Frame materials should be light, strong and easily adjusted, but retain their shape well. They need be flexible enough (under under the right conditions) to insert a lens, inert both to external agents and body fluids, and be cosmetically attractive (e.g. colours, surface finish, retention of surface finish). No material can meet all these requirements perfectly however and so frame manufacturers are working hard to keep up with lens technology to provide us with the best choices of features and benefits for both function and fashion. As newer materials are introduced, there is an ever growing choice for every patient, from the person looking for lightweight simplicity, to the sports participant in need of increased flexibility, safety and fit.
There are two basic types of plastics – thermoplastics which can be re-formed with heat and those which can’t be, commonly called thermosetting. Almost all plastic spectacle frames are made from plastics which soften with heat, although they are not all thermoplastic. Frames are occasionally made from nylons or from composite materials, most of which do not soften properly on heating, although they may be thermoplastic.
This is probably the most common plastic spectacle frame material. It is tolerably light & strong, mechanically stable at normal temperatures, easily worked and relatively inert. It tends to whiten (this may be additives leaching from it) where in contact with body fluids, particularly at the bridge and temples. Acetate frames are often cut from sheets, although injection/vacuum moulding is becoming increasingly common, and they usually have full-length wire reinforcement and any colour is usually throughout the material, often as different coloured and/or transparent layers fused together.
These materials are similar to acetate but are slightly stronger , more flexible and have a slightly lower density. Frames are usually made by moulding, are surface dyed and are sometimes painted (“enamelled”).
Also known as “acrylic” and by many other trade names (PMMA, “Perspex”, “Plexiglass”), it is almost obsolete as a frame material, but “clinical quality” is particularly hypoallergenic. Fronts are almost always supra-type, as it can be very difficult to glaze a full rim. Perspex has a slight “memory” when heated to its softening point, but this is lost with further heating.
These materials are used in sunglasses, sports spectacles and safety spectacles. They are very strong, have a very soft surface but can be very flexible and they are usually made by moulding. They are resistant to most common solvents other than phenols. “Co-polyamides” may be either mixtures/co-polymers of different polyamides or co-polymers witother plastics materials. The name of one product, SPX made by Silhouette, has become almost a generic name for this type of material, although there are now many others on the market.
Rolled gold, invented by jewellers, describes a material created by bonding a layer of 14Kt gold to a durable base alloy on to which a final coating of 24Kt gold is applied. This technique means that the gold moves with the base metal as one uniform material and the thickness of the gold is up to 6 times that of gold plated frames. Therefore, there is unlikely to be any visible erosion of the gold within a normal life span.
Gold Plating is when frames are dipped in an electrolytic plating solution to which an electric current is introduced, creating a molecular bond. Gold Flashing is a poor cousin of gold plating. Produced in virtually the same manner but more quickly. Given the extremely thin coating, flash gold would be highly susceptible to corrosion and discolouration without the protection of a varnish coating.
Metal frames usually consist of a number of different materials: base metals (i.e. the structural metal of the frame), plating (often several different layers), frequently an organic lacquer or coating, and the plastic side tips and nose-pads. These are usually cellulose acetate or a closely related plastic or silicone rubber. Before coating (except for rolled gold), the wire is “drawn” through rollers to reduce it to the required section. This “work hardens” most alloys. Joints between the components of most metal frames can either be made by soldering (using a “hard” solder – e.g. silver solder) or by welding.
This is probably close to the ideal spectacle frame material. It is particularly light, strong, inert and (in its pure form) hypoallergenic. It is currently more expensive for spectacle frames, nominally due to extraction and manufacturing costs. It is relatively difficult to plate with more “attractive” metals such as gold, and although this is quite common, some of the coating processes involve the use of a nickel intermediate layer. It is also sometimes coloured by a process called ion plating. One point of particular importance to patients with allergies – “titanium” seldom means pure titanium. Screws, nose-pads and side-tips are very frequently excluded so "Pure" is usually taken to mean that 98% or more of the putatively titanium parts is titanium (not of the whole frame). There are other common descriptors, such as “β titanium” in which only about 75-80% is titanium, the rest being a selection from aluminium, chromium, iron, molybdenum, niobium, silicon, tin, vanadium and zirconium. Nickel and cobalt are also common additives to “titanium alloys”.
This is 12-25% nickel, but mostly copper. It is mechanically quite a good material for spectacle frames, although it goes dull very quickly if not plated/coated, and rapidly turns green in contact with body fluids. It is easily worked and soldered and is probably the most common material for spectacle frames.
This is similar to nickel silver, but is 68% nickel (BS 3072, 1989; 3076, 1989). The higher nickel content than nickel silver reduces the rate of corrosion. Monel is often used outside the UK to describe lower nickel content alloys and this “abuse use” may be continued into the UK with imported frames.
This is a relatively difficult material to work and to plate and although It is relatively uncommon as a spectacle frame material, its use is increasing. Many “stainless” alloys also can have quite a high nickel content. The surface leaching of nickel from some of the nickel-containing alloys is minimal even when unplated, but not all have been tested.
Optyl was the name of the principal company to manufacture frames from this material and is still the name usually attached to it. If heated and allowed to cool without holding it, it returns to the original shape in which it was made. This effect can also annoyingly occur if the frames are left in sunshine (commonly in a car window). Epoxy frames are usually translucent, but opaque colours have also been available.
There are a great many other materials in use today used in the manufacture of spectacle frames such as tortoiseshell, wood and many metal alloys which are distinct to certain suppliers. This is not an exhaustive guide to every possible material but intended to simply give an overview of the most commonly used.