All movements in injection molding process, whether hydraulic or electric, generate pressure. Appropriate control of required pressure produces a finished product of reasonable quality. Pressure regulation and metering system is on hydraulic injection molding machine, and all movements are performed by oil circuit responsible for following operations:

 

Screw rotation in plasticizing stage.
Slider forehearth (nozzle near nozzle bushing).
Axial movement of injection screw during injection and pressure holding.
Close substrate on shot rod until toggle lever is fully extended or piston clamping stroke is complete.
Activate ejector table of assembly ejector to eject part.
On full voltage machines, all movements are performed by brushless synchronous motors with permanent magnets. Rotary motion is converted into linear motion by ball bearing screws, which have always been used in machine tool industry. Efficiency of the entire process depends in part on plasticizing process, in which screw plays a crucial role.
Screw must ensure that material is melted and homogenized. This process can be adjusted with the help of back pressure to avoid overheating. Mixing elements must not generate too high a rotational speed, otherwise polymer will degrade. Each polymer has a different maximum rotational speed, and if this limit is exceeded, molecules will stretch and polymer backbone will break. However, focus remains on controlling forward axial movement of screw during injection and holding.
Subsequent cooling process, including intrinsic stress, tolerances and warpage, is important to ensure product quality. It's all about mold quality, especially when it comes to optimizing cooling runners to ensure effective closed-loop temperature regulation. System is completely self-contained and does not interfere with mechanical adjustments. Mold movements such as mold closing and ejection must be precise and efficient. Velocity profiles are usually employed to ensure accurate approach of moving parts. Contact maintenance force is adjustable.
Therefore, it can be concluded that product quality is mainly determined by system that controls forward movement stage of screw without considering energy consumption and mechanical reliability, and additional conditions are same (such as quality of mold). On hydraulic injection molding machines, this adjustment is accomplished by sensing oil pressure. Specifically, oil pressure activates a set of valves through control panel, and fluid acts through manipulator, which 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 a common proportional valve. Proportional tension is applied to desired proportion of fluid, which causes fluid to create pressure in injection barrel, causing injection screw to move at a forward speed. Semi-closed loop system uses a closed loop proportional valve.
Loop is closed at location of closing port, which controls flow rate of oil by moving within valve. Closed loop system closes when screw translates at speed. A speed sensor (usually a potentiometer type) is used in a closed loop system to periodically detect tension drops. Oil flowing out of proportional valve can be adjusted to compensate for speed deviation that occurs. Closed-loop control relies on dedicated electronics integrated into machine. Closed-loop pressure control ensures uniform pressure during injection and hold phases, as well as uniform back pressure throughout cycle.
Proportional valve is adjusted by detected pressure value, and deviation is compensated according to set pressure value. In general, hydraulic pressure can be monitored, but detecting melt pressure in a nozzle or mold cavity is another effective method. A more reliable solution is to manage proportional valves by reading nozzle or cavity pressure readings. Adding temperature detection on basis of pressure detection is particularly beneficial to process management.
Knowing actual pressure material can withstand also helps predict actual weight and size of molded part based on set pressure and temperature conditions. In fact, by changing holding pressure value, more material can be introduced into mold cavity to reduce part shrinkage to meet design tolerances (including preset injection shrinkage). Near melting conditions, semi-crystalline polymers show great changes in specific volume. In this regard, overfilling mold will not hinder ejection of part.

 

Average hydraulic pressure generated by centrifugal pump can reach 140 bar, which is especially suitable for injection molding. At the other stages of cycle, requirements are significantly lower, except for specific cases where rapid plasticization is required (eg PET injection-stretch-blow one-step injection molding machines).
To reduce energy consumption, variable displacement pumps and accumulator cylinders can be used during peak discharge periods. Fixed displacement pumps move same amount of oil per revolution, so pump selection is based on the amount of oil that needs to be moved at a given time. Speed of three-phase motor is generally 1440 rpm, and it is usually required to install double pumps. Only in plasticizing process (power up to 100%), utilization rate of oil pump reaches maximum. During standstill process, 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 various quality grades. Two or more sets of proportional valves are installed on injection press to accurately control following aspects:
Mold opening speed (two stages), mold closing speed (two stages), mold closing safety, injection (3-10 stages), feeding (3-5 stages), suction and ejector (two stages).
Mold opening pressure, mold closing pressure, mold safety, mechanical fixture (barrel or toggle), injection (one time for filling stage, 3-10 times for subsequent stages), screw rotation speed (level 3-5).
Carriage approach speed (speed at which mechanical nozzle approaches injection liner on stationary half of mold) and movement speed of ejector pin (ejection table 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 is modified according to required discharge requirements in form of pressure drop. Valve must have a fast, repeatable and low-hysteresis discharge response to tension or general commands. In fact, aim of the current research is to improve frequency response to enable dialogue between power equipment (hydraulic side) and electronic equipment operating at frequencies in kilohertz (kHz).
Since effective discharge depends on effect of degree of polymerization (DP) on valve, oil temperature in hydraulic circuit must be maintained in the range of 45-55℃ (usually with a closed-loop regulation system), depending on fluid viscosity and geometry of transition port.
Without a proper adjustment 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 of drive train means faster injection speeds. Precise control of high-tech servo-actuated valves essentially eliminates hysteresis and enhances repeatability of all functions.