Improving Mold Surface Performance With Nickel-Plated Coatings

Producing optical-quality parts through injection molding presents unique challenges, especially for applications requiring precise surface clarity and minimal defects. This is particularly critical in manufacturing curved lenses for eyewear, where even minor imperfections can distort light transmission and compromise visual performance.

To address these challenges, the Masato Research Group at UMass Lowell’s Department of Plastics Engineering collaborated with Gesswein to evaluate nickel-plated coatings on injection molds. The objective was to enhance mold surface performance and achieve the optical clarity needed for high-precision applications while maintaining process efficiency.

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The Challenge: Maintaining Optical Quality in Injection Molding

Injection molding optical parts demands strict control over surface quality and heat transfer. Residual stresses, birefringence and weld line notches can all compromise optical clarity. Also, increasing mold temperatures can improve surface replication, leading to longer cooling times and increased energy consumption.

Nickel-plated coatings were selected to address these challenges due to their ability to provide surface protection and localized thermal management, ensuring the mold surface met optical-quality standards.

The Solution: Application of Nickel-Plated Coatings

Gesswein provided nickel plating and polishing of the mold cores and cavities as part of this effort. The coating was applied to improve heat distribution at the mold surface, reduce defects and protect the mold from wear and corrosion.

Here are three keys areas of improvement from using nickel-plated coatings:

1. Surface finish: The nickel-plated coating enhanced the mold surface quality by reducing surface roughness, achieving a mirror-like finish essential for optical-grade parts. This ensured the molded lenses closely replicated the mold surface, minimizing optical distortions and achieving high clarity.
2. Flow: The nickel-plated coating created a thermal barrier, reducing heat transfer during molding. This enabled better flow behavior and improved surface replication, particularly in thin-walled parts.
3. Residual stress: By enabling localized heat retention, the coating reduces thermal gradients, which are often a cause of residual stress. Optical parts molded with nickel-plated surfaces exhibited lower birefringence and higher clarity.

Nickel-plated coatings were selected to address these challenges due to their ability to provide surface protection and localized thermal management, ensuring the mold surface met optical-quality standards.

Testing and Outcomes

The UMass Lowell team conducted a comprehensive evaluation of nickel-plated molds to assess their impact on surface quality, residual stress and thermal behavior during injection molding. Optical-grade polycarbonate lenses were chosen as the test parts due to their stringent requirements for clarity and precision, particularly for eyewear applications.

UMass Lowell’s injection molding machine used for experiments, such as evaluating nickel-plated mold coatings.

The experiments involved molding trials using both nickel-plated and uncoated molds under varying process conditions, including different injection speeds, cooling times and wall thicknesses. The analysis combined experimental observations and quantitative measurements, such as photoelastic stress analysis and melt pressure monitoring, to assess the effectiveness of the coatings.

UMass Lowell’s Masato Research Group and Gesswein evaluated nickel-plated mold coatings to improve surface performance, ensuring optical clarity and efficiency.

Key findings from the study include the following:

1. Surface quality assessment: The surface finish of the molded lenses was analyzed. Across all molding conditions, nickel-plated molds produced lenses with superior optical clarity and fewer surface defects. This enhancement resulted in smoother lens surfaces, reduced roughness and improved optical performance.
2. Residual stress analysis: Residual stress was measured using photoelastic stress analysis. Lenses molded with nickel-plated tools exhibited significantly lower stress levels, as indicated by diminished fringe patterns under polarized light. This reduction in residual stress was attributed to the nickel plating’s thermal barrier effect and the lessened cooling-induced gradients.
3. Thermal behavior via melt pressure monitoring: Melt pressure data collected during molding confirmed that nickel plating reduced heat transfer at the mold surface. This enabled the polymer melt to retain heat longer, improving flow behavior and cavity filling. The enhanced thermal management facilitated the molding of complex thin-walled geometries with greater consistency.
4. Simulation findings: Numerical simulations using Moldex3D incorporated variations in thermal contact resistance (TCR) to model the effects of nickel plating on heat transfer. The simulations demonstrated that higher TCR values, representing the thermal barrier effect of the nickel plating, reduced melt pressure and residual stress by moderating thermal gradients during the molding process.

Practical Takeaways for Mold Designers and Manufacturers

The findings from this study offer clear insights for mold designers and manufacturers aiming to improve their processes:

• Surface quality is key: The smoothness and durability of nickel-plated molds make them an excellent choice for achieving optical-grade finishes, particularly for demanding applications such as eyewear lenses.
• Thermal control matters: Managing heat transfer through coatings, such as nickel plating, can enhance flow behavior, improve surface replication and reduce cooling-induced stresses without requiring excessive mold temperature adjustments.
• Integrated approach benefits performance: Combining experimental and simulation-based analyses can guide the effective application of coatings to molds, ensuring optimal part quality and manufacturing efficiency.
• Broad applicability: While this study focused on optical-grade polycarbonate lenses, the benefits of nickel plating extend to a wide range of precision-molded parts, particularly those requiring consistent surface quality and reduced stress.

Summary

This study demonstrated that nickel-plated molds can significantly enhance the performance of injection molding processes, particularly for optical-quality parts such as curved lenses for eyewear. The technical results highlighted notable improvements in surface quality, with better replication of polished surfaces and reduced residual stress in molded parts. Melt pressure monitoring and numerical simulations using Moldex3D confirmed that the thermal barrier effect of nickel plating optimized cavity filling and minimized thermal gradients, improving part clarity and flow behavior in thin-walled geometries.

The collaboration between UMass Lowell and Gesswein was instrumental in achieving these outcomes. Gesswein’s expertise in mold surface finishing and UMass Lowell’s research capabilities combined to provide valuable insights into the practical application of nickel plating in injection molding.

Importantly, this connection between academia and industry was facilitated by MoldMaking Technology Editorial Director Christina Fuges, whose efforts bridged the gap between research and application, making this collaboration possible.

Overall, the findings validate the effectiveness of thermal barrier coatings and underscore the importance of partnerships between universities and industry in advancing manufacturing technologies and addressing real-world challenges.