1 process flow
These 6 stages directly determine the molding quality of the product, and these 6 stages are a complete continuous process.
Filling is a step in the entire injection molding cycle. The time starts from the time the mold is closed and the mold is filled to about 95%. In theory, the shorter the filling time, the higher the molding efficiency; however, in actual production, the molding time (or injection speed) is subject to many conditions.
High-speed filling. The shear rate is high during high-speed filling, and the viscosity of the plastic decreases due to the shear thinning effect, which reduces the overall flow resistance; the effect of local viscous heating will also reduce the thickness of the cured layer. 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 great, and the cooling effect of the thin wall is not obvious, so the utility of the rate prevails.
Low speed filling. When the heat transfer is controlled at low speed, the shear rate is low, the local viscosity is high, and the flow resistance is large. Due to the slower replenishment rate of the thermoplastic and the slower flow, the heat conduction effect is more obvious, and the heat is quickly taken away by the cold mold wall. Coupled with a smaller amount of viscous heating phenomenon, the thickness of the cured layer is thicker, and the flow resistance of the thinner part of the wall is further increased.
Due to the flow of the fountain, the plastic polymer chains in front of the flow wave are aligned to an almost parallel flow wavefront. Therefore, when the two plastic melts meet, the polymer chains on the contact surfaces are parallel to each other; coupled with the different properties of the two melts (the residence time in the mold cavity is different, and the temperature and pressure are also different), the melt fusion area is Microstructure strength is poor. Place the part at an appropriate angle under the light and observe with the naked eye. It can be seen that there are obvious bonding lines, which is the formation mechanism of the weld mark. Welding marks not only affect the appearance of plastic parts, but their microstructures are loose, which can easily cause stress concentration, which reduces the strength of the part and causes fracture.
Generally speaking, the strength of a weld mark that produces a weld in a high temperature region is better. Because under high temperature conditions, the polymer chains have relatively good mobility and can penetrate and intertwine with each other.In addition, the temperature of the two melts in the high temperature region is close, and the thermal properties of the melts are almost the same, which increases the strength of the welded region. In low temperature areas, the welding strength is poor.
The role of the holding stage is to continuously apply pressure, compact the melt, and increase the density (densification) of the plastic to compensate for the shrinkage behavior of the plastic. During the holding process, the back pressure is high because the cavity is filled with plastic. In the process of holding and compacting, the screw of the injection molding machine can only slowly move forward slightly, and the flow speed of the plastic is also relatively slow. The flow at this time is called holding pressure flow. As the plastic is cooled and solidified by the mold wall and the melt viscosity increases rapidly during the holding pressure stage, the resistance in the mold cavity is very large. In the later stage of holding pressure, the material density continues to increase, and the plastic parts are gradually formed. The holding pressure phase must continue until the gate is solidified and sealed.
During the dwell phase, the plastic is partially compressible due to the relatively high pressure. In areas with high pressure, plastics are denser and denser; in areas with low pressure, plastics are looser and lower in density, so the density distribution changes with location and time. During the holding process, the plastic flow rate is extremely low, and the flow no longer plays a leading role; pressure is the main factor affecting the holding process. During the holding process, the plastic has filled the mold cavity. At this time, the gradually solidified melt serves as a medium for transmitting pressure. The pressure in the mold cavity is transmitted to the surface of the mold wall through the plastic, which tends to open the mold. Therefore, an appropriate mold clamping force is required for mold clamping. Under normal circumstances, the mold expansion force will slightly stretch the mold, which will help the exhaust of the mold; however, if the mold expansion force is too large, it will easily cause burrs, flashes, and even stretch the mold. Therefore, when selecting an injection molding machine, an injection molding machine with a sufficiently large clamping force should be selected to prevent the phenomenon of mold expansion and to effectively maintain pressure.
Under the new injection molding environment conditions, we need to consider some new injection molding processes, such as gas-assisted molding, water-assisted molding, foam injection molding, etc.
In injection molds, the design of the cooling system is very important. 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 due to external forces after demolding. Since the cooling time accounts for about 70% to 80% of the entire molding cycle, a well-designed cooling system can greatly reduce the molding time, improve injection productivity, and reduce costs. An improperly designed cooling system will lengthen the molding time and increase costs; uneven cooling will further cause warpage and deformation of plastic products.
According to the experiment, the heat entering the mold from the melt is roughly divided into two parts, one part is transmitted to the atmosphere through radiation and convection, and the remaining 95% is conducted from the melt to the mold. Due to the role of the cooling water pipe in the mold, heat is transferred 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 cooling liquid through thermal convection. A small amount of heat that is not taken away by the cooling water continues to be conducted in the mold, and then escapes into the air after contacting the outside world.
The molding cycle of injection molding is composed of mold clamping time, filling time, pressure holding time, cooling time and demolding time. Among them, the cooling time accounts for a large proportion, about 70%-80%. Therefore, the cooling time will directly affect the length and output of plastic products. During the demolding phase, the temperature of the plastic product should be cooled below the thermal deformation temperature of the plastic product to prevent the plastic product from being loosened due to residual stress or warped and deformed by the external force of the mold.
The factors that affect the cooling rate of the product are:
Design of plastic products. Mainly the wall thickness of plastic products. The larger the product thickness, the longer the cooling time. Generally speaking, the cooling time is approximately proportional to the square of the thickness of the plastic product, or proportional to the 1.6th power of the large runner diameter. That is, the thickness of plastic products is doubled, and the cooling time is increased by 4 times.
Mold material and its cooling method. Mold materials, including mold cores, cavity materials, and mold base materials, have a significant effect on cooling rates. The higher the thermal conductivity of the mold material, the better the effect of transferring heat from the 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 diameter and the larger the number, the better the cooling effect and the shorter the cooling time.
Coolant flow. The greater the cooling water flow rate (generally turbulent flow is preferred), the better the cooling water will remove heat by means of thermal convection.
Coolant properties. The viscosity and thermal conductivity of the coolant will also affect the thermal conductivity of the mold. The lower the viscosity of the cooling liquid, the higher the thermal conductivity and the lower the temperature, the better the cooling effect.
Plastic choice. Plastic is a measure 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 the plastic, the temperature is likely to change, so the heat is easily dissipated, the thermal conductivity is better, and the cooling time is shorter.
Setting of processing parameters. The higher the material temperature, the higher the mold temperature, the lower the ejection temperature, and the longer the cooling time required.
Design rules for cooling systems:
The designed cooling channel should ensure uniform and rapid cooling effect.
The cooling system is designed to maintain proper and efficient cooling of the mold. Cooling holes should be of standard size to facilitate processing and assembly.
When designing the cooling system, the mold designer must determine the following design parameters based on the wall thickness and volume of the plastic part-the position and size of the cooling holes, the length of the holes, the type of the holes, the configuration and connection of the holes, and the flow rate of the coolant Heat transfer properties.
Demolding is the last step in an injection molding cycle. Although the product has been cold-formed, demolding still has a very important impact on the quality of the product. Improper demolding methods may cause the product to be unevenly stressed during demolding and cause defects such as product deformation when ejected. There are two main ways of demolding: ejection from ejector and demolding from stripper. When designing the mold, you should choose the appropriate demolding method according to the structural characteristics of the product to ensure product quality.
For the mold used for ejection of the ejector, the ejector should be set as evenly as possible, and the location should be selected in a place with high ejection resistance and strong strength and rigidity of the plastic part, so as not to deform and damage the plastic part.
The stripper plate is generally used for demolding of deep-cavity thin-walled containers and transparent products that are not allowed to have traces of push rods. This mechanism is characterized by a large and uniform demolding force, smooth movement and no obvious traces.
2 process parameters
The injection pressure is provided by the hydraulic system of the injection system. The pressure of the hydraulic cylinder is transmitted to the plastic melt through the screw of the injection molding machine. Under the pressure of the plastic melt, the plastic melt enters the vertical flow path of the mold (which is also the mainstream flow for some molds), the mainstream flow, and the shunt through the nozzle of the injection molding machine. And enter the mold cavity through the gate, this process is the injection process, or the filling process. The existence of pressure is to overcome the resistance in the flow of the melt, or conversely, the resistance in the flow needs to be offset by the pressure of the injection molding machine to ensure the smooth filling process.
During the injection molding process, the pressure at the nozzle of the injection molding machine is high to overcome the flow resistance throughout the melt. Thereafter, the pressure gradually decreases along the flow length toward the wave front of the melt front. If the inside of the cavity is well ventilated, the pressure behind the melt front is atmospheric pressure.
There are many factors that affect the melt filling pressure, which can be summarized into three categories: (1) material factors, such as the type and viscosity of plastics; (2) structural factors, such as the type, number and location of the pouring system, and the cavity of the mold Shape and thickness of the product, etc .; (3) molding process elements.
The injection time mentioned here refers to the time required for the plastic melt to fill the cavity, excluding auxiliary time such as mold opening and closing. Although the injection time is short and the impact on the molding cycle is small, the adjustment of the injection time has a great effect on the pressure control of the gate, runner and cavity. A reasonable injection time is helpful for the ideal filling of the melt, and it is of great significance for improving the surface quality of the product and reducing the dimensional tolerance.
The injection time is much lower than the cooling time, which is about 1/10 to 1/15 of the cooling time. This rule can be used as a basis for predicting the entire molding time of the plastic part. In the mold flow analysis, the injection time in the analysis result is equal to the injection time set in the process conditions only when the melt is completely filled by the screw to fill the cavity. If the holding pressure of the screw is switched before the cavity is full, the analysis result will be greater than the process conditions.
Injection temperature is an important factor affecting injection pressure. The injection molding machine barrel has 5 to 6 heating sections, and each raw material has its own suitable processing temperature (for detailed processing temperature, please refer to the data provided by the material supplier). The injection temperature must be controlled within a certain range. If the temperature is too low, the melt will not be plasticized, which will affect the quality of the molded parts and increase the difficulty of the process. If the temperature is too high, the raw materials will be easily decomposed. In the actual injection molding process, the injection temperature is often higher than the barrel temperature. The higher value is related to the injection rate and the performance of the material, which can be as high as 30 ° C. This is due to the high heat generated when the melt is sheared through the injection port. There are two ways to compensate for this difference when doing mold flow analysis. One is to try to measure the temperature of the melt during air injection molding, and the other is to include the nozzle when modeling.
Holding pressure and time
Near the end of the injection molding process, the screw stopped rotating, but only advanced forward. At this time, the injection molding entered the pressure holding stage. During the dwelling process, the nozzle of the injection molding machine continuously feeds the cavity to fill the volume vacated by the shrinkage of the part. If the cavity is not filled after the pressure is maintained, the part will shrink by about 25%, especially the ribs will shrink due to excessive shrinkage. The holding pressure is generally about 85% of the filling pressure. Of course, it must be determined according to the actual situation.
Back pressure refers to the pressure to be overcome when the screw is reversed and backed up for storage. The use of high back pressure is beneficial to the dispersion of the pigment and the melting of the plastic, but at the same time it prolongs the screw retraction time, reduces the length of the plastic fiber, and increases the pressure of the injection molding machine. 20% of pressure. When injecting foamed plastic, the back pressure should be higher than the pressure formed by the gas, otherwise the screw will be pushed out of the barrel. Some injection molding machines can be programmed with back pressure to compensate for the reduction in screw length during melting, which reduces the input heat and reduces the temperature. However, because the results of such changes are difficult to estimate, it is not easy to make corresponding adjustments to the machine.
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