Injection molding mesh defects are common appearance and structural issues in plastic part production, often manifesting as irregular mesh patterns on the surface. This not only affects the aesthetics of the product but can also weaken the mechanical properties of the part. The occurrence of these defects is often closely related to the characteristics of the raw materials, process parameters, and mold design. For example, when molten plastic flows in the mold cavity, if it encounters excessive local resistance or uneven temperature distribution, unstable flow and fusion will occur at the melt front, forming a mesh-like pattern. In products with high appearance requirements, such as automotive interiors and electronic housings, mesh defects can cause the entire batch of products to be scrapped, making it crucial to promptly identify and address this issue.
The selection and pretreatment of raw materials are the basis for preventing grid defects. Different grades of plastic have different melt flow rates and thermal stability. If the selected raw materials are not fluid enough, it is easy to cause uneven molecular orientation due to excessive shear stress when filling complex cavities, forming a grid. In addition, moisture or volatiles in the raw materials are also important factors. When plastic particles contain moisture, they will be converted into gas at high temperatures, forming bubbles in the melt, and appearing as grid-like indentations after cooling. Therefore, the raw materials need to be strictly dried before production. For example, polyamide plastics need to be dried at 80-100°C for 4-6 hours to ensure that the moisture content is less than 0.05%; at the same time, raw materials with appropriate flow rates should be selected according to the structure of the plastic part. For thin-walled or complex structural parts, high-flow grades are preferred.
Optimization of process parameters is the core means to solve grid defects. If the injection molding temperature is too low, the melt viscosity will increase and the flow resistance will increase. During the filling process, the front material is prone to cooling too quickly and poor fusion, which will lead to the formation of a grid. At this time, it is necessary to gradually increase the barrel temperature and nozzle temperature. For example, the barrel temperature of polypropylene can be increased from 180°C to 200°C. At the same time, ensure that the mold temperature matches the melt temperature to avoid the cavity surface temperature being too low, resulting in rapid solidification of the melt. The setting of injection speed and pressure also needs to be precisely controlled. Too slow speed will cause the melt to cool excessively during the flow process, and too fast speed may cause turbulence and shear overheating, both of which may cause grid defects. It is recommended to use a segmented injection method, filling the dead corners of the cavity at a lower speed in the early stage, increasing the speed in the middle stage to ensure complete filling, and reducing the pressure in the later stage to reduce internal stress.
The rationality of the mold structure is also critical to preventing grid defects. If the gate position of the mold is inappropriate, the melt will form multiple flow fronts after entering the cavity. If the temperature of these fronts is too low when they converge, obvious weld marks will be produced, which will then develop into grid-like defects. Therefore, the gate position needs to be optimized according to the shape of the plastic part, and the melt should be filled in a single direction as much as possible to reduce the confluence of multiple melts. Inadequate design of the mold’s exhaust system is also one of the causes. If the air in the cavity cannot be discharged in time, it will be compressed to form high pressure, which will hinder the flow of the melt, resulting in local insufficient filling and the formation of grids. At this time, an exhaust groove should be added at the position where the melt is last filled. The groove width is usually 0.02-0.05mm and the depth does not exceed 0.03mm to ensure smooth exhaust of air while preventing plastic overflow.
Post-processing and quality monitoring are important steps in reducing the impact of mesh defects. For plastic parts that have already produced minor mesh defects, the appearance can be improved through surface treatment methods such as grinding and polishing, but care should be taken to avoid excessive grinding that may cause uneven wall thickness of the plastic part. If the defects are more serious, it is necessary to analyze the specific causes and readjust the production parameters or mold structure. Establishing a comprehensive quality monitoring system is also essential. During the production process, samples should be regularly taken for appearance inspection and mechanical property testing, and the correlation data between process parameters and defect generation should be recorded. Through big data analysis, the production process can be optimized to fundamentally reduce the incidence of mesh defects. In addition, strengthening the skills training of operators so that they can identify defects in a timely manner and take countermeasures is also an important guarantee for improving product quality stability.
