Shrinkage Value Selection for Injection Molded Soft Sealing Rings

Soft sealing rings are mostly made of TPU, TPE and TPV thermoplastic elastomers, which are widely used in waterproof and dustproof sealing scenarios. Different from rigid plastics, soft elastomers have obvious flow direction shrinkage difference, and their shrinkage rate is greatly affected by hardness, wall thickness, filler and molding process. Unreasonable shrinkage value in mold design will cause dimensional deviation of inner diameter and wire diameter, resulting in poor assembly, water leakage and seal failure. Scientific shrinkage selection and correction are essential for qualified sealing products.

1. Basic Shrinkage Reference of Common Soft Materials

Different elastic materials have fixed basic shrinkage ranges. TPU material with Shore A60–A95 has a stable shrinkage of 0.8% to 1.2%, and ultra-soft TPU can reach 1.5%. SEBS-based TPE has higher shrinkage: medium hardness Shore A40–A60 takes 1.4% to 1.8%, and super soft high-oil-filled TPE reaches 1.8% to 2.3%. TPV weather-resistant sealing material is stably controlled at 1.2% to 1.6%.

The actual shrinkage shall be based on the material supplier’s official data, and the empirical range is only used for preliminary mold design. Harder materials have lower shrinkage, while softer and oil-filled materials have larger shrinkage fluctuation.

2. Structural Correction of Shrinkage Value

Product wall thickness is the key correction factor. Wall thickness above 3mm has slow cooling and sufficient molecular relaxation, and the shrinkage value needs to increase by 0.2%–0.4%. Thin wall below 1.5mm cools quickly, and the shrinkage can be reduced by 0.1%–0.2%.

For inserts encapsulated sealing rings, the metal insert restricts local shrinkage, and the shrinkage value of the wrapped part should be reduced by 0.3%–0.6%. For special-shaped long strips, the flow direction shrinkage is larger, and the vertical direction is appropriately reduced to prevent oval deformation. The thin sealing lip cannot adopt the body shrinkage value alone to avoid dimensional tolerance deviation.

3. Mold Structure Influencing Factors

Gate type directly affects shrinkage stability. Small point gates freeze early with insufficient packing, resulting in larger product shrinkage, which needs a slight increase in design. Large fan gates have sufficient pressure transmission and smaller shrinkage. Multi-cavity molds have different filling distances, and the distal cavity has larger shrinkage, requiring targeted dimensional compensation.

Unreasonable runner and exhaust structure will cause insufficient filling and false shrinkage. The shrinkage value must be reserved with process tolerance, and the middle value of the material interval is preferred for mold opening.

4. Process Matching and Actual Calibration

Mold temperature and holding pressure are the most sensitive factors affecting soft glue shrinkage. Higher mold temperature increases shrinkage, and sufficient holding pressure reduces shrinkage deformation. In the trial molding stage, standard processes are adopted for production. After the product is cooled to room temperature for stress release, measure the inner diameter and wire diameter, calculate the actual shrinkage rate, and modify the mold cavity size.

Summary

The shrinkage value of soft sealing rings cannot adopt fixed empirical data. It is necessary to select the basic range according to the material hardness, correct the value according to wall thickness and mold structure, and finally verify and calibrate through trial molding measurement. Accurate shrinkage matching ensures stable size of sealing rings and reliable sealing performance.