The injection molding process of plastic parts mainly includes four stages: filling - holding pressure - cooling - demolding. These four stages directly determine the molding quality of the product, and these four stages are a complete continuous process.
1. Filling stage
Filling is the first step in the entire injection molding cycle. The time starts from the mold closing and injection molding until the mold cavity is filled to about 95%. In theory, the shorter the filling time, the higher the molding efficiency, but in practice, 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 local viscous heating effect will also make the solidified 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 the melt is often large, and the cooling effect of the thin wall is not obvious, so the rate effect prevails.
Low-speed filling
When heat conduction controls low-speed filling, the shear rate is low, the local viscosity is high, and the flow resistance is large. Since the hot plastic replenishment rate is slow and the flow is slow, the heat conduction effect is more obvious, and the heat is quickly taken away by the cold mold wall. In addition, due to a small amount of viscous heating phenomenon, the thickness of 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 arranged almost parallel to the flow wave front. Therefore, when the two plastic melts meet, the polymer chains on the contact surface are parallel to each other; coupled with the different properties of the two melts (different residence time in the mold cavity, different temperature and pressure), the microscopic structural strength of the melt intersection area is poor.
When the parts are placed at an appropriate angle under light and observed with the naked eye, it can be found that there are obvious joint lines, which is the formation mechanism of the weld mark. The weld mark not only affects the appearance of the plastic part, but also easily causes stress concentration due to the loose microstructure, which reduces the strength of the part and causes fracture.
Generally speaking, the weld marks produced in the high temperature zone have better strength, because under high temperature conditions, the polymer chains are more active and can penetrate and entangle each other. In addition, the temperatures of the two melts in the high temperature zone are relatively close, and the thermal properties of the melts are almost the same, which increases the strength of the weld zone; on the contrary, in the low temperature zone, the weld strength is poor.
2. Holding pressure stage
The function of the holding pressure stage is to continuously apply pressure, compact the melt, increase the density of the plastic (densification), and compensate for the shrinkage behavior of the plastic. During the holding pressure process, the back pressure is high because the mold cavity is already filled with plastic. During the holding pressure compaction process, 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.
Because in the holding pressure 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 holding pressure, the material density continues to increase, and the plastic parts are gradually formed. The holding pressure stage should continue until the gate is solidified and sealed. At this time, the mold cavity pressure in the holding pressure stage reaches the highest value.
During the pressure holding stage, due to the high pressure, the plastic shows some compressible characteristics. In the high pressure area, the plastic is denser and has a higher density; in the low pressure area, the plastic is looser and has a lower density, so the density distribution changes with position and time.
During the pressure holding process, the plastic flow rate is extremely low, and the flow no longer plays a dominant role; 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 at this time serves as a medium for transmitting pressure.
The pressure in the mold cavity is transmitted to the mold wall surface with the help of plastic, and there is a tendency to open the mold, so appropriate clamping force is required for 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, and even open the mold of the molded product.
Therefore, when selecting an injection molding machine, you should choose an injection molding machine with a sufficiently large clamping force to prevent mold expansion and effectively maintain pressure.
3. Cooling stage
In injection molding molds, the design of the cooling system is very important. This is because the molded plastic products can only avoid deformation due to external forces after cooling and solidification to a certain rigidity and 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 molding productivity and reduce costs.
An improperly designed cooling system will prolong the molding time and increase costs; uneven cooling will further cause warping and deformation of plastic products.
According to experiments, the heat entering the mold from the melt is generally dissipated in two parts, 5% of which is transferred to the atmosphere through radiation and convection, and the remaining 95% is conducted from the melt to the mold. Due to the effect of the cooling water pipe, the heat of the plastic product in the mold cavity is transferred to the cooling water pipe through the mold frame through heat conduction, and then carried away by the coolant through heat convection. A small amount of heat that is not carried away by the cooling water continues to be conducted in the mold, and dissipates into 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, cooling time accounts for the largest proportion, about 70% to 80%. Therefore, cooling time will directly affect the length of the molding cycle and the output of plastic products. During the demolding stage, the temperature of plastic products should be cooled to a temperature lower than the thermal deformation temperature of plastic products to prevent the relaxation of plastic products due to residual stress or warping and deformation caused by external forces during demolding.
The factors affecting the cooling rate of products are:
Plastic product design. 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 proportional to the 1.6th power of the maximum flow channel diameter. That is, when the thickness of the plastic product doubles, the cooling time increases by 4 times.
Mold materials and their cooling methods. Mold materials, including mold core, cavity materials and mold frame materials, have a great influence on the cooling rate. The higher the thermal conductivity coefficient 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 method. 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.
Coolant flow rate. The larger the cooling water flow rate (generally turbulent flow is preferred), the better the cooling water can carry away heat by 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 coolant, the higher the thermal conductivity, the lower the temperature, and the better the cooling effect.
Plastic selection. Plastic refers to the measure of the speed at which the plastic conducts heat from a hot place to a cold place. The higher the thermal conductivity of the plastic, the better the thermal conductivity, or the lower the specific heat of the plastic, the easier it is to change the temperature, so the heat is easy to dissipate, the better the thermal conductivity, and the shorter the cooling time required.
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.
Cooling system design rules:
The designed cooling channel must ensure uniform and rapid cooling.
The purpose of designing a cooling system is to maintain proper and efficient cooling of the mold. Cooling holes should use standard sizes to facilitate processing and assembly.
When designing a cooling system, the mold designer must determine the following design parameters based on 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 heat transfer properties of the coolant.
4. Demolding stage
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 uneven force on the product during demolding, causing product deformation during ejection and other defects. There are two main ways of demolding: ejector demolding and stripper demolding. When designing a mold, choose the appropriate demolding method based on the structural characteristics of the product to ensure product quality.
For molds that use ejector demolding, the ejector should be set as evenly as possible, and the position should be selected where the demolding resistance is the largest and the strength and rigidity of the plastic part are the largest to avoid deformation and damage to the plastic part. The stripping plate is generally used for demoulding deep-cavity thin-walled containers and transparent products that do not allow push rod marks. The characteristics of this mechanism are large and uniform demoulding force, smooth movement, and no obvious residual marks.
