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OV2006 contact lenses > manufacturing of CLs > Flashcards

manufacturing of CLs Flashcards

1
Q

what are the three techniques used to manufacture CLs?

A

. lathe cutting
. spin casting
. cast moulding

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2
Q

explain lathe cutting (SCL) process?

A

. special CL lathe ( fat rotating tool) to cut a hard button into required shape, polish carefully, then hydrate this to form the finished soft lens

. cut in a hard form , then hydrated to become SCL

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3
Q

why is lathe cutting (SCL) more expensive?

A

. the number and variability of manufacturing steps that are necessary for lathe cutting mean that manufacturing soft lenses using this process is more expensive than spin casting or spin moulding

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4
Q

what is the difference between back surface curvature and front surface curvature?

A

. when we are cutting a CL material , when the back surface is cut that will decide the shape of the lens

. back surface curvature decides the shape
. front surface curvature decides the power

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5
Q

what happens to the lens after hydration in lathe cutting

( SCL)?

A 
1 . after hydration, lens is rinsed with saline
2 . quality control checks: 
. check surface defects
. check base curve 
. check BVP
  1. seal in sterile vials
  2. autoclave
    . 120 deg for 15-20 minutes
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6
Q

what are the steps in lathe cutting (SCL) ?

A 
. the rod is cut into buttons
. back surface is cut
. front surface is cut
. then there is polishing involved 
. inspection 
. hydration
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7
Q

what are the advantages of lens cutting?

A

. any prescription or curvature: good for lenses with high power or high cyl
. thicker lenses: easier to handle
. excellent movement and centration

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8
Q

what are the disadvantages of lathe cutting (SCL)?

A

. labour intensive
. expensive
. difficult to reproduce : each lens is made individually

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9
Q

what is spin casting?

A

. male stainless steel tool to create ‘ female ‘ PVC moulds
. little polishing necessary
. spin casting works by injecting numerous liquid monomers into the concave moulds which spin at controlled rate
. centripetal force from spinning forms aspheric base curve of CL

  • front curve formed by curvature of male mould
  • UV light is applied to solidify the material
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10
Q

what determines the final lens parameters in spin casing ?

A

. speed of rotation combined with both the mould tool shape and the monomer dose determines the final lens parameters

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11
Q

what is the lens geometry in spin casting determined by?

A

. lens geometry is determined by spin speed and volume of monomers

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12
Q

what is the lens power in spin casting determine by?

A

. lens power is determined by spin speed

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13
Q

what is the centre thickness of the lens dependent on in spin casting?

A

. centre thickness of the lens is dependent on the amount of monomer injection

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14
Q

what are the advantage of spin casting (SCL)?

A

. smooth edge profile and overall surface - enhanced comfort during lens wear
. back aspheric lens design matches well with aspheric cornea - minimal lens movement with acceptable tear film dynamics
. easy to fit

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15
Q

what are the steps in spin casting ( SCL)?

A

. the male tool is impressed against heated liquid PVC
. this then cools and sets to form a solid plastic concave female mould
. multiple number of metal tools is used to produce the same number of plastic moulds simultaneously
. monomer is induced in the female moulds
. UV light is used
. little polishing from edges
. then hydration
. then same as lathe cutting

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16
Q

what is cast moulding (SCL) the dominant technology ?

A

. dominant technology in high volume lens manufacture , this is because in spin casting a series of highly polished steel tools is used to fabricate PVC moulds
. however in cast moulding , matching male and female moulds are required

17
Q

what is cast moulding (SCL)?

A

. takes place in automated continuous production line
. monomer in liquid form is introduced into a concave female mould which defines the shape of the lens front surface
. once the monomer is capsulated into the mould it is cured ( this is process where the male and female moulds are exposed to UV light or thermal radiation or both)
. female and male mould are clamped together , excess polymer is squeezed out before UV light is applied
. hydration
. then same as lathe cutting

18
Q

how are coloured tinted SCLs made?

A

. made by cast moulding
. pigments embedded in middle layer
. lenses are very thin and smooth

19
Q

how are opaque coloured SCLs made?

A

. made by lamination
. hand painted or printed
. stamped ink

20
Q

what are some manufacturing defects that have to be checked when lens are made ?

A
  1. edge defects
    - nick: small piece missing from edge
    - tear: material or full separation on material including edge
    - roughness: uneven edge profile
    - excess material or flash : surplus material
  2. body defects
    - split
    - blemish: hazy/ low transparent region, on or within lens
    - eccentric optic zone: optic zone not concentric with lens perimeter
    - multiple pieces
21
Q

how is lathe cutting process in ( RGP) ?

A

. the raw material of RGP is supplied to lens manufacture in the form of a flat cylindrical buttons
. buttons have handling tinted
. often a blue RGP is prescribed for left eye and a green RGP is prescribed for the right eye
. some lathes are used to generate both front and back surface of lens
. the button is then released and turned so the front surface can be cut to represent correct prescription and central thickness
. polishing particularly in edges
. lenses are checked and cleaned and sterilised individually

22
Q

how does lens geometry change with the power of the lens?

A

. low minus power - the thickness of lens is similar in centre and periphery ( slightly thicker in periphery )

. high minus power - the centre is thinner, while the thickness in the periphery is high - almost double

. high plus lens - centre thickness is high , lens might drop after blinking because of gravity - fit lens tighter to the eye

23
Q

what is back optic zone?

A

. area of CL which the patient has optically best vision

24
Q

how does the thickness of lens have an impact on weight of the lens ?

A

. the thickness centrally can cause the lens to drop after blinking because of gravity

25
Q

what happens with contact lens edge in RGPs?

A

. in RGPs contact lens edge doesn’t have to be in touch with cornea
. if edge touches the cornea, the tears sitting below the CL have no were to escape

26
Q

what is tear exchange and why is it important ?

A

. tear exchange is important in an RGP fitting - makes cl healthy option
. tear exchange - means with every blink the tears behind the CL are being replaced with fresh ones

27
Q

what is edge lift ?

A
  • edge needs to come off the peripheral cornea

- we can change the geometry of RGP lens to have a bigger or smaller edge lift

28
Q

why do we manipulate edge lift of RGP?

A
  • manipulation of edge lift away from the cornea can contribute significantly to improved patient comfort and help with stabilising issues, prevents binding of contact to cornea
  • edge of RGP can be flattened or steepened
29
Q

what is the good thing about RGPs?

A

. the manufacturer can make any lens you want
. there are great tools to help you predict how the lens will fit in the eye
. predictability of RGP makes it a intersting lens choice

30
Q

what are support available when fitting CLS?

A
  • fitting guide
  • software
  • online calculators

OV2006 contact lenses (11 decks)

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