Metal frames tend to be thinner, lighter,
and more subtle in design and appearance. Thanks to new coloration techniques,
metal frames are also available in a wide variety of colors. Choices in
metal frame materials include:
Plastic frames are available in a wide
variety of both subtle and dramatic colors and styles. They are typically
more casual in appearance than metal frames. The two most common plastic
materials used in frames are:
cellulose acetate (also called ""zyl")
and cellulose propionate.
Rimless glasses should only be worn by people with very low powers of prescription, preferably myopic (or nearsighted) patients. This is because of the actual physics involved in lens fabrication. A farsighted (or hyperopic) person wears a lens that is convex in shape so it is thickest in the middle and thinnest on the edge creating magnification. Because the rimless frame requires us to cut a groove all the way around the edge, it will be necessary to surface a thicker lens than normal, resulting in a less attractive heavier pair of glasses!
The hottest metal frame material today is titanium. Titanium is an extremely strong, lightweight metal used to manufacture a wide variety of products from aircraft and satellites to golf clubs and cameras. Titanium frames may be composed of pure titanium or titanium alloys (sometimes called "beta titanium") that contain small amounts of vanadium, aluminum, and other metals to increase flexibility. Titanium frames are very light and durable. The strength of titanium allows manufacturers to create frames with very thin, minimalistic designs. Titanium frames are also hypoallergenic and non corrosive so they won't react with your skin. And titanium frames are available in a wide variety of colors.
Frames made of titanium or titanium alloys are more expensive than most other metal frames. Though titanium ore is abundant in the earth's crust, the high melting point of titanium (approx. 3,000 degrees F) makes it much more difficult to purify and process than other metals.
Stainless steel is also a very popular frame material. It is an alloy composed mainly of iron with smaller amounts of chromium, manganese, and nickel. Stainless steel frames are durable, corrosion resistant, and maintain their luster well. They are also strong and flexible in thin designs, and are relatively easy to adjust.
Monel is another popular material. It is an alloy of nickel and copper. Monel is very durable and can be adjusted into a wide variety of shapes without losing its strength. Monel frames are corrosion resistant and are available in a wide variety of colors and styles. (A small percentage of people are allergic to nickel. If you suspect you are or if you easily experience skin reactions to metal, you may want to choose a different frame material.)
Many frames are made with varying amounts of gold. Solid gold frames are among the most expensive frames available. Solid gold is not pure gold, which is too soft (and too expensive) to be manufactured into eyewear. It is a term used to describe any number of gold alloys that include smaller amounts of other metals for added strength and durability.
The unit used to measure the purity of gold is the karat. Pure gold is 24 karat gold. Solid gold frames are usually made of 12 karat (50% gold) or 18 karat (75% gold) gold alloys. The color of the alloy depends on the type and quantity of the metals added to the gold (usually nickel or copper).
Solid gold frames retain their luster for years. They hold their adjustment well, and are hypoallergenic and non corroding. Smaller amounts of gold are used to produce gold-filled frames. These consist of a base metal surrounded by an outer coating of gold alloy. At least five percent of the frame's weight must be due to gold alloy that is 10 karat or higher in order for it to be considered a gold-filled frame. Gold-filled frames have the same beautiful luster as solid gold frames at a significantly lower cost. And like solid gold frames, they are non corrosive and hypoallergenic.
Gold plated frames have a thinner layer of gold than gold-filled frames. There is also no minimum purity requirement for the gold alloy in these frames. Gold plated frames have a beautiful gold luster when new, but the thin outer gold alloy may wear off over time, exposing the base metal.
Other metal materials used to
manufacture eyeglass frames include aluminum, cobalt, and nickel silver
(an alloy of copper, nickel, and zinc). In many cases, the manufacturer
does not specify the composition of the frame beyond classifying it as
"metal." (Buyer beware.)
Plastic frames are usually less expensive than metal frames. There are two major categories of plastics used in frames: cellulose propionate and cellulose acetate. (The trade name for cellulose acetate is zylonite, commonly referred to as "zyl").
Cellulose propionate frames are created by a
process called injection molding. In this process, liquid plastic is injected
into a mold under high pressure and heat.
The mold is then cooled to produce a solid plastic frame. The production costs are relatively low for this type of frame, but there can be a five-fold difference in cost between one injection molded frame and another. Cellulose propionate frames are lightweight and relatively inexpensive. However, they eventually fade in color and become brittle.
By comparison, cellulose acetate (zyl) frames are made from a solid piece of plastic. These frames typically cost more than injection molded frames because of the labor required to mill the frames from solid sheets of the plastic material. Like cellulose propionate frames, zyl frames tend to discolor and get brittle with age.
A newer formulation of cellulose acetate is called super acetate. This plastic material is lighter and more durable than zyl, and is available with longer lasting finishes.
Combination frames have both metal and plastic parts. By using a combination of materials, these frames can exhibit both strength and durability of metal frames and the adjustability of plastic frames