Diameter to Centre Thickness Ratio

2:1 Difficult to  mould, loose tolerancing recommended;

3:1 Moderately easy to mould, commercial tolerancing;

5:1 High Precision tolerancing achievable.

Smaller, Lighter, Faster, Cheaper

Power (Collection Efficiency)

Design freedoms provided by polymer optics permit an approximate 1.78 energy collection advantage over glass: Example: A high index SFL6, all spherical glass element will have an f# of around 0.8 maximum relative aperture (effective useful relative aperture is 1.0) for an effective focal length of 15 mm. A bi-aspherical moulded polycarbonate element can be designed with f#  0.6 maximum relative aperture for a 15 mm efl. This lens will collect 1.78 times the energy of the glass element.

Colour Correction:

Aspherics and Diffractives provide correction with fewer elements.

Challenges of Mould Insert Fabrication:

Improvements in tool fabrication techniques permit construction of complex tooling architectures with improved micro-finishes.

Compared to Glass, some benefits of polymer mouldings:

• Mechanical mounting flanges, self-registration, alignment features, snap features, and tabs provide an ability to automate assembly;
• Mechanical repeatability to high accuracy, in comparison to ground and polished glass lenses;
• Aspheric, conic or toroidal surfaces are as easy to produce as spherical;
• Moulded micro-structured surfaces - such as diffractives.

Performance:

Issues of dN/dT (index vs. temperature) and the restricted refractive index range of polymers can be corrected by either compensation in  mechanical design or employing micro-structured surfaces for temperature and chromatic dispersion.

Dispersive diffractives may be used to correct colour.

The high index and negative Abbe values of diffractive surfaces help reduce monochromatic aberrations and may be used to athermalize the design.

Optical Performance:

Glass has variable surface accuracies after finishing whereas injection moulded optics have repeatable surface accuracies - lens to lens repeatability of 0.5% is typical. Complex shapes used in laser scanner systems that cannot be manufactured in glass can be moulded and coated as one unit. Optical accuracy may be held to 5 waves or less. Micro finishes of less than 70 Angstroms are possible - hence low scatter.

Good reasons for using polymers:

Polymers provide a repeatable level of performance at lower cost.

• Raw material is less expensive;
• Variety of mounting and alignment features can be moulded as part of the optic element - permitting automated assembly;
• Multicavity moulds reduce production costs if there is enough volume to justify higher tooling investment;
• Lens to lens repeatability - less than 0.5%;
• Surface accuracy variations less than 1 fringe, depending on size;
• Dimensional tolerances in the tenths of thousandths of an inch.

Aspherical surfaces: it costs no more to mould an asphere than it does a spherical element.

Diffractives: are an example of a product difficult and expensive to fabricate in glass - whether it be a rotationally symmetric kinoform surface, multiphase binary stepped surface or grayscale diffractive structure.

Benefits of polymer optics: Lower material weight; Integrated mounting features; Optical glasses typically are 2 to 2.5 times heavier than corresponding polymer lenses; Aspheric and diffractive surfaces permit reduced numbers of elements. 

Temperature Effects: Although the refractive index changes with temperature (and humidity) to a greater extent than glass, the more significant effect in polymers is thermal expansion - causing the radii and thickness of the optic to change - thus shifting the point of focus. Typical glasses have coefficients of expansion of 0.7 X 10-7 /K, whereas plastics such as Acrylic have 7 x 10-7/K, a factor of 10 greater. Thermal compensation designs (athermalization) are possible using diffractives or compensating mechanical structures in the lens housing/spacing. Example: The use of a higher temperature coefficient non-optical polymer such that lens separation and image distances physically change but the image plane remains constant. 

Alternatively the combination of glass/polymer elements (hybrids) employing a thin polymer layer, reduces overall expansion.

Mechanical Design Guidelines

Thickness: Shrinkage compensation is less complex for a thin lens and moulding cycle times are shortened - reducing cost. Thicker cross sections will increase cycle times and increase cost as well as make it more difficult to hold tight surface figure. Extreme variations in thickness may create uneven flow characteristics that may induce uneven cooling and change optical figure. Rule of thumb: Keep the diameter to thickness ratio greater than 6:1 and flange width to 2.5 times the edge thickness.

Structural Elements: Flanges; Spacers; Tabs; Slots; Brackets and Snaps may be integrated to the optical element - resulting in a single-piece design that eliminates mounting hardware and simplifies assembly and alignment. 

The shape and size of the structural elements outside the optic surfaces are important - allowing shrinkage effects to occur in the flange area and not in the critical optic aperture - thus preserving optical performance to the edge of the specified critical aperture.

Clear Aperture: If no flange is permitted, the clear aperture should be no closer to the edge than 1.5 times the edge thickness.

Symmetry: within the moulding aids flow characteristics.

Flat Surfaces: have a tendency to sink when the polymer cools. If possible, add a surface of power - even as little as a meter (radius of curvature). If a flat surface is required to compensate for sag longer cycle times and other process changes are required to create acceptable flatness.

Gate: Consider factors that determine which type of gate design is best suited to the optic element configuration and surface figure required. These range from fan gates to small buried sub-gates which automatically shear off as the mould opens.

Gate Vestige: When detaching the part from the runner some vestige of material is left on the part. This can be compensated by creating a small flat section at the gate area - which could also be used as a clocking feature if rotational alignment is necessary. 

 Draft Angle: (minimum of 0.5 degrees)  The degree to which side walls are tapered - in order to extract the part from the cavity.

Number of Cavities: Increasing the number of cavities per shot will lower the part cost but increases the mould cost. This will increase the degree of difficulty of balancing moulding characteristics of these cavities. The size, shape of part, volume of parts and tightness of optical tolerances desired, will limit the practical number of cavities used for a particular part configuration.

Shrinkage: Thermoplastic shrink rates for commonly used materials vary from 0.001" to 0.006"/F. It is important to compensate for shrinkage precisely. However it is often not possible to model shrinkage. 

Polymer is compressed in the cavity at injection and more material is added to compensate for shrinkage during cooling. Where there is uncertainty, moulds are usually built with 'steel safe' to allow for minor corrections after the first round of parts has been made and measured. This enables material to be gradually removed from the tool in order to approach the required final part dimensions.

Housings/Structures: Maintain a uniform wall cross-section thickness, even at the corners; Avoid overly thin / thick cross sections or combinations of extremes in cross sections. Avoid sharp corners which localizes stress concentrations.

Holes:  The flow of material around holes can cause weld lines. Shear in the polymer material during the fill of the cavity induces polymer orientation and birefringence.

High power negative lenses are problematic. Flow of material around the thicker edge section fills first and last in the center - causing weld lines where the flow fronts meet - resulting in distortion of surfaces and poor optical figure control.

Assembly Considerations: Press fits; Snap fits; Cementing; Heat staking; Ultrasonic welding.

Limitations of Glass: Advantages of glass optical elements are temperature stability, Abbe values and a greater index range for colour compensation over a wide wavelength range. Cost and repeatability are limitations however.

When not to use Polymer: If optical figures tighter than 1 fringe are required and if extreme temperature ranges are required - 40 C or greater operating range.

A durable 'hard coat' may be required to provide better scratch resistance. The 'hard coat' application involves a spin-on chemical solution followed by curing. Acrylic hardcoat treatment additionally enables surface adherence for the AR coating that otherwise was not obtainable. A chemical 'hard coat' of a few µm thickness provides most of the scratch resistance.

Polymers diffuse moisture continuously.