Whether hydraulic or electrically trained, all movements during the injection molding process generate pressure. Proper control of the required pressure can produce a finished product of reasonable quality.
Pressure regulation and metering systems
On hydraulic injection molding machines, all movements are performed by the oil circuit responsible for:
1. Screw rotation in the plasticizing stage.
2. Slide forehearth (nozzle close to injection bushing).
3. The axial movement of the injection screw during injection and packing.
4. Close the substrate on the injection rod until the toggle rod is fully extended or the piston clamping stroke is completed.
5. Start the ejector platform that assembles the ejector bar to eject the parts.
On a full voltage machine, all movements are performed by a brushless synchronous motor equipped with a permanent magnet. Ball bearing screws, which have always been used in the machine tool industry, transform rotary motion into linear motion. The efficiency of the entire process depends partly on the plasticizing process, in which the screw plays a crucial role. Mitsubishi's latest solution for the production of all-electric models consists of a filling screw (second threaded holding roller) and a screw tip with mixing elements. In this way, plasticizing capacity and mixing effect can be maximized, screw length can be shortened, and high-speed operation can be achieved.
The screw must ensure that the material is melted and homogenized. This process can be adjusted with the help of backpressure to avoid overheating. The mixing elements must not produce excessively high flow rates, which would lead to polymer degradation. Each polymer has a different maximum flow rate, and if this limit is exceeded, the molecules stretch and the polymer backbone breaks. However, the focus is still on controlling the forward axial movement of the screw during injection and packing. The subsequent cooling process, including internal stress, tolerances and warpage, is important to ensure product quality. This is all determined by the quality of the mold, especially when it comes to optimizing the cooling forehearth and ensuring effective closed-loop temperature regulation. The system is completely self-contained and does not interfere with mechanical adjustment. Mold movements such as mold closure and ejection must be precise and efficient. Velocity distribution curves are often used to ensure that moving parts are accurately approached. Contact retention is adjustable. Therefore, it can be concluded that the quality of the product is mainly determined by the system that controls the forward movement phase of the screw, without considering energy consumption and mechanical reliability, and the additional conditions are the same (such as mold quality). On hydraulic injection molding machines, this adjustment is achieved by detecting the oil pressure. Specifically, the oil pressure is activated by the control panel and a set of valves is activated, and the fluid is activated by 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. Open loop systems rely on shared proportional valves. The proportional tension is applied to the required proportion of the fluid, so that the fluid creates pressure in the injection barrel and allows the injection screw to move at a certain forward speed. The semi-closed-loop system adopts a closed-loop proportional valve. The loop closes where the closing port is located, which controls the proportion of oil flow through movement within the valve. The closed-loop system closes at the screw translation speed. Speed sensors (usually potentiometer types) are used in closed-loop systems to periodically detect tension drops. The oil flowing out of the proportional valve is adjusted to compensate for the resulting speed deviation. Closed-loop control relies on dedicated electronics integrated into the machine. Closed-loop pressure control ensures uniform pressure during the injection and packing phases, as well as uniform backpressure throughout cycles. The proportional valve is regulated by the detected pressure value, and deviations are compensated according to the set pressure value. In general, the hydraulic pressure can be monitored, but detecting the melt pressure in the nozzle or mold cavity is another effective method. A more reliable solution is to manage proportional valves by reading nozzle or cavity pressure readings. The addition of temperature detection to pressure detection is particularly beneficial for process management. Knowing the actual pressure that the material can withstand also helps predict the actual weight and dimensions 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 part shrinkage and meet design tolerances, including preset injection shrinkage. As the melting conditions approach, semicrystalline polymers show a great change in specific volume. For this, overcharging does not prevent the ejection of the part.
Hydraulic equipment and discharge volume and pressure regulation
The centrifugal pump produces an average hydraulic pressure of 140 bar, which is particularly suitable for injection molding. At all other stages of the cycle, the requirements are significantly lower, except in specific cases where rapid plasticization is required (e.g. PET injection molding machines).
To reduce energy consumption, variable displacement pumps and pressure reservoirs can be used during peak discharge periods. Fixed-displacement pumps move the same amount of oil with each rotation, so the selection of an oil pump is determined by the amount of oil that needs to be moved at a given time. The speed of the three-phase motor is generally 1440 rpm, and it is usually required to be equipped with a double pump. Only during the plasticizing process (up to 100% power) can the oil pump be used to the maximum. During standstill, the machinery does not need energy consumption, and even if it does, it is a power loss.
All injection molding machines use proportional servo valves of different quality grades. Two or more sets of proportional valves are installed on injection presses for accurate control of:
Mold opening speed (two stages), mold closing speed (two stages), mold closing safety, injection (3-10 stages), dosing (3-5 stages), suction and ejector rod (two stages).
Mold opening pressure, mold closing pressure, mold safety, mechanical fixture (barrel or elbow), injection (once in the filling stage, 3-10 times in the subsequent stage), suction and back pressure (3-5 stages). Screw rotation speed (3-5 steps).
The speed of the slide proximity (the speed at which the mechanical nozzle is close to the injection pad on the mold fixing half) and the speed of movement of the ejector bar (ejector speed) can also be adjusted. The auxiliary motor sends the amplified signal (output signal) to the valve through a weak input signal, so that the servo valve performs the adjustment function. In servo valves, weak input electrical signals are converted into hydraulic output signals, which are improved in the form of pressure drops according to the required discharge requirements. Valves must respond quickly, repeatably and with low hysteresis to tension or general commands. In fact , the aim of the current study is to improve the frequency response , allowing a dialogue between power equipment ( hydraulic side ) and electronics operating at frequencies of several kilohertz ( kHz ) . Since the effective discharge depends on the degree of polymerization (DP) on the valve, the oil temperature in the hydraulic line must be kept within the range of 45-55°C (usually using a closed-loop adjustment system), depending on the fluid viscosity and the geometry of the transition. Without a proper regulation system inside the valve, a rise in temperature will lead to a decrease in the viscosity of the melt; If a balanced opening threshold is equiponsible, the discharge volume can be increased. Increasing the amount of oil discharged from the drivetrain means that the injection speed is increased. Precise control of high-tech servo drive valves virtually eliminates hysteresis and enhances repeatability of all functions.
Force determination of all-electric presses
Since there is no vector fluid that causes motion on an all-electric injection molding machine, hydraulic pressure detection is not possible. Therefore, load cells are usually used, and elastic deformation is measured with a telescopic meter to directly determine its strength. The manufacturer of all-electric injection molding machines has developed a variety of elastic parts and equipped them with corresponding telemeters. Another difference lies in the back pressure and its control, which can be achieved by increasing resistance to the axial movement generated by the injection motor, while the other motor causes the screw to rotate and the subsequent material to plasticize. Previously, some machine manufacturers used measurement systems mounted in injector transducers, but abandoned them due to "insufficient functionality and reliability".
Advantages of nozzle pressure measurement
The importance of pressure regulation in the injection and packing process has been demonstrated above. Therefore, the accuracy and repeatability of pressure detection are critical factors. In closed-loop systems, pressure detection is important and only by ensuring accurate pressure detection can the regulator bring the actual pressure close to or equal to the set value. In open-loop systems, the accuracy and repeatability of pressure detection is even more important because it is directly connected to the drivetrain. Today, open-loop systems are still in use and are more widely used in high-tonnage models. In general, setpoint-based velocity control is performed during injection (i.e., the velocity change is measured by a potentiometer or magnetoshrinkage sensor), which is converted to pressure regulation. Pathways can be activated according to quotas (quota pathways) or pressures. In any case, a pressure-actuation path must be used when the pressure actuation path also acts as a "cut-off" to limit the filling pressure, prevent overflow formation, and damage to the mold. Once the path is formed, the subsequent pressure holding process is regulated by pressure (profiles are no exception). The pressure of the hydraulic press is generally detected in the hydraulic line and rarely in the mold nozzle. For injection molding , the probe point must be as close as possible to the cavity. Therefore, the mold pressure measurement is best performed at the nozzle, even if it is not too direct, it can be carried out within the hydraulic line.
Unlike mold pressure detection, in-nozzle detection also controls the plasticizing process by adjusting the back pressure. Mold pressure detection enables conversion when the pressure close to the injection actually reaches the set value and is maintained for the time required for the material to be molded. The determination can be performed directly or via a probe (e.g. piezoelectric sensors). Direct probing inside the mold is very effective, the only limitation is that it can leave marks on the molded part. Indirect detection is often affected by probe structure and clearance, for example, excessive tolerances can cause material to pour, resulting in insufficient detection accuracy.
Nozzle pressure detection is less effective than cavity pressure detection because the material still has to go through a flow line (cold or hot). However, nozzle pressure detection has certain advantages, mainly including: detection on the material; No mold modification is required; No marks are left on the molded parts. By controlling the melt pressure, preferably in the mold cavity, the risk of overfilling (and subsequent overfill) at the initial pressure is avoided. This increases the effectiveness of control, avoids material burning, prevents underfilling, shortens cycle times and enhances repeatability.
There are some technical problems with producing sensors that ensure system reliability and are easy to use.
If uniform adjustment of the back pressure is required, the process-related difficulties are indeed not small.
The sensors used for nozzle pressure detection must meet the following requirements:
▲Do not interfere with the molding process.
▲It can ensure detection accuracy under high pressure (2500 bar) and high temperature (350-400 °C).
▲Small size, solid structure, easy to replace in case of failure.
▲When in contact with mold filling material, it has excellent wear resistance.
▲ It can ensure the effectiveness of detection for a long time (when friction and pollution occur after long-term use, it can ensure that the measurement is error-free, error-free and hysteresis).
Provides high-speed sampling (2-5 microseconds) and standardized communication protocols such as CAN Open version CANbus or DeviceNet.
Therefore, the problem is more complex. It is not difficult to understand that until now, hydraulic presses still have sensors in hydraulic circuits, and full electric motors use force detection, neither of which uses melt sensors. For many years, melt sensors have been widely used in extruders, but extruders have low requirements in terms of detection range, accuracy, response time and structural robustness (the mechanical fatigue stress on the sensor film is much greater when mounted on an injection machine than the static stress on the extruder).
