Injection multi-stage molding process lecture

First, the principle of multi-stage injection plastic flow:
1. The first injection glue is fed at low speed, the nozzle is removed from the cold material head and then the second stage of injection rate is increased to fill the mold cavity to shorten the time for the plastic flow to the end of the gate part, so that the plastic viscosity in the filling is maintained to the minimum curing, but it is difficult to control the correct holding pressure switching point at high speed, so it is necessary to use multi-stage deceleration to effectively control and master the holding pressure switching point.
2. The action principle of segmented injection:
The flow proportional valve in the hydraulic system is controlled by electro-optical control to instantly obtain the injection velocity at a point to achieve a segmented rate of fire.
Second, the relationship between segmented injection rate and pressure:
1. In the segmented injection molding process, no matter how many stages of the filling process change, its injection pressure has only one pressure, that is, one pressure, while the holding stage maintains only one speed regardless of several stages of pressure holding.
2. Example of segmented injection:
(1) The same finished product can be molded with a smaller clamping force, which can extend the life of the machine and mold;
(2) Segmented deceleration and mastering the correct holding pressure switching point can effectively ensure the stability of quality;
(3) When the flow of the molded plastic is too good, to prevent the burr from occurring, take low-speed injection glue, but the raw material must not be cooled and solidified as the principle, and the molten resin is quickly filled with the cavity after the meat thickness is increased, and the flow mark (is the striped appearance of the molten grease gradually centered on the gate) is caused by the junction formed between the resin that initially flowed into the cavity cooled too quickly and the resin that flowed second.
(4) If the glue inlet (that is, the gate) part is thickened, the rate of injection is too fast to cause turbulence, and the cold material is easy to remain in the channel and form flow marks, so the cold material head should be pushed open at a slow and low pressure glue feed, so that the plastic behind can enter smoothly.
(5) In the injection molding process, the nozzle part is in contact with the mold, because the mold cooling water cooling mold temperature is lower than that of the nozzle
The partial heat is taken away by the mold, and the nozzle is prone to produce cold heads, which are injected into the mold and will be blocked at the gate and cause rhyolines or silver bar marks, which can be improved by detecting segmented injection.
(6) Precision and small parts, fine gate size, and the gate balance of most mold cavities is extremely difficult and difficult, and the gate can be overcome by opening the gate of the same size and using multi-stage injection technology.
(7) The change of the junction switching position of the first low-speed injection and the second high-speed injection can correct the local displacement of the weld line, such as moving the part that is obvious in appearance to a less obvious position (such as the sticker).
(8) The phenomenon of depression and poor fusion is opposed to each other in molding, and this method can be improved at the same time; The rate of fire in the dent part of the product drops sharply, and the rate of fire is quickly increased after filling the surface layer to cold solidification to fill the mold cavity, and the welded line part should be taken to prevent poor fusion (generally the surface sag of the molded product occurs in the thick of the meat, which is caused by the volume shrinkage when the molten resin is cooled and cured).
(9) Segmented reduction in the holding process can reduce the residual stress of the molded product.
(10) The molding conditions of thin molded products and long flow distance require high pressure to be successfully completed, but high-speed and high-pressure injection glue is easy to cause residual stress at the gate part, thereby affecting the quality, so high-speed glue feeding, medium-speed filling, and low-speed pressure holding are adopted to eliminate residual stress, thereby preventing the deformation of the finished product.
(11) Mold cooling to improve: if the moving mold temperature is low, the fixed mold temperature is high, the molded product will not be inwardly warped, and if the moving mold temperature is high, the molded product may be externally warped.
(12) Scorching (caused by the compression and combustion of gas in the mold cavity) is most likely to occur at the die parting line or welding, and the surface of the resin shows traces of black carbonization, and the air gas needs to be smoothly discharged from the mold cavity, and the rate of fire must be reduced.