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1. The injection molding process can be simply expressed as follows:

End of the last cycle-mold closing-filling-pressure maintaining-glue returning-cooling-mold opening-demoulding-start the next cycle.

In the filling and pressure maintaining stage, the pressure in the cavity rises with time. After filling the cavity, the pressure will remain in a relatively static state to make up for the lack of glue caused by shrinkage. In addition, this pressure can prevent the colloid from flowing back due to the decrease of injection amount, which is the pressure holding stage. After the pressure holding stage is completed, the pressure in the cavity gradually decreases, which can be reduced to zero in theory with time, but it is not zero in practice. Therefore, after demoulding, the memory stress of the product needs post-treatment. The so-called stress is the force produced by the free movement of Fourier sub-chain or chain segment, that is, bending deformation, stress cracking, shrinkage cavity and so on.

Second, the main parameters of injection molding process

1. Plastic injection temperature and melt temperature play a major role in melt fluidity. Because plastics have no specific melting point, the so-called melting point is a temperature range in the molten state. The different structure and composition of plastic molecular chain have different effects on its fluidity. Rigid molecular chains are obviously affected by temperature, such as PC and PPS, while the fluidity of flexible molecular chains, such as PA, PP and PE, is not obviously affected by temperature changes.

2. The injection speed is the speed of the melt in the barrel (mm/s) (also the forward speed of the screw). The injection speed determines the appearance, size, shrinkage and flow distribution of the product. Generally speaking, it is slow-fast-slow again, that is, the melt flows through the main runner, branch runner and gate at a relatively high speed to achieve the purpose of balanced injection, and then the whole mold is filled by rapid mold filling.

3. Injection pressure is the resistance required for the melt to overcome the advance, which directly affects the size, weight and deformation of the product. Different plastic products need different injection pressures. For PA, PP and other materials, increasing the pressure will significantly improve their fluidity, and the injection pressure determines the density of the product, that is, the appearance luster.

4. Mold temperature: Some plastic materials need higher mold temperature because of their high crystallization temperature and slow crystallization speed, others need higher or lower temperature for controlling size and deformation, for example, PC generally needs more than 60 degrees, while PPS sometimes needs more than 160 degrees in order to achieve better appearance and improve fluidity, so mold temperature can improve the appearance, deformation and size of products.

Third, the meaning of injection molding professional parameters

1, injection quantity

Injection amount refers to the amount of melt injected into the mold by the screw of the injection molding machine during injection molding.

Injection quantity = screw propulsion volume *ρ*C

ρ is the density of the injection molding material.

C of crystalline polymer is 0.85, and C of amorphous polymer is 0.93.

Injection molding machines cannot be used to process products with injection quantity less than110 or more than 70%.

2. Measuring stroke (preformed stroke)

After each injection procedure, the screw is located at the front end of the injection cylinder. When the preforming procedure arrives, the screw starts to rotate and the material is sent to the head of the screw. The screw rod retreats under the reaction of the material until it touches the limit switch. This process is a measurement process.

The injection quantity is related to the accuracy of metering stroke. If it is too small, the injection quantity is not enough; if it is too large, the residual material in front of the barrel after each injection is too large, and the melt temperature is uneven or overheated.

There is a temperature difference between the longitudinal temperature and the radial temperature of the melt in the metering practice after preform, and the screw revolution, preform back pressure and barrel temperature will have a great influence on the melt temperature and temperature difference.

3. Extension.

Anti-delay means that after the screw rod is measured in place, it will retreat straight for a certain distance, which will increase the specific volume of the measuring cavity and decrease the internal pressure, thus preventing the fluid from flowing out of the measuring cavity.

Another purpose of tape casting prevention is to reduce the pressure and internal stress of the nozzle runner system when the injection nozzle does not retreat for pre-molding, and it is easy to extract the material handle when opening the mold. If the delay is large, bubbles will be entrained in the metering chamber, and there is no delay for materials with high viscosity.

The above parameters can meet the quality requirements by reasonable adjustment, such as the size can be achieved by injection pressure, mold temperature, injection speed and back pressure.

Four, how to adjust the injection molding process parameters

? Temperature control

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 by the set point. In this simplest system, when the temperature reaches the set value, it will be turned off, and after the temperature drops, the power will be turned on again. This system is called switch control because it is either on or off.

? temperature

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 sampling lines.

On most injection molding machines, the temperature is detected by thermocouples. Thermocouples are basically made up of two different wires connected at the tail. If one end is hotter than the other, it will produce tiny telecommunications; The higher the temperature, the stronger the signal.

? melting temperature

The melting temperature is very important, and the shooting pot temperature used is only a reference. The melting temperature can be measured at the nozzle or by air injection. The temperature setting of injection cylinder depends on melting temperature, screw speed, back pressure, injection quantity and injection cycle.

If you have no experience in processing a certain grade of plastic, please start with the minimum setting. For the convenience of control, the shooting pot is divided into several zones, but not all the zones are set at the same temperature. If it is operated for a long time or at high temperature, please set the temperature in the first zone to a lower value, which will prevent the plastic from melting and shunting prematurely. Before injection molding, ensure that the hydraulic oil, hopper sealer, mold and ejector cylinder are at the correct temperature.

? injection pressure

This is the pressure that causes the plastic to flow, which can be measured by the sensor on the nozzle or hydraulic pipeline. It has no fixed value, the more difficult it is to fill the mold, the higher the injection pressure, and 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, it may be necessary to adopt high injection pressure to keep the injection speed at the required level. After the mold is full, there is no need for high pressure. However, when injecting some semi-crystalline thermoplastics (such as PA, POM), the structure will deteriorate due to the sudden change of pressure, so it is sometimes unnecessary to use secondary pressure.

? Mode locking pressure

In order to counteract the injection pressure, it is necessary to use the locking pressure. We should consider the projected area and calculate an appropriate value, instead of automatically selecting the maximum available value. Seen from the direction of clamping force, the projected area of injection molded parts is the largest. For most injection molding cases, it is about 2 tons per square inch, or 3 1 meganewton per square meter. But this is only a low value and should be taken as a very rough empirical value, because once the injection molding has any depth, then the sidewall must be considered.

? back pressure

This is the pressure that must be generated and exceeded before the screw retreats. Although the adoption of high back pressure is beneficial to the uniform dispersion of pigments and the melting of plastics, it also prolongs the return time of the intermediate screw, reduces the length of fibers contained in the filled plastics and increases the stress of the injection molding machine. So 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.

? Injection molding 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 adopt a higher firing rate in order to completely fill the mold and produce a smoother surface when the molten glue is not solidified. When filling, a series of programmed emission rates are used to avoid defects such as injecting or trapping gas. Injection can be carried out under open-loop or closed-loop control system.

? injection pressure

Nozzle pressure is the pressure inside the nozzle. It is pressure that causes the plastic to flow. It has no fixed value, but it increases with the increase of mold filling difficulty. There is a direct relationship among nozzle pressure, pipeline pressure and injection pressure. On the screw injection molding machine, the nozzle pressure is less than the injection pressure by about 10%. The pressure loss of piston injection molding machine can reach about 10%. In the piston injection molding machine, the pressure loss can reach 50%.