PC and LSR Overmolding Process Analysis

Overmolding technology that combines polycarbonate (PC) and liquid silicone rubber (LSR) has become a staple in manufacturing high-performance components for consumer electronics, automotive, medical devices and daily necessities industries. PC’s excellent mechanical rigidity, impact resistance and dimensional stability complement LSR’s outstanding flexibility, sealing performance, biocompatibility and high-temperature resistance, creating composite parts with integrated rigid and flexible structures. The success of PC-LSR overmolding, however, hinges on precise control of material pretreatment, mold design and injection process parameters—any oversight in these links can lead to bonding failure, flash, or substrate deformation, directly affecting product quality and production efficiency. This article delves into the key technical points and control measures of the PC-LSR 

overmolding process, providing practical references for industrial production.

Surface pretreatment of the PC substrate is the foundational step to ensure reliable bonding between PC and LSR. Unlike thermoplastic-thermoplastic overmolding, PC and LSR have poor inherent interfacial adhesion, so targeted physical or chemical treatment is mandatory to activate the PC surface. Plasma treatment is the most widely used physical method in industrial production: using low-temperature plasma to etch the PC surface, it removes organic contaminants and forms micro-nano rough structures, increasing the contact area with LSR while introducing polar functional groups that enhance chemical bonding. The process parameters must be strictly controlled—typically using argon-oxygen mixed gas, with a treatment time of 10–30 seconds and a power of 100–300W—to avoid over-treatment that causes PC surface aging and brittleness. For components with complex structures that are difficult to reach via plasma, a dedicated LSR primer for PC is the optimal choice. The primer should be uniformly coated on the PC bonding surface with a thickness of 5–15μm, and cured at room temperature for 30–60 minutes or at 60–80℃ for 10–15 minutes; it is critical to avoid primer accumulation, which can cause bonding delamination.

Precision mold design is the core guarantee for the PC-LSR overmolding process, with key focus on mold temperature control, gating system design and parting surface precision. First, the mold must adopt a partitioned temperature control system to adapt to the distinct processing temperature requirements of PC and LSR. 

The PC molding section requires a mold temperature of 80–120℃ to reduce internal stress and ensure dimensional stability of the substrate, while the LSR injection section needs a constant temperature of 160–180℃ to realize rapid vulcanization and cross-linking of LSR. The temperature difference between the two sections must be isolated with thermal insulation materials to prevent mutual interference. Second, the LSR gating system should use pinpoint or submarine gates, positioned at the non-critical structural areas of the part to avoid gate marks affecting appearance and bonding strength; the runner should be designed with a circular cross-section to reduce LSR flow resistance and ensure uniform filling. Third, the mold parting surface and core-pin fit must reach a precision of 0.005–0.01mm, and a gas-assisted sealing structure can be added if necessary to prevent LSR flash—one of the most common defects in this process. For two-shot overmolding molds, high-precision positioning pins and guide pillars are essential to ensure the PC substrate is accurately positioned during the LSR injection stage, avoiding misalignment that leads to uneven bonding.

The optimization of injection process parameters is the key to coordinating the performance of PC and LSR and avoiding production defects, and the process is divided into two sequential steps: PC substrate injection and LSR overmolding. In the PC injection stage, the material temperature is controlled at 260–320℃ (adjusted according to PC viscosity grade), with an injection pressure of 80–120MPa and a holding pressure of 50–70MPa. The holding time is set based on the part thickness to ensure full filling and reduce internal stress; the cooling time must be sufficient (generally 20–60 seconds) to make the PC substrate fully solidified and its temperature drop to below 60℃, preventing thermal deformation caused by the high temperature of subsequent LSR vulcanization. In the LSR overmolding stage, the injection pressure is controlled at 30–50MPa—moderate pressure to ensure LSR fully fits the PC bonding surface without causing PC substrate deformation due to excessive pressure. The vulcanization time is 10–30 seconds, determined by the LSR layer thickness (1mm thickness requires about 5–8 seconds of vulcanization time). After LSR injection and vulcanization, the mold is cooled to 40–60℃ before demolding to reduce the thermal stress at the bonding interface and improve bonding strength.

Common process defects and targeted solutions are also critical to the stability of PC-LSR overmolding. Bonding failure is the most severe defect, mainly caused by inadequate PC surface treatment, expired primer or improper vulcanization parameters. The solution is to establish a strict pretreatment process inspection system, use primer within the validity period, and calibrate mold temperature and vulcanization time regularly. Flash is mostly due to insufficient mold clamping force, excessive LSR injection volume or poor parting surface precision, which can be resolved by increasing clamping force appropriately, optimizing injection volume and regrinding the parting surface to improve fit precision. PC substrate deformation is often caused by uneven mold temperature or excessive LSR injection pressure, and can be addressed by optimizing the mold cooling circuit, increasing the number of cooling water channels on the PC side, and reducing the LSR injection pressure while ensuring full filling.

In conclusion, the PC-LSR overmolding process is a systematic engineering that integrates material science, mold manufacturing and precision injection molding. The core of achieving high-quality composite parts lies in three aspects: thorough and standardized surface pretreatment of the PC substrate to lay a solid foundation for interfacial bonding, precise mold design with partitioned temperature control and high-precision fitting to avoid structural defects, and fine-tuned injection process parameters to coordinate the processing characteristics of PC and LSR. With the continuous upgrading of industrial demands for high-performance composite parts, the PC-LSR overmolding process will continue to develop toward higher precision and automation. Strict control of each process link is the key to realizing stable mass production and meeting the application requirements of various high-end industries.