Weld Line Free Injection-Compression Minus Power Lens Elements
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Solution Overview
Problem
Conventional injection molding techniques are inadequate for producing thin plus and minus lenses for augmented reality eyewear due to high flow resistance and residual stresses, leading to weld lines, uneven shrinkage, and warpage, which are cosmetic defects and affect optical performance.
Innovation Solution
An injection-compression process using mold inserts made of borosilicate glass with specific thermal diffusivity and flatness, combined with a high flow polycarbonate resin containing UV absorbers, to prevent weld lines and minimize residual stresses, ensuring flat and concave surfaces with precise thickness and optical quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional injection molding techniques are used to produce thin minus lenses, then the manufacturing process is simple, but weld lines and residual stresses occur leading to poor optical quality
Solution Approach 1:
The mold cavity is pre-configured with a larger initial volume to accommodate the thermoplastic material before compression, ensuring proper filling without weld lines. The mold inserts are positioned and sized in advance to control the final lens thickness and geometry, preventing residual stresses before they can form.
Solution Approach 2:
The process transitions from conventional constant-volume injection molding to a variable-volume injection-compression process. The cavity volume changes during molding: initially larger to allow complete material filling, then compressed to final dimensions. This parameter change eliminates weld lines and controls shrinkage to prevent warpage while maintaining manufacturability.
2Strength
If high flow resistance materials are used in thin lens production, then material strength is maintained, but residual stresses and warpage increase
Solution Approach 1:
The injection-compression process changes the density and molecular orientation parameters of the thermoplastic material during molding. By compressing the material after injection, the process achieves higher density and better molecular alignment, which increases strength while simultaneously reducing residual stresses and preventing warpage in thin lens applications.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The method effectively produces minus power lens elements with reduced warpage and residual stresses, meeting stringent geometrical and optical requirements, including UV protection, without the need for new machinery.
Implementation Method 1
injecting a melt of thermoplastic material at a temperature higher than a glass transition temperature (Tg) of the thermoplastic material
Implementation Method 2
the two facing mold inserts are moved toward one another to define a final molding cavity whose volume is less than that of the initial molding cavity
Implementation Method 3
After cooling and opening of the molding cavity, the weld line free minus power lens element is recovered
Implementation Method 4
the melt of thermoplastic material comprises at least one UV absorber
Data Source
AI summary
A method for injection molding of a weld line free minus power lens element comprises injecting a melt of thermoplastic material at a temperature higher than a glass transition temperature (Tg) of the thermoplastic material in an initial molding cavity delimited by two facing mold inserts, wherein the melt of thermoplastic material comprises at least one UV absorber. During the injecting, the two facing mold inserts are moved toward one another to define a final molding cavity whose volume is less than that of the initial molding cavity. After cooling and disassembling of the two facing mold inserts, the weld line free minus power lens element is recovered. One of the two facing mold inserts comprises a flat surface facing the initial molding cavity, thereby to form a flat surface on one side of the weld line free minus power lens element. The other of the two facing mold inserts comprises a convex surface facing the initial molding cavity, thereby to form a concave surface on an opposite side of the weld line free minus power lens element.