CASE RECORD 51
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Lens manufacturing process
The starting point for freeform production is a semi-finished lens, also known as a ‘puck’ as it resembles an ice hockey puck.
These are produced in a casting process that is also used to manufacture stock lenses. Liquid monomers are first poured into moulds.
Various substances are added to the monomers, for example initiators and UV absorbers. The initiator triggers a chemical reaction that leads to hardening or “curing” of the lens, while the UV absorber increases the UV absorption of the lenses.
Moulds are required for the casting process. The moulds feature the negative shape of the resulting surface of the plastic lens. The two moulds are held together by a sealing ring known as a ‘gasket’.
The gasket ensures that a defined distance between the two moulds is maintained. The liquid monomer is poured into the moulds and hardens to form a polymer, i.e. the plastic lens.
Lenses with different powers can be made from one semi-finished lens. The curvature of the front and back surfaces indicates whether the lens will have a plus or minus power.
After filling, the moulds are exposed to a thermal process for many hours – this is “baking” the semi-finished lens. The input of heat activates the initiator and triggers a chemical reaction (polymerisation or polyaddition) which, with the release of heat (exothermic reaction), causes the material to harden or “cure.” Here, the starting materials bond to become long molecular chains.
Depending on the refractive index required, different starting materials and chemical reactions are used. While only one starting monomer is used for CR 39, for example, two monomers are needed for lenses with the indices 1.6 to 1.74. These two monomers join in a chemical reaction known as polyaddition. The manufacturing process for high-index materials is considerably longer and lasts for up to 48 hours.
Stock lenses have been so named as they are not produced to prescription but are kept in stock until an appropriate order is received from the optician. These lenses can be produced using the casting technique. The technology is essentially the same as that used for semi-finished lenses. After the lens blanks or “pressings” have been cured, they are provided with a hard protective lacquer and coating.
Spectacle preparation process
After the optician has sent the individual order to the lab, the preparation process begins.
Each lens is calculated individually and the data required for production is generated. The data is saved on a chip integrated into a ‘job’ tray.
The two semi-finished lenses – for left and right – are automatically removed from inventory and placed in the tray.
Blocking is needed to ensure that the lens is mounted securely and in the correct position in the machine tools. The semi-finished lens is already finished on the front surface, a protective lacquer is applied and a blocker is attached.
The material used to bond the lens and the blocker is a metal alloy with a low melting point. The semi-finished lens is therefore “welded” into position for the subsequent machining process (forming, polishing and engraving)
In freeform technology the front surface of the semi-finished lens has already been finished and displays an optical power. The adaptation of the lens design to the needs of each individual wearer is performed on the back surface in order to provide the wearer with optimum vision.
This forming process is conducted in an automated 5-axis CNC machining process.
It takes approximately 90 seconds to generate the optical power.
The data required for the forming process are received from a central server. The shaping process includes diameter reduction, bevelling using a milling technique and fine turning with a natural diamond.
The minimum surface roughness resulting from the fine turning process enables direct polishing without compromising the lens shape or radii.
Polishing and engraving
In the “soft” polishing process the surface is polished in 60-90 seconds in a method that ensures no marks of any kind remain. The freeform surface previously generated remains intact.
De-blocking and cleaning
The blocking material is melted in hot water, allowing the lens to be gently removed from the tool. The metal alloy is completely recycled.The cleaning process is performed in several steps in a brush washing unit which is similar to a car wash. Various cleaning agents and high-purity water are used, and finally the lenses are dried.
Whether single, graduated, customised or medically prescribed tints are required, plastic lenses can be tinted in any desired colour. The dyes used correspond to those used for textiles. In a dipping process the dyes penetrate deep into the surface of the plastic material. In glass lenses the tints are applied in layers consisting of metal oxides, therefore limiting the choice of colours.
In the dipping process a lacquer is applied to the lens to make it scratch-resistant. Specially adapted lacquers are used for the various plastic materials with different refractive indices. The hard protective coating is approximately two microns thick.
After ultrasonic cleaning the anti-reflection coatings are applied in a vacuum deposition process. The individual lenses are arranged in special racks known as ‘calottes’.
A modern coating has up to 9 individual layers – the overall package is around 400 nanometres thick. The last layer makes the surface of the lens extremely smooth, making it resistant to both dirt and water. This coating is approximately 10 nanometres thick.
Each lens is inspected thoroughly before delivery. The quality check includes a visual inspection for dust, damage and the right residual reflection colour. Machines are used to check whether each individual lens meets the specifications: refractive power, axis, cylinder, thickness, design, diameter, etc.
The packaging and despatch processes are steered by means of a bar code.
Printing of the delivery note, allocation of the lenses to the delivery note, automated packaging and insertion of the warranty card are performed.
The automated assignment of the orders into boxes for collection by a transport company is the last process to be performed at the lab.
The finished lenses are despatched daily via an express delivery service, by road or air.
Glazing – the term used to describe the insertion of the lenses in the frame – is generally performed at the lens manufacturers, or by the Optician. The lenses are checked to ensure they are accurately centred to provide the best visual quality.