Temperature measurement and control are very important in injection molding. Although it is relatively easy to make these measurements, most injection molding machines
do not have sufficient temperature pick-up points or lines.
On most injection molding machines, the temperature is sensed by thermocouples. A thermocouple basically consists of two different wires connected at the end. If one end is hotter than the other, a tiny electrical signal will be generated; the hotter it is, the stronger the signal.
Thermocouples are also widely used as sensors for temperature control systems. On the control instrument, the desired temperature is set and the display of the sensor is compared to the temperature generated at the set point.
In this simplest system, the power is turned off when the temperature reaches the setpoint and turned back on when the temperature drops. This system is called on/off control because it is either on or off.
The melt temperature is very important and the shot cylinder temperature used is only a guide. The melt temperature can be measured at the injection nozzle or by using the air injection method. The temperature setting of the injection cylinder depends on the melt temperature, screw speed, back pressure, injection volume, and injection cycle.
If you are not experienced in processing a particular grade of plastic, start with the lowest setting. Injection cylinders are divided into zones for ease of control, but not all are set to the same temperature.
If operating for long periods of time or at high temperatures, set the temperature in the first zone to a lower value, which will prevent premature melting and shunting of the plastic. Make sure the hydraulic fluid, hopper closers, molds, and injection cylinders are all at the correct temperature before injection begins.
This is the pressure that causes the plastic to flow and can be measured with a sensor on the injection nozzle or hydraulic line. It has no fixed value, and the more difficult the mold is to fill, the more the injection pressure increases. There is a direct relationship between injection line pressure and injection pressure.
First stage pressure and second stage pressure
During the filling phase of the molding cycle, high injection pressure may be required to maintain the molding speed at the required level. After the mold is filled, high pressures are no longer required. However, in the case of semi-crystalline thermoplastics (such as PA and POM), a sudden change in pressure can deteriorate the structure, so sometimes it is not necessary to use the second stage pressure.
In order to counteract the injection pressure, it is necessary to use the clamping pressure. Instead of automatically selecting the maximum value available, calculate a suitable value considering the projected area. The projected area of the injection port is the maximum area visible 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 a low value and should be taken as a very rough empirical value because, once the injection molded part has any depth, then the sidewalls must be taken into account.
This is the pressure that must be generated and exceeded before the screw retreats. Although high backpressure is beneficial to the uniform distribution of color and melting of plastic, it also prolongs the return time of the middle screw, reduces the length of the fibers contained in the filled plastic, and increases the stress of the injection molding machine
; therefore, the lower the backpressure, the better, and in no case should it exceed 20% of the injection pressure (maximum quota) of the injection molding machine.
Nozzle pressure is the pressure inside the nozzle. It is approximately the pressure that causes the plastic to flow. It does not have a fixed value but increases with the difficulty of filling the mold. There is a direct relationship between nozzle pressure, line pressure, and injection pressure. On screw injection molding machines
, the nozzle pressure is about ten percent less than the injection pressure. In the case of piston injection molding machines, the pressure loss can reach about ten percent. In the case of piston injection molding machines, the pressure loss can reach 50 percent.
This refers to the filling speed of the mold when the screw is used as the punch. When injecting thin-walled products, high injection speeds are necessary to allow the mold to be completely filled before the melt solidifies, producing a smoother surface. A range of programmed injection speeds is used for filling to avoid defects such as jetting or trapped air. The injection can be performed with either an open-loop or closed-loop control system.
Regardless of the injection speed used, the speed value must be recorded on a log sheet along with the injection time, which is the time required for the mold to reach a predetermined first stage of injection pressure and is part of the screw advance time.