Detailed explanation of various injection pressures of plastic molds
Plastic molds require various pressures during injection molding to complete injection molding, and finally form finished plastic parts. Today we will focus on introducing various pressures encountered in the process of plastic parts processing.
1. Injection pressure
The plastic melt is transported to the nozzle in the melting box, and then injected into the mold cavity from the nozzle. This series of actions requires pressure to complete. This is the injection pressure, which is the pressure that causes the plastic to flow. It can be used in the nozzle or hydraulic line. on the sensor to measure. It does not have a fixed value, and the more difficult it is to fill the mold, the higher the injection pressure. The injection line pressure is directly related to the injection pressure.
During the filling phase of the injection cycle, high injection pressure may be required to maintain injection speed at the required level. Once the mold is filled, high pressure is no longer required. However, when injection molding some semi-crystalline thermoplastics (such as PA and POM), due to the sudden pressure change, the structure will deteriorate, so sometimes it is not necessary to use the secondary pressure.
2. Clamping pressure
The clamping pressure is the pressure that keeps the mold in a locked state. In order to resist the injection pressure, the clamping pressure must be used. Do not automatically select the maximum value available, but consider the projected area and calculate a suitable value. The projected area of the injection molded part is the largest area seen from the direction of application of the clamping force. For most injection molding cases, it's about 2 tons per square inch, or 31 meganewtons per square meter. However, this is only a low value and should be taken as a very rough rule of thumb, since as soon as the injection moulded part has any depth, the side walls must be taken into account.
3. Back pressure
This is the pressure that needs to be generated and exceeded before the screw retreats. Although the use of high back pressure is conducive to the uniform distribution of the color material and the melting of the 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 Therefore, the lower the back pressure, the better, and under no circumstances should it exceed 20% of the injection pressure (maximum rating) of the injection molding machine.
4. Nozzle pressure
Nozzle pressure is the pressure inside the nozzle. It's roughly 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 a screw injection molding machine, the nozzle pressure is about ten percent less than the injection pressure. In the piston injection molding machine, the pressure loss can reach about ten percent. In the case of piston injection molding machines, the pressure loss can reach 50%.