There is a huge difference between molding and grinding/polishing optics, and the situation is complicated when working with polymer materials. Compared to glass, polymers have exaggerated response to high stress and temperature. Good optical design with such materials hinges on a working knowledge of the molding process and its effects. In a previous post, we advised on general considerations when working with polymers. Now we will share some “pro tips” to assist you when creating optical designs.
1. Understand the molding process before choosing dimension ratios
For optical elements that are manufactured using traditional grinding and polishing methods, the ratio of the part's diameter to its center thickness is a critical design consideration because that ratio determines the strain response. Grinding and polishing processes place localized stress on a component's surface, which means that a part that has a small diameter-to-thickness ratio is prone to break or distort.
By contrast, for molded optics, the pressure across a part's surface is essentially uniform during manufacturing. The important design consideration is instead the ratio of a component's center thickness (CT) to its edge thickness (ET). A ratio of 1:1 is ideal from a manufacturing standpoint, but optical elements almost always have one or more curved surfaces. A rule of thumb is "Always keep CT/ET<5 for positive lenses and always keep ET/CT<3 for negative lenses." It is preferable to keep CT/ET≈3 for positive lenses and ET/CT≈2 for negative lenses. It is interesting to note that symmetric lenses tend to mold to lower irregularity than Asymmetric ones. Thus a strongly positive meniscus lens can be difficult to mold.
2. Determine how cooling will affect your surface tolerances
During cooling polymer optics shrink, sometimes by a significant factor. The shrinkage is predictable, though, and therefore it can be accounted for during the design of the negative mold. An important consideration, though, is that the concave optical surface will generally "stick" to the negative molding surface during cooling. The extended contact time with the mold means that a concave molded optical surface maintains its surface figure better than a convex molded surface does. That knowledge is a valuable factor when determining surface tolerances. For bi-concave and bi-convex lenses it is on average true that the steeper surface will tend to stick to the mold.
3. Relax surface tolerances on thin optics
An interesting feature of molded optical elements with a similar center and edge thicknesses (e.g. thin lenses) is that simultaneously pressing both sides creates a coupled relationship between the surfaces. Specifically, a surface irregularity on one side creates an opposing irregularity on the other side. In effect, surface form and power errors of the opposite faces "cancel out." Because of this, it is preferable to set a fabrication tolerance on the part's transmitted wavefront rather than on the surface quality of the opposing surfaces. This small tolerance change simplifies the design and reduces unnecessary part rejection, both of which represent cost savings.
4. Add alignment features
It is easy to incorporate alignment features into the design of a molded optical element. Such features include things like face flats that extend beyond the optical surface or edge markings that identify the part. Face flats are excellent for aspheric optical elements. Measuring the parallelism of the flats reveals any wedge between the opposing surfaces. That knowledge decouples wedge error from decentration error, and that makes it easier to "tune in" the mold alignment.
5. Match the material to the application
Before getting deep into your design, talk to your manufacturer about the actual purpose of your optical elements. The system's usage environment will limit the choice of materials. Because materials respond differently during molding, and that distinct behavior restricts design options such as surface curvature and center thickness. For precision molded optics, COP and COCs (Zeonex E48R for instance) and optical polyester are best. Polystyrene and PMMA (acrylic) are acceptable for some applications. Polycarbonate is only appropriate for impact-resistant parts because the material is highly birefringent.
Fabricating precision mold masters takes time. Strong optical design choices simplify the transition from design through manufacturing. That directly saves you money, and it also means a faster turnaround.
To make sure your polymer optics use the best design for your product needs, download our free checklist: