chapter 3, Manufacturing Methods

Excerpt

Having discussed some of the available optical plastic materials, we now consider several of the methods of manufacturing optical components from them. The choice of manufacturing method can depend on several factors, such as part size, shape, features, and production quantities. Over the course of a project, several manufacturing methods may be used as the product transitions from the prototype/development stage to full production. In this chapter, we examine the four main plastic optic manufacturing methods.

3.1 Casting

Casting is the process of filling a mold with a liquid (or semi-liquid) material and allowing, assisting, or forcing the liquid to solidify. Once the material is sufficiently hardened, the part is removed from the mold. Depending on the casting process, part requirements, or application, the component may be ready to use as-is from the mold, or it may be subjected to additional processes.

We discussed in a previous chapter the emergence of acrylic materials early in the second half of the 1930s. On August 2, 1937, Harold Moulton filed for a U.S. patent titled “Ophthalmic Lens,” which was granted in 1941 and assigned to the American Optical Company. In the opening paragraphs of the patent, he states that his invention pertains to “improved means and methods of making contact lens or lenses … which are exceedingly light in weight, clear and transparent, and of tough, noncorrosive and infrangible material.” The manufacturing method he refers to is casting.

Casting, along with grinding and polishing, are the oldest methods of producing plastic optics. Before the advent of modern diamond-turning equipment, these methods were frequently used together; even today, in certain applications, they still are. Casting dominates the market for the production of ophthalmic lenses. It is also used to encapsulate light-emitting diodes (LEDs), to fabricate Fresnel lenses, and also to create windows for a number of products, such as ATMs, bar-code scanners, security sensors, and touch screens. Casting works well for applications that require replication of microfeatures, such as lens arrays or diffractive structures. High-quality diffraction grating replicas have been created by casting for decades.

Outside of acrylic, most materials that are regularly cast are of the thermoset variety. The best known thermoset material is CR-39. Chemically, CR-39 is an allyl diglycol carbonate monomer, which was patented in 1945 by Muskat and Strain and assigned to the Pittsburgh Plate Glass Company, now known as PPG Industries. At the time of the invention, PPG owned a subsidiary called the Columbia Southern Chemical Company.