The injection molding process mainly includes six stages such as compacting --- filled-pressure holding-cooling-mold opening-demoulding. These 6 phases directly determine the quality of the product, and these 6 phases are a complete continuous process. This chapter focuses on the three stages of filling, holding, cooling.
Here is the content list:
Filling phase
Pressure holding phase
Cooling phase
The filling is the first step in the entire injection cycle process, and the time from the mold closure begins with the mold, to the mold type cavity to fill about 95%. In theory, the shorter the fill time, the higher the molding efficiency; however, in actual production, the molding time (or injection velocity) is subject to a lot of conditions.
High-speed fill: High-speed filling time is high, the plastic has a decrease in viscosity due to shear and thinning so that the overall flow resistance is lowered; the local viscous heating effect is thinner. Therefore, in the flow control phase, the fill behavior tends to depend on the volume of volume to be filled. That is, in the flow control phase, since the high-speed filling, the shear and thin effect of the melt is often large, and the cooling of the thin wall is not obvious, so the rate of the rate accounts for the upper air.
Low-speed fill: When the heat transfer controls low speed, the shear rate is low, the local viscosity is high, and the flow resistance is large. Since the thermoplastic supplement rate is slow, the flow is slow, the heat conductivity is more obvious, and the heat rapidly tapes. Plus a small amount of viscous heating phenomenon, the curing layer thickness is thick, and further increases the flow resistance of the wall portion.
The role of the holding stage is to continuously apply pressure, compact melt, increase plastic density (encyclopedia) to compensate for the contraction behavior of plastics. During the pressing process, since the mold cavity has been filled with plastic, the backpressure is high. During the pressing compaction process, the injection molding machine screw can only slowly move forward, and the flow rate of the plastic is also slow, and the flow is referred to as pressure flow. Due to the inhibition stage, the plastic is cured by the mold wall, and the increase in melt viscosity is also very fast, so the resistance in the mold cavity is large. In the later stage of the holding, the material density continues to increase, the plastic parts are gradually formed, and the pressure phase should continue until the gate curing seal, at which time the cavity pressure in the pressing stage reaches the highest value.
In injection molding molds, the design of the cooling system is very important. This is because the molded plastic article is only cured to a certain rigidity, and the plastic article can be deformed due to external force after being demolded. Since the cooling time accounts for about 70% to 80% of the entire molding cycle, the well-designed cooling system can significantly shorten the molding time, improve injection productivity, and reduce costs. Improper design of the design will make the molded time, increase the cost; cooling unevenness will further result in warpage deformation of plastic products.
According to the experiment, a portion of 5% is radiated, and the flow is transferred to the atmosphere, and the remaining 95% is conducted from the melt to the mold. The plastic article acts in the mold due to the cooling water pipe, heat is passed through the plastic in the mold cavity through the heat conduction to the cooling water pipe, and then the heat convection is taken away by the cooling liquid. The heat that is not taken away from the cooling water continues to be conducted in the mold, and it is lowered in the air after contacting the outside.
The molding cycle of injection molding is composed of a molding time, a filling time, a compressive time, a cooling time, and a demolding time. Among them, the maximum specific gravity of the cooling time is approximately 70% to 80%. Therefore, the cooling time will directly affect the growth and yield size of the plastic product molding cycle. The temperature of the plastic product should be cooled to a thermal deformation temperature below the plastic article to prevent warpage and deformation caused by the relaxation or demolding external force caused by residual stress due to residual stress.
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