Today, we are going to talk about the application of the gas–assisted injection molding process.
The gas-assisted injection molding process utilizes the principle of high-efficiency pressure transmission of gas, so that the long process parts are quickly filled without sink marks, so as to eliminate deformation, reduce injection pressure, reduce product weight, increase design freedom, save plastic and reduce Cost and other purposes.
The gas-assisted injection molding process is usually related to factors such as injection volume, injection speed, and holding pressure, gas pressure and gas injection speed, and delay time in practice.
The principle of gas-assisted injection molding process
Use high-pressure inert gas (nitrogen) to inject into the molten plastic to form a vacuum section and push the molten material forward to realize the processes of injection, pressure holding, and cooling.
Because the gas has high-efficiency pressure transmission, it can keep the pressure in all parts of the air channel consistent, which can eliminate internal stress, prevent product deformation, and greatly reduce the pressure in the cavity, so there is no need for high pressure during the molding process. Clamping force can also reduce product weight and eliminate sink marks.
Gas-assisted equipment of gas-assisted injection molding process
The gas-assisted equipment of the gas-assisted injection molding process includes a gas-assisted control unit and a nitrogen generator. It is another system independent of the injection molding machine, and its only interface with the injection molding machine is the injection signal connection line.
After the injection molding machine transmits an injection signal injection start or screw position to the gas-assisted control unit, it starts a gas injection process, waits for the next injection process to give another injection signal, starts another cycle, and so on.
The gas used in gas-assisted injection molding must be an inert gas (usually nitrogen), the maximum pressure of the gas is 35MPa, and the maximum pressure of the gas is up to 70MPa, and the nitrogen purity is ≥98%.
The gas-assisted control unit is a device that controls the gas injection time and pressure. It has a multi-group gas circuit design that can control the gas-assisted production of multiple injection molding machines at the same time. The gas-assisted control unit has a gas recovery function to reduce gas consumption as much as possible.
Gas-assisted injection molding process control
Gas injection parameters
The gas-assisted control unit is a device that controls the gas pressure in each stage. The gas-assisted parameters have only two values: gas injection time (seconds) and gas injection pressure (MPa)
3.2. The gas-assisted injection molding process is to inject high-pressure gas at the same time as the plastic melt is injected into the mold.
There is a complex two-phase interaction between the melt and the gas. Therefore, process parameter control is very important. The control methods of each parameter are as follows:
Gas-assisted injection molding process uses the so-called short shot method, in which a certain amount of material (usually 70-95% during the full shot) is injected into the mold cavity, and then gas is injected to achieve a full filling process. The amount of melt injection has the greatest relationship with the size of the mold air passage and the structure of the mold cavity.
The larger the cross-section of the airway, the easier the gas to penetrate, and the higher the hollowing rate, which is suitable for using a larger short shot rate. At this time, if too much material is used, it is easy to cause melt accumulation, and sink marks will appear in places with a lot of material.
If the material is too small, it will cause a blow-through. If the direction of the air passage is exactly the same as that of the flow material, it is most conducive to the penetration of the gas, and the hollowing rate of the air passage is the largest. Therefore, in the mold design, keep the air passage and the flow direction as consistent as possible.
Injection speed and holding pressure
Under the circumstance of ensuring that there are no defects in product performance, use a higher injection speed as much as possible to make the melt fill the cavity as soon as possible. At this time, the temperature of the melt remains high, which is conducive to gas penetration and mold filling.
The gas still maintains a certain pressure after pushing the melt to fill the mold cavity, which is equivalent to the pressure holding stage in traditional injection molding. Therefore, the gas-assisted injection molding process can save the pressure holding process of the injection molding machine.
However, some products still need to use certain injection molding pressure to ensure the quality of product performance due to structural reasons. But do not use high holding pressure, because if the holding pressure is too high, the gas needle will be sealed, the gas in the cavity cannot be recovered, and it is easy to blow up when the mold is opened.
High holding pressure will also hinder gas penetration. Increasing the holding pressure of injection molding may cause larger sink marks in the performance of the product.
Gas pressure and gas injection speed
The gas pressure has the greatest relationship with the fluidity of the material. Materials with good fluidity (such as PP) use lower gas injection pressure.
High gas pressure, easy to penetrate, but easy to blow through; low gas pressure may cause insufficient filling, insufficient filling of sink marks on the surface of the product; high gas injection speed can fill the cavity when the melt temperature is high.
For molds with long processes or small air passages, increasing the gas injection speed is conducive to the filling of the melt and can improve the surface quality of the product. However, if the gas injection speed is too fast, blow-through may occur, and for products with large air passages, it may be possible. Will produce surface flow marks and gas lines.
The delay time is the time period from the start of injection of the injection molding machine to the start of the gas injection by the gas-assisted control unit, which can be understood as a parameter of the reverse mapping of the rubber and the synchronization of the gas injection.
The delay time is short, that is, when the melt is still at a higher temperature, the gas injection is obviously beneficial to gas penetration and mold filling, but the delay time is too short, the gas is easy to diverge, the hollowing shape is not good, and the hollowing rate is also not enough.
Gas-assisted mold of the gas-assisted injection molding process
The gas-assisted mold of the gas-assisted injection molding process is not much different from the traditional injection mold, only the air inlet element (called the air needle) is added, and the air passage design is increased.
The air passage can be simply understood as the passage of the gas, that is, the part through which the gas flows after entering. Some of the air passages are part of the product, and some are the glue positions specially designed to guide the airflow.
The gas needle is a key component of the gas-assisted mold, which directly affects the stability of the process and the quality of the product. The core part of the air needle is made up of many small gaps that are too large and will be blocked by the melt, and the air output will decrease instead.
Gas-assisted injection molding process
The gas-assisted injection molding process has four steps
The first step of the gas-assisted injection molding process is resin filling: the mold is partially filled with melt.
The second step of the gas-assisted injection molding process is gas filling: nitrogen is injected into the hot melt as required.
The gas flows rapidly in the high-temperature and low-pressure areas. The direction of gas flow is usually the direction of least resistance. According to the design, the airway should be placed in a place that is convenient for guiding the gas to the low-pressure area. The hot melt material at the thick section of the plastic part is replaced with pressurized gas, and the pressurized gas is used to complete the filling of the plastic;
The third step of the gas-assisted injection molding process of gas pressure keeping: due to the joint action of the melt and the gas, after the mold is filled, the nitrogen stays in the gas flow channel of the plastic part, and it has enough pressure to compact the plastic part.
Then the resin cools and shrinks, and the gas presses the resin that has not yet solidified into the voids caused by the shrinkage. Use the holding pressure to eliminate the sink marks on the surface of the plastic part, and ensure that the mold has a good surface quality in the next molding cycle to form a plastic part with a good surface quality;
The fourth step of the gas-assisted injection molding process of gas discharge: all the gas needed in the whole process must be discharged before the mold is opened.
If the pressure gas is not discharged in time, the plastic part will swell or even burst.