Contact Lens Manufacturing

Stages 1 and 2 – Manufacture of Moulding Tools.

A typical injection-moulding machine is designed to accommodate a mould tool for the product that is to be produced, i.e. the moulds from which the contact lens is to be produced. Fig. 29.2 shows how large and sturdy the moulding machines must be to support the type and size of the moulding tool used.

The mould tool that creates the final male and female parts is mounted in the moulding machine in two principal halves, one on a fixed mount and the other on a movable mount, and presents two principle moulding faces to each other in the open state and creates a series of cavities when closed ( Fig. 29.3 ). The moulding machine is controlled in such a way that the mould halves are tightly and accurately aligned in the closed position. The movable mount opens once the product is moulded so that they can be removed. A common material used to create the moulds for contact lenses is polypropylene. This is because it is low cost and has negligible toxicity and easy moulding control.

The majority of contact lens companies choose to use two injection-moulding machines, configured to create either female or male casts. The former is used to form the front surface of the contact lens and the latter the back surface. This is due to the inherent requirement in moulding that more accurate replication can be achieved if all the parts being moulded are as similar as possible. Usually there is a significant design difference between the male and the female moulds, and this could cause some replication variance if they are both moulded on the same tool.

During Stage 1, the mould tool halves are brought together to form the cavity shape and locked at high pressure to secure the tools.

Molten Plastic Injection.

Once the mould tool halves are closed and locked, molten plastic is injected into the mould tool at high temperature and pressure via a screw mechanism to ensure that every insert tool cavity is completely filled and packed to a controlled density. The number of insert tool cavities may total six or eight, but because of the growth in daily disposable lenses, the need for vastly increased numbers has resulted in mould tools with over 30× cavities. These cavities are linked via carefully designed channels in the mould tool face and serve to channel the molten plastic to the insert cavity during the moulding process ( Fig. 29.4 ).

Once the plastic has been injected and packed to its required force within the tool cavities, it cools and solidifies to make the cast-mould. Once cooled, the mould tool halves are moved apart to allow access to the cast parts.

Stages 3–5 – Dosing the Moulds.

During the filling and assembly process, the female and male moulds are automatically controlled in order to preserve accurate orientation and consistency, and then moved to a station whereby a quantity of contact lens material monomer is dosed into the female cast-mould. Immediately after dosing, the male cast-mould is oriented into the female mould to form an individual contact lens cavity containing the monomer. The two moulds are precisely squeezed together at a predetermined pressure to ensure that the monomer fully fills the cavity and the resultant contact lens form will have the correct centre thickness and edge form. It is usual for some excess monomer to be expelled from the mould into channels positioned around the edge of the lens.

If the lens being manufactured is a stabilised toric lens, the front and back casts may be rotated relative to each other to determine the axis of the lens.

Stage 6 – Polymerisation or Curing.

The dosed and assembled mould sets are then transferred from the filling and assembly area to a polymerisation station. During this process, an initiator is mixed in with the liquid monomer and subjected to a specific energy type resulting in polymerisation to make up the final contact lens. The judicious use of a cross-linker (or chain stabilisation means) ensures that the final contact lens conforms exactly to the physical geometry of the male and female moulds and retains preferred handling and fitting characteristics once fully hydrated into its soft form.

Stage 7 – Removing Lenses From the Mould.

Once fully cured, the two mould halves are automatically separated to reveal the contact lens in one half. The lens is removed by a variety of methods ranging from mechanical pinching, suction methods and direct hydration of the lens from the mould.

Stages 8 and 9 – Inspection and Hydration.

The demoulded lens is in a xerogel state and may be quite fragile. It is common for an inspection process to be included at this stage to ensure that the moulding and curing process are running in a successful and controlled manner. Inspection includes:

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  dry lens inspection for moulding-related defects

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  sampling fully hydrated lenses

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  checking general lens clarity and for edge and surface damage, bubbles or voids

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  more detailed sample-based inspection of parameter conformity and lens strength.

The demoulded lenses are then hydrated and extracted in a hydrating fluid, such as water or saline, either in an automated series of water baths or in the final packaging with sequenced hydrating medium changes. This process should also remove any unpolymerised monomers to ensure that the final contact lens is as pure as possible.

Stage 10 – Quality Inspection.

Lenses are visually inspected in the wet state for:

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  surface blemishes and edge defects

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  lens profile and lens-to-lens consistency

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  any other defect that would compromise the quality of the lens.

Inspection may be carried out on a sample-based system using a wet cell projection device such as the Optimec JCF Contact Lens Analyser ( Fig. 29.5 ) or an automated visual inspection system that detect blemishes using camera and computer imaging systems ( Fig. 29.6 ). Such systems are becoming more sophisticated and can also include the analysis of more complex lens designs such as multifocals. An example of such a system is the Omnispect inspection station ( Fig. 29.7 ). Quality checks usually include:

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  spherical power and any high-order aberrations that may be included in the design

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  cylindrical power and axis in the case of toric lenses

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  back optic zone radius (BOZR), although the use of radius to assess mass-produced soft lens base curvature is gradually giving way to the more practical full sag height means of categorising the back surface. This method is more accurate for the prediction of fit as it utilises the diameter of the lens as well.

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  total diameter (TD)

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  lens edge profile assessment

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  material properties verification.

Mass-produced lenses are inspected via sampling the batch size, and grading of pass/fail criteria are usually set by an internationally recognised acceptable quality limit (AQL) standard.

Stages 11–14 – Final Packaging.

The finished contact lens is then placed into a preformed plastic (usually polypropylene) single-use blister pack with a predetermined amount of saline solution and heat-sealed around the periphery of the contact lens compartment, using an aluminium foil lid.

The blister-packed lenses are then autoclaved, so the sealing must be carried out in such a way as to maintain the contents of the pack in a hermetically sealed environment after it has been sterilised in an autoclave. A ballasting system prevents the blister packs from rupturing during the severe pressure and temperature changes in the autoclave. There are alternative methods of sterilisation such as ultraviolet (UV) light irradiation, but steam is the preferred method in most countries.

Finally, the parameters are marked on the foil lid by printing or laser marking, and the lenses are packed into the secondary boxes in their respective configurations and then stocked ready for dispatch.

Stages 1 and 2 – Specially Designed Concave Plastic Mould.

This is similar to that found in the cast-moulding process but with the following additional form requirements to support accurate processing of the moulds via centripetal forces in a glass tube:

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  mould roundness

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  parallelism

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  accuracy of diameter.

Spin-casting relies on an accurate fit relationship between the mould and the tube in which it will be processed.

Stage 3 – Introducing the Monomer.

The modern-day process consists of:

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  dosing an exact amount of monomer into a mould

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  stacking the moulds vertically into a transparent and rotatable tube

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  saturating with an inert gas to reduce the oxygen level to an anaerobically acceptable level

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  rotating the tube around its vertical axis at a predetermined speed so that the monomer spreads out against the front surface of the mould towards the edge. The front surface lens shape is fixed by the mould geometry, and the back surface shape and lens power by a combination of:

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    monomer dose amount

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    rate of spin

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    viscosity of the monomer

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    gravitational forces.

Stages 4 and 5 – Polymerisation.

Once the required speed is achieved and the monometric shape is stabilised, a source of energy is activated to begin the polymerisation process followed by the removal of excess monomer. This energy source is usually UV light in the spin-casting process.

Limited spin-casting moulds are used to achieve a complete power range, as a variable speed of rotation can control the power. However, there is a trade-off between the number of moulds used and the back surface variation across the complete power range. In order to add more plus to the power of the lens, the mould has to rotate more slowly to produce a flatter back surface radius in relation to the front. Likewise, a faster rotation will result in a more negative lens, which can affect the fitting characteristics. Using a larger number of moulds would reduce this effect, but increase manufacturing costs.

The back surface of these lenses is aspheric rather than spherical, which can affect both the fitting and optical characteristics of the lens. Depending on the design and how many front surface moulds are used, the back surface asphericity will form as a prolate shape in plus and low-minus lenses and will then change to an oblate shape in higher-minus powers.

Stages 6 and 7 – Edge Polishing and Lens Inspection.

In traditional spin-casting, the final lens may require additional edge polishing in order to overcome some of the inherent process shortcomings. These can include defects due to vibration, prism and poorly formed edges ( Fig. 29.9 ).