Three methods of gas-assisted injection molding
Gas-assisted injection molding is an advanced molding technology developed on the basis of traditional injection molding. By injecting high-pressure gas into the melt, the gas pressure is used to push the melt to fill the cavity, maintain pressure and compensate for shrinkage, effectively solving defects such as shrinkage marks and warping of thick-walled products, while reducing the weight of the product and shortening the molding cycle. Depending on the gas injection method and the action stage, gas-assisted molding is mainly divided into three basic methods: gas penetration molding, gas pressure-maintaining molding and gas back-pressure molding. Each method is suitable for different product structures and molding requirements. Mastering the characteristics and application scenarios of these three methods can give full play to the advantages of gas-assisted molding and improve product quality and production efficiency.
Gas penetration molding is the most commonly used gas-assisted molding method, suitable for parts with uneven wall thickness or with ribs or bosses. Its core principle is to use high-pressure gas to create continuous gas channels within the melt, pushing the melt into the mold cavity and maintaining pressure. The molding process consists of four stages: First, 70%-90% of the melt is injected to fill the thin-walled areas of the cavity. Next, high-pressure gas (typically 10-30 MPa) is injected through the gate or a dedicated gas needle. The gas penetrates the melt along the path of least resistance (generally the thick-walled areas), forming a gas channel. The gas continues to expand, pushing the remaining melt to the end of the cavity, completing the filling. Finally, the gas pressure is maintained to maintain pressure and compensate for melt shrinkage. This method can significantly reduce sink marks in thick-walled areas. For example, in parts such as television housings and automobile armrests, the hollow structure formed by gas penetration reduces weight and avoids surface concavity. The key to gas penetration is to control the melt pre-injection volume and gas pressure. Excessive pre-injection volume will hinder gas penetration, while too little will easily cause gas to break through the melt and form bubbles.
Gas-hold molding is primarily used to eliminate surface sink marks and improve dimensional stability. It is suitable for products with relatively uniform wall thickness, such as covers and panels. Its characteristic process is that gas is injected after the melt completely fills the mold cavity. The gas does not penetrate the melt, but only compensates for melt shrinkage through gas pressure, replacing traditional plastic pressure-holding. The molding process is as follows: the mold cavity is first filled 100% with the melt. At this point, the melt begins to shrink during cooling, potentially causing sink marks. High-pressure gas is then injected through the gate. The gas pressure acts on the melt surface, pushing it toward the cavity walls to compensate for shrinkage. After the pressure-holding period is complete, the gas is released and the mold is opened for removal. Compared to traditional pressure-holding, gas-holding provides more uniform pressure distribution, effectively avoiding internal stress and warpage caused by uneven pressure-holding. For example, in the molding of PC display panels, gas-holding can improve surface flatness by over 20% and reduce the incidence of sink marks to zero. This method has high requirements for gas pressure control. The pressure needs to be slightly higher than the melt injection pressure, usually 15-25MPa, and the holding time needs to match the melt solidification rate.
Gas backpressure molding is a specialized gas-assisted molding method primarily used to improve melt flow and reduce internal stress at the gate. It is suitable for plastics with poor flow properties (such as PC and POM) or thin-walled, complex products. The principle is to inject low-pressure gas (typically 0.5-3 MPa) into the mold cavity before injecting the melt to create backpressure. During injection, the melt must overcome the backpressure to enter the cavity, allowing it to fully plasticize under high pressure, thereby improving melt density and uniformity. The molding process is as follows: After the mold is closed, low-pressure gas is injected into the cavity to establish backpressure. The melt is then injected, slowly filling the cavity under the action of the backpressure to avoid turbulence and bubbles. Once the melt has filled to a certain level, the backpressure is released, and high-pressure gas is injected simultaneously to maintain pressure and compensate for shrinkage. This method reduces melt shear stress and internal stress at the gate. For example, in POM gear molding, gas backpressure can reduce internal stress on the gear tooth surface by 30%, improving wear resistance and service life. The back pressure value needs to be adjusted according to the viscosity of the plastic. High-viscosity plastics require a higher back pressure, while low-viscosity plastics require a lower back pressure to avoid filling difficulties caused by excessive back pressure.
The choice of the three gas-assisted molding methods needs to be determined comprehensively based on the product structure, material properties and quality requirements. Gas penetration molding is preferentially used for products with uneven thick walls, which can maximize weight reduction and eliminate sink marks; gas pressure holding molding is suitable for products with uniform wall thickness that require high surface quality, with simple operation and low cost; gas back pressure molding is suitable for high viscosity or thin-walled complex products, which can improve melt fluidity and the intrinsic quality of the product. In actual production, multiple methods are sometimes used in combination, such as first using gas back pressure to improve filling, and then maintaining pressure through gas penetration, taking into account multiple advantages. Regardless of which method is used, special gas-assisted equipment (such as gas generators, pressure controllers) and precise mold exhaust systems are required to ensure stable gas pressure and smooth exhaust. In addition, the mold design needs to consider the position and shape of the gas channel to prevent gas from breaking through the melt or forming bubbles inside the product, affecting performance.
The application of gas-assisted molding requires careful coordination of process parameters to achieve optimal results. For gas penetration molding, precise control is required of the pre-injection volume (typically 75%-85%), gas injection timing (0.5-1 second before the melt front reaches the thick-walled area), and gas pressure (initial pressure slightly higher than the injection pressure, gradually decreasing during the holding phase). The key to gas-holding molding is the gas injection delay: injection should occur when the melt begins to shrink but has not yet fully solidified. Injecting too early will cause melt overflow, while injecting too late will not compensate for shrinkage. For gas backpressure molding, the backpressure value and injection speed must be coordinated. If the backpressure is too high, the injection speed should be increased to ensure smooth melt filling. During the mold trial phase, parameters should be adjusted by observing the gas channel morphology, surface quality, and dimensional accuracy of the part. For example, if gas penetration is uneven, the gate position should be optimized or gas-assisted channels should be added. If gas lines appear on the surface, the gas injection speed should be reduced or the melt temperature should be adjusted. With technological advancements, intelligent gas-assisted systems can automatically adjust parameters by monitoring melt flow and gas pressure in real time, further improving molding stability and part quality.
