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Plastic parts of the injection molding process mainly include filling - pressure - cooling - demoulding and other four stages, the four stages directly determine the quality of the product molding, and the four stages are a complete continuous process.

 

Filling stage

 

The filling is the first step in the entire injection cycle, from when the mold is closed to when the mold cavity is filled to about 95%. Theoretically, the shorter the filling time, the higher the molding efficiency, but in practice, the molding time or injection speed is subject to many conditions.

 

High-speed filling

 

When filling at high speed, the shear rate is higher, and the viscosity of plastic decreases due to the effect of shear thinning, so that the overall flow resistance is reduced; Local viscous heating effects can also thin the cured layer. Therefore, in the flow control stage, the filling behavior often depends on the volume to be filled.

 

In other words, in the flow control stage, the shear-thinning effect of melt is often great due to high-speed filling, while the cooling effect of thin walls is not obvious, so the efficiency of speed prevails.

 

Low-speed filling

 

Low-speed filling controlled by heat conduction has a lower shear rate, higher local viscosity, and greater flow resistance. As the hot plastic replenishment rate is slow, the flow is slow, so that the heat conduction effect is obvious, and the heat is quickly carried away by the cold mold wall. With less viscous heating, the solidified layer is thicker, which further increases the flow resistance at the thinner wall.

 

Due to the fountain flow, the plastic polymer chain in front of the flow wave is almost parallel to the flow wavefront. Therefore, when the two strands of plastic melt glue meet, the polymer chain of the contact surface is parallel to each other. In addition, the two strands have different properties (different retention times in the cavity, different temperatures, and pressure), resulting in poor microstructure strength in the intersection area of the melt.

 

Under the light, the parts are placed at the appropriate Angle to observe with the naked eye, and it can be found that there are obvious joint lines, which is the formation mechanism of weld marks. Weld marks not only affect the appearance of plastic parts but also cause stress concentration due to the loose microstructure, which reduces the strength of this part and causes a fracture.

 

Generally speaking, the strength of weld marks is better in the high-temperature zone, because the activity of the polymer chain is better at high temperatures, which can penetrate and entangle each other. In addition, the temperature of two strands of melt in the high-temperature zone is close, and the thermal properties of melt are almost the same, which increases the strength of the weld zone. On the contrary, the welding strength is poor in low-temperature areas.

 

The holding period

 

The function of the holding phase is to compensate for the shrinkage behavior of the plastic by applying constant pressure to compact the melt and increasing the density (densification) of the plastic. In the process of holding pressure, the backpressure is high because the cavity has been filled with plastic. In the process of compaction, the screw of the injection molding machine can only slowly move forward, and the flow speed of the plastic is relatively slow. At this time, the flow is called the pressure flow.

 

Because in the pressure holding stage, the plastic by the mold wall cooling solidification speed, melt viscosity increases quickly, so the mold cavity resistance is very large. In the later stage of holding pressure, the material density continues to increase, and the plastic parts gradually take shape. The holding pressure stage should continue until the gate solidifies and seals. At this time, the pressure of the cavity in the holding pressure stage reaches the maximum value.

 

During the compression stage, the plastic exhibits partially compressible properties due to the high pressure. In the area of higher pressure, the plastic is more dense and dense; In the lower pressure area, the plastic is looser and less dense, thus causing the density distribution to change with location and time. Plastic flow rate is very low in the process of pressure preservation, and flow no longer plays a leading role; Pressure is the main factor affecting the process of pressure preservation.

 

During the pressure holding process, the plastic has filled the cavity, and the gradually solidifying melt acts as the pressure transfer medium. The pressure in the mold cavity is transmitted to the surface of the mold wall with the help of plastic, which tends to open the mold. Therefore, an appropriate clamping force is required to lock the mold. Die force in normal circumstances will slightly split the mold, for the mold exhaust has a helping role; But if the deadly force is too large, easy to cause molding products to burr, overflow, and even open the mold.

 

Therefore, in the selection of injection molding machines, we should choose the injection molding machine with enough clamping force, in order to prevent the phenomenon of expanding mold and can effectively maintain pressure.

 

The cooling stage

 

Cooling system design is very important in injection molding. This is because molding plastic products only cooling solidified to a certain rigidity, after demoulding to avoid plastic products because of external force and deformation. As the cooling time accounts for about 70% ~ 80% of the whole molding cycle, a well-designed cooling system can greatly shorten the molding time, improve injection productivity and reduce costs.

 

The improper cooling system will lengthen the molding time and increase the cost; Uneven cooling will further cause the warping deformation of plastic products.

 

According to the experiment, the heat from the melt into the mold can be roughly divided into two parts, one part of 5% is transferred to the atmosphere by radiation and convection, and the other 95% is transferred from the melt to the mold. Due to the cooling water pipe in the mold, heat is transmitted from the plastic in the mold cavity to the cooling water pipe through heat conduction through the mold frame and then taken away by the cooling liquid through thermal convection. A small amount of heat that is not taken away by cooling water continues to conduct in the mold and is scattered in the air after contacting the outside world.

 

The molding cycle of injection molding consists of mold closing time, filling time, holding time, cooling time, and demolding time. Among them, cooling time accounts for the largest proportion, about 70% ~ 80%. Therefore, the cooling time will directly affect the length of the plastic product molding cycle and production size. In demoulding stage, the temperature of plastic products should be cooled to lower than the thermal deformation temperature of plastic products, in order to prevent the plastic products due to residual stress relaxation phenomenon or demolding force caused by warping and deformation.


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