Engineering Plastic Injection Molding Melt Molding Process Control

Detailed engineering plastics injection molding melt molding process control: after plasticization of plastic filling, holding pressure, backflow, cooling, demolding, and other processes are melt molding process. Although this process is short, the melt in the melt changes, and this change has an important impact on the quality of engineering plastics plastic parts, the following four aspects to explain.

Engineering plastics injection molding melt molding process control

(1) Mold filling. When the mold is filled, there is no pressure in the mold cavity, and as the melt continues to enter, the melt pressure rises rapidly to the maximum when the mold cavity is filled. The mould filling time is related to the moulding pressure, the mould filling time is long, the melt which enters the mould first is cooled more, and the viscosity increases, then the melt needs to enter the mould cavity under higher pressure. At this time, due to the higher shear stress on the melt, the degree of molecular orientation is greater. This phenomenon is retained until the material temperature is lowered to the softening point, then there are frozen oriented molecules in the product, and the product becomes anisotropic. If the product is used in an environment with a large temperature difference, cracks will appear. Moreover, the thermal stability of the product is poor because the softening point of plastic decreases with the increase of molecular orientation. If the mold is filled at high speed, the melt will generate more frictional heat when it passes through the nozzle, runner, and gate with a small cross-section, which will increase the material temperature, so that when the pressure reaches the maximum, the temperature of the melt can be maintained at a higher value, which will reduce the molecular orientation and increase the fusion strength of the product. In the actual mold filling, the pressure of the melt depends on many factors such as material properties, melt temperature, mold structure, pouring system structure, mold temperature, etc.

(2) Holding pressure. This is the period of time from when the melt fills the mold cavity to when the screw or plunger retreats. At this time, the melt pressure and nozzle pressure are relatively stable, and the pressure-holding pressure is basically unchanged. At the same time, the mold cools down due to the cooling system, so that the melt temperature drops and shrinks accordingly. At this time, because the melt is still under the holding pressure, the melt in the barrel will certainly enter the mold cavity to fill the gap left by the shrinkage. The density of the product increases. In this pressure-holding stage, pressure-holding for improving the density of the product, reducing shrinkage, and overcoming the product surface defects have a role. In addition, because the melt is still flowing and the temperature is dropping, the oriented molecules are easily frozen, so this stage is also the main stage of macromolecular orientation formation, with the extension of the holding time, the greater the degree of molecular orientation. But too long holding time will make the product produce large internal stress, making the product brittle, thus reducing the quality of the product.

(3) Backflow. When the screw or plunger is back, the pressure is over. At this time, the pressure in the mold cavity is higher than in the runner, so the melt in the mold cavity will flow backward so that the pressure in the mold cavity drops rapidly until the material at the gate cools and solidifies. If the melt at the gate is already frozen when the screw or plunger is backed up, the backflow does not occur. Therefore, the amount of backflow is determined by the time of the pressure-holding phase. But regardless of the solidification of the melt at the gate before or after the screw (or plunger) retreats, the solidification pressure and temperature are always important factors in determining the average shrinkage of the product, and these factors are influenced by the time of the holding phase. Since there is a flow of melt during the backflow phase, the orientation of the plastic molecules occurs. However, due to the low flow rate of the melt and the low flow velocity, this orientation is relatively small and the area affected is not large. On the contrary, because the temperature of the melt is still relatively high in the backflow stage, some molecules that have been oriented may also be disoriented due to Brownian motion.

(4) Cooling, demoulding. This stage refers to the solidification of the gate and the subsequent solidification and cooling of the melt in the mold cavity until the plastic part is ejected from the mold cavity. The plastic in the mold in this stage is to continue to solidify, cooling so that the product in the mold release has sufficient rigidity and does not deform. In this stage, the plastic in the mold cavity can not enter and exit the mold cavity, but there may still be a small amount of flow in the mold cavity, there may still be a small amount of molecular orientation. Because the temperature, pressure, and volume of the plastic inside the mold are not constant at this stage, the pressure inside the mold may not be equal to the external pressure when the product is demolded. This pressure difference is called residual stress, if the residual pressure is positive, it is more difficult to release the mold, the product is easy to be scratched or damaged; when the residual pressure is negative, the product will shrink inward to produce depressions, wrinkles, internal vacuum bubble formation. Only when the residual stress is close to zero, the molding will be smooth and the product quality will be the most ideal.