Summary of four knowledge points of injection molding process

The injection molding process of plastic parts mainly includes four stages: filling, pressure maintaining, cooling and demoulding. These four stages directly determine the molding quality of products, and these four stages are a complete continuous process.

1. Filling phase

Filling is the first step in the whole injection cycle. The time starts from the closing of the mold 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 are restricted by many conditions.

High speed filling. When filling at high speed, the shear rate is high, and the viscosity of plastic decreases due to shear thinning, which reduces the overall flow resistance; Local viscous heating will also make the thickness of 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 high-speed filling, the shear thinning effect of melt is often great, while the cooling effect of thin wall is not obvious, so the effect of rate prevails.

Low speed filling. When the heat conduction control is low-speed filling, the shear rate is low, the local viscosity is high, and the flow resistance is large. Due to the slow supplement rate and slow flow of thermoplastics, the heat conduction effect is obvious, and the heat is quickly taken away by the cold mold wall. With 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.

Due to the fountain flow, the plastic polymer chain in front of the flow wave is arranged almost parallel to the flow wave front. Therefore, when two strands of plastic melt glue meet, the polymer chains on the contact surface are parallel to each other; In addition, the properties of the two strands of melt adhesive are different (different residence time in the mold cavity, different temperature and pressure), resulting in poor microstructure strength in the intersection area of melt adhesive. When the parts are placed at an appropriate angle under the light and observed with the naked eye, it can be found that there are obvious bonding lines, which is the formation mechanism of fusion 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 breaks.

Generally speaking, the strength of the weld line where the fusion occurs in the high temperature zone is better, because in the case of high temperature, the polymer chain has better activity and can penetrate and wind each other. In addition, the temperature of the two melts in the high temperature zone is relatively close, and the thermal properties of the melt are almost the same, which increases the strength of the fusion zone; On the contrary, in the low temperature region, the welding strength is poor.

2. Pressure maintaining stage

The function of the pressure maintaining stage is to continuously apply pressure, compact the melt and increase the plastic density (densification) to compensate for the shrinkage behavior of the plastic. In the pressure maintaining process, the back pressure is high because the mold cavity has been filled with plastic. In the process of pressure maintaining and compaction, the screw of the injection molding machine can only move forward slowly, and the flow speed of plastic is also relatively slow. At this time, the flow is called pressure maintaining flow. In the pressure maintaining stage, the plastic is cooled and solidified 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 material density continues to increase and the plastic parts are gradually formed. The pressure maintaining stage should continue until the gate is cured and sealed. At this time, the mold cavity pressure in the pressure maintaining stage reaches the highest value.

In the pressure maintaining stage, due to the high pressure, the plastic presents partial compressibility. In the area with high pressure, the plastic is dense and the density is high; In the area with low pressure, the plastic is loose and the density is low, so the density distribution changes with position and time. In the process of pressure maintaining, the flow velocity of plastic is very low, and the flow no longer plays a leading role; Pressure is the main factor affecting the pressure holding process. During the pressure maintaining process, the plastic has filled the mold cavity, and the gradually solidified melt is used as the medium to transfer pressure. The pressure in the mold cavity is transmitted to the mold wall surface with the help of plastic, which has the trend of supporting the mold. Therefore, an appropriate clamping force is required for mold locking. The die expansion force will slightly open the die under normal circumstances, which is helpful for the exhaust of the die; However, if the mold expansion force is too large, it is easy to cause rough edges, overflow and even open the mold. Therefore, when selecting the injection molding machine, the injection molding machine with enough mold locking force should be selected to prevent mold expansion and effectively maintain the pressure.

3. Cooling stage

In the injection mold, the design of cooling system is very important. This is because only when the molded plastic products are cooled and solidified to a certain rigidity, can the deformation of the plastic products caused by external forces be avoided after demoulding. Because 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 the injection molding productivity and reduce the cost. Improper design of cooling system will prolong the forming time and increase the cost; Uneven cooling will further cause warpage and deformation of plastic products.

According to the experiment, the heat entering the mold from the melt is generally distributed in two parts, one part is 5% transmitted to the atmosphere through radiation and convection, and the other 95% is transmitted from the melt to the mold. Plastic products in the mold due to the role of the cooling water pipe, the heat is transmitted from the plastic in the mold cavity to the cooling water pipe through the mold base through heat conduction, and then taken away by the coolant through heat convection. A small amount of heat not taken away by the cooling water continues to be transmitted in the mold and overflows in the air after contacting the outside world.

The molding cycle of injection molding consists of mold closing time, filling time, pressure holding time, cooling time and demoulding time. Cooling time accounts for the largest proportion, about 70% ~ 80%. Therefore, the cooling time will directly affect the molding cycle and output of plastic products. In the demoulding stage, the temperature of plastic products shall be cooled to be lower than the thermal deformation temperature of plastic products, so as to prevent the relaxation of plastic products caused by residual stress or warpage and deformation caused by external demoulding force.

The factors affecting the cooling rate of products are: plastic product design. Mainly the wall thickness of plastic products. The greater the thickness of the product, the longer the cooling time. Generally speaking, the cooling time is approximately proportional to the square of the thickness of the plastic product, or to the 1.6 power of the maximum channel diameter. That is, the thickness of plastic products is doubled and the cooling time is increased by 4 times.

Die material and its cooling mode. Mold materials, including mold core, cavity and mold base materials, have a great impact on the cooling rate. The higher the thermal conductivity of mold material, the better the effect of heat transfer from plastic per unit time, and the shorter the cooling time.

Cooling water pipe configuration. The closer the cooling water pipe is to the mold cavity, the larger the pipe diameter, the more the number, the better the cooling effect and the shorter the cooling time.

Coolant flow. The larger the cooling water flow (generally it is better to achieve turbulence), the better the effect of cooling water taking away heat by thermal convection.

Properties of coolant. The viscosity and heat transfer coefficient of coolant will also affect the heat transfer effect of the die. The lower the viscosity of the coolant, the higher the thermal conductivity and the lower the temperature, the better the cooling effect.

Plastic selection. Plastic is a measure of the velocity at which plastic conducts heat from a hot place to a cold place. The higher the thermal conductivity of plastics, the better the heat conduction effect, or the lower the specific heat of plastics, the temperature is easy to change, so the heat is easy to escape, the better the heat conduction effect and the shorter the cooling time.

Machining parameter setting. The higher the material temperature, the higher the die temperature, the lower the ejection temperature and the longer the cooling time.

Design rules of cooling system:

The designed cooling channel should ensure uniform and rapid cooling effect.

The cooling system is designed to maintain proper and efficient cooling of the die. Cooling holes shall be of standard size to facilitate processing and assembly.

When designing the cooling system, the die designer must determine the following design parameters according to the wall thickness and volume of the plastic part – the position and size of the cooling hole, the length of the hole, the type of hole, the configuration and connection of the hole, as well as the flow rate and heat transfer properties of the coolant.

4. Demoulding stage

Demoulding is the last link in an injection molding cycle. Although the product has been cold solid formed, demoulding still has a very important impact on the quality of the product. Improper demoulding method may lead to uneven stress of the product during demoulding and deformation of the product during ejection. There are two ways of demoulding: ejector rod demoulding and stripper plate demoulding. When designing the mold, the appropriate demoulding method shall be selected according to the structural characteristics of the product to ensure the product quality.

For the mold with ejector rod demoulding, the ejector rod shall be set as evenly as possible, and the position shall be selected where the demoulding resistance is the largest and the strength and stiffness of plastic parts are the largest, so as to avoid deformation and damage of plastic parts.

The stripper plate is generally used for the demoulding of deep cavity thin-walled containers and transparent products that are not allowed to have push rod traces. This mechanism is characterized by large and uniform demoulding force, stable movement and no obvious traces.