Calculating the pitch of injection-molded threaded rings and threaded cores is crucial for ensuring the precise fit of plastic part threads. This requires comprehensive consideration of the plastic’s shrinkage characteristics, thread accuracy, and the molding process. The basic formula for calculating thread pitch is: Mold thread pitch = Part thread pitch × (1 + Shrinkage). Shrinkage must be determined experimentally rather than relying solely on theoretical values. For example, if the design pitch of a PP part is 2mm and the measured shrinkage is 1.5%, the mold thread pitch should be 2 × 1.015 = 2.03mm. Ignoring shrinkage differences in the calculation can lead to poor fit. For example, a PA66 threaded part failed to account for the reduced shrinkage caused by glass fiber filling (from 2% to 0.8%). The mold pitch was over-calculated, resulting in excessive looseness during assembly and a rework rate of 12%.
The thread precision grade determines the tolerance range of the pitch dimension. The appropriate grade should be selected based on the application scenario and factored into the calculation. Common connecting threads (such as M10) typically use 6g/6H precision with a pitch tolerance of ±0.03mm. A water pipe fitting mold maintains a pitch tolerance of ±0.02mm, maintaining a stable fit clearance of 0.05-0.1mm and ensuring sealing performance. Precision transmission threads (such as trapezoidal threads) require Grade 5 precision with a pitch tolerance of ±0.015mm. A gearbox thread core, calibrated through 3D measurement, achieves a pitch deviation of ±0.01mm, reducing transmission noise by 3dB. The upper deviation of the plastic part’s pitch should be factored in with shrinkage fluctuations. For example, a PC thread with a shrinkage fluctuation of ±0.3% and a design pitch value of 1.5mm requires a mold pitch tolerance of ±0.01mm (1.5 × 0.003 + 0.005) to avoid over-tolerances during mass production.
There are differences in the pitch calculation of different thread types (metric, imperial, trapezoidal), and the parameters need to be adjusted according to the corresponding standards and specifications. Metric coarse thread (such as M12×1.75) is calculated according to GB/T 196. For PP plastic parts with a pitch of 1.75mm, the mold pitch = 1.75×(1+1.2%)=1.771mm; the pitch of imperial thread (such as 1/4″-20) is 20 threads per inch, which is 1.27mm when converted to millimeters. For a certain ABS imperial thread plastic part, the mold pitch = 1.27×(1+0.8%)=1.280mm. The pitch of trapezoidal thread (such as Tr30×6) is 6mm. Due to the high load-bearing requirements, the shrinkage rate needs to take the measured maximum value. The mold pitch of a certain POM trapezoidal thread is 6×(1+2.5%)=6.15mm Design ensures sufficient strength after assembly. Special threads (such as G1/2″ pipe threads) require consideration of taper, requiring segmented compensation along the axial direction when calculating the thread pitch. One plumbing pipe mold achieved a 95% thread sealing surface fit through segmented calculations.
The impact of the molding process on thread pitch size must be corrected in calculations, especially for post-shrinkage in crystalline plastics. A PA66 threaded part will shrink by 0.02mm within 24 hours after molding. An additional 0.02mm compensation must be added during mold calculations, i.e., mold pitch = theoretical calculated value + 0.02mm, to ensure the final part pitch remains within ±0.01mm of the designed value. Fluctuations in injection pressure can cause pitch deviation. A test showed that increasing the pressure from 120MPa to 140MPa increases the pitch of a PP thread by 0.015mm. Therefore, a 0.01mm adjustment margin must be allowed during calculations. For multi-cavity molds, pitch consistency across cavities must be ensured by allocating tolerances during calculations. For an 8-cavity threaded mold, pitch variation between cavities is controlled to within 0.01mm by allocating a total tolerance of ±0.03mm to each cavity, typically ±0.005mm.
Wear compensation for threaded rings and cores is a crucial consideration for long-term production. Wear allowance must be accounted for during calculations to extend mold life. The thread pitch of a standard threaded mold wears approximately 0.005mm per 10,000 mold cycles. A design for an M8 threaded core adds a 0.01mm wear allowance, allowing the mold to maintain acceptable accuracy after 20,000 mold cycles. High-load threads (such as automotive bolts) wear more quickly, requiring an additional 0.02mm allowance. This design on one production line extended mold maintenance cycles from 50,000 to 80,000 mold cycles. Wear calculations must consider material hardness. Nitrided threaded cores (HRC60 hardness) show 60% less wear than untreated cores (HRC45), allowing for a reduced compensation allowance during calculations. Regularly measuring the actual pitch size and comparing it with the calculated value, one workshop established a wear trend curve after every 10,000 mold cycles. This provides more accurate compensation data for subsequent mold design, reducing pitch calculation errors from ±0.015mm to ±0.008mm.
