High-resolution digital imaging, low-light-level biomedical devices, and automotive sensing are just a few of today’s hot technologies demanding both low-cost and high-performance optical systems. Critical in this effort is the mid- and high-volume requirement for aspheric optical components. Unfortunately, CNC polishing methods are expensive and take too long to produce each component, which has pushed precision glass molding to the front of asphere manufacturing technologies.
Glass Molding Challenges
Precision glass molding has been around since the 1980s. The first high-end photo compact disks relied on precision glass molding to reduce cost without sacrificing quality. Virtually all point-and-shoot digital cameras incorporate at least two molded-glass aspheres in their zoom optics. Until about five or six years ago, however, this technology was limited to high-volume markets because few manufacturers had the equipment for precision glass molding because the tooling was expensive, particularly for optics larger than 12 mm in diameter.
Over the past five years, some precision optics companies, including Edmund Optics (EO), have invested significantly in R&D, capital equipment, and engineering resources capable of supporting mid-volume precision glass molding. Critical for this endeavor was the invention of low-cost tooling, improved release-coating methods, and the implementation of advanced metrology such as aspheric interferometry and surface profilometry. Now, precision glass molding has become a cost-effective solution for the mid-volume markets, with typical tolerances that include deviations from an aspheric profile of less than 0.250 μm, and center thickness variation less than ±10 μm.
- Heat must be uniform across the tool and its surrounding mass, and it must be precisely increased during molding. The process must eliminate temperature differences across the mold, tool, and glass at temperatures approaching 700 °C. By implementing advanced IR heating methods, companies such as Edmund have developed a way to control temperature variations to within 1 °C across the entire tooling package.
- Position of the preforms in the mold must be controlled accurately before heating begins. The preforms typically are loaded by robots in high-precision molding processes to improve positioning accuracy and take human error out of the equation.
- Force applied to the glass must be controlled in much the same way as the heat, with precise control over a number of steps during the molding cycle.
In addition to these technical considerations, cost continues to be an overriding factor. A cost-effective glass molding production process must include ways to replicate mold tooling accurately, rapidly, and inexpensively.
A related issue is the ability of the process to maintain the surface tooling finish. The upkeep of the surface tooling may be the most difficult issue that faces molded glass optic manufacturers. Glass tends to stick to materials that are strong enough to withstand the pressures and heat of molding. Therefore, manufacturers apply a thin-film coating to the tooling to act as a release agent, but the coatings typically don’t last long — only 200 cycles or less. To combat this issue, glass suppliers are developing low-temperature glass, which reduces the stresses on the tooling and also reduces overall cycle time. Other developments, such as eliminating the coating completely while maintaining tool finish, are under study by Edmund Optics’ Glass Molding group and should be ready to implement in production by the end of 2006.
The growing availability and low cost of glass molding represents a milestone for optical designers. Glass molding is now capable of making components with irregularities as small as a half-fringe and aspheric accuracy to 0.25 μm. Designers finally have the accuracy they require to make aspheric elements as ubiquitous as their spherical counterparts.