Soft CL Manufacture Flashcards
What are the 3 main method of soft CL Manufacture? What are the combos?
- Lathe cutting
- Spin-casting
- Cast moulding
* Moulding/lathing combo
* Spin-casting/ lathing combo
Describe soft CL manufacture - lathe cutting?
- A rod of material/polymer is made, made as xerogel (anhydrite– made w/o water, just the polymer) – lathed as rigid, hard material
- Rod cut into buttons
- An anhydrous “button” of lens material (xerogel) is lathed in a controlled atmosphere
- Button is mounted on a spinning shaft & shaped w/ computer-controlled precision (diamond tipped) cutting tools
- After front & back surfaces are shaped w/ the cutting tool
- Lens is then removed from lathe & hydrated to soften it
- Sealed in normal saline
- Autoclaved at 121 ˚C for 15 minutes
o Anhydrous – without water
o Xerogel – a hydrogel that has not been hydrated [etymology = xero- (“dry”) gel]
o Autoclave – a pressure chamber that is used to sterilize items. Items placed inside autoclave are exposed to high temperature steam
- Think of it like a miniature potter’s wheel that rotates at 8-12,000 rpm to shape lens
o Lathes are used in metal work, glass spinning etc. to make symmetrical objects
What are the advantages of soft CL 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 manufacture of custom or extreme range lenses that are not amenable to mass production
- Suitable for most materials
- A lathe is a finite investment (not v expensive)
What are the disadvantages of soft CL lathe cutting?
- More steps & more time consuming than moulding
- Takes a few mins to make one lens
- Higher unit cost – due to time & expertise required
- Requires strict humidity control
- Cleaning / polishing required on completion before lens is finished
- Variable surface finish – often need polishing to get good surface finish
- Lenses must be soaked to remove impurities (unpolymerized monomers) prior to sterilization and packaging
Describe soft CL manufacture - spin casting?
- Liquid CL monomers are poured into mould
- Mould is spun in a controlled environment
- Rotational velocity, surface tension and gravity define back curvature and therefore BVP
- Mould shape defines front surface
- UV light applied to polymerize the monomers
- Edges then buffed and polished
- Lenses are then hydrated
- Otto Wichterle (1913-1998) made 1st pHEMA but found could not make pHEMA lenses by cast moulding developed spin casting technique
What are the advantages of soft CL spin casting?
- Produces a very smooth (back) surface as not had to be against a mould
- Back aspheric surface matches aspheric profile of cornea
- Least likely to produce (back) surface defects as it has been cast
What are the disadvantages of soft CL spin casting?
- Slower & more complex process than moulding (more steps)
- Therefore, more expensive
- Not as scalable as moulding – harder to scale up
- Requires edge polishing
Describe soft CL manufacture - injection moulding?
- The soft CL monomers is in liquid form
- Is injected into computer-designed moulds under pressure
- UV light or heat is applied to initiate polymerization
- Then lenses are removed from moulds
- The lenses are then hydrated to soften them
- Lenses are verified, packaged and sterilised in the same way as the spin cast lenses
- Female part of mould makes front surface, male part of mould makes back surface
What are the advantages of soft CL injection moulding?
- Very quick to produce
- Low cost per lens – due to scale they are being produced in
- Easy to produce many lenses at once
- No polishing required
- Most widely used method
What are disadvantages of soft CL injection moulding?
- Strict environmental control of temperature and humidity – can’t have any particulates in air
- Expensive equipment, huge investment required to setup
- Number of moulds limits parameters i.e. good for stock lenses
o Can’t have bespoke moulds & tools for v small no. of specialist/high-powered lenses
Describe the combined methods of manufacture in soft CLs?
- Used for specialist lenses
- Moulding/lathing combo:
o Moulding back surface
o Lathing front surface - Spin-casting/lathing combo:
o Spin-casting back surface
o Lathe front surface
What are the 7 final steps in soft CL manufacture?
Applicable to soft lenses
1. Anhydrous lens inspected (10x mag)
2. Lens hydrated in saline
3. Hydrated lens inspected (10x mag)
4. Lens inserted into blister pack (glass vial for lathe cut lenses)
5. Blister pack sealed and labelled
6. Blister pack and lens autoclaved
7. Lenses dispatched
Describe defects during soft CL manufacture?
- Done by eye for specialist lenses (particularly lathe cut lenses)
- For mass produced lenses (particularly mould cast lenses) done by computer
- For image: edge defects in bold text, body defects in plain text
o Lens would be rejected for all of these - Flash: excess material been put into mould & gone over edge
- Edge roughness
- Nicks in lens – cause discomfort
- Tear in lens – normally extend through lens
- Lens in 2 fragments
- Blemishes on lens, splits on lens
- Eccentric (not centred) optic zone
What are the quality assurances of soft CL manufacture?
- Inspected dry (10x mag)
- Hydrated in saline
- Inspected wet (10x mag)
- Packaged either in blister pack or glass vial (lathe cut lenses) then autoclaved
- Dispatched to practitioner/user
