Injection molding, injection molding process
Injection molding, injection molding process
The process flow
These 6 stages directly determine the forming quality of products, and these 6 stages are a complete and continuous process.
Filling stage
Filling is the first step in the injection cycle from the time the mold is closed to the time when the mold cavity is about 95% filled. Theoretically, the shorter the filling time, the higher the molding efficiency; But in the actual production, the molding time (or injection speed) to be subject to many conditions.
High speed filling. When filling at high speed, the shear rate is high, and the viscosity of plastic decreases due to the shear thinning effect, so that the overall flow resistance is reduced. Local effects of viscous heating can also make the cured layer thinner. Therefore, in the flow control stage, the filling behavior often depends on the volume to be filled. That is, in the flow control stage, due to the high filling speed, the shear thinning effect of the melt is often large, while the cooling effect of the thin wall is not obvious, so the effect of the rate is the upper hand.
Low speed fill. At low filling speed, the shear rate is lower, the local viscosity is higher, and the flow resistance is larger. Due to the slow replenishment rate of hot plastics and the slow flow, the heat conduction effect is more obvious, and the heat is quickly taken away by the cold mold wall. With a small amount of viscous heating phenomenon, the curing layer thickness is thick, and further increase the flow resistance at the thin wall.
The plastic polymer chain in front of the flow wave is almost parallel to the flow wave due to the flow of the fountain. Therefore, when the two strands of plastic glue meet, the polymer chains on the contact surface are parallel to each other; In addition, the properties of the two molten adhesives are different (the retention time in the mold cavity is different, the temperature and pressure are also different), which results in the poor structural strength in the intersection area of the molten adhesives. When the parts are placed at an appropriate Angle under the light and observed by the naked eye, it can be found that there are obvious joint lines, which is the formation mechanism of weld marks. Welding marks not only affect the appearance of plastic parts, but also their loose microstructure, easy to cause stress concentration, so that the strength of the part of the fracture.
Generally speaking, the strength of weld marks generated in the high temperature zone is better. In the case of high temperature, the polymer chain has relatively good activity and can penetrate and wind each other. In addition, the temperature of the two strands of melt in the high temperature region is relatively close, and the thermal properties of the melt are almost the same, which increases the strength of the fusion region. On the contrary, in the low temperature area, the welding strength is poor.
The holding period
The role of the holding stage is to continuously apply pressure to compact the melt and increase the density (densification) of the plastic to compensate for the shrinkage behavior of the plastic. In the pressure-holding process, the back pressure is high because the cavity is filled with plastic. In the process of holding pressure compaction, the screw of the injection molding machine can only slowly move forward slightly, and the flow speed of the plastic is relatively slow. At this time, the flow is called holding pressure flow. Because in the pressure holding stage, the plastic by the mold wall cooling solidification speed up, melt viscosity increases quickly, so the resistance in the mold cavity is very large. In the later stage of the pressure holding, the material density continues to increase, and the plastic parts are gradually formed. The pressure holding stage should continue until the gate is solidified and sealed. At this time, the pressure of the cavity in the pressure holding stage reaches the highest value.
In the pressure-holding stage, the plastic exhibits partial compressibility because the pressure is quite high. In the high pressure area, the plastic is more dense and the density is higher. In the lower pressure area, the plastic is looser and the density is lower, which causes the density distribution to change with location and time. In the process of holding pressure, the flow rate of plastic is very low and the flow no longer plays a leading role. Pressure is the main factor affecting the process of holding pressure. In the pressure-holding process, the plastic has already filled the mold cavity, and the gradually solidified melt acts as the medium to transfer the pressure. The pressure in the mold cavity is transferred to the surface of the mold wall with the help of plastic, and there is a tendency to open the mold, so appropriate clamping force is needed for clamping. Under normal circumstances, the expansion force will slightly open the mold, which will help the exhaust of the mold. But if the mold force is too large, it is easy to cause the rough edges of the molding products, overflow, and even open the mold. Therefore, when choosing the injection molding machine, the injection molding machine with sufficient clamping force should be selected to prevent the phenomenon of rising mold and to effectively maintain pressure.
In the new injection molding environment, we need to consider some new injection molding process, such as gas assisted molding, water assisted molding, foam injection molding and other injection molding,