The main parameters involved in the gas-assisted injection molding process
As an advanced injection molding technology, the gas-assisted injection molding process eliminates sink marks, reduces the weight of plastic parts, and shortens the molding cycle by injecting high-pressure gas into the melt. The core of this process lies in the precise control of various parameters, of which the gas injection pressure is one of the most critical. When the gas pressure is too low, it cannot effectively fill the cavity inside the plastic part or promote the flow of the melt, which can easily lead to defects such as sink marks and dents. Excessive pressure may cause flash and bubbles in the plastic part, and even damage the mold. For example, when producing large TV casings, the gas injection pressure is usually controlled between 8-12MPa. If the pressure drops below 6MPa, obvious sink marks will appear on the casing surface. If it exceeds 15MPa, flash will form at the mold parting surface.
Gas injection time is another important parameter that affects the effectiveness of the gas-assisted process. It is directly related to the distribution of gas in the melt and the filling state of the melt. If the injection time is too early, the gas will enter the mold cavity too early and mix with the melt to form bubbles. If the injection time is too late, the melt will have cooled and solidified, and the gas will not be able to diffuse effectively, and it will not be able to eliminate sink marks. When producing small plastic parts such as car door handles, the gas injection time is usually set between 0.5 and 1 second. An auto parts factory once extended the injection time to 1.5 seconds, resulting in a large number of bubbles inside the plastic parts and a product scrap rate of 15%. By adjusting the injection time to 0.8 seconds, the scrap rate dropped to below 1%, fully demonstrating the importance of injection time.
The impact of melt temperature on the gas-assisted process is also not negligible. When the melt temperature is too high, the material has good fluidity and the gas can easily diffuse within the melt, but the cooling time will be prolonged, affecting production efficiency. When the temperature is too low, the melt viscosity increases, making it difficult for the gas to push the melt to flow, and localized material shortages can easily occur. Taking the production of plastic chairs made of PP material as an example, the melt temperature is usually controlled at 200-220°C. If the temperature is below 190°C, the gas cannot effectively fill the cavities inside the chair legs, resulting in insufficient leg strength. When the temperature is above 230°C, scorch marks will appear on the surface of the chair, affecting the appearance quality. Therefore, reasonably setting the melt temperature according to the characteristics of different materials is the basis for ensuring the stability of the gas-assisted process.
Mold temperature is also a parameter that needs to be focused on in the gas-assisted process. If the mold temperature is too low, the melt will cool down quickly in the mold cavity, making it difficult for the gas to diffuse, which can easily lead to cold spots or shrinkage marks on the surface of the plastic part; if the temperature is too high, the cooling time will be prolonged, which will reduce production efficiency and may cause warping and deformation of the plastic part. When producing laptop computer cases, the mold temperature is usually set at 60-80°C. An electronics factory once tried to increase the mold temperature to 90°C. Although the surface shrinkage was eliminated, the cooling time was extended from 20 seconds to 30 seconds, and the production efficiency dropped by 30%. By optimizing the mold temperature to 70°C, while ensuring product quality, the cooling time was controlled at 22 seconds, achieving a balance between quality and efficiency.
Gas holding time and holding pressure are the last key links to ensure the effectiveness of the gas-assisted process. If the holding time is too short, the gas cannot fully fill the space created by the melt cooling and shrinkage, which can easily lead to sink marks on the plastic parts; if the holding time is too long, it will increase the production cycle and reduce efficiency. The holding pressure is usually slightly lower than the injection pressure. If the holding pressure is too high, it may cause internal stress in the plastic part; if the pressure is too low, it will not have the holding effect. When producing water dispenser buckets, the gas holding time is generally set at 5-8 seconds, and the holding pressure is 80%-90% of the injection pressure. A water dispenser factory increased the wall thickness uniformity of the bucket by 20% by extending the holding time from 5 seconds to 6 seconds and adjusting the holding pressure to 85% of the injection pressure, greatly improving the service life of the product.
