Process flow of automotive injection molding parts

Most of the parts on the interior and exterior of the center console injection molding. The interior plastic parts generally include instrument panel accessories, seat accessories, floor accessories, roof accessories, steering wheel accessories, door interior accessories, rearview mirrors and various buckles and fixings; the exterior plastic parts include front and rear lights, air intake grilles, fenders, and rearview mirrors. The following is the process flow and related important parameters of automotive injection molding parts.

1 Definition

The injection molding process refers to the process of making semi-finished parts of a certain shape by filling, holding pressure, cooling, demolding and other operations of molten raw materials.

2 Process flow

The injection molding process is as follows:
1 Filling stage
Filling is the first step in the entire injection molding cycle. The time starts from the start of injection molding when the mold is closed until the mold cavity 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. Filling can be divided into high-speed filling and low-speed filling.
1) High-speed filling
During high-speed filling, the shear rate is high, and the viscosity of the plastic decreases due to shear thinning, which reduces the overall flow resistance; local viscous heating will also make the thickness of 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, while the cooling effect of the thin wall is not obvious, so the effect of rate prevails.
2) 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. Coupled with a small amount of viscous heating, the thickness of the solidified layer is thicker, which further increases the flow resistance at the thinner wall.
2 Holding stage
The function of the holding 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 process, the back pressure is high because the mold cavity is already filled with plastic. During the pressure holding and 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 pressure holding flow. Because in the pressure holding stage, the plastic is cooled and solidified by the mold wall faster, and the melt viscosity increases rapidly, so the resistance in the mold cavity is very large. In the later stage of pressure holding, the material density continues to increase, and the plastic parts are gradually formed. The pressure holding stage should continue until the gate is solidified and sealed. At this time, the cavity pressure in the pressure holding stage reaches the highest value.
In the pressure holding stage, due to the high pressure, the plastic shows partial compressibility. 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 leading role. Pressure is the main factor affecting the pressure holding process.

3 Cooling stage

In the injection molding mold, the design of the cooling system is very important. This is because only when the molded plastic products are cooled and solidified to a certain rigidity can the plastic products be prevented from being deformed by 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 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. 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 cooling water pipe in the mold, the heat of the plastic product is transferred from the plastic in the mold cavity 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, the cooling time accounts for the largest proportion, about 70% to 80%. Therefore, the cooling time will directly affect the length of the molding cycle and the output of plastic products. During the demoulding 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 being relaxed due to residual stress or warping and deformation caused by external demoulding force.

4 Demolding stage

Demolding is the last step in an injection molding cycle. Although the product has been cold-formed, demoulding still has a very important impact on the quality of the product. Improper demoulding methods may cause uneven force on the product during demoulding and deformation of the product during ejection. There are two main ways of demoulding: ejector demoulding and stripper demoulding. When designing a mold, choose a suitable demoulding method based on the structural characteristics of the product to ensure product quality. For molds that use ejector demoulding, the ejector should be set as evenly as possible, and the position should be selected where the demoulding resistance is the largest and the strength and rigidity of the plastic part are the largest, so as to avoid deformation and damage to the plastic part. The stripper plate is generally used for the demoulding of 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.
Three important parameters
The important parameters related to injection molding are as follows:
1 Injection molding pressure
The injection molding pressure is provided by the hydraulic system of the injection molding system. The pressure of the hydraulic cylinder is transmitted to the plastic melt through the screw of the injection molding machine. Under the pressure, the plastic melt enters the vertical flow channel (also the main flow channel for some molds), the main flow channel, the branch flow channel of the mold through the nozzle of the injection molding machine, and enters the mold cavity through the gate. This process is the injection molding process, or it is called the filling process. The existence of pressure is to overcome the resistance in the flow process of the melt, or conversely, the resistance in the flow process needs to be offset by the pressure of the injection molding machine to ensure the smooth progress of the filling process. During the injection molding process, the pressure at the nozzle of the injection molding machine is the highest to overcome the flow resistance of the melt throughout the whole process. Afterwards, the pressure gradually decreases along the flow length to the front end of the melt wave front. If the exhaust inside the mold cavity is good, the final pressure at the front end of the melt 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 plastic;
2) Structural factors
Such as the type, number and position of the gating system, the shape of the mold cavity and the thickness of the product;
3) Process elements of molding
2 Injection time
The injection time 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 very short and has little impact on the molding cycle, the adjustment of the injection time has a great effect on the pressure control of the gate, runner and cavity. Reasonable injection time helps the melt to fill ideally, and is of great significance for improving the surface quality of the product and reducing the dimensional tolerance. The injection time should be 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 total molding time of plastic parts. When performing 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 pushed by the screw to fill the cavity. If the screw pressure holding switch occurs before the cavity is filled, the analysis result will be greater than the setting of the process conditions.
3 Injection temperature
Injection temperature is an important factor affecting injection pressure. The barrel of the injection molding machine has 5 to 6 heating sections, and each raw material has its appropriate processing temperature (for detailed processing temperatures, 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 well, affecting the quality of the molded part and increasing the difficulty of the process; if the temperature is too high, the raw material will easily decompose. 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, and can reach up to 30°C. This is caused by the high heat generated by the shear when the melt passes through the injection port. There are two ways to compensate for this difference when performing mold flow analysis. One is to try to measure the temperature of the melt when injecting into the air, and the other is to include the nozzle in the modeling.
4 Holding pressure and time
When the injection molding process is about to end, the screw stops rotating and only moves forward. At this time, the injection molding enters the holding stage. During the pressure holding process, the nozzle of the injection molding machine continuously feeds material into the cavity to fill the volume vacated by the shrinkage of the part. If the cavity is filled without pressure holding, the part will shrink by about 25%, especially the ribs will shrink too much and form shrinkage marks. The pressure holding pressure is generally about 85% of the maximum filling pressure, which should be determined according to the actual situation.

5 Back pressure

Back pressure refers to the pressure that needs to be overcome when the screw reverses and retreats to store material. The use of high back pressure is conducive to the dispersion of colorants and the melting of plastics, but it also prolongs the screw retraction time, reduces the length of plastic fibers, and increases the pressure of the injection molding machine. Therefore, the back pressure should be lower, generally not exceeding 20% ​​of the injection pressure. When injecting foam plastics, 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 program the back pressure to compensate for the reduction in screw length during melting, which will reduce the input heat and reduce the temperature. However, since the result of this change is difficult to estimate, it is not easy to make corresponding adjustments to the machine.

Conclusion

Due to the development requirements of lightweight and low energy consumption of automobiles, the material composition of automobile parts has changed significantly from plastic to steel. Judging from the application of automobile plastics at home and abroad, the amount of center console injection molding has become an important indicator of the level of automobile production technology.