Whether it is a hydraulic or electric injection molding machine, all movements during the injection molding process will generate pressure. Only by properly controlling the required pressure can a finished product of reasonable quality be produced.
Pressure control and metering system
On a hydraulic injection molding machine, all movements are performed by the oil circuit responsible for the following operations:
1. Screw rotation during the plasticization phase (back pressure can be determined and even controlled).
2. Slide channel (nozzle close to the sprue bushing).
3. Axial movement of the injection screw during injection and holding pressure.
4. Closing the substrate on the injection rod until the elbow is fully extended or the piston mold closing stroke is completed
5. Start the ejector table equipped with the ejector pin to eject the part.
On a full-electric machine, all movements are performed by a brushless synchronous motor with permanent magnets. The rotational movement is converted into linear movement by a ball bearing screw that has always been used in the machine tool industry. The effect of the entire process depends in part on the plasticization process, in which the screw plays a very critical role. Mitsubishi's latest solution for the production of all-electric models consists of a filling screw (two-flight roller) and a screw tip with mixing elements. This maximizes plasticizing capacity and mixing, shortens screw length and allows high speeds.
The screw must ensure melting and homogenization of the material. This process can be adjusted with the help of back pressure to avoid overheating. The mixing elements must not generate too high a flow rate, otherwise the polymer will degrade. Each polymer has a different maximum flow rate, if this limit is exceeded, the molecules will stretch and the polymer backbone will break. However, the focus remains on controlling the forward axial movement of the screw during injection and holding pressure.
The subsequent cooling process, including inherent stresses, tolerances and warpage, is very important to ensure product quality. This is all determined by the quality of the mold, especially when optimizing the cooling channel to ensure effective closed-loop temperature regulation. The system is completely independent and does not interfere with mechanical adjustments.
The mold movements such as closing and ejection must be precise and efficient. Usually a velocity profile is used to ensure the accurate approach of the moving parts. The contact maintenance force can be adjusted. It can be concluded that, without considering energy consumption and mechanical reliability, and under the premise of the same additional conditions (such as mold quality), the product quality is mainly determined by the system that controls the forward movement phase of the screw. On hydraulic injection molding machines, this regulation is achieved by detecting the pressure. Specifically, the oil pressure activates a set of valves through the control board, and the fluid acts through the manipulator, is regulated and released.
Injection speed control includes open-loop control, closed-loop control and closed-loop control options. Open-loop systems rely on shared proportional valves. Proportional tension is applied to the required proportion of fluid, which causes the fluid to generate pressure in the injection barrel and moves the injection screw at a certain forward speed.
Closed-loop systems use closed-loop proportional valves. The loop is closed at the location of the closing port, which controls the flow rate of oil by moving in the valve. The closed-loop system is closed at the translation speed of the screw. A speed sensor (usually a potentiometer type) is used in the closed-loop system to detect the tension drop regularly. The material flowing out of the proportional valve can be adjusted to compensate for the speed deviation that occurs. Closed-loop control relies on dedicated electronic components integrated with the machine. Closed-loop pressure control ensures uniform pressure during injection and holding phases, as well as uniform back pressure in each cycle.
The proportional valve is adjusted by the detected pressure value to compensate for deviations from the set pressure value. Generally, the hydraulic pressure can be monitored, but another effective method is to detect the melt pressure in the nozzle or cavity. A more reliable solution is to manage the proportional valve by reading the nozzle or cavity pressure reading. Adding temperature detection to pressure detection is particularly beneficial for process management.
Knowing the actual pressure that the material can withstand also helps to predict the actual weight and size of the molded part based on the set pressure and temperature conditions. In fact, by changing the holding pressure value, more material can be introduced into the mold cavity to reduce part shrinkage and meet the design tolerance (including the preset injection shrinkage). Semi-crystalline polymers show a large specific volume change when approaching the melting condition. In this regard, overfilling the mold will not hinder part ejection.
Hydraulic equipment and discharge and pressure regulation
Centrifugal pumps generate an average hydraulic pressure of up to 140 bar, which is particularly suitable for injection molding. At all other stages of the cycle, the requirements are significantly lower, except for specific situations where rapid plasticization is required (e.g. PET injection stretch blow molding machines), where the requirements are higher.
To reduce energy consumption, variable displacement pumps and pressure storage cylinders can be used during peak discharge periods. Fixed displacement pumps move the same amount of oil per rotation, so the oil pump selection is determined by the amount of oil required to be moved in a specific time. The speed of a three-phase motor is generally 1440 rpm, and a double pump is usually required. Only during the plasticization process (power reaches 100%) is the utilization rate of the oil pump maximized. During the pause process, the machine 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 the injection press to accurately control the following aspects:
Mold opening speed (two levels), mold closing speed (two levels), mold closing safety, injection (3-10 levels), feeding (3-5 levels), suction and ejector (two levels).
Opening pressure, closing pressure, mold safety, mechanical clamp (barrel or elbow), injection (once in the filling phase, 3-10 times in the subsequent phases), suction and back pressure (3-5 levels), screw rotation speed (3-5 levels).
The slide approach speed (the speed at which the mechanical nozzle approaches the injection liner on the fixed half of the mold) and the movement speed of the ejector (ejector table speed) can also be adjusted. The auxiliary motor sends the amplified signal (output signal) to the valve through the weak input signal, allowing the servo valve to perform the regulation function.
In the servo valve, the weak input electrical signal is converted into a hydraulic output signal, which is modified according to the required discharge requirements in the form of a pressure drop. The valve must respond quickly, repeatably and with low hysteresis to the discharge of tension or general commands. In fact, the current research aims to improve the frequency response and enable the dialogue between the power equipment (hydraulic side) and the electronic equipment running at a frequency of several kHz.
Since effective discharge depends on the effect of 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 with a closed-loop regulation system), depending on the fluid viscosity and the geometry of the transition port. Without a proper regulation system in the valve, the temperature rise leads to a decrease in the melt viscosity; with a balanced opening value, the discharge volume can be increased. Increasing the discharge oil volume of the drive system means that the injection speed is increased. Precise control of high-tech servo drive valves can basically eliminate hysteresis and enhance the repeatability of all functions.
Force measurement on all-electric presses
Since there is no vector fluid to induce movement on all-electric injection molding machines, hydraulic pressure detection is not possible. Therefore, load sensors are usually used to measure the elastic deformation with an extensometer to directly determine its force. All-electric injection molding machine manufacturers have developed a variety of elastic components and equipped with corresponding extensometers. Another difference is the back pressure and its control, which can be achieved by adding resistance to the axial movement generated by the injection motor, while the other motor causes the screw rotation and subsequent material plasticization. Previously, some machine manufacturers used a measuring system with a transducer installed in the nozzle, but later abandoned this system due to "lack of functionality and reliability".
Advantages of nozzle pressure measurement
The above has demonstrated the importance of pressure regulation during injection and holding pressure. Therefore, the accuracy and repeatability of pressure detection are critical factors. In closed-loop systems, pressure detection is very important, only by ensuring accurate pressure detection can the regulator make the actual pressure close to or equal to the set value.
In open-loop systems, due to the direct connection to the transmission system, the accuracy and repeatability of pressure detection are even more important. Today, open-loop systems are still in use and are more widely used in high-tonnage machines.
Generally speaking, speed control based on the set value is carried out during the injection process (that is, the speed change is measured by a potentiometer or magnetostrictive sensor) and converted to pressure regulation after the measurement. The passage can be activated based on quota (quota passage) or pressure. In any case, the pressure-activated passage must be used when it also acts as a "cut-off" to limit the filling pressure and prevent flash formation and mold damage. Once the path is formed, the subsequent holding process is regulated by pressure (also for profiles).
The pressure of hydraulic presses is usually detected in the hydraulic circuit and rarely in the mold nozzle. For injection molding, the detection point must be as close to the mold cavity as possible. Therefore, the mold pressure measurement is best carried out at the nozzle. Even if it is not direct, the pressure measurement can also be carried out in the hydraulic circuit.
Unlike mold pressure detection, detection in the nozzle can also control the plasticization process by adjusting the back pressure. Mold pressure detection can be switched when the pressure close to the injection actually reaches the set value and maintains this pressure for the time required for the injection of the material. The measurement can be carried out directly or by a probe (e.g. piezoelectric sensor). Direct detection in the mold is very effective. The only limitation is that it leaves marks under the molded part. Indirect detection is often affected by the probe structure and clearance. For example, large tolerances can cause material to pour out, resulting in insufficient detection accuracy.
Nozzle pressure detection is less effective than cavity pressure detection because the material still has to pass through a flow path (either cold or hot). However, nozzle pressure detection has certain advantages, mainly including: detection is performed on the material; no modification of the mold is required; no trace is left on the molded part. The risk of overfilling at the initial pressure (and subsequent flashing) can be avoided by melt pressure control (preferably in the mold cavity). This can improve the effectiveness of control, avoid material burning, prevent underfilling, shorten cycle time and enhance repeatability.
There are indeed some technical problems in producing sensors that can ensure system reliability and are easy to use. If uniform back pressure is required, the process-related difficulties are indeed not small.
The sensor used for nozzle pressure detection must meet the following requirements:
1. It must not interfere with the molding process.
2. It can ensure detection accuracy at high pressure (2500 bar) and high temperature (350-400).
3. It must be small and solid, and easy to replace in the event of a malfunction.
4. It must have excellent wear resistance when in contact with the mold material.
5. Long-term detection effectiveness (when friction and contamination occur after long-term use, it can ensure that the measurement is free of deviation, error and lag).
6. Provide high-speed sampling (2-5 microseconds) and standardized communication protocols, such as CAN open version CANbus or DeviceNet.
Therefore, the problem is more complicated. It is not difficult to understand that to date, hydraulic presses still configure sensors in the hydraulic circuit, and all-electric motors use force detection, and neither uses melt sensors. For many years, melt sensors have been widely used in extruders, but extruders have lower requirements for detection range, accuracy, response time and structural solidity (compared with static stress on extruders, the mechanical fatigue stress on the sensor film when installed on injection molding machines is much greater).
