Calculation of thread core size
Thread cores are critical components used in injection molds to form the threaded structure of plastic products. Their dimensional accuracy directly impacts the thread quality and assembly performance of plastic products. Calculating thread core dimensions requires comprehensive consideration of factors such as the plastic material’s shrinkage, the thread’s precision level, and molding process parameters to ensure that the plastic product achieves the desired thread dimensions after cooling. Improper calculation of thread core dimensions can result in overtightening or loosening of the plastic product’s threads, impacting assembly efficiency and even causing product failure. Therefore, mastering the correct thread core dimension calculation method is crucial.
The shrinkage rate of the plastic material is the most crucial factor in calculating thread core dimensions. After injection molding, plastic shrinks during cooling, causing the product to shrink in size. This shrinkage varies significantly among different plastic materials. For example, the shrinkage rate of polyethylene (PE) is typically 1.5%-3.0%, while that of polyoxymethylene (POM) is 1.8%-3.5%. Polystyrene (PS) has a relatively small shrinkage rate, generally between 0.4%-0.7%. Therefore, when calculating thread core dimensions, the thread dimensions of the plastic product should be increased based on the specific shrinkage rate of the plastic material to compensate for the shrinkage after cooling. The basic calculation formula is: Thread core size = Plastic product thread size × (1 + shrinkage rate). For example, if a plastic product requires an M10 × 1.5 thread and the plastic material used has a 2% shrinkage rate, the major diameter of the thread core should be 10 × (1 + 2%) = 10.2 mm.
The thread’s precision grade and fit requirements are also crucial factors in calculating thread core dimensions. The thread’s precision grade is primarily determined by the tolerance band and engagement length, with different precision grades corresponding to different dimensional tolerance ranges. When calculating thread core dimensions, it’s important to determine reasonable dimensional tolerances based on the precision grade of the plastic product’s threads to ensure the product’s threads meet assembly requirements. For example, for a 6g external thread, the tolerance band is negative. When calculating the major, intermediate, and minor diameters of the thread core, it’s necessary to add shrinkage to the basic dimensions and consider the impact of the tolerance band to ensure the core dimensions are within the allowable tolerance range. Furthermore, the longer the thread engagement length, the higher the core dimensional accuracy requirements to avoid assembly difficulties caused by excessive dimensional deviation.
The structure and molding process parameters of a thread core also affect its dimensional calculation. Thread cores can be categorized as either integral or modular. Integral cores offer a simpler structure but are more difficult to machine, making them suitable for small threads. Modular cores, composed of multiple parts, are easier to machine and replace, making them suitable for large or complex threads. When calculating the dimensions of cores with different structures, factors such as assembly clearance and fit accuracy must be considered. Furthermore, process parameters during the injection molding process (such as melt temperature, mold temperature, and holding pressure) can also affect the plastic’s shrinkage, which in turn affects the calculated dimensions of the thread core. For example, high mold temperatures slow plastic cooling, increasing shrinkage. In this case, the thread core size may need to be appropriately increased. Conversely, high holding pressures increase the plastic’s density, reducing shrinkage, and thus reducing the core size. Therefore, in actual calculations, it is necessary to adjust the shrinkage based on the specific process parameters to improve the accuracy of the core dimensions.
With the continuous development of injection mold technology, the methods for calculating thread core dimensions are also constantly improving. Today, computer-aided engineering (CAE) software can simulate and analyze the shrinkage process of plastics and accurately predict the shrinkage of the product, providing a more reliable basis for calculating thread core dimensions. At the same time, the application of three-dimensional modeling technology makes the design and dimensioning of thread cores more intuitive and accurate, reducing the errors caused by manual calculations. In addition, the application of some new materials (such as fiber-reinforced plastics) has the characteristics of anisotropic shrinkage, which also poses new challenges to thread core dimension calculation, requiring the use of more complex calculation models for analysis. In the future, with the integration of intelligent design and manufacturing technologies, thread core dimension calculation will be automated and intelligent, further improving the efficiency and accuracy of mold design.
