How Colored Contact Lenses Are Made

Colored Contact Lenses are made from polymers. (Polymers are mixtures of materials formed by connecting molecules of different chemicals.)
Hard contact lenses are made from certain polymethyl methacrylate (PMMA) variants. Soft contact lenses are made from polymers such as polyhydroxyethyl methacrylate (pHEMA), which are hydrophilic, i.e., they absorb water and retain their shape and optical function.

There are dozens of soft contact lens materials currently registered with the FDA, each with different water content, oxygen permeability and modulus of elasticity for different products and different wearing needs.
In all manufacturing processes, the formulation of the finished soft lens depends on the liquid material monomer selected.

The liquid material monomers are polymerized by means of light or thermal irradiation or light heating. This process leads to the joining or cross-linking of the monomer molecules, which are then hydrated (hydrated) by immersion in physiological saline, transforming the lens from a solid to a soft semi-finished product.

The following molding techniques are commonly used.

01 Centrifugal casting method

It consists of a mixture of two liquid monomeric raw materials injected behind a rotatable mold. The shape of the mold determines the front surface morphology of the lens and the rear surface of the lens depends on the rotating surface formed by the speed of rotation of the mold. The increased centrifugal force drives the material to flow around the mold to form a thin lens.

During the centrifugal rotation process, the solid lens is produced by a photopolymerization reaction at a certain temperature and under UV light.

The lenses are modified at the edges or specially polished when they are dehydrated and dry, then they are immersed in saline to fully hydrate them and are extracted in boiling water to remove impurities. When the hydration is complete, the lenses are treated with surface cleaners and, before being packaged in saline, they are subjected to quality assurance control checks.

The higher the water content of the lens material, the faster the water evaporates during the centrifugal rotation process, so the application of this production process is limited and is not applicable to any material other than HEMA.

The size, shape and refraction of the lenses can be changed by changing the curvature of the mold, by changing the speed of rotation, by controlling the amount of material monomer drops, by using different material monomers, by controlling the speed and time of polymerization.

Advantages: The process has good reproducibility production, can be mass produced, smooth lens surface, parabolic inner surface, easy and comfortable fit to the corneal surface morphology.

Disadvantages: low diopter lenses are not well shaped, difficult to wear operation, ionic materials make lenses with charged surface, easy to adsorb proteins and other lens precipitates, and lenses are easy to adhere.

02 Turning molding method

The turning process was used for the production of hard lenses, which required the pre-polymerization of HEMA material before cutting. This is the second process used commercially for the production of soft lenses. The basic principle of this process is to cut the lens in the dry state and then hydrate it.

The material is first polymerized and made into button-shaped blanks approximately 8m thick and 15mm in diameter, then the blanks are fixed on a lathe, excess material is removed during cutting of the front and back surfaces of the lens, and finally the finished product is polished.
The dry lens must undergo a hydration process, allowing the lens to fully absorb water and expand in a balanced manner, so that the parameters of the final lens conform to the predetermined design of the diameter of the posterior optical zone, the total diameter, the number of diopters and avoid distortion. Modern computer numerical control (CNC) turning technology can improve the accuracy of the turning process, but the process still has the disadvantage of large errors and poor repeatability.
Advantages: the surface of the lens is not charged, it is not easy to adhere and adsorb protein deposits, easy to handle and good positioning. Customizable for different diopters and toric lenses, visual stability, long lens design life up to 18 months.

Disadvantages: low yield and high cost. Unable to produce disposable or frequently replaced lenses.

03 Dry-set molding method

The first step in the centrifugal casting method is the production of different molds for cast lenses. In the casting process, the same set of molds can be used for different diopters, whereas in the dry molding process, different sets of molds are required for the production of lenses with different diopters. The metal master molds used for the production of plastic molds need to be strictly checked before use

The dry molding process consists in injecting the liquid material into the mold, the front and back surface of the lens depending on the shape of the mold; when the material is injected into the mold, heat is radiated to polymerize the material until it is fully reacted and completely polymerized; some edges of the lens require additional polishing to fit the finished product; after the edges are polished, the lens is hydrated by immersion in physiological saline.

As in the case of turning and centrifugal casting, the lenses must expand in a preestimated and balanced manner during the hydration process to ensure accurate lens parameters and avoid distortion. Even though the degree of expansion can be pre-set and standardized, the lenses molded and turned during the hydration process are still subject to inconsistencies in the expansion of the lenses, which affects the reproducibility of the production.

The traditional molding process is limited by cost because it requires a large number of different molds to produce lenses of different diameters and designs. It is also limited by the ability of the polymer material to be produced accurately, as the number of monomers decreases when the polymerization process changes from liquid to solid, which reduces productivity and repeatability.

Advantages: high efficiency, low cost, high volume production, suitable for soft disposable lenses.

Disadvantages: uneven thickness of lenses, slightly poor oxygen permeability, poor strength, not durable

04 Wet molding method

The wet molding method is now widely used to produce short-cycle contact lenses.

Unlike the dry-set molding method
The production mold for each lens is used once
Rapid polymerization of material monomers by optical radiation
Lenses remain soft throughout the production process
No edge polishing is required
Each lens is inspected with an X10 magnifying glass

It is worth mentioning that in 1987, Johnson & Johnson used a similar process (Stabilized Soft Mold Method - SSM) to produce the world's first disposable contact lenses.

Today, the improved MAXMIZE process retains the original advantages of SSM, but with additional technological breakthroughs, such as full automation, system integration, and increased production speed.

Quality control is very important for contact lenses because they are medical devices.

Lenses are inspected after each stage of the manufacturing process, under magnification for any abnormalities. They are also measured by shade maps. The magnified shade of the lens is projected on a screen with a graph printed on it to measure the diameter and curvature. Any errors in the shape of the lens are shown in the shadows. The process can be performed automatically by a computer.

After the lenses have passed inspection, they are disinfected. The lenses are boiled in a mixture of water and salt for several hours to soften the lenses.

Next, the lenses are packaged: the standard packaging for lenses is a glass vial filled with saline and corked with rubber or metal. Nowadays, they are more often packaged in vacuum aluminum foil.

The production process for what appears to be a small contact lens is extremely precise. This is because it is the basic element that determines the comfort of wearing contact lenses.

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