Analysis and treatment of injection molding product defects
Injection molded products inevitably develop various defects during the production process. These defects not only affect the product’s appearance quality but may also reduce its mechanical properties and safety. Common injection molding defects include flash, sink marks, bubbles, weld marks, and warpage. The occurrence of each defect is closely related to factors such as raw material characteristics, mold design, and process parameters. For example, flash often appears at the mold parting surface, appearing as excess flakes on the edge of the product, which not only affects the appearance but can also cause assembly difficulties. Sink marks, on the other hand, often appear in areas with thicker walls, appearing as depressions. In severe cases, they can weaken the product’s structural strength. In-depth analysis of the causes of these defects and the implementation of targeted treatment measures are key to ensuring the quality of injection molded products.
Flash defects are primarily related to mold precision, clamping force, and injection pressure. When the mold parting surface is not tightly fitted or the clearance between the cavity and core is too large, the molten plastic, under high pressure, can easily overflow the cavity, forming flash. Insufficient clamping force is also a significant factor. If the clamping force is less than the expansion force generated by the injection pressure, the mold parting surface will be forcibly stretched, causing the plastic melt to leak. Regarding raw materials, plastics with high fluidity (such as polyethylene and polypropylene) are more prone to flash under the same process parameters. To address flash defects, first inspect the mold parting surface for wear and foreign matter. If wear is present, repair it by grinding, and if foreign matter is present, remove it promptly. Alternatively, reduce the injection pressure and melt temperature to reduce the plastic’s fluidity and filling pressure. If adjustments to these parameters do not improve the situation, inspect the clamping mechanism to ensure that the clamping force meets process requirements and, if necessary, replace worn clamping mechanism parts.
Sink mark defects are often closely related to the plastic’s shrinkage characteristics and the mold filling and holding process. When the wall thickness of a part is uneven, the plastic in thicker areas cools more slowly, making it more likely to not receive sufficient melt replenishment due to volumetric shrinkage, leading to sink marks. Furthermore, insufficient holding pressure or a short holding time can prevent the melt from being adequately replenished during the cooling and shrinkage phase, leading to sink marks. To address sink marks, the first step is to optimize the product design to ensure uniform wall thickness. If thick walls are necessary, reinforcing ribs or a gradual transition can be used to minimize localized shrinkage differences. Regarding process parameters, the holding pressure and holding time can be appropriately increased to ensure continuous melt replenishment during the cooling process. Furthermore, the melt and mold temperatures can be lowered to accelerate the cooling rate and minimize volumetric shrinkage during the cooling phase. For crystalline plastics (such as polyoxymethylene and nylon), the degree of crystallinity must be controlled by adjusting the cooling rate to avoid sink marks caused by excessive crystallization shrinkage.
The causes of bubble defects can be categorized as gas entrapment and raw material decomposition. If air, moisture, or volatiles enter the barrel during raw material melting, these gases can become trapped within the melt during injection molding, forming bubbles upon cooling. Insufficient raw material drying is a common cause, especially for hygroscopic plastics (such as polyamide and polycarbonate), where moisture vaporizes at high temperatures, creating bubbles. Furthermore, excessively high injection speeds can cause the melt to flow violently within the mold cavity, entraining air and forming bubbles. To address bubble defects, the raw material must be thoroughly dried. For hygroscopic plastics, drying should be performed at 80-120°C for 4-6 hours to ensure a moisture content below 0.05%. Process control measures can include reducing the injection speed and adopting a staged injection method to avoid turbulent melt flow within the mold cavity. Furthermore, improving barrel venting and increasing venting time during the melt stage can facilitate gas discharge. If bubbles result from raw material decomposition, the barrel temperature should be lowered to prevent overheating and plastic degradation.
Weld marks and warpage are common defects that affect the mechanical properties of products. Weld marks are produced at the point where two melts converge in the mold cavity. Due to the decrease in temperature during convergence, the molecular chains are insufficiently diffused, resulting in a decrease in bonding strength and easy breakage when subjected to force. Improper mold gate position and too slow injection speed are the main reasons. When dealing with weld marks, the gate position can be optimized to allow the melt to form an orderly flow in the mold cavity and reduce the convergence of multiple melts; at the same time, the injection speed and melt temperature can be increased to enhance the diffusion capacity of the molecular chains. As for warpage defects, its essence is the release of internal stress caused by uneven shrinkage of various parts of the product, which is related to the mold temperature distribution, cooling time and holding pressure parameters. During processing, the mold cooling system needs to be adjusted to ensure uniform cooling of all parts of the product; at the same time, the holding pressure parameters should be optimized to reduce the generation of internal stress. If necessary, annealing treatment can be used to eliminate residual stress inside the product and reduce the degree of warpage.
