Week 9 - Contact lens Manufacture Flashcards
(36 cards)
What are the 3 main cutting methods, and 2 combo methods for SCL manufacturing?
- Lathe cutting
- Spin-casting
- Cast moulding
• Moulding/lathing combo
• Spin-casting/lathing combo
What are the 3 SCL Lathe cutting - Terms?
• Anhydrous - without water
• Xerogel - a hydrogel that has not been hydrated [etymology = xero- (“dry”) gel]
• Autoclave - a pressure chamber that is used to sterilise items. Items placed inside the autoclave are exposed to high temperature steam
How is SCL Lathe cut?
• Anhydrous “button” of lens material (xerogel) is lathed in a controlled atmosphere
• The button is mounted on a spinning shaft and shaped with computer/controlled precision tools
• After front/back surfaces are shaped with cutting tool, lens is removed from lathe and hydrated to soften it
• Sealed in normal saline
• Autoclaved at 121°C for 15minutes
What is a Lathe?
Think of it like a miniature potter’s wheel that rotates at 8-12,000 pm to shape the lens. Lathes are used in metal work, glass spinning etc. to make symmetrical objects
What are the 6 advantages of lathe cutting?
• Established technology
• Easily adapted for CL manufacturing
• Few limitations on parameters that can be lathed i.e. virtually any radius or power can be made
• Reserved for the manufacture of custom or extreme range lenses that are not amenable to mass production
• Suitable for most materials
• A lathe is a finite investment
What are 7 disadvantages to SCL Lathe cutting?
• More steps and more time consuming than moulding
• Takes a few minutes to make one lens
• Higher unit cost
• Requires struct humidity control
• Cleaning/polishing required on completion
• Variable surface finish
• Lens must be soaked to remove impurities (unpolymerized monomers), prior to sterilisation and packaging
What is SCL Spin Casting?
• Liquid contact lens monomers are poured into mould
• Mould is spun in a controlled environment
• Rotational velocity, surface tension and gravity defines back curvature and therefore BVP
• Mould shape defines front surface
• UV light applied to polymerise the monomers
• Edges then buffed and polished
• Lenses are then hydrated
Who was Otto Wichterle?
• Made the first pHEMA but found that could not make pHEMA lenses by cast moulding
• Developed a spin-casting technique. Used a childrens’ mechanical construction kit!
What are the 3 advantages and 4 disadvantages to SCL Spin casting?
Advantages
• Produces a very smooth (back) surface
• Back aspheric surface matches aspheric profile of the cornea
• Least likely to produce surface defects
Disadvantages
• Slower and more complex process than moulding (more steps)
• Therefore more expensive
• Not as scalable as moulding
• Requires edge polishing
What is SCL Injection moulding?
• Soft CL monomers in liquid form
• Injected into computer designed moulds under pressure
• UV light or heat is applied to initiate polymerization, then lenses are removed from mold
• Lenses are then hydrated to soften them
• Lenses are verified, packed and sterilised in the same way as the spin cast lenses
What are the SCL injection moulding advantages?
• Very quick
• Low cost per lens
• Easy to produce many lenses at once
• No polishing required
• Most widely used method
SCL Injection moulding - Disadvantages
• Strict environmental control of temperature and humidity
• Expensive equipment, huge investment required to setup
• Number of moulds limits parameters i.e. good for stock lenses
What are the two combined methods?
• Moulding/lathing combo
- Moulding the back surface
- Lathing the front surface
• Spin-casting/lathing combo
What are the 7 final steps of any SCL manufacturing?
- Anhydrous lens inspected (10x mag)
- Lens hydrated in saline
- Hydrated lens inspected (10x mag)
- Lens inserted into blister pack (glass vial for lathe cut lenses)
- Blister pack sealed and labelled
- Blister pack and lens autoclaved
- Lenses dispatched
What are the 4 edge defects and 4 body defects that can be found?
• Edge defects:
- Excess material (flash)
- Roughness
- Nick
- Tear
• Body defects:
- Eccentric optic zone
- multiple pieces
- split
- blemish
what are the 2 main methods of RGP manufacturing?
- Lathe cutting
- Cast moulding
How is RGP Lathe cutting done?
• Very similar to soft lenses but less margin for error
• Separately configured lathes for front/back surface
• Engraving/marking is common on RGP, e.g. R/L or the BOZR
• Edge polishing is then done
• Lens stored dry
What are the advantages and disadvantages for RGP lathe cutting?
• Advantages and disadvantages are very like those for soft lenses.
How is RGP Cast moulding done?
• Similar to the soft lens method in that the lens is left to solidify in the
• The mould is then broken apart to reveal the lens in its final form which doesn’t routinely require further finishing.
• Advantages and disadvantages same for soft lenses
What are the first two toric manufacturing methods?
- Toric machining: dual-axis lathing tool on a stationary lens button to produce two radii. Computer controlled now, can produce any shape required (the most common).
- Crimping: lens blank is crimped across the front surface then worked into a sphere, releasing the crimping the leaves a toric surface (almost obsolete now)
What are the 3rd and 4th method for toric manufacturing?
- Moulding - as for standard spherical lenses, just requires a separate mould for each Rx (therefore limited range)
- Dual-axis flying cutter: cutter is set to a particular axis, as this passes by the rotating lens button it creates the toric shape (less stable on the eye than toric machining)
What is Prism Ballast stabilisation in toric manufacturing?
• Can be done on both RGP or SCL
• Balances the thickness profile, to minimise rotational effects
• 1 or 1.5 prism base down traditionally, although now can be placed peripherally (newer designs)
• This is all done by software
What is Dynamic stabilisation in toric manufacturing?
• Depends on the interaction between the lids and front surface of the lens
- Dual thin zones or double slab off: Top and bottom portions of the lens are chamfered (thinner sloping edge)
too
- Truncation is now rarely used with soft lenses as difficulty in mass production moulding methods needed for disposable lenses; removes a 1-1.5mm chord from the lower edge of the lens giving excellent stability but can be uncomfortable. Edge removed with an emery board or a diamond impregnated tool and polished. Usually combined with a prism.
What is multifocal segmented designs?
- Two or three distinct portions that make up an alternating lens may be either fused (insert of higher RI) or solid portions with a range of alternative segment shapes
- Probably lathe cut but difficult to find info as this lens type is now quite rare
- Stabilised with prism and/or truncation
- More commonly RGP
