Injection molding size deviation and solution
Injection molding dimensional deviations refer to deviations between the actual dimensions of a plastic product and its designed dimensions that exceed the allowable range. This is a common quality defect in injection molding production. These deviations not only affect the assembly performance and functionality of plastic products but, in severe cases, can also lead to product scrapping and increase production costs. The occurrence of injection molding dimensional deviations is closely related to a variety of factors, including raw material characteristics, injection molding process parameters, mold design, and manufacturing precision. Targeted solutions are required to ensure the dimensional accuracy of plastic products.
First, raw material properties significantly influence dimensional tolerances during injection molding. The shrinkage of plastic raw materials is one of the main causes of dimensional tolerances. Different types of plastics exhibit varying shrinkage rates, and even the same plastic can vary depending on its formulation, molecular weight distribution, and additive content. For example, the shrinkage of polyethylene (PE) typically ranges from 1.5% to 3.5%, while that of polyvinyl chloride (PVC) is between 0.5% and 1.5%. Failure to accurately predict the plastic’s shrinkage during mold design, or selecting raw materials with shrinkage rates that do not match the designed values, can lead to dimensional tolerances in plastic products. Furthermore, raw material properties such as fluidity and thermal stability can also affect dimensional accuracy. Raw materials with poor fluidity struggle to fill the mold cavity, resulting in undersized parts. Raw materials with poor thermal stability are prone to degradation at high temperatures, resulting in defects such as bubbles and shrinkage cavities, which indirectly affect dimensional accuracy. Therefore, addressing dimensional tolerances requires strict raw material quality control, selecting materials with stable shrinkage rates, and incorporating dimensional compensation based on the raw material’s actual shrinkage rate during mold design.
Secondly, improper injection molding process parameter settings are a common cause of dimensional deviations. Parameters such as injection pressure, injection speed, holding pressure, holding time, melt temperature, and mold temperature all affect the dimensional accuracy of plastic products. Insufficient injection pressure or too low a holding pressure prevents adequate shrinkage during cooling and shrinkage of the melt, resulting in undersized parts. Excessive injection pressure, on the other hand, can cause elastic deformation in the mold, resulting in oversized parts. Excessive injection speeds create turbulent melt flow in the mold cavity, leading to uneven local pressure distribution and dimensional fluctuations. Too slow a speed prevents adequate mold filling, affecting part size. Too short a holding time prevents adequate shrinkage, leading to shrinkage. Too long a holding time increases production cycle time and may lead to excessive internal stress in the part. Melt and mold temperatures significantly influence the shrinkage of the plastic. Excessively high temperatures increase shrinkage, resulting in undersized parts. Excessively low temperatures reduce melt fluidity, making mold filling difficult and affecting dimensional accuracy. Therefore, it is necessary to optimize the process parameters through mold trials, establish a reasonable parameter range, ensure that parameters such as injection pressure, holding pressure, temperature, etc. match the raw material characteristics and product structure, and reduce dimensional deviation.
Mold design and manufacturing precision are fundamental to ensuring the dimensional accuracy of plastic products. The dimensional accuracy of the mold cavity and core directly determines the dimensions of the plastic product. Inadequate machining precision of mold components, such as excessive dimensional, shape, or positional errors, can result in dimensional deviations of the plastic product. For example, an undersized mold cavity will result in an undersized product; a misaligned core will lead to uneven wall thickness, affecting dimensional accuracy. The accuracy of the mold’s parting surface also affects product dimensions. Poor fit at the parting surface can lead to flash, indirectly affecting dimensional measurement results. Furthermore, an improperly designed mold cooling system can lead to uneven cooling of the product, resulting in differential shrinkage and dimensional deviation. For example, uneven distribution of cooling channels can cause excessively rapid cooling in certain areas of the product, increasing shrinkage and resulting in undersize. Therefore, during the mold design phase, it is important to determine reasonable mold manufacturing tolerances based on the dimensional accuracy requirements of the plastic product. Generally, the dimensional tolerance of mold components should be within 1/3-1/5 of the product’s dimensional tolerance. At the same time, the mold structure is optimized and the cooling water channels are arranged reasonably to ensure uniform cooling of the product; the parting surface accuracy is improved and the flash generation is reduced, providing a good mold foundation to ensure the dimensional accuracy of the product.
Environmental factors and equipment conditions during the production process can also affect the dimensional accuracy of plastic products. Fluctuations in ambient temperature and humidity can cause thermal expansion and contraction of plastic raw materials and molds, affecting product dimensions. For example, rising ambient temperature increases mold temperature, leading to increased plastic shrinkage and smaller product dimensions. High humidity also causes hygroscopic plastic raw materials to absorb moisture, causing changes in melt flow and impacting mold filling. The operating condition of the injection molding machine also significantly impacts dimensional accuracy. Leaks in the injection system and parallelism errors in the clamping mechanism can lead to unstable injection pressure and clamping force, resulting in dimensional fluctuations in the product. Therefore, the production environment’s temperature and humidity must be controlled, generally maintaining the ambient temperature between 20-25°C and the relative humidity between 50% and 60%. Regular maintenance of the injection molding machine is also essential, including checking the operating condition of the injection system and clamping mechanism to ensure that the equipment meets production requirements. For example, regular calibration of the injection pressure sensor and cleaning of the barrel and screw can minimize the impact of equipment factors on dimensional accuracy.
Finally, an effective testing and feedback mechanism is crucial for addressing dimensional deviations during injection molding. During the production process, a comprehensive dimensional inspection system should be established, with regular spot checks or full inspections of plastic products to promptly identify dimensional deviations. The choice of inspection tools should be determined based on the product’s dimensional accuracy requirements, such as using vernier calipers and micrometers for routine dimensional inspections and coordinate measuring machines for high-precision dimensional inspections. Any dimensional deviations discovered during inspection should be promptly analyzed and addressed. For example, if dimensional deviations are caused by raw material shrinkage variations, the holding pressure and time can be adjusted; if mold wear is the cause, the mold should be repaired or replaced; and if process parameter fluctuations are the cause, the process parameters should be optimized. Furthermore, a quality traceability system should be established to record information such as raw material batches, process parameters, and test results to provide a basis for subsequent quality analysis and improvement. Through continuous testing, analysis, and improvement, the dimensional accuracy of plastic products can be continuously improved, the incidence of dimensional deviations can be reduced, and stable and reliable product quality can be ensured.
