Temperature Control
Thermocouples are also widely used as sensors in temperature control systems. On the control instrument, the desired temperature is set and the sensor display is compared to the temperature produced at the set point. In the simplest system, when the temperature reaches the set point, the power is turned off and the power is turned back on when the temperature drops. This system is called on-off control because it is either on or off.
Melt Temperature
The melt temperature is very important and the shot cylinder temperature used is only a guide. The melt temperature can be measured at the nozzle or using the air jet method. The temperature setting of the shot cylinder depends on the melt temperature, screw speed, back pressure, shot size and injection cycle.
If you have no experience in processing a specific grade of plastic, start with the lowest setting. For ease of control, the shot cylinder is divided into zones, but not all are set to the same temperature. If the operation time is long or at high temperature, set the temperature of the first zone to a lower value. This will prevent the plastic from melting and diverting prematurely. Before injection molding begins, make sure the hydraulic oil, hopper closer, mold and shot cylinder are at the correct temperature.
Injection pressure
This is the pressure that causes the plastic to flow and can be measured by a sensor on the nozzle or hydraulic line. It does not have a fixed value, and the more difficult the mold filling, the greater the injection pressure. There is a direct relationship between injection line pressure and injection pressure.
First stage pressure and second stage pressure
During the filling stage of the injection cycle, high injection pressure may be required to maintain the injection speed at the required level. After the mold is filled, high pressure is no longer required. However, when injecting some semi-crystalline thermoplastics (such as PA and POM), the structure will deteriorate due to the sudden change in pressure, so sometimes it is not necessary to use the second stage pressure.
Clamping pressure
To counter the injection pressure, the clamping pressure must be used. Do not automatically select the maximum value available, but consider the projected area and calculate a suitable value. The projected area of the injection molded part is the maximum area seen from the direction of application of the clamping force. For most injection molding situations, it is about 2 tons per square inch, or 31 meganewtons per square meter. However, this is only a low value and should be used as a very rough empirical value, because once the injection molded part has any depth, the side wall must be considered.
Back pressure
This is the pressure that needs to be generated and exceeded before the screw retreats. Although the use of high back pressure is conducive to uniform color distribution and plastic melting, it also prolongs the return time of the middle screw, reduces the length of the fiber contained in the filling plastic, and increases the stress of the injection molding machine; therefore, the lower the back pressure, the better, and in any case it cannot exceed 20% of the injection pressure (maximum rating) of the injection molding machine.
Nozzle pressure
The nozzle pressure is the pressure inside the nozzle. It is approximately the pressure that causes the plastic to flow. It has no fixed value, but increases as the difficulty of mold filling increases. There is a direct relationship between nozzle pressure, line pressure and injection pressure. On a screw injection molding machine, the nozzle pressure is about 10% less than the injection pressure. On a piston injection molding machine, the pressure loss can reach about 10%. On a piston injection molding machine, the pressure loss can reach 50%.
Injection speed
This refers to the speed at which the mold is filled when the screw is used as a punch. When injecting thin-walled products, a high injection speed must be used to completely fill the mold before the molten plastic solidifies and produce a smoother surface. A series of programmed injection speeds are used during filling to avoid defects such as spraying or air entrapment. Injection can be performed under open-loop or closed-loop control systems.
No matter which injection speed is used, the speed value must be recorded on the record sheet together with the injection time. The injection time refers to the time required for the mold to reach the predetermined first-stage injection pressure, which is a part of the screw advancement time.
