order to reduce energy consumption, variable displacement pumps and pressure storage cylinders can be used during peak discharge periods. The fixed displacement pump moves an equal amount of oil per revolution, so the selection of the pump depends on the amount of oil that needs to be moved during a specific time. Three-phase motor speed is generally 1440 rev / min, usually requires the installation of dual pumps. Only during the plasticization process (power up to 100%), the utilization of the oil pump is maximized. During the pause process, the machine does not require energy consumption, and even if it is needed, it is a power loss.
All injection molding machines use proportional valve valves of various quality grades. Two or more proportional valves are installed on the injection press to accurately control the following aspects:
Opening speed (two stages), mold closing speed (two stages), closed mold safety, injection (grades 3-10), feeding (level 3-5), suction and ejector (two stages).
Open mold pressure, mold closing pressure, mold safety, mechanical fixture (cylinder or toggle), injection (once filling stage, subsequent stages 3-10 times), suction and back pressure (grade 3-5). Screw rotation speed (3-5 levels).
The speed of the slider close to the speed (the speed at which the mechanical nozzle is close to the injection pad on the mold fixing mold half) and the speed of the ejector (the ejection speed) can also be adjusted. The auxiliary motor sends the amplified signal (output signal) to the valve through a weak input signal, causing the servo valve to perform the adjustment function.
In the servovalve, the weak input electrical signal is converted to a hydraulic output signal, which is modified in the form of pressure drop according to the required discharge requirements. The valve must respond quickly, reproducibly, and with low hysteresis to the tension or general purpose commands. In fact, the purpose of the current study is to improve the frequency response, allowing dialogue between power equipment (hydraulic sides) operating at frequencies of several kilohertz (kHz) and electronic equipment.
Since effective discharge depends on the degree of polymerization (DP) on the valve, the oil temperature in the hydraulic circuit must be maintained in the range of 45-55 ° C (usually using a closed-loop adjustment system), depending on the viscosity of the fluid and the transition port. Depending on the geometry. There is no proper adjustment system in the valve, and the temperature rise will cause the viscosity of the solution to decrease; if it is equipped with a balanced opening value, the discharge amount can be increased.
Increasing the amount of oil discharged from the transmission system means that the injection speed is accelerated. Precise control of high-tech servo-actuated valves virtually eliminates hysteresis and enhances the repeatability of all functions.
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