Injection molding process for plastic parts

 

The injection molding process mainly includes four stages, such as filling, pressure holding, cooling and demolding. These four stages are very important and can directly determine the molding quality of injection molding products, and these four stages are a complete continuous process.

 

First, the filling stage

 

Filling is the first step in the cycle of the entire injection molding process, and the time is calculated from the beginning of the injection of the mold closure to the filling of the mold cavity 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: the shear rate of high speed filling is high, and the viscosity of plastic decreases due to the effect of shear thinning, so that the overall flow resistance decreases; Local viscous heating effects can also thin the cured layer thickness. Therefore, in the flow control phase, the filling behavior often depends on the volume to be filled. That is, in the flow control stage, due to the high-speed filling, the shear thinning effect of the melt is often large, while the cooling effect of the thin wall is not obvious, so the utility of the rate dominates.

 

Low-speed filling: When the low-speed filling is controlled by heat conduction, the shear rate is lower, the local viscosity is higher, and the flow resistance is greater. Due to the slow filling rate of thermoplastics and the slow flow, the heat conduction effect is more obvious, and the heat is quickly taken away by the cold mold wall. In addition to a small amount of viscous heating, the thickness of the solidified layer is thicker, which further increases the flow resistance at the thinner part of the wall.

 

Generally speaking, the strength of the fusion marks produced in the high temperature region is better, because under the high temperature situation, the polymer chain has better activity and can penetrate and wrap each other. In addition, the temperature of the two melts 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 region, the fusion strength is poor.

 

Two, pressure holding stage

 

The role of the holding stage is to continuously apply pressure to compress the melt and increase the plastic density (densification) to compensate for the shrinkage behavior of the plastic. During the pressure holding process, the back pressure is high because the mold cavity is already 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 also relatively slow. At this time, the flow is called the pressure flow. Because in the press-holding stage, the plastic is accelerated by the cooling of the mold wall, and the melt viscosity increases quickly, so the resistance in the mold cavity is very large. In the later stage of pressure retention, the density of the material continues to increase, and the plastic parts are gradually formed. The pressure retention stage should continue until the gate is cured and sealed. At this time, the pressure of the mold cavity in the pressure retention stage reaches a higher value.

 

During the pressureholding phase, the plastic exhibits partially compressible characteristics due to the considerable high pressure. In the high pressure area, the plastic is denser and has higher density. In the lower pressure area, the plastic is loose and has a lower density, which causes the density distribution to change with location and time. The flow rate of plastic is very low in the process of holding pressure, and the flow no longer plays a dominant role. Pressure is the main factor affecting the pressure holding process.

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Third, the cooling stage

 

The design of the cooling system is very important in injection molding molds. This is because molding plastic products are only cooled and solidified to be rigid enough to avoid deformation of plastic products due to external force after demolding. Because the cooling time accounts for about 70% ~ 80% of the whole molding cycle, a well-designed cooling system can significantly shorten the molding time, improve injection molding productivity, and reduce costs. A poorly designed cooling system will prolong the molding time and increase the cost; Uneven cooling will further cause warping deformation of plastic products.

 

Four, demoulding stage

 

Demolding is the last step in the injection molding cycle. Although the product has been cold solid molding, but demoulding or the quality of the product has a very important impact, improper demoulding method, may lead to uneven force in the product in demoulding, extrusion caused by product deformation and other defects. There are two main ways of demoulding: rod demoulding and stripper demoulding. According to the structural characteristics of the product, the appropriate demolding method should be selected to ensure the quality of the product.