Injection Cracks (Cracks) and Solutions
Injection cracks, also known as crazing, are common defects in the injection molding process. These microscopic cracks appear on or within the product, severely impacting its appearance and mechanical properties. The occurrence of these defects is often closely related to multiple factors, including raw material properties, molding process parameters, and mold design. Cracks can appear as irregular, reticular, radial, or linear patterns, and are more likely to occur at corners, at sudden changes in wall thickness, or in areas of stress concentration. Cracking not only reduces the strength and service life of a product but can also cause it to break during use, posing a safety hazard. Therefore, effective measures must be taken to address this issue.
Improper selection and handling of raw materials is one of the key causes of injection molding cracking. First, impurities or excessive moisture content in the plastic raw materials can affect the material’s melt fluidity and uniformity, easily causing stress concentration during the molding process, which in turn causes cracks. For example, if highly hygroscopic plastics such as polyamide (PA) are not fully dried before molding, the moisture will vaporize at high temperatures, forming bubbles and internal stress, causing the product to crack. Secondly, uneven molecular weight distribution of the raw materials or improper dosage of additives (such as plasticizers and stabilizers) can also cause cracking. An overly wide molecular weight distribution will make the material’s melt viscosity unstable, while excessive additives or poor compatibility with the substrate will destroy the material’s continuity, reduce its mechanical properties, and increase the risk of cracking.
Improper setting of molding process parameters is the main factor causing injection molding cracks. Excessive injection pressure and long holding time will cause large residual stresses inside the product. When the residual stress exceeds the material’s bearing limit, cracks will appear. For example, during the injection process, if the injection speed is too fast, the flow rate of the molten material in the mold cavity will be uneven, which will easily form eddies and stress concentration at the corners, causing cracks on the product surface. In addition, melt temperature and mold temperature are also key factors. If the melt temperature is too low, the material will not melt fully, the fluidity will be poor, filling will be difficult, and the molding pressure and internal stress will increase; if the mold temperature is too low, the product will cool too quickly, the surface will shrink unevenly, and large thermal stress will be generated, which will cause cracks.
The rationality of mold design also has an important impact on the occurrence of injection molding cracks. Improper mold gate position, number and form will cause turbulent flow and large shear stress of the molten material during the filling process, increasing the possibility of cracking. For example, if the gate is located far away from the thick-walled part of the cavity, the molten material will experience a longer flow path during the filling process, be subjected to greater shear force, and generate internal stress. A poor exhaust system of the mold is also a common problem. If the gas in the cavity cannot be discharged in time, bubbles and pressure will form in the molten material, which will easily cause cracks when the pressure is released. In addition, if the surface roughness of the mold cavity is high or there are sharp angles, undercuts and other structures, the product will be subjected to additional friction and stress during the demolding process, causing surface cracks.
The solution to injection molding cracking requires multiple approaches, including raw material processing, process parameter optimization, and mold improvement. First, it is necessary to strengthen raw material quality control, select high-purity, stable-performance plastic raw materials, and fully dry them according to their properties. For example, hot air drying or vacuum drying can be used for hygroscopic plastics to ensure that the moisture content is within the allowable range. Second, the molding process parameters should be optimized, and the injection pressure and holding pressure should be appropriately reduced, and the holding time should be shortened to reduce residual stress within the product. The melt temperature and mold temperature should be reasonably increased to improve the material’s fluidity and cooling uniformity. For example, for crystalline plastics, the mold temperature can be appropriately increased to promote uniform crystal growth and reduce internal stress.
In terms of mold design and maintenance, the gate location and number should be appropriately set to ensure that the molten material can smoothly and evenly fill the mold cavity and avoid excessive shear stress. Venting slots should be added to the mold to improve exhaust efficiency and promptly expel gas from the cavity. The mold cavity surface should be polished to reduce surface roughness, and sharp angles should be converted to rounded transitions to reduce stress concentration during demolding. Furthermore, during the product design phase, sudden changes in wall thickness should be avoided as much as possible, and a gradual wall thickness design should be adopted to reduce stress differences during the cooling process. For products that have already cracked, internal stress can be eliminated through annealing treatments, such as placing the product in an oven at a certain temperature for a period of time and then slowly cooling it to relieve internal stress and reduce crack propagation. By taking a combination of these measures, the problem of injection molding cracking can be effectively prevented and resolved, improving product quality and reliability.
