Injection molding

High-transparent nylon represents one of the most remarkable developments in advanced engineering plastics in recent years. Compared with conventional nylon, it not only requires excellent mechanical strength and heat resistance but also demands a delicate balance between high light transmittance and low birefringence at the molecular level. Achieving this balance relies on the regularity of molecular chains, controlled crystallinity, and extremely low impurity content. Traditional nylons often suffer from optical scattering due to the refractive index difference between crystalline and amorphous regions, which limits transparency. To overcome this, researchers have modified monomer structures, introduced copolymer units, and adjusted crystallization kinetics to optimize optical performance at the molecular scale. During the optical design phase, high-transparent nylon typically adopts aliphatic and cycloaliphatic copolymer structures to reduce intermolecular polarity and suppress crystallization. The incorporation of cycloaliphatic rings enhances molecular rigidity and minimizes birefringence during light transmission. As a result, transmittance in the visible spectrum can reach 88–92%, comparable to PMMA and PC. At the same time, nylon’s superior toughness and thermal stability enable it to maintain optical performance under high temperature and impact, giving it unique advantages in automotive, electronic, and optical applications. Processing conditions play a decisive role in determining transparency. Since crystallinity strongly affects optical clarity, precise control of cooling rate and mold temperature is essential during injection molding. Rapid cooling suppresses crystallization and increases the amorphous fraction, improving transparency, though overly fast cooling may induce internal stress. Hence, temperature zoning and gradual cooling are often employed. Proper drying before molding is also critical, as moisture can disrupt hydrogen bonding and cause optical defects. Today, transparent nylon is widely used in optical lenses, automotive lamp covers, sensor windows, and 3D-printed optical components. Especially in automotive lighting, it is gradually replacing PC and PMMA due to its excellent heat aging resistance and impact strength. Future research will focus on orientation-controlled amorphous transparent nylon, low-hygroscopicity grades, and recyclable bio-based transparent nylons, aiming to achieve a balance between optical performance and sustainability.

Nylon materials are highly susceptible to internal stress during injection molding, primarily due to molecular orientation, uneven cooling shrinkage, and poor additive dispersion. Excessive internal stress can lead to deformation, cracking, and deterioration of performance. To address this issue, modification technologies play a critical role. On the molecular level, incorporating flexible segments or impact modifiers helps reduce brittleness and mitigate stress concentration. Commonly used toughening agents include elastomers, thermoplastic elastomers, or graft-modified materials, which form phase-separated structures within the nylon matrix, effectively absorbing and redistributing stress. Glass fiber reinforcement significantly improves the strength and rigidity of nylon, yet it can also introduce internal stress. Controlling fiber length, content, and distribution is essential. While long fibers provide higher strength, they also induce greater shrinkage differences during cooling. Short fibers can improve dimensional stability, and surface treatments with coupling agents can enhance interfacial compatibility, thus minimizing stress concentration. From a processing perspective, mold design and molding parameters are equally important. Gate position, cooling system design, and molding temperature and pressure curves determine stress distribution within the part. Proper gate design ensures uniform melt flow and reduces molecular orientation. Higher mold temperatures extend relaxation time for molecular chains, lowering residual stress. Post-molding annealing is another effective approach, allowing molecular chains to rearrange under conditions near nylon’s glass transition temperature, thereby relieving residual stress from rapid cooling. In terms of additive systems, lubricants and nucleating agents can also be applied. Lubricants improve melt flowability and reduce friction-induced defects, while nucleating agents regulate crystallization rate and grain size, ensuring uniform shrinkage during cooling and minimizing stress concentration. All in all, reducing internal stress in nylon injection molded parts requires a combination of material modification and process optimization. Toughening, reinforcement, lubrication, and crystallization control can enhance stress distribution on a molecular level, while appropriate molding parameters and post-processing further stabilize performance. This integrated approach not only enhances the application value of nylon but also lays the foundation for its adoption in high-performance engineering applications.