Injection shrinkage is a common quality defect in injection molding production, manifesting as depressions, sink marks on the surface of the plastic part, or voids inside the part. This not only affects the product’s appearance, but can also weaken its structural strength. This phenomenon often occurs in areas with uneven wall thickness, corners, or near ribs. It is essentially caused by the uneven volume shrinkage of the melt during the cooling and solidification process. After the plastic melt is injected into the mold cavity, the outer layer first contacts the low-temperature mold and rapidly cools and solidifies, while the internal melt remains in a high-temperature molten state. As the temperature drops, the internal melt shrinks due to a lack of sufficient material replenishment, resulting in shrinkage. For example, when producing thick-walled plastic parts, if the melt is not fully filled or the pressure is insufficient during the holding stage, it is very easy for obvious sink marks to appear in the thick wall area, and in severe cases, it may even lead to product scrapping.
The causes of injection molding shrinkage can be analyzed from three dimensions: raw materials, equipment parameters, and mold design. In terms of raw materials, the shrinkage rate of plastic is a key factor. The shrinkage rates of different types of plastics vary greatly. For example, the shrinkage rate of polyethylene is usually between 1.5% and 3.6%, while the shrinkage rate of ABS is only 0.4% to 0.7%. Materials with high shrinkage rates are more prone to shrinkage. Improper equipment parameter settings are also an important contributing factor. If the injection speed is too slow, the melt will cool too quickly during the filling process, making it difficult to replenish subsequent materials. If the holding pressure and holding time are insufficient, the volume shrinkage of the melt during cooling cannot be offset. Mold design defects should not be ignored either. For example, an unreasonable gate position will result in an excessively long melt flow path, or improper venting groove design will prevent the gas in the cavity from being discharged. These will affect the filling effect of the melt and cause shrinkage.
To address shrinkage caused by raw materials, both material selection and pretreatment are crucial. First, plastics with appropriate shrinkage should be selected based on product requirements. For products requiring high precision, low-shrinkage materials are preferred. Furthermore, plastic raw materials must be thoroughly dried before use, especially for highly hygroscopic materials like PA and PC. Excessive moisture content can create bubbles after melting, which can burst during cooling and lead to shrinkage. Drying is typically performed in a hot air dryer at a controlled temperature of 80-120°C for 4-6 hours, ensuring the raw material’s moisture content is below 0.05%. Furthermore, the appropriate addition of fillers, such as glass fiber, can effectively reduce the plastic’s shrinkage and minimize shrinkage.
Optimizing equipment process parameters is the core means of solving injection molding shrinkage. During the injection stage, the injection speed and injection pressure should be appropriately increased to ensure that the melt can quickly fill the mold cavity and avoid insufficient filling due to rapid cooling. For example, for thick-walled plastic parts, a multi-stage injection process can be used. In the initial stage, the mold cavity is quickly filled to 80%, and then the speed is reduced to slowly fill the remaining part to reduce pressure loss during the melt flow. During the holding stage, the holding pressure and holding time need to be adjusted according to the thickness of the plastic part. Generally, the holding pressure is 60%-80% of the injection pressure, and the holding time should continue until the gate solidifies. In addition, appropriately increasing the mold temperature can slow down the cooling rate of the melt, allowing the melt more time to compensate for shrinkage. For crystalline plastics such as PP and PE, the mold temperature should be controlled at 40-60℃.
The optimized design of the mold plays an important role in preventing shrinkage. In the gate design, the melt flow path should be shortened as much as possible. For large plastic parts, a multi-point pouring method can be used to ensure that the melt can evenly fill the cavity. The gate size needs to match the wall thickness of the plastic part. Thick-walled plastic parts should use a larger gate, such as a fan gate or a disc gate, to extend the pressure holding and shrinkage compensation time. Cavity venting design is also critical. A venting groove is opened at the position where the melt is last filled. The groove depth is controlled at 0.02-0.05mm and the width is 5-10mm to ensure that the gas in the cavity is smoothly discharged to avoid inadequate filling due to gas obstruction. In addition, for plastic parts with large differences in wall thickness, the structure can be optimized by increasing transition radius, setting reinforcing ribs, etc. to make the wall thickness uniform and reduce shrinkage caused by uneven shrinkage.
