# Effect of Punch Geometry on Stress and Strain Distribution During Contact Lens Demolding

## TL;DR

This paper studies how punch geometry affects stress and strain during contact lens demolding, finding that an optimized punch design improves manufacturing yield and performance.

## Contribution

A novel punch design with a central arc-shaped groove is introduced to enhance demolding performance in contact lens manufacturing.

## Key findings

- The optimized punch design reduced mold deformation and improved stress dispersion during demolding.
- The new punch achieved an 82% yield, 13% higher than a flat punch without a groove.
- The design reduces development costs and improves manufacturing throughput.

## Abstract

This study optimized the punch-assisted demolding technique for the separation of contact lenses, incorporating finite-element analysis to evaluate the effects of punch geometry (punch material: 304L stainless steel) on the stress and strain distributions of polypropylene lens molds. The simulation results revealed that the punch surface featured a flat base with a central arc-shaped groove (groove diameter: 7 mm, depth: 0.75 mm), which exhibited optimal stress dispersion characteristics during the demolding process, effectively reducing mold deformation. Experimental validation over 100 demolding cycles confirmed that the use of the aforementioned punch resulted in the manufactured lens having high central stability and reduced van der Waals forces during demolding, allowing smoother lens release and facilitating improved demolding performance. Comprehensive evaluation based on defect inspection and centering stability indicated that a yield of 82% was achieved with the optimized punch, with this yield being 13% higher than that obtained with a flat punch lacking an arc groove (69%). These results indicate that the optimized punch design not only reduces development costs but also enhances manufacturing yield and throughput, demonstrating strong potential for application in contact lens production.