Effective Improvement Solutions for Poor Adhesion of TPU Overmolding Injection in China
TPU secondary overmolding achieves bonding by molecular fusion between molten soft TPU and hard plastic substrate. Poor adhesion defects manifest as peeling under hand force, separation, local blistering and interface delamination, which are frequent quality problems in China’s overmolding injection industry. Many on-site improvements only adjust process parameters while ignoring material compatibility and interface conditions, making stable bonding hard to achieve. The failure causes cover substrate material selection, product structure, surface pretreatment, mold design and injection process.
Substrate material compatibility control
Material compatibility between substrate and TPU is the foundation of bonding. Prioritize substrate grades directly suitable for overmolding. Commonly compatible substrates include PC, PC/ABS and PA. Ordinary PP and PE show poor compatibility with TPU. Direct overmolding without modified formula easily leads to delamination. Even for materials of the same series, confirm whether sliding agents, release agents or glass fiber fillers are added to the substrate. Precipitated additives form an isolating layer on the substrate surface and block molecular fusion. Raw materials shall be stored with moisture-proof measures. Moisture absorbed by TPU generates water vapor under high injection temperature, forming tiny pores at the bonding interface and damaging adhesion strength. Moist hard substrates also produce interface bubbles and reduce bonding force. Raw materials must be dried strictly in accordance with material specifications before use. TPU drying temperature is controlled from 70℃ to 85℃. Corresponding drying parameters shall be set for hard substrates based on material grades to avoid moisture entering the mold cavity.
Optimization of overmolding product structure
Pure molecular bonding on flat structures delivers relatively low adhesion strength. Reasonably designed mechanical interlocking structures greatly improve overall adhesion. Grooves, through holes, undercuts and serrated textures are designed on the hard substrate’s overmolding area to allow molten TPU to fill into these structures and form mechanical locks as supplement to molecular bonding. Grooves cannot be too deep to prevent sink marks after overmolding. Add fillets at corners to avoid stress concentration and later cracking. Avoid narrow sharp gaps in overmolding zones, as TPU filling becomes difficult and local adhesion failure occurs. Add a glue stopping step at the overmolding boundary to prevent TPU flash outside non-overmolding areas and limit glue layer thickness. Over-thick glue layers increase internal stress, and shrinkage during cooling pulls the bonding interface and causes delamination.
Surface pretreatment methods for substrates
Oil stains, release agents and dust on hard substrate surfaces are the most common triggers for poor adhesion. Minimize internal release agent addition during hard substrate injection, and avoid frequent spraying external mold release agent on mold surfaces. Residual release agent on substrates forms an isolating film so TPU cannot fuse with the base material. Plasma surface activation treatment can be adopted for molded hard workpieces to raise substrate surface energy and improve TPU wettability. For small-batch production, wipe the overmolding area with alcohol and fully dry the parts to avoid solvent residue. Polishing only serves as an auxiliary measure. Dust from excessive polishing will stay on the interface and reduce bonding effect if not fully cleaned. Blow and remove all dust after polishing. Treated substrates should enter overmolding molding quickly. Long-time storage will absorb oil contaminants from air and weaken activation effect.
Key points of mold structure design
The overmolding mold cavity shall realize stable positioning of hard plastic blanks. Excessive positioning clearance leads to substrate offset during injection, resulting in uneven overmolding thickness and local adhesion failure. Moderate mold insert polishing grade is required. Mirror polishing is not recommended for overmolding zones. Slightly rough surfaces expand TPU attachment area, while overly rough surfaces trap dirt and bring cleaning troubles. Gates are arranged close to the bonding interface to shorten melt flow distance, keep high temperature at the bonding interface and improve wettability. Vent slots are opened at melt convergence terminals inside the cavity to exhaust air in time. Burnt brittle layers formed by trapped air will completely lose bonding capacity. Mold temperature setting needs to fit both two materials. Too low mold temperature rapidly cools TPU melt and prevents sufficient wetting on substrate surfaces. Too high mold temperature causes thermal deformation of hard substrates and positioning deviation.
Adjustment of overmolding injection process parameters
Raise TPU barrel temperature to guarantee full melting and good fluidity, allowing melt to fully wet substrate surfaces. The temperature cannot exceed the upper limit of material specifications to prevent TPU degradation, which reduces material mechanical performance and bonding strength. Use medium-high injection speed to fill the cavity quickly and avoid cold material layers forming at the interface due to early solidification of melt front. Set proper packing pressure and packing time to keep TPU tightly attached to substrate surfaces and reduce interface gaps caused by cooling shrinkage. Excessive packing pressure deforms substrates while insufficient packing triggers shrinkage delamination. Control the temperature of hard substrates before loading. Cold substrates quickly take away melt heat and reduce wetting effect. Preheat hard blanks before putting them into molds to narrow temperature difference between substrate and TPU melt. Maintain proper cooling time after molding. Insufficient cooling leaves high residual stress inside TPU, which slowly causes delamination and peeling after stress release during storage.
Production and finished product verification control
Keep the production environment clean during mass production. Store hard semi-finished products with dust protection. Operators must wear gloves when touching bonding areas to prevent grease contamination. Clean mold cavities regularly to remove low-molecular precipitates accumulated on mold surfaces, which will weaken adhesion. Conduct peel tests for each batch of finished products after molding by manual peeling or tensile testing. Judge whether failure occurs at material interface or inside TPU bulk material. If fracture appears fully on the material interface, molecular bonding is insufficient and adjustments should focus on material matching, surface activation and temperature parameters. If fracture happens within TPU itself, the bonding strength meets requirements. Long-term storage test is also essential. Some products show good bonding just after molding but peel after high-low temperature cycling or water immersion. Add environmental aging tests to expose potential adhesion defects in advance. In summary, poor TPU overmolding adhesion cannot be solved merely by adjusting machine parameters. Collaborative optimization covering material compatibility, mechanical lock structure, interface cleaning & activation, mold design and molding process is required. Prioritize checking substrate surface contamination and material matching problems, then improve mechanical interlocking with structural design, together with reasonable temperature and injection parameters to eliminate isolating layers on the interface. Stable and reliable overmolding bonding can be realized to lower the risk of peeling failure for mass-produced parts in China’s plastic manufacturing industry.
