Design Essentials of TPU Elastomer Overmolding Mold
TPU features high elasticity, excellent wear resistance and soft tactile performance, which is widely applied in secondary overmolding products with rigid plastic substrates. TPU melt boasts favorable fluidity, yet it suffers large fluctuation of shrinkage rate and slow cooling & solidification speed. During molding, defects such as incomplete overmolding, bubbles, delamination & poor adhesion and overflow flash tend to occur easily. Different from single-material injection molds, overmolding molds need to satisfy both rigid substrate positioning and soft plastic molding requirements. This article sorts out core design principles focusing on sealing, anti-shifting positioning, venting, cooling and gating systems.
1. Substrate Positioning and Anti-shift Structural Design
When rigid substrates are placed inside mold cavities for secondary overmolding, injection pressure may push substrates to shift or warp, resulting in uneven overmolding thickness and local substrate exposure. Profile simulation positioning is the preferred solution. For large-area products, multiple positioning pillars and positioning grooves should be added. The fitting clearance for positioning is controlled between 0.02mm and 0.05mm. Excessive clearance causes melt penetration, while insufficient clearance leads to difficult part loading. Thin-wall rigid components require additional top pressing blocks arranged outside overmolding zones with moderate clamping force to avoid substrate deformation under compression. For special curved substrates, multi-point support shall be adopted to reduce suspended areas instead of single-point positioning only. Sharp edges on positioning structures should be avoided to prevent indentation on rigid substrates, and fillet transition must be implemented. Small mass-production molds can be equipped with pre-positioning inclined guides to facilitate rapid loading by workers or manipulators and shorten cycle time.
2. Parting Line and Overmolding Shut-off Structural Specifications
TPU melt has strong fluidity and tends to overflow through clearances and form extensive flash, so shut-off design serves as the core of overmolding mold development. For flat shut-off surfaces, the width of sealing zone shall be set from 3mm to 5mm, and a micro-taper of 0.5°~1° shall be machined on sealing surfaces to improve fitting tightness. A 0.2~0.3mm anti-overflow step shall be reserved at substrate edges for surrounding shut-off structure, blocking melt spreading along substrate surfaces. Long and winding sealing paths should be eliminated, since narrow gaps continuously generate fine flash. For slide and insert matching areas, fitting clearance must be controlled within 0.015mm. Wear plates shall be installed on moving inserts while reserved vent gaps; complete clearance lock for sealing is forbidden. Simplify parting layout for large-area overmolding parts, reduce multiple splicing seams from numerous inserts and lower the difficulty of flash control.
3. Gating System and Gate Type Selection
TPU is shear-sensitive; high-speed shear may trigger material degradation and yellowing, so gates cannot be designed too small. Commonly used gate types include side gate, fan gate and submarine gate. Fan gates are preferred for appearance components, delivering steady melt propagation and minimizing weld lines and bubbles. Submarine gates can be adopted for small overmolding structures, yet the cross-section area must be enlarged properly to prevent overheating caused by intense shear. Gates should be arranged on thick overmolding sections, far away from bonding boundaries between hard and soft materials, avoiding direct melt impact leading to substrate displacement. Round runners are adopted to improve melt flow capacity, and all runner corners adopt fillet transition to eliminate stagnant dead zones. Tiny pinpoint gates are not recommended, as persistent heavy shear causes TPU discoloration and deterioration of mechanical performance. For multi-gate layout, control melt convergence positions away from stress-bearing and appearance zones, and arrange venting structures properly at weld line positions.
4. Specialized Venting System Design
Bubbles and cavities are frequent defects in TPU overmolding, mostly induced by insufficient venting. Air wrapped by melt and moisture adsorbed on substrate surfaces cannot be exhausted smoothly, bringing internal voids and surface pits on overmolded layers. Vent grooves shall be opened at melt terminal zones and flow convergence areas. Vent depth is controlled between 0.01mm and 0.02mm, with width ranging from 8mm to 15mm. Matching gaps of inserts and slides act as auxiliary vent channels; over-fitting shall not seal off gaps completely. Additional ejector pins are installed at deep ribs and thin overmolding areas, utilizing unilateral clearance of ejector pins as micro vent passages. Vent inserts are added at cavity terminals for convenient adjustment of vent depth by grinding at later stages. It is necessary to distinguish bubbles induced by trapped air and moisture; mold vent structures shall cooperate with material drying process to eliminate defects jointly.
5. Cooling System and Shrinkage Deformation Control
TPU cools far slower than rigid plastics such as ABS and PC. Insufficient cooling extends molding cycle, and continuous post-shrinkage may cause debonding and sink marks. Cooling channels should be arranged as close as possible to TPU molding surfaces, with the distance between channel and cavity surface maintained from 8mm to 12mm. Contour-conforming cooling channels are prioritized. Zoned channels are arranged in areas with dramatic thickness variation of overmolding layers. Mold temperature machine interfaces are reserved on molds to realize precise temperature regulation. Mold designers shall refer to shrinkage data provided by material suppliers at early design phase. The shrinkage range varies significantly for TPU with different hardness, generally 0.6% ~ 1.4%. Shrinkage compensation should be enlarged moderately for thick overmolding sections. Fillets are added at positions with abrupt thickness transition to reduce stress concentration and warpage induced by uneven shrinkage.
6. Ejection Structure and Auxiliary Bonding Design
TPU features high elasticity and is prone to tensile deformation, so concentrated single-point ejection force should be avoided in ejection layout. Multiple ejector pins are evenly arranged for large-area overmolding components; ejector pin diameter cannot be too small, and ejector blocks can be adopted when necessary to reduce local pressure. Ejector pins shall be arranged away from thin plastic zones to avoid penetration and whitening during ejection. To improve bonding strength between overmolding material and rigid substrate, fine knurls, shallow grooves and through holes are machined on contact surfaces of rigid substrates to enhance mechanical interlocking effect. Sufficient draft angle is reserved at design stage. Large-area mirror polishing on cavity surfaces is not recommended; moderate textured polishing facilitates ejection and prevents extensive vacuum sticking of TPU. Undercut release inserts are installed for slender overmolding ribs to avoid tensile fracture of finished products.
Conclusion
Design of TPU overmolding molds cannot directly copy schemes of ordinary two-color molds or single-material injection molds. Core challenges focus on anti-shift positioning of substrates, precise shut-off for flash suppression, adequate venting for bubble elimination and rational gating to avoid shear degradation of raw materials. Positioning, shut-off, venting, cooling and ejection structures are mutually correlated. Defects such as debonding, bubbles, flash and deformation will occur in mass production if any structural design is neglected. Meanwhile, mold structures need to match material characteristics of TPU, coordinate with optimized molding parameters, balance bonding strength, surface quality of finished products and production cycle efficiency, and guarantee stable long-term mass production.
