An Introduction of Injection Molding Process
The injection molding
process mainly includes 6 stages including mold clamping, filling, pressure holding, cooling, mold opening and demoulding. These 6 stages directly determine the molding quality of the product, and these 6 stages are a complete continuous process. This chapter focuses on the four stages of filling, holding pressure, cooling, and demolding.
1. Filling stage
Filling is the first step in the entire injection molding cycle, from the time the mold is closed and the injection is started until the mold cavity is filled to approximately 95%. In theory, the shorter the filling time, the higher the molding efficiency; but in actual production, the molding time (or injection speed) is subject to many conditions.
High-speed filling. During high-speed filling, the shear rate is high, and the viscosity of the plastic decreases due to the effect of shear thinning, which reduces the overall flow resistance; the local viscous heating effect will also reduce the thickness of the cured layer. Therefore, during the flow control phase, the filling behavior often depends on the size of the volume to be filled. That is, in the flow control stage, due to high-speed filling, the shear thinning effect of the melt is often large, and the cooling effect of the thin wall is not obvious, so the effect of velocity prevails.
Fill at low speed. Heat conduction control When filling at low speed, the shear rate is lower, the local viscosity is higher, and the flow resistance is higher. Due to the slow replenishment rate and slow flow of the thermoplastic, the heat conduction effect is more obvious, and the heat is quickly taken away by the cold mold wall. Combined with a smaller amount of viscous heating, the solidified layer is thicker, which further increases the flow resistance at the thinner wall.
Due to the fountain flow, the plastic polymer chains in front of the flow wave are aligned almost parallel to the flow wave front. Therefore, when the two strands of plastic melt meet, the polymer chains on the contact surface are parallel to each other; in addition, the properties of the two strands of melt are different (the residence time in the mold cavity is different, and the temperature and pressure are also different), resulting in the fusion area of the melt. Microscopically, the structural strength is poor. 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 the weld line. The weld line not only affects the appearance of the plastic part, but also its microstructure is loose, which is easy to cause stress concentration, so that the strength of the part is reduced and fracture occurs.
Generally speaking, the strength of the weld line that produces the weld in the high temperature region is better. Because at high temperature, the mobility of polymer chains is relatively good, and they can penetrate and intertwine with each other. In addition, the temperature of the two melts in the high temperature area is relatively close, and the thermal properties of the melts are almost the same, which increases the strength of the welding area. In the low temperature area, the welding strength is poor.
2. Holding stage
The function of the packing stage is to continuously apply pressure to compact the melt and increase the density of the plastic (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 holding pressure and compaction, the screw of the injection molding machine can only move forward slowly and slightly, and the flow speed of the plastic is also relatively slow. The flow at this time is called holding pressure flow. In the pressure holding stage, the plastic is cooled and solidified faster 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 the holding pressure, the material density continues to increase, and the plastic parts are gradually formed. The pressure holding stage continues until the gate is cured and sealed. At this time, the cavity pressure in the pressure holding stage reaches the highest value.
During the packing stage, the plastic exhibits partially compressible properties due to the relatively high pressure. In areas of higher pressure, the plastic is denser and denser; in areas of lower pressure, the plastic is looser and lower in density, resulting in a change in density distribution with location and time. During the pressure-holding process, the flow rate of the plastic is extremely low, and the flow no longer plays a leading role; the pressure is the main factor affecting the pressure-holding process. During the pressure-holding process, the plastic has filled the mold cavity, and the gradually solidified melt is used as the medium for transmitting the pressure. The pressure in the mold cavity is transmitted to the surface of the mold wall through the plastic, and there is a tendency to open the mold, so an appropriate clamping force is required for mold clamping. Under normal circumstances, the mold expansion force will slightly open the mold, which is helpful for the exhaust of the mold; but if the mold expansion force is too large, it is easy to cause burrs, overflow of the molded product, and even open the mold. Therefore, when selecting an injection molding machine, an injection molding machine with sufficient clamping force should be selected to prevent mold expansion and effectively maintain pressure.
In the new injection molding environment, we need to consider some new injection molding processes, such as gas-assisted molding, water-assisted molding, foam injection molding, etc.
3. Cooling stage
In injection molding molds, the design of the cooling system is very important. This is because the molded plastic product can only be cooled and solidified to a certain rigidity, and the plastic product can be prevented from being deformed by external force after demolding. Since the cooling time accounts for about 70% to 80% of the entire molding cycle, a well-designed cooling system can greatly shorten the molding time, improve injection productivity, and reduce costs. Improperly designed cooling system will prolong 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 entering the mold is generally dissipated in two parts, one part is 5% transferred to the atmosphere by radiation and convection, and the remaining 95% is conducted from the melt to the mold. Due to the cooling water pipe in the mold, the heat is transferred from the plastic in the mold cavity to the cooling water pipe through the mold frame 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 dissipates into the air after contacting the outside world.
The molding cycle of injection molding consists of mold clamping time, filling time, pressure holding time, cooling time and demolding time. Among them, the cooling time accounts for the largest proportion, which is about 70% to 80%. Therefore, the cooling time will directly affect the length of the molding cycle and the output of plastic products. In the demolding stage, the temperature of the plastic product should be cooled to a temperature lower than the thermal deformation temperature of the plastic product to prevent the plastic product from loosening due to residual stress or warping and deformation caused by the external force of demolding.
The factors that affect the cooling rate of the product are:
1) 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 approximately proportional to the square of the thickness of the plastic product, or to the 1.6th power of the maximum runner diameter. That is, the thickness of the plastic product is doubled, and the cooling time is increased by 4 times.
2) The mold material and its cooling method. The mold material, including the mold core, cavity material, and mold base material has a great influence on the cooling rate. 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.
3) The configuration of cooling water pipes. The closer the cooling water pipe is 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.
4) Coolant flow. The larger the cooling water flow rate (generally better to achieve turbulent flow), the better the effect of cooling water to remove heat by thermal convection.
5) The nature of the 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.
6) Plastic choice. Plastic refers to a measure of how quickly plastic conducts heat from a hot place to a cold place. The higher the thermal conductivity of the plastic, the better the thermal conduction effect, or the lower the specific heat of the plastic, the easier the temperature is to change, so the heat is easy to dissipate, the thermal conduction effect is better, and the cooling time required is shorter.
7) Processing parameter setting. 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:
1)The cooling channel is designed to ensure that the cooling effect is uniform and rapid.
2)The purpose of designing the cooling system is to maintain proper and efficient cooling of the mold. Cooling holes should be of standard size for ease of machining and assembly.
3)When designing a cooling system, the mold designer must determine the following design parameters according to the wall thickness and volume of the plastic part - the location and size of the cooling hole, the length of the hole, the type of hole, the configuration and connection of the hole, and the flow rate and flow rate of the cooling liquid. heat transfer properties.
4. Demoulding stage
Demoulding is the last link in an injection molding cycle. Although the product has been cold-set, demoulding still has a very important impact on the quality of the product. Improper demolding method may cause uneven stress on the product during demoulding, and cause defects such as product deformation during ejection. There are two main ways of demoulding: ejector demoulding and stripping plate demoulding. When designing a mold, an appropriate demoulding method should be selected according to the structural characteristics of the product to ensure product quality.
For the mold with ejector ejector, the ejector pins should be set as uniform as possible, and the location should be selected at the place with the largest ejection resistance and the highest strength and stiffness of the plastic parts, so as to avoid deformation and damage of the plastic parts. The stripper plate is generally used for the demoulding of deep cavity thin-walled containers and transparent products that do not allow traces of push rods.
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