Irregular Injection Molding Sinks and Solutions
Irregular injection molding pits are a common surface defect on products. These irregularly shaped depressions not only affect the product’s appearance but can also weaken its structural strength, leading to breakage and other issues during use. The occurrence of these pits is related to a variety of factors, including raw material properties, the injection molding process, and mold design. Comprehensive analysis and effective solutions are essential.
First, improper raw material selection and pretreatment are key causes of irregular sinks. If the plastic raw material contains excessive volatiles, moisture, or low-molecular-weight impurities, they will volatilize due to the high temperatures during the injection molding process, forming gases. These gases cannot be expelled promptly as the melt cools and solidifies, resulting in sinks on the surface of the product. Therefore, rigorous raw material pretreatment is essential before injection molding. Highly hygroscopic materials, such as PA and PC, require thorough drying, typically using a hot air dryer at 100-140°C for 4-8 hours to keep the moisture content below 0.05%. This prevents vaporization at high temperatures, which can cause bubbles and sinks. For raw materials containing volatiles, the barrel temperature can be appropriately increased to allow them to fully escape before injection. Furthermore, the hopper seal can be strengthened to prevent the ingress of impurities. Furthermore, selecting raw materials with high purity and uniform particle size can reduce sink defects caused by raw material quality issues.
Secondly, improper injection molding process parameter settings are the main cause of irregular sinks. Both excessively high and low melt temperatures can affect the molding quality of the product. Excessively high temperatures can lead to excessive degradation of the raw material, producing gases and low-molecular substances, which can form sinks. Excessively low temperatures can lead to poor melt fluidity, inadequate mold filling, and uneven shrinkage during cooling, which can easily cause sinks. Therefore, the barrel temperature should be set appropriately based on the raw material characteristics. For example, the barrel temperature for PP materials is generally 180-220°C, and for ABS materials, 200-250°C. Mold trials should be conducted to ensure stable melt temperatures. Insufficient injection and holding pressures are also common causes of sinks. Insufficient pressure prevents adequate shrinkage during cooling, resulting in sinks. The injection and holding pressures should be appropriately increased to ensure that the melt fully fills the mold cavity and provides effective shrinkage. Generally, injection pressure should be maintained between 80-150 MPa, with holding pressure at 60%-80% of the injection pressure. Furthermore, extending the holding and cooling times allows the product to fully solidify within the mold, reducing sinks caused by shrinkage.
Mold design flaws can also cause irregular sinkholes in parts. Improper gate and runner design can cause turbulence and stagnation in the melt during filling, leading to localized pressure deficiency or uneven cooling, which can form sinkholes. Therefore, the location and number of gates should be optimized, and a multi-point injection method should be adopted to ensure uniform melt filling of the cavity and avoid insufficient shrinkage in areas too far from the gate. Runners should have circular or trapezoidal cross-sections to ensure smooth melt flow and minimize pressure loss. The design of the mold’s cooling system is also crucial. Uneven cooling can lead to inconsistent shrinkage in various parts of the part, resulting in sinkholes. Cooling channels should be arranged appropriately according to the part’s shape, ensuring a uniform distance from the cavity surface (generally 15-25mm). The cooling water flow and temperature should also be stable to ensure consistent cooling across the part. Furthermore, the mold cavity surface should have a moderate roughness. Excessive roughness can hinder melt flow and create localized sinkholes. Appropriate polishing should be performed, with an Ra value between 0.8-3.2μm.
Improper product structural design is also a major cause of irregular sinkholes. Uneven wall thickness can easily lead to sinkholes in thicker areas due to excessive cooling shrinkage. This is because thicker areas cool more slowly and shrink longer. Without sufficient melt replenishment, sinkholes form. Therefore, during the product design phase, wall thickness should be uniform, with variations within 20%. For necessary thick-walled structures, reinforcing ribs or localized wall thinning can be used to minimize shrinkage variations. Improperly designed ribs, bosses, and other structures can also cause sinkholes. Rib thickness should be minimal, generally 50%-70% of the thickness of the adjacent wall. Fillets should be incorporated into the joints to avoid stress concentration and uneven shrinkage. Furthermore, inaccurate shrinkage calculations, leading to improper mold sizing, can also cause sinkholes after molding. Mold dimensions should be accurately calculated based on the actual shrinkage of the raw material, allowing for sufficient shrinkage margins.
Finally, operation and equipment maintenance during the production process also play a crucial role in preventing irregular sinking. Operators should strictly follow process specifications during production, regularly check the stability of parameters such as barrel temperature, injection pressure, and hold time, and promptly adjust any anomalies. Regularly clean residues from the barrel and mold to prevent degraded materials or impurities from entering the melt and affecting molding quality. Strengthen equipment maintenance to ensure the hydraulic and temperature control systems of the injection molding machine are functioning properly and that injection screw wear is within acceptable limits. This is crucial to avoid fluctuations in process parameters due to equipment failure. For products that already have sinking, the cause should be promptly analyzed and addressed through adjustments to process parameters, improved mold design, or replacement of raw materials. Furthermore, the use of online detection technologies, such as infrared thermometers and pressure sensors, to monitor temperature and pressure changes during the molding process in real time, allowing for timely identification and action, can effectively reduce the occurrence of irregular sinking and improve product quality stability.
