The basic theory of injection molding process parameters

◆Temperature

Temperature measurement and control are very important in injection molding. Although it is relatively simple to make these measurements, most injection molding machines do not have adequate temperature pick-up points or lines.

On most injection molding machines, the temperature is sensed by thermocouples. A thermocouple 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 more it is heated, the stronger the signal.

◆Control of temperature

The thermocouple is also widely used as a sensor for a temperature control system. On the control instrument, the desired temperature is set 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 setpoint, it 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 glue temperature

The melt glue temperature is very important, and the injection cylinder temperature used is only a guideline. 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 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. Before injection starts, make sure the hydraulic oil, hopper closers, molds, and injection cylinders are all at the correct temperature.
 

◆Injection pressure

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 harder the mold is filled, the more the injection pressure increases. There is a direct relationship between the injection line pressure and the injection pressure.
 

◆Stage 1 Pressure and Stage 2 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 needed. However, when molding some semi-crystalline thermoplastics (such as PA and POM), the sudden change in pressure may deteriorate the structure, so sometimes it is not necessary to use the second stage pressure.

◆Clamping pressure

To counteract the injection pressure, it is necessary to use the clamping pressure. Instead of automatically selecting the maximum value available, a suitable value should be calculated by considering the projected area. The projected area of the injected 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 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.
 

◆Backpressure

This is the pressure that must be generated and exceeded before the screw retreats. Although the use of high back pressure is beneficial to the uniform distribution of color and plastic melting, 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 under no circumstances should it exceed 20% of the injection pressure (maximum quota) of the injection molding machine.

◆Injector pressure

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 as the difficulty of filling the mold increases. 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.
 

◆Injection speed

This refers to the filling speed of the mold when the screw is used as the punch. When injecting thin-walled products, it is necessary to use high injection speed to fill the mold before the melt solidifies and produces 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.