Main processes of mold injection molding

Posted on :Sep 26, 2022 By  GREFEE

What are the processes of injection molding process? The plastic injection molding process mainly includes four stages, which are filling, pressure maintaining, cooling and mold releasing, which are an integrate and continual progress that directly affect the molding quality of products.

Filling stage

Filling is the first store of the whole injection molding process, which starts from the beginning of injection molding when the mold is clamped to the mold cavity is about 95% filled. Theoretically, the shorter the filling time, the higher the molding rates. However, the molding time or the injection speed is limited by many conditions actually, not merely relies on the fast speed.

The shear rates of the speeding filling time is high. The viscosity of plastic decreases due to the effect of the shear thinning, which slows down the overall flow resistance. The local viscous heating effect will also make the curing thickness thinner. Thus, during the flow control stage, the filling generally depends on the size of the volume to be filled. In another word, in the flow control stage, due to high-speed filling, the shear thinning effect of melt is often high, but the cooling effect of thin wall is not evident, so the effect of rate prevails.

Low speed filling. The shear rate is low, the local viscosity is high, and the flow resistance is large. heat conduction control when filling at a low speed. Slow replenishment rate and slow flow of thermoplastics make the heat conduction effect obvious, so the heat is quickly taken away by the cold film wall. Asa well as a small amount of viscous heating, the thickness of the curing layer is thicker, which further increases the flow resistance at the thinner part of the wall.

Due to the flow of the fountain, the plastic polymer chain in front of the flow wave is arranged almost parallel to the flow wave front. Therefore, the polymer chains on the contact surface are parallel to each other when the two plastic melts come across. As well as the different properties of the two strands of melt (different retention time in the membrane cavity, different temperature and pressure), the microstructure and strength of the intersection area of melt are extremely low.

Under the sunlight, there are obvious bonding lines when observing the parts are placed at an appropriate angle with the naked eye, which is the formation mechanism of welding lines. Weld marks not only affect the appearance of plastic parts, but also cause stress concentration due to the loose microstructure, which lowers the strength of some parts and may further lead to fracture.Generally, the welding strength is great under some high temperate areas. Thus, under high temperature, the moving performance of the high molecular chain is excellent, which can be mutually penetrated and entangled. In addition, the temperature of the two melts under high temperature areas are almost similar. The thermal properties of melts are almost same, too. To increase the strength of the welding area, the welding strength of low temperature areas is poor.

Pressure holding stage

The function of pressure holding can maintain the pressure to compress the melt sufficiently and increase the density of plastic to compensate the shrinkage of plastic. During the stage, as the mold cavity is filled with plastic and its back pressure is high, so the screw of injection molding machine only can make slightly move forward during the hard compressing. The flow speed of plastic is also slow, which is called the pressure maintaining flow.

In the pressure maintaining stage, since the solidification of plastic is faster due to the cooling by the mold wall, and the melt viscosity increases rapidly, so the resistance in the mold cavity is very large. In the later stage of pressure maintaining, the density of the material continues to increase, and the plastic is gradually formed. The pressure maintaining stage should continue until the gate is sealed. At this time, the mold cavity pressure in the pressure maintaining stage reaches the highest value.

During the pressure holding stage, due to the ultra high pressure, the plastic can be partially compressible. In the area with a high pressure, the plastic is dense, while in the area with low pressure, the plastic is porosity with low density, and thus the distribution of density can change as the time. The flow speed of the plastic during the pressure holding process is low, the flow is no longer be the dominated role. Pressure is the main factor affecting the pressure holding process.

During the pressure holding stage, the plastic has filled the cavity. At this moment, the gradually solidified melt works as a medium to pass the pressure. The pressure inside the cavity is passed to the surface of the mold cavity by plastic, which is going to open the mold, thus it requires appropriate mold clamping force to clamp the mold. In the normal situation, the increased mold clamping force will spread out the mold, which is helpful with the ventilation of mold. If the force is too large, it will cause the burrs, overflow, or spread out the mold.

Thus, when selecting the injection molding machines, we should select the ones with large mold clamping force to prevent the expansion being happened and effectively carry out the pressure holding.

Cooling stage

For the injection molds, the design of the cooling system is essential, which because the deformation of products can only be prevented when it reaches  a certain rigidity through the cooling and solidification after the mold releasing. As the cooling time only takes up about 70 ~80% of the whole molding cycle, the deigns of cooling system can significantly reduce the molding cycle, increase the production rate of injection molding, and lower the cost. Improper design of cooling system will extend the molding time and increase the cost. Uneven cooling will further cause the deformation and warpage of the plastic products.

According to the experiment, the heat produced when the melt enters into the mold is diffused mainly through two means. 5% of the heat gets into the air through the radiation, and another 95% is transferred from the melt into molds. Due to the water cooling, the heat is transferred by the plastic in the mold cavity through heat conduction to the cooling water pipe through the mold base, and then taken away by the coolant through heat convection. A small amount of heat that has not been taken away by the cooling water continues to be transmitted in the mold, and is lost in the air after contacting the outside world.

The molding cycle of the injection molding is composed of mold clamping time, filling time, pressure holding time, cooling time and mold releasing time, among which the cooling time accounts up the largest promotion, which is about 70~80%. Thus, cooling time directly affects the length of the molding cycle, like abs injection molding process and the yield of products. The temperature of products during the mold releasing stage should be lower than that of the thermal deformation of products to prevent the loosen phenomenon due to the residual stress or the warpage and deformation due to the external force of the mold releasing.

Mold releasing stage

Mold releasing is the last step of the injection molding. Although the products have been cooled and formed, it still impacts significantly to the quality of products. Improper mold releasing methods will lead to uneven stress of the product during mold releasing and deformation of the product during ejection. There are two means to release the mold: ejector rod mold releasing and stripper plate mold releasing. The design of mold should be according to the structural characteristics of the products to select appropriate mold releasing modes to ensure the quality.

For molds that require ejector rod demoulding, the position of the ejector rod should be as even as possible, and the position should be at the places with largest resistance of mold releasing and strength and rigidity of plastic parts to avoid the deformation and damages.

While, the stripper plate is often used in deep cavity and thin wall containers or mold release of clear products without putter marks. The characteristic of this structure is that the mold release is large and even, smooth movement, and no obvious traces left.


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