Core Structural Design Essentials for TPU Overmolding Molds
TPU combines rubber elasticity and plastic processing performance. Dual-color hard-soft overmolding and secondary insert overmolding are mainstream molding technologies, yet overmolding easily causes incomplete filling, hard-soft delamination, flash, soft rubber shrinkage and position offset. Mold design must combine hard substrate material features, TPU melt fluidity and bonding mechanism, including parting layout, positioning structure, gating system, exhaust layout, cooling design and anti-shrinkage structure.
1. Parting Structure for Overmolding Molds
Overmolding molds are divided into integrated dual-color molds and secondary insert molds. Dual-color molds inject hard plastic and TPU successively via two injection barrels, with moving molds rotating stations to finish overmolding. Parting surfaces form complete sealing lines along hard plastic overmolding boundaries, with 0.015~0.02mm precise fitting clearance at sealing positions to stop molten TPU flashing and wrapping non-overmolding areas. Secondary insert molds independently design cavities for placing hard plastic semi-finished products, reserving limiting steps equal to hard plastic wall thickness on cavity inner walls to prevent blank offset and warpage. All parting lines avoid product appearance surfaces to prevent obvious parting marks left on soft TPU surfaces. Sealing inserts adopt S136 steel hardened to HRC 52~55 for balanced wear resistance and rust prevention, and full-surround overmolding products use segmented insert splicing for convenient local flash revision.
2. Anti-Offset Positioning & Bond Reinforcement Structure
Hard substrate displacement causes uneven overmolding thickness and local delamination. Insert overmolding molds install a central positioning pillar and more than three peripheral limiting inserts with 0.008mm reserved clearance for accurate restriction. Inverted positioning ribs and circular positioning holes are added on non-appearance areas of hard plastic to realize bidirectional fixation relying on original structural features. If hard substrates are smooth ABS or PC, dense micro pits and micro grooves are machined on overmolding areas, forming mechanical occlusion after TPU molding to improve bonding strength between hard and soft materials, with texture depth controlled at 0.1~0.2mm to avoid trapped air causing whitening on bonding surfaces.
3. Gating System Layout Adapted to TPU Fluidity
TPU cools and solidifies rapidly after melting, so fan gates and thin side gates are preferred for thin-walled overmolding areas. Fan gate width matches overmolding length with 0.3~0.5mm thickness for gentle filling and fewer weld lines. Strip overmolding structures use multi-point submarine gates hidden on non-appearance side walls to avoid exposed gate marks. Semi-circular polished runners are adopted with roughness below Ra0.8 to prevent cooled TPU from accumulating inside runners and blocking material flow. Dual-color molds configure two isolated runner systems to separate hard plastic and TPU materials completely and avoid cross contamination.
4. Dense Exhaust System for Overmolding Cavities
TPU decomposes tiny molecular gas under heating; trapped air inside closed cavities leads to scorching, reduced bonding strength and incomplete filling. Exhaust slots are opened every 12~15mm along overmolding sealing boundaries, with 0.01~0.015mm front depth and widened rear section for rapid exhaust. Extra exhaust inserts are added at melt convergence and final filling positions, and breathable steel inserts are embedded inside large flat overmolding areas to solve planar whitening caused by trapped air, avoiding breathable steel embedding on product stress bonding positions to prevent TPU permeating pores and forming burrs.
5. Independent Cooling & Anti-Shrinkage Structure
Hard plastic cavities and TPU overmolding inserts are equipped with mutually independent cooling circuits for separate temperature control. The optimal mold temperature for TPU molding is 40℃~65℃, and conformal water channels are arranged close to overmolding contours 8~12mm away from cavity surfaces to ensure uniform soft rubber cooling. Overmolding thickness is uniformly designed as 1.0~2.5mm with thickness difference below 0.3mm, adopting R0.5 arc transition at thickness changing positions to prevent warpage delamination caused by uneven shrinkage. A 0.2mm thin rubber edge sealing structure is reserved at hard-soft bonding edges to wrap hard plastic sides and block water vapor penetrating bonding interfaces.
6. Uniform Ejection Demolding Structure
Soft TPU is prone to stretch and tear under uneven ejection force, so molds adopt composite ejection of pins and ejection plates, arranging ejection pins close to TPU areas and flat ejectors at stress positions for uniform stress bearing. Large overmolding products install balance blocks to guarantee synchronous separation between products and cavities and avoid hard-soft separation caused by unilateral pulling. Cavity surfaces adopt matte polishing of Ra1.6 to prevent vacuum adhesion of TPU, and all soft rubber undercuts use slider core pulling instead of forced stripping to avoid TPU tearing and delamination defects.
Conclusion
TPU overmolding mold design focuses on stable bonding and flawless molding. Precise positioning prevents substrate offset, mechanical occlusion textures enhance hard-soft bonding strength, reasonable gating and dense exhaust eliminate incomplete filling and trapped-air delamination, independent temperature-controlled water channels balance TPU shrinkage rate, and uniform ejection avoids demolding separation. Adjust gate width and exhaust density according to TPU hardness: widen gates for high-hardness TPU above 90A to improve fluidity, and increase ejection pins for soft TPU below 60A to prevent deformation. Molds designed with above structures effectively lower defective rates of delamination, flash, shrinkage and whitening during mass production.
