The injection molding process generally includes four stages: filling, holding pressure, cooling, and demoulding. These four injection molding process stages directly determine the molding quality of the product, and these four injection molding process stages are a complete continuous process.
1. Filling stage of injection molding process
- (1) Filling is the first step in the entire injection molding process. The time starts from the mold is closed and the injection is completed, 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.
- (2) High-speed filling. The shear rate is high during high-speed filling, and the viscosity of the plastic decreases due to shear thinning, which reduces the overall flow resistance; the local viscous heating effect will also make the thickness of the cured layer thin. 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 great, and the cooling effect of the thin wall is not obvious, so the effect of velocity prevails.
- (3) 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. Due to the slower replenishment rate of the hot plastic 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 small amount of viscous heating, the thickness of the cured layer is thicker, which further increases the flow resistance at the thinner wall.
Due to the flow of the fountain, 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; plus the two strands of melt have different properties (different residence time in the mold cavity, different temperature and pressure),
As a result, the melt adhesive area has a poor microstructure strength. Observe the naked eye by placing the parts at an appropriate angle under the light, and you can find that there are obvious bonding lines. This is the formation mechanism of the weld line. The weld line not only affects the appearance of the plastic part, but also because of the looseness of the microstructure, it is easy to cause stress concentration, so that the strength of this part is reduced and fracture occurs.
Generally speaking, the strength of the weld line that produces welding in the high temperature region is better, because at high temperature, the polymer chain has better mobility and can penetrate through each other. In addition, the temperature of the two melts in the high temperature region is closer, the melt The thermal properties of 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.
2. Packing stage of injection molding process
The role of the pressure holding phase 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.
In the pressure-holding process, the back pressure is higher because the cavity is already filled with plastic. In the process of holding pressure and compaction, the screw of the injection molding machine can only move slightly forward slowly, and the flow speed of the plastic is relatively slow. The flow at this time is called holding pressure flow. In the pressure-holding stage, the plastic is accelerated by the cooling and solidification of the mold wall, and the melt viscosity increases quickly, so the resistance in the mold cavity is great.
In the later stage of holding pressure, the material density continues to increase, and the plastic parts are gradually formed. The holding pressure stage must continue until the gate is cured and sealed, at this time, the cavity pressure in the holding pressure stage reaches the highest value.
In the pressure-holding stage, due to the relatively high pressure, the plastic exhibits partial compressibility. In areas with higher pressure, plastics are denser and denser; in areas with lower pressures, plastics are looser and have lower densities, resulting in changes in density distribution with location and time.
The plastic flow rate is extremely low during the pressure holding process, 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. At this time, the gradually solidified melt serves as a medium for transmitting pressure. The pressure in the mold cavity is transferred to the surface of the mold wall by means of plastic, and there is a tendency to expand the mold, so an appropriate mold clamping force is required for mold clamping.
Under normal circumstances, the mold expansion force will slightly spread the mold, which will help the exhaust of the mold; but if the mold expansion force is too large, it may easily cause burrs, flashing of the molded product, and even spread the mold. Therefore, when choosing an injection molding machine, you should choose an injection molding machine with a sufficiently large clamping force to prevent mold expansion and effectively hold the pressure.
3. Cooling stage of injection molding apartment
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 by external force after demolding. Because 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 increase the molding time and increase costs; uneven cooling will further cause warpage and deformation of plastic products.
According to the experiment, the heat from the melt into the mold is generally distributed 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 in the plastic product, heat is transferred from the plastic in the mold cavity to the cooling water pipe through the mold frame by heat conduction, and then is 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 until it contacts the outside world and escapes into the air.
The molding cycle of injection molding consists of mold closing time, filling time, pressure holding time, cooling time and demolding time. Among them, the proportion of cooling time is the largest, about 70% to 80%. Therefore, the cooling time will directly affect the molding cycle length and output of plastic products. During the demoulding stage, the temperature of the plastic product should be cooled below the thermal deformation temperature of the plastic product to prevent the plastic product from slackening due to residual stress or warping and deformation caused by the external force of demolding.
The factors that affect the cooling rate of products are:
- (1) Design of plastic products. Mainly the wall thickness of plastic products. The thicker the product, the longer the cooling time. In general, 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, the thickness of plastic products is doubled, and the cooling time is increased by 4 times.
- (2) Mold material and its cooling method. Mold materials, including mold cores, cavity materials and mold base materials have 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) Configuration method of cooling water pipes. The closer the cooling water pipe is to the mold cavity, the larger the pipe diameter and the greater the number, the better the cooling effect and the shorter the cooling time.
- (4) Coolant flow. The greater the flow of cooling water (generally to achieve turbulence is better), the better the effect of cooling water to remove heat by convection.
- (5) The nature of the coolant. The viscosity and thermal conductivity of the coolant 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 selection. Plastic refers to a measure of the rate at which 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 the temperature changes, so the heat is easily dissipated, the thermal conductivity is better, and the cooling time required is shorter. Processing parameter setting. The higher the material temperature, the higher the mold temperature and the lower the ejection temperature, the longer the cooling time required.
Cooling system design rules:
- (1) The designed cooling channel should 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. Standard size of cooling holes should be used to facilitate processing and assembly.
- (3) 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 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 cooling Flow rate and heat transfer properties.
4. Demoulding stage
Demoulding is the last link in an injection molding cycle. Although the product has been cold set, the demolding still has a very important effect on the quality of the product. Improper demolding methods may result in uneven stress during demolding and deformation of the product when ejected. There are two main ways of demoulding: ejection of ejector rod and demoulding of ejector plate. When designing the mold, it is necessary to select the appropriate demoulding method according to the structural characteristics of the product to ensure the quality of the product.
For the mold with ejector ejection, the ejector setting should be as uniform as possible, and the location should be selected at the place where the ejection resistance is the largest and the strength and rigidity of the plastic part are the largest, so as to avoid deformation and damage of the plastic part. The stripping plate is generally used for the demolding of deep cavity thin-walled containers and transparent products that do not allow traces of push rods. This mechanism is characterized by a large and uniform demolding force, smooth movement, and no obvious traces.