Demolding Optimization of Microstructures in Smart Watch Strap Molds

Smart watch straps are typically made from soft elastic materials such as TPU, TPE, liquid silicone rubber, and modified elastomers. These straps often feature dense micro-textures, anti-slip particles, breathable micro-holes, curved profiles, and flexible locking structures. While microstructures enhance appearance, comfort, and functionality, they also introduce significant demolding challenges. During high-volume production, microfeatures are prone to sticking, tearing, deformation, surface whitening, and incomplete texture replication. Since no release agents are allowed in consumer wearable products, demolding performance depends entirely on mold structure optimization, surface engineering, and ejection system design.

Draft angle is the most fundamental solution to reduce adhesion. For general strap surfaces, a draft angle of 1° to 3° is sufficient. However, for micro-textures, micro-ribs, and dense anti-slip patterns, draft angles should be increased to 3°–5° to overcome strong adhesion. For shallow textures less than 0.3 mm deep, a micro-taper transition maintains appearance while significantly improving releasability. Draft angles must be consistent along the demolding direction to avoid steps or reverse tapers that cause jamming.

III. Cavity Surface Treatment and Polishing OptimizationSurface roughness directly determines demolding resistance. Cavities and micro-textured areas must undergo unidirectional precision polishing along the demolding direction, achieving a surface roughness of Ra 0.025 or better. Transverse polishing marks create microscopic undercuts that trap plastic and cause tearing.

To further reduce adhesion, molds can be treated with chrome plating, nano-coatings, or low-friction functional coatings. These treatments lower surface energy, prevent plastic adhesion, and improve wear resistance. Laser texturing ensures uniform depth and consistency across micro-patterns, avoiding irregularities that lead to localized sticking.

IV. Balanced Ejection System DesignSmart watch straps are flexible and easily deformed. Therefore, ejection force must be evenly distributed. A combination of flat ejector pins, blade ejectors, and ejector blocks is commonly used to avoid concentrated force. Air ejection is highly effective for large micro-textured areas, as compressed air forms a thin air film between the part and the mold, breaking vacuum adhesion and preventing texture damage.

Non-uniform cooling causes uneven shrinkage, which increases wrap-around force on cores and micro-inserts. Cooling channels should follow the contour of the strap to ensure uniform temperature distribution. Controlled mold temperature reduces internal stress and improves shape retention. Sufficient cooling time ensures the material solidifies stably before ejection, reducing elasticity-induced adhesion.

Shrinkage compensation is applied based on material characteristics to avoid excessive gripping force. TPU and silicone materials have relatively high shrinkage rates, so mold dimensions must be precisely compensated to prevent tight adhesion in micro-cavities.

VI. Venting and Insert Structure OptimizationTrapped gas in micro-cavities creates negative pressure that causes severe sticking. Micro-vents with a depth of 0.01–0.03 mm are placed along flow fronts and parting lines to allow air escape without causing flash. For deep micro-holes, insert structures improve machinability and venting efficiency. Proper venting not only improves filling quality but also eliminates vacuum adhesion, one of the main causes of demolding failure in microstructured molds.

Through comprehensive optimization of draft angles, surface quality, ejection balance, cooling uniformity, and venting efficiency, consistent and high-yield demolding of microstructured smart watch straps can be achieved without relying on release agents.