Rgardless of whether it is a hydraulic or electric injection machine, all movements in injection molding process will generate pressure. Only by properly controlling required pressure can a finished product of reasonable quality be produced.

On hydraulic injection press, all movements are performed by oil circuit responsible for following operations:
1. Screw rotation in plasticization stage (back pressure can be determined or even controlled).
2. Sliding seat material path (nozzle is close to nozzle bushing).
3. Axial movement of injection screw during injection and pressure holding.
4. Close base material on shooting rod until toggle rod is fully extended or piston mold clamping stroke has been completed.
5. Start ejection stage of assembly ejector rod to eject components.
On a full-voltage machine, all movements are performed by a brushless synchronous motor equipped with permanent magnets. Through ball bearing screw that has been used in machine tool industry, rotary motion is converted into linear motion. Effect of the whole process partly depends on plasticizing process, among which screw plays a very critical role.
Screw must ensure that material is melted and homogenized. This process can be adjusted with help of back pressure to avoid overheating. Mixing element must not generate excessively high flow rates, otherwise it will cause polymer degradation. Each polymer has a different maximum flow rate. If this limit is exceeded, molecules will stretch and polymer backbone will break. However, focus is still to control forward axial movement of screw during injection and pressure holding.
Subsequent cooling process, including internal stress, tolerance and warpage, is very important to ensure product quality. All of this is quality of the whole mountain mold, especially when it comes to optimizing cooling channel and ensuring effective closed-loop temperature regulation. System is completely independent and will not interfere with mechanical adjustments. Mold movement such as mold closing and ejection must be precise and efficient.
Speed distribution curve is usually used to ensure that moving parts are accurately approached. Contact maintaining force can be adjusted. Therefore, it can be concluded that, without considering energy consumption and mechanical reliability, with same additional conditions (such as mold quality), product quality is mainly determined by system that controls forward movement of screw. On a hydraulic injection molding machine, this adjustment is achieved by detecting pressure.
Specifically, oil pressure activates a set of valves through control panel, fluid acts through manipulator, is adjusted and released.

 

Injection speed control includes options such as open loop control, closed loop control and closed loop control. Open loop system relies on a shared proportional valve. Proportional tension is applied to fluid of required ratio, so that fluid generates pressure in injection barrel, injection screw moves at a certain forward speed. Closed loop system uses a closed loop proportional valve.
Loop is closed at position where closed port is located, closed port controls flow rate of oil by moving in valve. Closed loop system is closed at screw translation speed. A speed sensor (usually a potentiometer type) is used in closed-loop system to detect tension drop regularly.
Material flowing out of proportional valve can be adjusted to compensate for speed deviation. Closed-loop control relies on dedicated electronic components integrated with machine. Closed-loop pressure control can ensure uniform pressure during injection and pressure holding stages, as well as uniform back pressure in each cycle.
Proportional valve is adjusted by detected pressure value, deviation compensation is performed according to set pressure value. Generally speaking, hydraulic pressure can be monitored, but detecting melt pressure in nozzle or cavity is another effective method. A more reliable solution is to manage proportional valve by reading nozzle or cavity pressure readings. Adding temperature detection on basis of pressure detection is particularly conducive to process management.
Knowing actual pressure that material can withstand can also help predict actual weight and size of molded part based on set pressure and temperature conditions. In fact, by changing holding pressure value, more materials can be introduced into mold cavity to reduce component shrinkage and meet design tolerances (including preset injection shrinkage). When approaching melting conditions, semi-crystalline polymers show a great change in specific volume. In this regard, overfilling will not hinder ejection of part.

Average hydraulic pressure generated by centrifugal pump can reach 140 bar, which is particularly suitable for injection molding. In the other stages of cycle, requirements are significantly lower, except for specific situations that require rapid plasticization (such as PET injection stretch-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. Fixed displacement pump moves same amount of oil every time it rotates. Therefore, oil pump selection is determined by amount of oil that needs to be moved in a specific time. Speed of a three-phase motor is generally 1440 rpm, and it is usually required to assemble a double pump. Only in plasticizing process (power up to 100%), utilization rate of oil pump reaches maximum. During pause process, machine does not need energy consumption, even if it does, it is also a power loss.
All injection molding machines use proportional servo valves with different quality levels. Two or more proportional valves are installed on injection press to accurately control following aspects:
Mold opening speed (two levels), mold closing speed (two levels), mold closing safety, injection (level 3-10), feeding (level 3-5), suction and ejector (two levels).
Mold opening pressure, mold closing pressure, mold safety, mechanical fixtures (barrel or toggle), injection (once in filling stage, 3-10 times in subsequent stages), suction and back pressure (level 3-5), screw Rotation speed (level 3-5).
Sliding seat approach speed (speed at which mechanical nozzle approaches injection liner on fixed half of mold) and movement speed of ejector rod (ejection speed) can also be adjusted. Auxiliary motor sends amplified signal (output signal) to valve through weak input signal, so that servo valve performs regulating function. In servo valve, weak input electrical signal is converted into a hydraulic output signal, which can be improved in the form of pressure drop according to required discharge requirements. Valve must make a fast, repeatable and low-lag discharge response to tension or general commands. In fact, purpose of current research is to improve frequency response and enable a dialogue between power equipment (hydraulic side) and electronic equipment operating at several kilohertz (kHz) frequencies.
Since effective discharge depends on function of degree of polymerization (DP) on valve, oil temperature in hydraulic circuit must be maintained within range of 45-55℃ (usually a closed-loop adjustment system is used), depending on fluid viscosity and geometry of transition port.
If there is no proper regulating system in valve, temperature rise will cause viscosity of solution to decrease; if it is equipped with a balanced opening threshold, output can be increased. Increasing output oil volume of transmission system means that injection speed will increase accordingly. Precise control of high-tech servo drive valves can basically eliminate hysteresis and enhance repeatability of all functions.

Since there is no vector fluid that causes motion on all-electric injection molding machine, hydraulic pressure detection cannot be performed. Therefore, a load sensor is usually used to measure elastic deformation with an extensometer to directly determine its strength.
Manufacturers of all-electric injection molding machines have developed a variety of elastic components and assembled corresponding extensometers in factory. Another difference is that back pressure and its control can be achieved by adding resistance to axial movement generated by injection motor, while the other motor causes screw to rotate and subsequent plasticization of material. In the past, some machinery manufacturers used a measurement system with a transducer installed in nozzle, but later abandoned system due to "lack of functionality and reliability."

Above has proved importance of pressure regulation in process of injection and pressure holding. Therefore, accuracy and repeatability of pressure detection are very critical factors. In a closed-loop system, pressure detection is very important. Only by ensuring accurate pressure detection can the regulator make actual pressure close to or equal to set value.
In an open-loop system, because it is directly connected to transmission system, accuracy and repeatability of pressure detection are more important. Now, open-loop system is still in use, and it is more widely used in high-tonnage models.
Generally speaking, speed control based on set value is performed during injection process (that is, speed change is measured by a potentiometer or a magnetostrictive sensor), and converted to pressure adjustment after measurement. Channels can be activated based on quotas (quota channels) or pressure.
In any case, when pressure-activated path is also used as a "cut" to limit filling pressure, prevent flash formation and mold damage, pressure-activated path must be used. Once passage is formed, subsequent pressure-holding process is regulated by pressure (profile is no exception). Pressure of hydraulic press is generally detected in hydraulic circuit, and rarely in mold nozzle. For injection molding, detection point must be as close as possible to mold cavity. Therefore, mold pressure measurement is best performed at nozzle, even if it is not straightforward, pressure measurement can be performed in hydraulic circuit.
Different from mold pressure detection, nozzle detection can also control plasticizing process by adjusting back pressure. When pressure close to injection actually reaches set value and maintains this pressure for time required for injection of material, the mold pressure detection can be switched.
Measurement can be carried out directly or through a probe (such as a piezoelectric sensor). Direct detection in mold is very effective, the only limitation is that it will leave traces under molded part. Indirect detection is often affected by structure and gap of probe. For example, too much tolerance can lead to material dumping, resulting in insufficient detection accuracy.
Nozzle pressure detection is less effective than cavity pressure detection because material has to pass through a section of flow path (either cold or hot). However, nozzle pressure detection has certain advantages, mainly including: detection is carried out on material; there is no need to modify mold; and no traces are left on molded part.
Through melt pressure control (preferably in mold cavity), risk of excessive mold filling (and subsequent formation of flash) under initial pressure can be avoided. As a result, effectiveness of control can be improved, scorching of materials can be avoided, insufficient mold filling can be prevented, cycle time can be shortened, and repeatability can be enhanced.
There are indeed some technical problems in producing sensors that can ensure system reliability and are easy to use. If it is required to adjust back pressure evenly, difficulty associated with process is indeed not small.

1. Do not interfere with molding process.
2. It can ensure detection accuracy under high pressure (2500 bar) and high temperature (350-400).
3. Small size, solid structure, easy to replace in case of failure.
4. It has excellent abrasion resistance when in contact with mold filling material.
5. Detection effectiveness can be guaranteed for a long time (when friction or pollution occurs after long-term use, it can ensure measurement without deviation, error, and hysteresis).
6. Provide high-speed sampling (2-5 microseconds) and standardized communication protocols, such as: CAN open version CANbus or DeviceNet.
Therefore, problem is more complicated. It is not difficult to understand that, so far, hydraulic presses still use sensors in hydraulic circuit, all electric motors use force detection, and neither of them uses melt sensors. For many years, melt sensors have been widely used on extruders, but extruders have lower requirements for detection range, accuracy, response time and structural solidity (compared with static stress on extruder, when installed on injection machine, mechanical fatigue stress on sensor film is much greater).