Dimensions and Control Issues of POM Plastic Parts (Steel Parts)
POM plastic parts, also known as steel parts, are widely used in the automotive, electronics, and machinery industries due to their excellent mechanical properties, wear resistance, and fatigue resistance. However, controlling dimensional accuracy has always been a prominent issue in the production of POM plastic parts. Dimensional deviations not only affect product assembly performance but can also lead to functional failure. Therefore, in-depth research on the dimensional variation patterns and control methods of POM plastic parts is of great practical significance.
The physical properties of POM material are important internal factors affecting the dimensional stability of its molded parts. POM is a crystalline polymer, and its degree of crystallinity directly affects the dimensional stability of the molded part. During the injection molding process, POM melt undergoes crystallization shrinkage as it cools. The higher the crystallinity, the greater the shrinkage. Due to POM’s rapid crystallization rate, under different cooling conditions, the crystallinity of different parts of the molded part may vary, resulting in uneven shrinkage and, in turn, dimensional deviation. Furthermore, while POM has low hygroscopicity, it still absorbs some moisture at high temperatures. While the dimensional impact is relatively minor, it still warrants attention in the production of precision parts. POM has a large coefficient of thermal expansion, and changes in ambient temperature can cause significant dimensional changes in the molded part, a factor that requires consideration during use and dimensional stability.
Injection molding process parameters significantly influence the dimensions of POM parts. Injection pressure is a key factor affecting part size. Sufficient injection pressure ensures the melt fully fills the mold cavity, minimizing undersize issues caused by insufficient filling. However, excessive injection pressure can generate significant internal stress within the part. The release of this stress during subsequent use can cause dimensional changes. Holding pressure and holding time also affect part dimensions. Appropriate holding pressure compensates for melt shrinkage during cooling and improves dimensional accuracy. However, excessive holding pressure or prolonged holding time increases internal stress in the part, negatively impacting dimensional stability. Melt and mold temperatures significantly influence the crystallization process of POM. Excessively high melt temperatures can cause POM decomposition and increase crystallization time. Excessively low mold temperatures can accelerate crystallization, leading to uneven crystallization and compromising dimensional accuracy.
Proper mold design is fundamental to controlling the dimensions of POM parts. The mold cavity size directly determines part size, so the shrinkage of the POM must be fully considered during mold design. The cavity size should be appropriately enlarged to compensate for the part’s cooling shrinkage. The mold’s cooling system design is crucial for achieving uniform part dimensions. An uneven cooling system can lead to varying cooling rates across the part, resulting in varying degrees of crystallinity and, consequently, dimensional deviations. For example, in parts with uneven wall thickness, thicker areas cool more slowly, leading to higher crystallinity and shrinkage, which can easily cause dimensional deviations. Furthermore, the mold’s gate design can influence part dimensions. The gate’s location and size influence the melt filling and pressure-holding processes. Improper gate design can lead to uneven pressure distribution across the part, further causing dimensional variations.
Environmental factors during the production process can also affect the dimensions of POM parts. Fluctuations in ambient temperature can cause POM parts to expand and contract, so maintaining a stable ambient temperature is crucial during production and storage. Especially in the production of precision parts, workshop temperatures should be kept within a certain range to minimize the impact of ambient temperature on part dimensions. Furthermore, while humidity has a minimal impact on POM, in certain circumstances, such as high humidity, it can still cause moisture absorption on the part surface. While this may not significantly affect overall dimensions, it can affect surface quality and indirectly impact assembly accuracy.
To effectively control the dimensions of POM molded parts, comprehensive control measures are required. Regarding material selection, the appropriate POM grade should be chosen based on the product’s intended use. Different POM grades vary in crystallinity and shrinkage, so the selection should be tailored to actual needs. Regarding process parameter optimization, optimal parameters such as injection pressure, holding pressure, holding time, melt temperature, and mold temperature should be determined through experiments to ensure part dimensional accuracy. In mold design, the shrinkage characteristics of POM should be fully considered, with appropriate cavity dimensions and cooling systems designed to ensure uniform cooling across all parts of the part. During production, environmental temperature and humidity should be strictly controlled, and molds should be regularly maintained to ensure mold accuracy. Furthermore, post-processing processes, such as annealing, can be used to eliminate internal stresses within the part and improve dimensional stability. These comprehensive measures can effectively control the dimensions of POM molded parts and improve product quality.
