ABDO Case Record form for the Pre-qualification Period Portfolio
Case number: 51 ABDO Membership number
Authorised manufacturing signature: Date of Signature:
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Manufacturing areas covered on this visit (please initial)
NB where areas are covered by video or presentation, please add title and date viewed
Case number: 51 ABDO Membership number 313204
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During my visit to the Norville factory in Gloucester I observed how lenses are glazed and surfaced as well as how tints and coatings are applied to the lenses.
Once the opticians practice has sent an order to the Norville lab that is when the preparation process begins. The data required for the production of the spectacles is generated and each individual lens is calculated. The job is put into a tray and the data required is processed and integrated into that specific job tray. The correct semi-finished lenses for both left and right are placed into the tray from inventory.
Lens Manufacturing processes:
Moulds: these are required for the casting process and feature the negative shape of the resulting surface of the plastic lens. The gasket, which is a sealing ring that holds the two moulds together, ensures that a distinct distance between the two moulds is maintained. Liquid monomers are poured into the moulds and harden to form polymers, resulting in the plastic lens.
Semi-finished lenses: they are also known as pucks and are the starting point for freeform production and are produced by a casting process. Freeform lens production involves the use of a computer numerically controlled milling machine which manufactures the freeform lenses. Moulds are used and are filled with liquid monomers with various substances such as initiators and UV absorbers added to the monomers. Chemical reactions are triggered by an initiator which in turn leads to ‘curling’ or hardening of the lenses and UV absorbers increase UV absorption of the lenses. The plus or minus powers are indicated by the curvature of the front and back surfaces of the lenses and lenses with different powers can be made from one semi-finished lens.
Polymerisation: the ‘baking’ of the semi-finished lens occurs once the moulds have been filled via exposure to a thermal process for many hours. The addition of heat activates the initiator, triggering the chemical reaction polymerisation/ polyaddition and with the release of heat via exothermic reactions the material starts to ‘cure’ (harden). The starting materials bond and become long molecular chains, with different starting materials and chemical reactions used depending on what refractive index is required of the lens. For CR39 lenses only 1 starter monomer is required whereas 1.6, 1.67 and 1.74 refractive index lenses require 2 monomers. For the higher index lenses the process is much longer and can take up to 48 hours. The two monomers join together via the chemical reaction polyaddition.
Stock lenses: Norville also houses stock lenses, which are not produced to prescription but are kept in stock until an appropriate order is received from an opticians practice. These lenses can be produced using the casting technique and the technology is essentially the same as what is used for semi-finished lenses. Once the lens blanks or “pressings” have been cured, they are provided with a hard protective lacquer and coating.
Semi-finished lenses are blocked once an order is started to be processed. Blocking ensures that the lenses are securely mounted and placed into the correct position within the machine. The semi-finished lens, already finished on the front surface, has a protective lacquer applied to it and then the blocker is attached. A metal alloy with a low melting point is used to bond the lens and the blocker. This results in the semi-finished essentially ‘welded’ into position ready for the machining processes of forming, polishing and engraving.
Forming: the semi-finished lens has a front surface which has already been finished, displaying an optical power. The prescription and lens design required on the order is worked onto the back surface of the lens. Freeform technology involves the adaptation of the lens design to the needs of each individual wearer worked onto the back of the lens which results in optimum vision for the wearer. A central server provides the data required for the forming process, with the shaping process including diameter reduction, bevelling using a milling technique and the use of a natural diamond for fine turning. The fine tuning process provides minimum surface roughness which enables direct polishing with compromising the lens radii or shape. An automated 5-axis CNC machining process is used for the forming process, with the optical power generated within minutes.
Polishing and engraving- the surface of the lens is polished in 60-90 seconds via the ‘soft’ polishing process and this method ensures that all marks are eradicated completely, with the freeform surface previously generated remaining intact.
Once the lenses have been through the processes of forming, polishing and engraving they need to be de-blocked and cleaned. This process occurs by melting the blocking material in hot water which allows the lens to be gently removed from the tool without damaging and marking it, with the metal alloy being completely recycled. A brush washing unit is used for cleaning via a process performed in several steps. High-purity water is used along with various cleaning agents and then the lenses are fully dried. The lenses are now ready for coating and/ or tinting.
Coating- if a coating is required on the lenses then this can be applied either by the lens being dipped or spun.
With the dipping process a lacquer is applied to the lens to make it scratch resistant. There are specially adapted lacquers which are used for the various plastic materials along with different refractive indices. The hard protective coating is approximately two microns thick. The dip coating process will coat both sides of the lens at the same time. Each batch of lenses are racked and given a reference number to track it through the process. The lenses are then washed in combinations of tap and deionised water with an automated transfer between the baths. Once they are placed in the relevant hard coating lacquer for the lens type they are then oven dried which enables handling, which can take 45-60mins. The coating is then checked for fault drips and marks and then the lenses are transferred into trays and oven cured for 3 hours. Following this the lenses are then cooled and relocated into their relevant job tray and dispatched to the next department. The total minimum transit time is 5 hours.
With the spin coating process at Norville the lenses are split into transitions and Polycarbonate, with the process only coating the inner surface of the lens. The lenses are inserted into the spin coater individually. The lacquer is applied when the lens is spinning to prevent drips and to provide an even coating. The coating is then cured with UV light. The lenses are dispatched out of the coater via a shoot to be inspected. They are then rinsed and dried and relocated to their relevant job tray to be dispatched to the next department.
Following ultrasonic cleaning the anti-reflection coatings are applied in a vacuum deposition process. Racks known as calottes arrange each individual lens. AR coatings are vacuum coated with modern coatings having up to 14 individual layers with the total amount around 400 nanometres thick. The last layer makes the surface of the lens extremely smooth which makes the lens resistant to water and dirt and this coating is approximately 10 nanometres thick. Norville has two types of MAR coating machines, which are a TLF and a Leybold. The TLF automatically flips the lenses to coat both surfaces.in the Leybold the lenses have to be manually flipped therefore produces smaller slower batches.
Tints- plastic tints can be tinted in any colour required, whether they are graduated, solid, customised or medically prescribed. The dipping process involves the use of dyes similar to what is used for textiles with the dyes penetrating deep into the surface of the plastic material. They are normally dipped and with plastic lenses in penetrates the lens.
If glass lenses require a tint then the tints are applied in layers consisting of metal oxides. This results in a more limited choice of colours for the glass lenses.
Glazing and Transit Through the Glazing Department:
Glazing is the term used to describe the insertion of lenses into the frame and 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.
At Norville the frame is picked and located into the job tray to be glazed. Here the lenses are checked for power and any faults and then dotted. The single vision lenses are dotted to indicate the optical centre and axis of the lens with three dots as well as being marked if they are right or left. If any prism is required in the order then this is also indicated through the markings. Straight top bifocals aren’t dotted as they are aligned for glazing in relation to the segment dividing line. Round bifocals have to be marked in a similar manner to single vision lenses to indicate the axis required in the order. PPLs are remarked with a temporary marking. Using the engraved markings a fitting cross is placed onto the lens equidistant between the markings and either 2mm or 4mm up depending on PPL lens design. Occupational lenses are marked similar to PPLs except the fitting cross is placed level with the engraved markings. Each and every lens both right and left are marked up before the glazing process begins.
The Norville glazing department has Brio Edgers and Weco Edgers as well as a Mei edger for specialised edging. Each lens, whether for a full frame, rimless frame or supra frame needs to be traced. By tracing the lenses it allows the prescription lens to be decentred as per order and to be blocked. Now the glazing machine knows the shape to which the lens needs to be cut however not necessarily the profile to place on the lens edge, what material is to be cut and if the lens is to be drilled. If the frame has a high base curve then the lenses have to be made to match and require more sophisticated edging techniques.
Lens profiles that are used for glazing include:
V Bevel- this will locate within a corresponding groove designed into the frame. The lenses will be sprung into the frame with heat if plastic (unless they are cold glaze plastic frames) and if the frame is metal it is opened to insert the lens by unscrewing the closing block on the rim edge.
Mini bevel- very similar to the V bevel however it is normally smaller and has a larger drop on the back edge which allows the glazing of difficult full rim frames.
High wrap- allows the fitting of high wrap lenses into high wrap frames. The high wrap edging can be combined with mini bevel functions.
Flat edge and supra groove- the lens is profiled with a flat edge and then a groove is cut into the lens to allow the fit of a nylon cord or fine rimmed metal frame.
Flat edge and drilled for rimless- the lens is firstly flat edged and then drilled to allow the lens to be mounted into a rimless frame.
All of the above profiles can be polished by the edger or by hand afterwards.
As soon as the lens edging has been established and any drill holes have been programmed then the lens material needs to be considered because different lenses have to be treated differently by the edger.
Glass lenses will need a different wheel to cut them and Norville use a different machine altogether. Polycarbonate lenses have to be cut dry and are not ideal for supra glazing because the lens is laminated. Some of the lenses may have to have their size reduced using a drill or router.
The lenses are placed into the edger and are clamped to keep them secure. Feelers measure the front and back curves of the lens to establish the thickness and to calculate where to place the relevant groove or bevel. The edging process produces a sharp edge on the front and/or back of the lenses. This is called a safety bevel and is removed by hand or by the edger after the edging process. The glazing button is removed and the glasses are assembled once the operator is happy with the sizing and shaping of the lens.
Once the frame and lenses have been assembled the finished lenses have to be checked before leaving the factory. The jobs are always checked to the original order as at this stage of the process there may have been several errors that could have occurred. The checking process includes:
Making sure the frame is the correct style, colour, etc.
That the frame set up is straight when open and closed
The lenses are secure but not under glazed, which causes excess pressure and that the lenses are not over glazed.
Appropriate base curves have been used.
The full prescription has been checked and matches the order.
The horizontal and vertical centration is checked.
That all the required lens materials and treatments, tints, coatings, etc. have been applied.
That the cosmetic appearance is acceptable, such as bifocal segments being level and that there are no scratches and defects.
A final stamp check is applied to the original order if everything is acceptable and then the spectacles are ready to be cleaned. A final cosmetic check is performed during the cleaning process. Each lens is inspected thoroughly before delivery, with the quality check including a visual inspection for dust, damage and the right residual reflection colour. The frame is then placed into a matching case where required and sent to invoice and dispatch. The packaging and despatch processes are steered by means of a bar code. The printing of a delivery note, allocation of the lenses to that delivery note, automated packaging and insertion of a warranty card are all performed. The spectacles from Norville to our practice are delivered securely via a courier containing the relevant order details so that we can match them up with the correct patient record for checking and verifying in practice. The automated assignment of the orders into boxes for collection by the courier is the final process performed at the factory.
I confirm that I have checked this report and my student attended the visit.
Signedby Supervisor or Lecturer Date Checked and Signed