Two-color overmolding combines substrate and overmold layers to achieve both functionality and comfort in products like tool handles, automotive seals, and electronic sheaths. Its core value lies in the firm bonding of the overmold layer to the substrate and the integrity of the finished product. In actual production, improper material matching, flawed mold design, or uncontrolled process parameters can lead to issues such as delamination, air bubbles, and sink marks in the overmold layer, directly affecting product performance and qualification rates. Key considerations for two-color overmolding must cover the entire process of material compatibility, mold design, and process control, with intelligent technologies integrated to enhance stability—critical for ensuring production efficiency and product quality.
The compatibility of substrate and overmold materials directly determines bonding strength; industry consensus requires the difference in solubility parameters to be within 1.5, as excessive differences cause delamination. Rigid substrates typically use ABS or PC/ABS alloys, paired with elastic overmold materials like TPU or TPEE. For PA substrates, nylon-compatible TPE is necessary to avoid interfacial peeling due to polarity differences. Additionally, the hardness of the overmold material should match product requirements—elastic materials with Shore hardness 40A–90A enable uniform coating, while overly soft materials deform easily and overly hard ones reduce bonding strength.
Elastic overmold materials (e.g., TPU) require pre-drying at 80–100°C to a moisture content below 0.05% to prevent air bubbles from damaging overmold layer density during melting and injection. Hygroscopic substrate resins (e.g., PA, PC) also need drying to avoid poor overmold adhesion caused by surface moisture. Processed materials must be used within 4 hours to prevent reabsorption of moisture.
The industry is increasingly adopting bio-based TPE and halogen-free flame-retardant overmold materials, which differ in melt flowability from traditional materials. Selection requires pre-testing of processing windows to avoid incomplete filling due to insufficient flowability.
Overmold layer thickness should be controlled at 0.5–3 mm—too thin causes incomplete coating, while too thick leads to sink marks from uneven cooling shrinkage. Mold positioning accuracy must reach within 0.02 mm to prevent overmold misalignment from substrate shifting during secondary injection. Exhaust systems need optimization for overmold cavities: exhaust grooves should be 0.1–0.2 mm wide and no more than 0.05 mm deep to expel gas quickly without material overflow, reducing air bubble defects.
Surface roughness (Ra) of substrate and overmold cavities must be below 0.2 μm; uneven polishing causes overmold sticking or surface scratches. Molds require rust and wear resistance treatment, especially for overmold cavities, to avoid corrosion affecting demolding and product appearance.
Molds need independent temperature control systems: substrate cavity temperature is maintained at 40–60°C, and overmold cavity temperature is 10–20°C higher. Excessive temperature differences cause rapid cooling of the overmold melt, leading to insufficient bonding with the substrate.
Melt temperature for secondary injection (overmold layer) should be 10–20°C higher than primary injection (substrate) to ensure the overmold melt slightly etches the substrate surface for physical interlocking. Injection pressure is adjusted based on overmold thickness (typically 80–120 MPa), with holding pressure at 50–70% of injection pressure to prevent overmold detachment from shrinkage.
The process strictly follows "primary injection of substrate → cooling and setting → secondary injection of overmold layer." Substrate cooling time must ensure sufficient rigidity to avoid deformation during secondary injection; overmold injection should occur when the substrate is not fully cooled (surface temperature 60–80°C) to use residual heat for improved interfacial bonding. Intervals exceeding 30 seconds reduce bonding strength.
Mainstream production uses melt pressure and temperature sensors for real-time monitoring, with parameter fluctuations controlled within ±5°C (temperature) and ±10 MPa (pressure). Automated systems enable timely adjustments to avoid defects from delayed manual intervention.
Finished products require testing of overmold-substrate bonding strength (industry standard: shear strength ≥15 MPa) and appearance inspection for bubbles, sink marks, and flash (sink mark depth on the overmold surface ≤0.1 mm to ensure product sealing).
Delamination stems from incompatible materials or contaminated substrate surfaces—replace matching materials or add substrate surface cleaning steps. Air bubbles require enhanced material drying or reduced injection speed. Sink marks are resolved by extending holding time or increasing mold temperature.
