Basic theroy of Temperature ,injection pressure and Injection speed
Basic theroy of Temperature ,injection pressure and Injection speed
Temperature measurement and control is very important in injection molding. Although these measurements are relatively simple, most injection molding machines do not have enough temperature sampling points or lines.
On most injection molding machines, the temperature is induced by thermocouples. A thermocouple is basically composed of two different wires connected at the tail. If one end is hotter than the other, it will produce a small electrical signal; The more heated, the stronger the signal.
Thermocouples are also widely used as sensors in temperature control systems. On the control instrument, set the required temperature, and the display of the sensor will be compared with the temperature generated at the set point. In this simplest system, when the temperature reaches the set point, it will be turned off, and the power will be turned on again after 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 temperature of the injection cylinder used is only guiding. The melt temperature can be measured at the nozzle or by air jet method. The temperature setting of the injection cylinder depends on the melt temperature, screw speed, back pressure, injection volume and injection cycle.
If you have no experience in processing a particular grade of plastic, start with the lowest setting. For ease of control, the injection cylinder is divided into zones, but not all are set to the same temperature. If operating for a long time or at high temperature, set the temperature of the first zone to a lower value, which will prevent premature melting and diversion of plastics. Before injection molding, ensure that the hydraulic oil, hopper closure, mold and injection cylinder are at the correct temperature.
Injection pressure
This is the pressure that causes the plastic to flow and can be measured by sensors on the nozzle or hydraulic line. It has no fixed value, and the more difficult it is to fill the mold, the injection pressure also increases. The injection line pressure is directly related to the injection pressure.
First stage pressure and second stage pressure
During the filling phase 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 some semi crystalline thermoplastics (such as PA and POM) are injected, the structure will deteriorate due to the sudden change of pressure, so it is sometimes unnecessary to use the secondary pressure.
Clamping pressure
In order to resist the injection pressure, the mold locking pressure must be used. Do not automatically select the maximum value available, but consider the projection area and calculate a suitable value. The projected area of injection molded parts is the maximum area seen from the application direction of clamping force.
For most injection molding cases, it is about 2 tons per square inch, or 31 trillion Newtons per square meter. However, this is only a low value and should be taken as a rough empirical value, because once the injection molded part has any depth, the side wall must be considered.
Back pressure
This is the pressure 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 filled plastic, and increases the stress of the injection molding machine; Therefore, the lower the back pressure, the better. Under no circumstances can it exceed 20% of the injection pressure (maximum quota) of the injection molding machine.
Nozzle pressure
Nozzle pressure is the pressure inside the nozzle. It's about the pressure that causes the plastic to flow. It has no fixed value, but increases with the difficulty of mold filling. There is a direct relationship between nozzle pressure, linear pressure and injection pressure.
The nozzle pressure is about 10% less than the injection pressure. In the piston injection molding machine, the pressure loss can reach about 10%. In the piston injection molding machine, the pressure loss can reach 50%.
Injection speed
This refers to the filling speed of the die when the screw is used as a punch. When injection molding thin-walled products, high injection rate must be adopted, so as to completely fill the mold when the melt adhesive is not solidified and produce a smoother surface. A series of programmed firing rates are used during filling to avoid defects such as jet or trapped gas. Injection can be carried out under open-loop or closed-loop control system.
No matter which injection speed is adopted, the speed value and injection time must be recorded on the record sheet. 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 propulsion time.
