一. 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 collection points or lines.
On most injection molding machines, the temperature is sensed by a thermocouple. A thermocouple basically consists of two different wires connected at the end. If one end is hotter than the other, there will be a tiny telecommunication; The more heat, the stronger the signal.
Control of temperature
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 generated at the set point. In this simplest system, when the temperature reaches a set point, it is turned off, and the power is turned back on when the temperature drops. Such a system is called an on-off control because it is either on or off.
Temperature of melt
The temperature of the melt is important, and the temperature of the injection cylinder used is only indicative. The melt temperature can be measured at the nozzle or 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 there is 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 the operation is long or at high temperatures, set the temperature in zone 1 to a lower value, which will prevent premature melting and diverting of the plastic. Ensure that the hydraulic oil, the hopper sealer die, and the injection cylinder are at the correct temperature before the injection begins.
二. Injection molding pressure
This is the pressure that causes the plastic flow, which can be measured by the sensor in the nozzle or the hydraulic line. It has no fixed value, and the more difficult the mold is to fill, the injection pressure also increases. There is a direct relationship between the injection line pressure and the injection pressure.
Phase I stress and phase II stress
During the filling phase of the injection cycle, it may be necessary to use high injection pressure to maintain the injection speed at the required level. After the mold is filled, high pressure is no longer required. However, in the injection of some semi-crystalline thermoplastics (such as PA and POM), due to the sudden change in pressure, the structure will deteriorate, so sometimes it is not necessary to use the secondary pressure.
Clamping pressure
In order to combat the injection pressure, the clamping pressure must be used. Do not automatically choose the maximum value available, but consider the projection area and calculate a suitable value. The projected area of the injection molded part is the largest area seen from the direction of the application of the mod-locking force. For most injection molding cases, it's about 2 tons per square inch, or 31 megavonts per square meter. However, this is a low value and should be considered as a rough rule of thumb, since once the injection part has any depth, the sidewalls must be considered.
Back pressure
This is the pressure that must be generated and exceeded before the screw recedes. Although the high back pressure is conducive to the uniform distribution of color 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, under no circumstances can exceed 20% of the injection molding machine injection pressure (the highest quota).
Nozzle pressure
Mouth pressure is the pressure on the surface of the mouth. It's about the pressure that causes the plastic to flow. It does not have a fixed value, but increases with the difficulty of mold filling. There is a direct relationship between nozzle pressure, line pressure and injection pressure.
The nozzle pressure is about 10 percent 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%.
三. Injection molding speed
This refers to the filling speed of the die when the screw is used as a punch. Injection molding thin-walled products, must use a high firing rate, so that the melt is not solidified completely fill the mold, the production of smooth surface filling using a series of programmed firing rate, to avoid the production of jet or trapped gas and other defects. Injections can be performed under either 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, which is the time required for the mold to reach the predetermined first stage of injection pressure and is part of the screw advance time.