How to adjust injection molding process parameters?

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The temperature

Temperature measurement and control are very important in injection molding. Although these measurements are relatively simple, most injection molding machines do not have sufficient temperature points or wiring.

 

In most injection machines, the temperature is sensed by a thermocouple.


A thermocouple is basically two different wires coming together at the end. If one end is hotter than the other, a tiny telegraph message will be generated. The more heat, the stronger the signal.

 

Temperature control

Thermocouples are also widely used as sensors in temperature control systems. On the control instrument, the required temperature is set, and the sensor display is compared to the temperature generated at the set point.

 

In the simplest system, when the temperature reaches a set point, 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.


Injection pressure

This is the pressure that causes the plastic to flow and can be measured by sensors in the nozzle or in the hydraulic line.


It has no fixed value, and the more difficult it is to fill the mold, the injection pressure also increases, and there is a direct relationship between injection line pressure and injection pressure.

 engineering plastic

Stage 1 pressure and stage 2 pressure

During the filling phase of the injection cycle, high injection pressure may be required to maintain injection rate at the required level.


High pressure is no longer required after the mold is filled.


However, in the injection molding of some semi-crystalline thermoplastics (such as PA and POM), the structure will deteriorate due to the sudden change in pressure, so sometimes there is no need to use secondary pressure.

 

Clamping pressure

To combat injection pressure, the clamping pressure must be used. Instead of automatically selecting the maximum value available, consider the projected area and calculate a suitable value.The projected area of an injection piece is the largest area seen from the application direction of the clamping force.For most injection molding cases, it is about 2 tons per square inch, or 31 megabytes per square meter. However, this is a low value and should be considered as a rough rule of thumb, because once the injection piece has any depth, the side walls must be considered.

 

Back pressure

This is the pressure that the screw needs to be generated and surmounted before it falls back. The high back pressure is conducive to uniform color distribution and plastic melting, but at the same time, it extends 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, under no circumstances can exceed the injection molding machine pressure (maximum quota) 20%.

 

Nozzle pressure

Nozzle pressure is the pressure to shoot into the mouth. 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, line pressure and injection pressure.


In a screw injection machine, the nozzle pressure is approximately 10 % less than the injection pressure. In piston injection molding machine, the pressure loss can reach about 10 %. The pressure loss can be as much as 50 percent with a piston injection molding machine.

 

Injection speed

This refers to the filling speed of the die when the screw is used as the punch. High firing rate must be used in the injection molding of thin-walled products, so that the melt glue can fill the mold completely before solidification to produce a smoother surface.A series of programmed firing rates are used to avoid defects such as injection or gas trapping. The injection can be carried out in an open-loop or closed-loop control system.

 

Regardless of the injection rate used, the speed value must be recorded on the record sheet together with the injection time, which is the time required for the mold to reach the predetermined initial injection pressure, as part of the screw propulsion time.


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