Manufacturing

• Casting

• Injection moulding

• Compression moulding

• Embossing

• Polymer coating

Shrink rates occur in the range of 0.001 to 0.006 in./in. Compensation of shrink rates is important and may be achieved empirically -  by initially starting with a mould of smaller dimensions than the designed dimensions of the actual optic. The undersized tool (with steel safe) is then run and fabricated components produced - from which shrinkage measurements may be accurately established. The tool insert may then be machined accurately accounting for polymer shrinkage. In this way and through Process Control techniques the dimensions of the polymer moulding may be consistently maintained. 

Spherical and plano inserts are generally fabricated from chromium-alloy stainless steel. 50 to 54 Rockwell C-hardness rating range prior to polishing. 

Aspheric inserts: A best-fit curve is generated on a stainless steel substrate which is then subjected to a nickel plating process - electrolytic or electroless.  This process deposits a layer of nickel - up to 0.5 mm thick. Diamond turning produces the final aspherical surface in the nickel. Diamond turned inserts typically exhibit RMS surface roughness values of less than 50 Angstroms.

Diamond grinding provides another method of producing mould tool inserts but now may be used on ferrous metals such as stainless steel. Less surface accuracy is achieved compared to the diamond-turned product. 

Component Tolerances: Repeatability is one advantage of the injection moulding process. Tolerances are dependent upon part geometry, size, mould material, mould tool insert design and construction.

Coatings: Vapour deposition is used to apply antireflective, conductive, mirror and beam splitter coatings. 

Antireflection coating - A Magnesium Fluoride l/4 single layer coating on a polymer surface reduces reflectance from about 4% to 1.5%. Broadband coatings comprising three or four layers may reduce reflectances to less than 0.5% across the visible spectrum. Narrowband, multi-layer antireflection coatings may yield surface reflectance to less than 0.2%.

Reflective coatings - Typical coating metals include aluminum, silver and gold. Aluminum coatings provide surface reflectances greater than 88% across the visible spectrum and gold coatings greater than 95% from 700 to 1000 nm.

Aspherical surfaces: Prudent placement of an aspheric surface can reduce element count in some designs or relax certain fabrication tolerances in others. The location of an aspherical element may be critical. Ideal shapes for plastic optical components are those that maintain a near uniform wall thickness - due to consideration of shrinkage and birefringence. Strong meniscus, bi-convex and bi-concave shapes should be avoided to achieve high-quality, high-yielding polymer optics.

Birefringence - For systems in which polarization control is paramount the optical designer must properly choose the location and shape of components. Various process parameters may be adjusted for use of polymers with fairly low birefringence - yielding components that exhibit qualitatively high extinction ratio components as viewed through crossed polarizers.

Thermal effects - The thermal differential index of refraction coefficient of optical polymers is approximately an order of magnitude greater than that of glass. For this reason high-performance lens systems that require large temperature-band operating conditions are more suited for hybrid glass-polymer designs. Although the temperature band for all-plastic lenses may be quite limited, this characteristic depends strongly upon resolution criteria.  A comparable design can often be achieved with the introduction of a minimal amount of glass elements.

Mechanical design - Most polymer optical components have a flange around the circumference of the optic that performs several functions: - prevents cosmetic defects, provides additional mechanical rigidity and provides a mechanical mounting surface. Occasionally the flange incorporates an integrated spacer.

Typical components suited to polymer lens manufacture: - Low cost and repeatability are the most significant benefits of polymer optics. There are other potential benefits that are achievable: Complex apertures and component geometries; Off-axis aspheres; Surface aperture recesses; Multiple-surfaces (>3) in a single component.

Examples of unique polymer applications include: off-axis scanning parabolas; gold-coated barcode scanning octagons; medical arthroscope prisms - which comprise refracting, beam-deviating and reflecting surfaces all within one component. Mounting surfaces may be incorporated into components.

For design examples, see D. Buralli and G.M. Morris (1991), "Design of diffractive singlets for monochromatic imaging," Appl. Opt. 30 (16) 2151-2158

The single-point diamond turned 'insert' fabrication process for diffractive elements has improved considerably since the early 1990s. This improvement in quality combined with advances in precision moulding has yielded components with diffraction efficiencies greater than 95 percent at the desired wavelength and incident angle.

Although injection moulding produces high-fidelity diffractive elements performing close to theoretical predictions, diffraction efficiency vs. incident wavelength and angle still detrimentally affect contrast in visible broadband systems. These effects dictate proper optical system design and analysis for successful product implementation.

Polymer optics offer several advantages in many optical systems: lower-cost, aspherical surfaces, integrated components and complex aperture or multi-surface elements. The successful application of polymer optics to an engineering problem results from an integration of the opto-mechanical design process, tooling construction, component fabrication and surface coating deposition.