Calculation of molding dimensions for injection molded threaded rings
Injection molding threaded rings are key mold components for the threaded structure of plastic parts. Their dimensional accuracy directly determines the fit and reliability of the finished thread. Calculating the molding dimensions of the threaded ring requires comprehensive consideration of the plastic’s shrinkage, thread accuracy, and mold manufacturing tolerances. Scientific calculation formulas and parameter selection ensure that the finished thread meets design requirements after cooling. This process involves the coordinated calculation of multiple parameters and is an essential step in mold design.
The basic dimensions of the threaded ring are calculated based on the nominal dimensions of the product thread, with the key consideration being the shrinkage characteristics of the plastic. Since plastic shrinks after molding and cooling, the size of the threaded ring must be larger than the nominal dimensions of the product thread to compensate for this shrinkage. The calculation formula is: Basic Ring Dimension = Product Thread Nominal Dimension × (1 + Shrinkage Factor). The shrinkage factor depends on the type of plastic. For example, the shrinkage factor of polyethylene (PE) is 1.5% to 3.5%, that of polypropylene (PP) is 1.0% to 2.5%, and that of polyoxymethylene (POM) is 1.5% to 3.0%. For example, if PP material is used for an M10×1.5 thread and the shrinkage factor is 2%, the major diameter of the ring is 10 × (1 + 2%) = 10.2 mm. The pitch and minor diameters should also be calculated using this formula to ensure effective compensation in all diameter directions.
The shrinkage factor also needs to be considered when calculating the pitch of a threaded ring, but the calculation method differs from that used for diameter. Pitch is a linear dimension, and the ring dimension is calculated as follows: Ring pitch = Part thread pitch × (1 + shrinkage factor). For example, if the part thread pitch is 1.5mm and the shrinkage factor is 2%, then the ring pitch = 1.5 × (1 + 2%) = 1.53mm. It is important to note that pitch accuracy requirements are high; excessive deviation can result in a loose or tight fit. Therefore, the shrinkage factor selection during calculations requires greater precision, and the actual shrinkage factor can be determined through testing if necessary. For multi-start threads, the ring dimension of the lead (pitch × number of turns) must also be calculated to ensure an accurate helical trajectory.
The tooth angle of the threaded ring is another important parameter that affects thread fit. Its calculation must take into account the plastic’s fluidity and elastic recovery after molding. Normally, the tooth angle of the ring should be consistent with that of the product’s thread. However, for some plastics with greater toughness, the ring angle can be reduced by 0.5° to 1° to compensate for the elastic recovery after the product cools. For example, the tooth angle of a common triangular thread is 60°, while the tooth angle of the ring can be designed to be 59° to 59.5°. Furthermore, the top and bottom fillet radii of the thread profile also need to be adjusted accordingly. The fillet radius of the ring should be slightly larger than the design value of the product’s thread to avoid defects such as sharp corners and flash at the top or bottom of the thread due to plastic shrinkage.
The tolerance grade of the threaded ring must match the precision requirements of the product’s threads, generally adhering to the principle of “mold tolerance lower than product tolerance.” For example, if the product’s thread precision is 6g, the ring’s tolerance grade should be IT6 or IT7 to ensure that the product still meets the precision requirements after shrinkage. The ring’s tolerance band is typically determined by the upper deviation, which is the basic dimension plus the positive deviation. For example, for an M10×1.5 ring with a major diameter of 10.2mm, if the tolerance is IT7 (upper deviation + 0.019mm), the actual range of the ring’s major diameter is 10.2 to 10.219mm. Furthermore, the ring’s cumulative error, including cumulative pitch error and half-angle error, must be controlled to prevent errors from compounding and affecting thread precision.
In actual calculations, mold wear and maintenance factors must also be considered. For mass-produced molds, the ring size can be appropriately enlarged by 0.01-0.03mm to compensate for wear after long-term use. Furthermore, for threads with specific fit requirements, such as those with metal bolts, the ring size must be adjusted through actual mold trials. If necessary, group assembly should be used to ensure the fit clearance is within the designed range. For example, if after mold trials, it is found that the product thread and bolt fit are too tight, the ring’s median diameter can be appropriately reduced by 0.02mm, the ring re-machined, and the mold retested until the ideal fit is achieved. Accurate calculations of threaded ring forming dimensions are only possible by comprehensively considering factors such as shrinkage, tolerance, and wear.
