Injection molding Process
- Injection molding process. The injection molding process of plastic parts mainly includes four stages: filling, holding pressure, cooling and demolding. These four stages directly determine the molding quality of products, and these four stages are a complete and continuous process.
- Filling stage. Filling is the first step in the whole injection cycle, from the time when the mold is closed to the time when the injection starts, until 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 restricted by many conditions. High speed filling. When filling at high speed, the shear rate is high, and the viscosity of plastics decreases due to shear thinning, which reduces the overall flow resistance. The influence of local viscous heating will also make the thickness of 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-speed filling, the shear thinning effect of the melt is often very large, while the cooling effect of the thin wall is not obvious, so the utility of the rate has the upper hand. Low speed filling. When low-speed filling is controlled by heat conduction, the shear rate is lower, the local viscosity is higher, and the flow resistance is larger. Due to the slow replenishment rate and slow flow of thermoplastic, 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 phenomenon, the thickness of solidified layer is thicker, which further increases the flow resistance at the thinner wall. Because of the fountain flow, the plastic polymer chain in front of the flowing wave is aligned to the almost parallel flowing wave front. Therefore, when two strands of plastic melt glue meet, the polymer chains of the contact surfaces are parallel to each other; In addition, the properties of the two melts are different (the residence time in the mold cavity is different, the temperature and pressure are also different), which leads to the poor structural strength of the melt-glue intersection area. When the parts are placed at an appropriate angle under light, it can be found that there are obvious joint lines, which is the formation mechanism of weld lines. The weld line not only affects the appearance of plastic parts, but also easily causes stress concentration due to the looseness of microstructure, which reduces the strength of this part and leads to fracture. Generally speaking, the strength of the weld line produced in the high-temperature area is better, because the polymer chains have better mobility and can penetrate and entangle each other. In addition, the temperatures of the two melts in the high-temperature area are close, and the thermal properties of the melts are almost the same, which increases the strength of the welding area. On the contrary, in the low temperature area, the welding strength is poor.
- Pressure maintaining stage. The function of holding pressure stage is to continuously apply pressure, compact the melt and increase the density (densification) of plastics to compensate the shrinkage behavior of plastics. During the pressure maintaining process, the back pressure is high because the mold cavity has been filled with plastic. In the process of pressure-keeping compaction, the screw of 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-keeping 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 density of materials continues to increase, and the plastic parts are gradually formed. The pressure maintaining stage should last until the gate is solidified and sealed, at which time the cavity pressure in the pressure maintaining stage reaches the highest value. In the pressure-holding stage, the plastic is partially compressible due to the high pressure. In the area with high pressure, the plastic is denser and denser; In the area with low pressure, the plastic is loose and its density is low, which causes the density distribution to change with location and time. In the process of maintaining pressure, the flow rate of plastic is extremely low, and the flow no longer plays a leading role; Pressure is the main factor affecting the pressure maintaining process. During the pressure maintaining process, the plastic has filled the mold cavity, and the gradually solidified melt is used as the medium for transmitting pressure. The pressure in the mold cavity is transmitted to the surface of the mold wall by means of plastic, which tends to open the mold, so proper clamping force is needed for mold clamping. Under normal circumstances, the mold expansion force will slightly open the mold, which is helpful for the exhaust of the mold; However, if the mold expanding force is too large, it is easy to cause burrs and flash of molded products, and even open the mold. Therefore, when choosing an injection molding machine, an injection molding machine with sufficient clamping force should be selected to prevent mold expansion and maintain pressure effectively.
- Cooling stage. The design of cooling system is very important in injection mold. This is because the molded plastic products can only be cooled and solidified to a certain rigidity, and the plastic products can be prevented from being deformed by external forces after demolding. 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 the injection productivity and reduce the cost. Improper design of cooling system will lengthen the molding time and increase the cost; Uneven cooling will further cause the warpage of plastic products. According to the experiment, the heat from the melt into the mold is generally distributed in two parts, one part of which is 5% transferred to the atmosphere by radiation and convection, and the other 95% is transferred from the melt to the mold. Because of the effect of cooling water pipe in the mold, the heat of the plastic in the mold cavity is transferred to the cooling water pipe through the mold base through thermal conduction, and then taken away by the cooling liquid through thermal convection. A small amount of heat that is not taken away by the cooling water will continue to be conducted in the mold, and will be 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, the 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 demolding stage, the temperature of plastic products should be cooled below 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 demolding external force. The factors that affect the cooling rate of products are: design of plastic products. Mainly the wall thickness of plastic products. The thicker the product, the longer the cooling time. Generally speaking, the cooling time is proportional to the square of the thickness of the plastic product, or to the 1.6 power of the maximum diameter of the runner. That is, the thickness of plastic products is doubled, and the cooling time is increased by 4 times. Materials and their cooling methods. Materials of mold, including mold core, cavity material and mold base material, have great influence on cooling rate. The higher the thermal conductivity of the mold, the better the effect of transferring heat from the plastic per unit time, and the shorter the cooling time. Configuration of cooling water pipes. The closer the cooling water pipes are to the mold cavity, the larger the pipe diameter and the more the number, the better the cooling effect and the shorter the cooling time. Coolant flow rate. The larger the flow rate of cooling water (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 thermal conductivity of the coolant will also affect the thermal conductivity of the mold. The lower the viscosity of the coolant, the higher the thermal conductivity and the lower the temperature, the better the cooling effect. Selection of plastics. Plastic refers to the measurement of the speed at which plastic conducts heat from a hot place to a cold place. The higher the thermal conductivity of plastic, the better the thermal conductivity, or the lower the specific heat of plastic, and the temperature changes easily, so the heat is easy to dissipate, the better the thermal conductivity, and the shorter the cooling time. Setting of processing parameters. 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 the uniform and rapid cooling effect. The cooling system is designed to maintain proper and efficient cooling of the mold. The cooling holes shall be of standard size to facilitate processing and assembly. When designing the cooling system, the mold designer must decide 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 the hole, the configuration and connection of the hole, and the flow rate and heat transfer properties of the cooling liquid.
- Demolding stage. Demolding is the last link in an injection molding cycle. Although the product has been cold-solidified, demolding still has a very important impact on the quality of the product. Improper demolding methods may lead to uneven stress on the product during demolding and deformation of the product during ejection. There are two main demolding methods: ejector demolding and stripper demolding. When designing the mold, we should choose the appropriate demolding method according to the structural characteristics of the product to ensure the product quality. For the mold with ejector pins for demolding, the ejector pins should be set as evenly as possible, and the position should be selected at the place where the demolding resistance is the largest and the strength and rigidity of the plastic parts are the largest, so as to avoid deformation and damage of the plastic parts. The stripper plate is generally used for demolding deep-cavity thin-walled containers and transparent products that are not allowed to have push rod marks. This mechanism is characterized by large and uniform demolding force, smooth movement and no obvious traces left.
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