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CET 18 -March 2002

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CET 18 -March 2002

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									                                                                                               made in the UK due to its flammability




                                                                                                                                               CONTINUING EDUCATION AND TRAINING
                                                                                               (Figure 1). Its flash point is at 70°C, which
                                                                                               is just above the softening point, 65°C.




       FRAME
                                                                                               SPX
                                                                                               SPX is a co-polyamide, which is a
                                                                                               lightweight, strong and mouldable nylon
                                                                                               derivative. Co-polyamides are derived
                                                                                               from two different monomers joined in



      MATERIALS
                                                                                               the same polymer chain. It is easily
                                                                                               coloured then lacquered to give a high
                                                                                               polish. It is highly elastic and impact-
                                                                                               resistant and has a harder surface than
                                                                                               acetate therefore resistance to abrasion is
                                                                                               greater. It is classed as a hypoallergenic
                                                                                               material. SPX is resistant to most solvents,
         There are many materials used in spectacle                                            cosmetics and oils. It softens at 95°C;
                                                                                               excessive heat will cause it to shrink,
           frame manufacture both from natural and                                             therefore it is recommended that it be
                                                                                               accurately cold glazed. Over-glazing of
            man-made sources. These may be used                                                SPX will cause it to craze and/or crack.
                                                                                               However, should the lenses be under-
          alone or in combination, explains Stephen                                            glazed, unlike acetate, it is not possible to
                                                                                               shrink the rim in a controlled fashion.
                                        Golding                                                SPX has a disadvantage in that it is more
                                                                                               susceptible to temperature shock. Due to
                                                                                               this most of the breakages occur in the
                                                                                               winter months. In the search for thinner
                                                                                               frames SPX used to be inline glazed.
                                                                                               Inline glazing was a process where the
     rame materials must meet certain           materials, fusing them together forming        frame had a bevel in the rim. The lens


F    criteria to enable them to be used
     successfully in the manufacture of
     frames. They must be lightweight,
yet strong, easily adjustable yet maintain
their shape, be machined in such a way
                                                sheet acetate. This is then machined into
                                                shape, before being barrel polished.
                                                Acetate is reasonably lightweight and
                                                mechanically stable at room temperature,
                                                non-flammable, softening at 57°C. It will,
                                                                                               had a supra groove running around it.
                                                                                               This is generally only found on the older
                                                                                               styles. Sides are reinforced.

                                                                                               Epoxy resin
as to allow easy glazing, be inert to both      however, warp easily above this. Acetate       Epoxy resin or Optyl is a thermo-setting
body fluids and external chemicals and          has poor resistance to common solvents.        plastic, softening at 80°C, with a
colour fast.                                    Acetone is used as a solvent to repair         thermoplastic memory, enabling it to set
   There are two types of plastics              acetate frames (also found in nail varnish     into any new shape, but return to its
materials available, thermoplastic and          remover). Frames when in contact with          original form when heat is applied. It is
thermosetting. The materials which frame        the skin can whiten especially at the          brittle when cold. Colour is applied by
manufacturers generally use are                 bridge and temple areas. Sides are             dying with a top layer of varnish. It is
thermoplastic. These are capable of being       reinforced. Joints are generally heat          resistant to burning and cannot be
reformed under heat. Thermosetting              inserted or they may be riveted.               shrunk. The resin is vacuum injection
plastics set or harden in the final stages of                                                  moulded with the joints inserted in the
adjustment, fixing their shape. Such            Cellulose propionate                           mould. It is approximately 30 per cent
frames will not soften when reheated,           Cellulose propionate is made from              lighter than acetate. The sides can either
but will burn or melt. This makes them          cellulose flakes and proprionic acid. In its   have a short reinforced side or a light
unsuitable for frame manufacture, as            raw state it comes in clear granules.          coated metal side (LCM). LCM sides were
dispensing opticians will have to make          These are liquified and injected into          introduced to allow the sides to be
adjustments to the frames.                      heated moulds. Joints are usually pre-         adjusted easily without the need for heat.
   Production of plastics frames can be         inserted in the moulds. Finished frames        The short reinforced sides required
carried out in a number of ways:                are then barrel polished. Colour is            enough heat to make them soften, then
   Injection moulding, where the liquid         applied by dying, transfer printing or by      they had to be held in shape whilst
monomer or polymer is pumped into a             hand painting. The finished frame is then      cooling. They used to be supplied with a
mould, Here joints are usually placed into      lacquered to maintain the polished             cloth cover to stop the dispensing
the mould prior to moulding.                    appearance. The softening temperate is         optician from burning their fingers.
   Machining or routing is where the eye        67°C. Excessive heat above this causes
shape is cut from sheet material. Joints        shrinkage. Propionate has good elasticity      Polymethylmethacrylate
are either heat sunk or riveted. Frames         but stretching the frame will cause it to      Polymethylmethacrylate (Perspex) is a
can also be made by moulding, followed          cleave.                                        stable, lightweight (specific gravity
by machining.                                                                                  1.2g/cm3)1 acrylic resin with a refractive
                                                Cellulose nitrate                              index of 1.49 giving it its crystal
Cellulose acetate                               Cellulose nitrate is made from cotton          properties. It is a hard, rigid material, but
Cellulose acetate is the most common            linters and nitric acid with camphor used      brittle. Frames made of acrylic are often
material used since being developed in          as a plasticiser, which give it its            of a supra, design due to the high
1894. It is produced from cotton linters        characteristic mothball smell. Old frames      temperature required to spring in lenses
and acetic acid. It is usually made into        have a typical dark urine yellow colour.       and the rigidity of the material (Figure 2).
sheet form, although it may be made             Nitrate frames were more robust than           The sides are usually sinuous and non-
from extruded strips. Colour is generally       acetate although they became very brittle      reinforced. Colour is laminated onto a
applied by laminating colours on base           with age. Nitrate frames are no longer         clear base. The joints are often fixed by
                                                                                                           MARCH 2002 DISPENSING OPTICS        1
CONTINUING EDUCATION AND TRAINING




                                    ▲ Figure 1: Three frames at approximately 5, 10 and 20 seconds after ignition.

                                    mushroom pinning. It is dermatologically           manufacture. It is lightweight and              three times that of gold plating making it
                                    inert.                                             flexible, being twice as strong as nylon        abrasion resistant. Titanium is very
                                                                                       and five times stronger than steel. It is       flexible, being 20 per cent more elastic
                                    Carbon fibre                                       thermally stable and self-extinguishing. It     than nickel silver.
                                    Carbon Fibre is a misnomer, as pure                is resistant to solvents but its external          In general, welding of titanium and its
                                    carbon fibre is not used for frame                 surface is affected by ultra violet light.      alloys can be readily performed, but it is
                                    manufacturing. What is called carbon                                                               necessary to exclude reactive gases,
                                    fibre is a composite material of nylon             Nickel silver                                   including oxygen and nitrogen from the
                                    impregnated with carbon fibres. The                Nickel silver is an alloy of copper, nickel     air, and to maintain cleanliness. Titanium
                                    typical composition is 80 per cent nylon           and zinc, its nickel content being              oxide, which is formed when titanium is
                                    and 20 per cent carbon fibre. They are             between 12 -25 per cent. It is an easily        heated to high temperatures, is brittle.
                                    naturally a dark grey colour with the              worked, flexible material, which requires       Argon gas is used during the welding
                                    colour being applied by lacquer; because           plating to prevent it discolouring. Where       process.
                                    of this the colours tend to be dark, bold          there is contact with the skin it goes             TIG welding, also known as Gas
                                    shades. Some of the frames have closing            green. It is often used as a core material      Tungsten Arc Welding (GTAW), is used to
                                    blocks whilst others have full rims, which         for rolled gold. Nickel is, however, one of     produce high quality welds. In TIG
                                    require cold accurate glazing. The sides           the most common materials to cause an           welding an arc is formed between a non-
                                    are usually of another material, which is          allergic reaction. Experience has shown         consumable tungsten electrode and the
                                    typically of a metal composition. The              this to be more of a problem with               metal being welded. Gas is fed through
                                    sides are usually attached by a screw              females, possibly due to the fact they          the torch to shield the electrode and
                                    fitting (Figure 3).                                have had more sensitising contact with          molten weld pool.
                                                                                       other nickel-based products, such as
                                    Kevlar                                             jewellery and clothes fastenings2. There is     Memory metals
                                    Kevlar is a polyaromatic amide, a nylon-
                                                                                       now a European Directive (94/27/EC),            Memory metals are made of various
                                    based derivative, developed in 1965. It is
                                                                                       which specifies the upper limit for nickel      alloys, typically 40 per cent nickel and 60
                                    occasionally used in spectacle frame
                                                                                       release in articles, which have direct and      per cent titanium. Cobalt and titanium
                                                                                       prolonged contact with the skin.                are also used. Due to the flexibility they
                                                                                                                                       are difficult to adjust. The flexibility is
                                                                                       Monel                                           about 8x that of spring steel. It is
                                                                                       Monel is another nickel-based alloy,            corrosion and tarnish-resistant. Most
                                                                                       developed in 1905. It has much higher           commonly the bridges and sides are
                                                                                       nickel content than nickel silver. It is        made of the alloy with the rims being of
                                                                                       made up from 66 per cent nickel, 31.5           a more rigid material, typically Monel.
                                                                                       per cent copper plus iron, manganese            They are usually attached together by
                                                                                       and silicon. It is corrosion-resistant and is   means of a cup soldered to the rim and
                                                                                       capable of being left in seawater for 24        the bridge fitting into the cup, being
                                                                                       hours without degradation. However due          made secure with a superglue3. Memory
                                                                                       to the high nickel content there can be a       metals are not new having been
                                                                                       higher incidence of allergic reaction. It is    developed for the US military in 1961.
                                    ▲ Figure 2: Perspex supra design.                  commonly used as rims with the bridges
                                                                                       being a memory metal.                           Aluminium
                                                                                                                                       Aluminium is a lightweight material,
                                                                                       Titanium                                        which is cold to touch. The colour is
                                                                                       Titanium is one of the most expensive           applied by anodising the surface, then
                                                                                       frame materials to produce due to its           dying. The anodising process is where an
                                                                                       extraction costs and manufacture. It is         electric current is passed through the
                                                                                       generally considered to be hypo                 aluminium (anode), which is suspended
                                                                                       allergenic, particularly in its pure form,      in a sulphuric acid bath. The aluminium
                                                                                       although when it is clad a layer of nickel      is oxidised. The length of time the
                                                                                       is used. It has low electrical and thermal      aluminium is in the bath determines the
                                                                                       conductivity. Weight for weight it is           thickness of the oxide layer. The oxidised
                                                                                       approximately half that of nickel silver. It    layer can then be dyed with a water-
                                                                                       is common for titanium frames to be an          based dye. It is stain and tarnish-resistant.
                                                                                       alloy of titanium, vanadium, aluminium          As it cannot be soldered or brazed, joints
                                    ▲ Figure 3: Side attachment on a carbon fibre      and manganese. This is known as Beta            have to be pinned. Some joints are milled
                                    frame.                                             titanium. The surface hardness is about         from a solid block giving the appearance
2                                   DISPENSING OPTICS FEBRUARY 2002
of a soldered joint. It was traditionally        Due to the risk of erosion where the




                                                                                                                                              CONTINUING EDUCATION AND TRAINING
used for sides on plastics frames, but is        sides meet the front, metal-to-metal joints
now used as in full frames or fronts with,       will be used. As it is a natural product the
for example, titanium sides.                     colouring will vary from frame to frame
                                                 with the rarer lighter colours being the
Stainless steel
Stainless steel is an alloy of iron,             most expensive. It is also identifiable by
chromium and nickel. It is a strong,             the green mottled patches. Shell
lightweight, non -corrosive metal, hence         adjustment needs to be done over a
the name. It is produced either from wire        steam bath at 75°C, but should an
or plates depending on the design. It is         accident happen, it is possible to repair
easy to adjust and not easily damaged. It        shell by splicing together the pieces,
rarely causes skin irritation. It discolours     placing in a saline bath and leaving for 24
easily when heated.                              hours.
Rolled gold                                         Another natural material often
Rolled gold is produced by bonding a             confused with shell is horn due to the
skin of gold onto base metal, usually that       metal-to-metal joints. This is commonly
of nickel silver. It has good resistance to      sourced from buffalo horn. Its warm
corrosion. Rolled gold maintains its             natural colours range from cream to
appearance even after repair and is              black. Like shell it is lightweight and also
unlikely to cause skin problems unless           hypoallergenic, not reacting to skin
constantly touching, or if the integrity of      secretions. It expands under heat without
the gold is compromised. The gold blocks         warping. It has a hard surface, which
are cold hammered and rolled to a
                                                 takes a good polish but it can flake easily
suitable size for drawing into frame-
making wire, which is then sold already          and will become brittle if left to dry out.
profiled to manufacturers. Rolled gold
will be stamped with the gold content,           Wood
eg, 1/20 14kt, which denotes 20 parts per        Wood is used in frame manufacture
thousand of 14-carat rolled gold.                typically in sides covering a metal core,
                                                 but fronts have also been made. Woods
Electroplating                                   used include boxwood, beech, lime,
A number of frames are electroplated to          cedar, bamboo and yew. The patient
give them their final finish. The base           needs to be warned, that some woods,
material is usually that of nickel silver. A
                                                 such as yew, can be toxic if eaten. Wood
number of materials have been used gold,
palladium, rhodium and ruthenium (all            is a fibrous material that is hard and
members of the platinum group) and               lightweight. It often has a yellow tinge.
chromium. If the plating is damaged then         Wood will alter in size depending on the
perspiration can act as an electrolyte           humidity, so no good for the sauna!
between the base (+ve) and the plating
(-ve) like a battery. The small current             Dispensing opticians are often guilty of
causes copper salts in the nickel silver to      concentrating on the advances in lens
form ‘verdigris’ which forces off more           technology to the detriment of frame
plating as it occupies a greater space. To       materials. A good working knowledge of
protect the frames further they are often
                                                 frames and their properties is an essential
treated with a lacquer, which may be
applied as a liquid or as a powder, which        part of the dispensing optician’s role,
is then heated until it liquifies. Common        especially now that more patients are
plastic polymers used for covering frames        experiencing or are becoming aware of
include polyurethanes,                           allergic responses to external elements.
polymethylmethacrylate, and epoxy
resins. Polymerisation can be before the         References
heating process or as a result of it.            1 www.teg.co.uk/nickel, www.dermnet.org.nz/index.html
                                                 www.aocd.org/skin/dermatologic diseases/nickel allergy.html
Other materials                                  2 Bridle D, Titanium - truth and fiction.
Some naturally occurring products have
                                                 Dispensing Optics, November 2000
been, and still are being used in spectacle
manufacture. These include shell, horn           3 Various internet sites
and wood.                                        4 Further Reading : Obstfeld: Spectacle frames
                                                 and their dispensing
Tortoise shell                                   Photographs by the author with assistance
Tortoise shell frames are formed from the
belly plates of the hawksbill turtle. As it is   of Ben Turley (Digital imaging, Manchester
an endangered species it is no longer            Royal Eye Hospital) for the Nitrate stills.
allowed to be killed for the shell.
                                                 Stephen Golding FBDO(hons)LVA, senior
However, there are still supplies in the
world and old frames still surface. It is        dispensing optician, Manchester Royal Eye
manufactured by forging the plates               Hospital. ABDO practical examiner, Council
together, under heat and pressure. It            Member for Area 3, and, Low Vision
displays thermoplastic characteristics.          Committee member. ■
                                                                                                               MARCH 2002 DISPENSING OPTICS       3

								
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