Ординатура / Офтальмология / Английские материалы / Essentials of Ophthalmic Lens Finishing, 2nd edition_Brooks_2003

C H A P T E R 6 C E N T R AT I O N O F S E G M E N T E D M U LT I F O C A L L E N S E S

34.Franklin-style bifocals are horizontally decentered for edging using which of the following?

a.Distance decentration

b.Segment inset

c.Total segment inset

35.To center an occupational double segment exactly as indicated on the order during the layout process, which of the following steps should be taken?

a.Center first the lower segment and mark the lens. Then center the upper segment and re-mark. Block the lens exactly between the two marks.

b.Center the upper segment first. Before marking, note whether the lower segment is in the prescribed location.

c.Center the lower segment as indicated. Ignore the upper segment.

36.A double-segment flat-top bifocal (double D) has an ordered segment height of 18 mm. Which of the following frames has a vertical depth that is suitable for this segment style?

(Hint: More than one response may be correct.)

a.B = 30

b.B = 35

c.B = 40

d.B = 45

e.None of the above

137

37.Which bifocal(s) can be marked for blocking without first spotting?

a.Executive bifocals

b.Flat-top bifocals

c.Round-segment bifocals

d.Curve-top bifocals

38.You would like to alter the segment inset of an already-surfaced lens before edging it. Which of the following lenses listed can you slightly rotate around the distance OC to alter the lens as described?

a.A flat-top 25 segment with a power of –2.00 D sphere

b.A 22-mm round segment with a power of –2.00 –2.50 × 15

c.An executive or Franklin-style segment with a power of +1.25 –0.25 × 30

d.An Ultex A segment (38-mm semicircular segment) with a power of –7.25 D sphere

between two felt pads in the edger. Because no block was used, it was never really “blocked” in the true sense of the word. Hand positioning the lens between the felt pads is an impractical method for ensuring accuracy of edged lens parameters. Instead the lens is prepositioned in a centering fork.

With the lens properly aligned in the centering fork, the adapter is slipped into the accepting portion of the edger. The second felt pad might then be pressed into place and tightened. Then the centering fork may be removed as the lens is clamped in at the correctly centered cutting line orientation.

The pressure blocking system has fallen into disuse. Besides the danger of lens slippage, such a system does not allow for periodic removal of a lens from the edger to check for size. Once the lens is removed, it is practically impossible to put back in the edger in its exact previous position.

Blocking with Suction

Another method of holding the lens in place is by means of a small suction cup (Figure 7-1). This suction cup is constructed with an adapter on the back surface that allows it to fit into the spindle assembly within the lens edger.

The advantage of a section cup system is that it may be used with no preparation. Although it is not often used for mass production purposes, it lends itself to smaller facilities where lenses are edged on a more periodic basis. In Europe, where most of the edging is done on location at the dispensary, this system was more widely used than in the United States.

The holding surface of the suction cup must be kept free of oil or foreign substances. Suction cups must be applied dry to a dry lens to prevent slippage and a corkscrewing effect. These cups may be reused often but must be inspected regularly for cracks in the material caused by drying.

Metal Alloy Blocking

A metal alloy blocking system is another system that enjoyed widespread usage in the past. It is practically out of use now. In this system a metal alloy with a low melting point is heated and molded to the lens. The metal cools and forms a small metal block that adheres to the lens (Figure 7-2).

FIGURE 7-1 The adapter attached to the suction cup may vary in its configuration, depending on the edger manufacturer’s design.

Metal alloy will not adhere directly to a lens surface and withstand the edging process. The surface must first be precoated. The coating was applied to the front surface of the lens in the form of a spray or with a brush or dab-on applicator.

Alloy blocking has the major disadvantage of using heavy metals in the alloy such as lead and cadmium. The environmental problems associated with heavy metals are a growing disincentive for working with a metal alloy blocking system.

Precast FreeBlock Blocking

This system of blocking uses a precast waxlike lens block that adheres to the surface of the lens when heated. The adhesion of these blocks is excellent as they conform nicely to the lens surface and prevent slippage. FreeBlocks are used in Gerber-Coburn Step Two blocking. The Step Two system incorporates centration and blocking in one “Step Two Finish

140

Blocker” that heats and presses the block against the lens. Reuse of the blocks is not recommended.

Adhesive Pad Blocking

Another method for blocking lenses utilizes adhesive pads.1 A block made of metal or plastic is fastened to the lens by means of a double-sided adhesive pad (Figure 7-3).

1Adhesive pad blocking is sometimes referred to as LEAP blocking. 3M was the first company to introduce this type of system, which is known as the 3M LEAP System.

FIGURE 7-2 The shape of the metal block can be molded to any one of several different chucking systems.

C H A P T E R 7 B L O C K I N G O F L E N S E S

BLOCKS

Block designs vary according to shape of the chuck used to hold the lens in the edger. In other words, the back of the block may be available in a variety of shapes to adapt to the chuck of the edger being used. This could be a problem for laboratories with edgers from different manufacturers. With only a few exceptions, most edger manufacturers are able to supply a variety of chuck designs so that chucks may be interchanged. This way the same block design may be used throughout the laboratory.

Metal Blocks

When the block is metal, the side contacting the adhesive pad must approximate the front curve of the lens. Blocks can be classified as low, regular, and high base, depending upon the steepness of curvature. They are color coded. A common color is black for low base blocks, gray for regular base blocks, and gold for high base blocks (Figure 7-4). Low base blocks are for lenses with base curves from approximately 0.00 to 4.25 D, regular base from 4.50 to 8.00 D, and high base above 8.50 D. These numbers are front surface base curve numbers. They are not lens refractive powers.

The front curve of a lens steepens as lenses increase in plus power and flattens as the power moves toward the minus direction. For lenses of equal refractive powers, aspherics are generally flatter than regular lenses. So aspheric plus lenses will not require a high base lens block as quickly as a nonaspheric lens of equal refractive power.

Plastic Blocks

Some blocks are made from either semirigid or flexible plastic material. These types of blocks are designed to conform to the front curvature of the lens because the flange of the block is designed to take on the lens shape (Figure 7-5). Plastic blocks sometimes require a larger adhesive pad.

Block

Adhesive pad

Lens

FIGURE 7-5 Flexible plastic lens blocks offer the advantage of conforming well to the front surface of the lens, regardless of how steep or flat the surface is.

Plastic blocks continue to appear in new designs and are rapidly replacing metal blocks.

BLOCKS FOR HALF-EYE LENSES AND NARROW B FRAMES

Frames that are small in their vertical dimension, such as some children’s frames and half-eye glasses, can be troublesome during edging. An ordinary lens block is larger than the edged lens will be. When this happens, the edger begins grinding the block and the lens. To prevent it, the lens block has to be cut off in the vertical dimension and used just for these types of frames (Figure 7-6). Switching the chucks being used in the lens edger is also necessary because these are also too large.

FIGURE 7-6 These three half-eye blocks are regular lens blocks with the top and bottom edges removed to allow for a very narrow lens. If a smaller half-eye block is needed, it is likely that a smaller chuck in the edger will also be required.

PREVENTING BLOCK SLIPPAGE

If slippage occurs during edging, the first thing to check for is an incorrect base block. If a high minus lens is edged on a regular base block, the lens adheres tightly only at the edges (Figure 7-7, A). If a high plus lens is edged on a regular base block, the lens holds

142

A

B

C

C H A P T E R 7 B L O C K I N G O F L E N S E S

Best adherence

Poor adherence

Poor adherence

Best adherence

Best adherence

well only in the center of the block (Figure 7-7, B). The lens block should parallel the curve of the lens surface for maximum adhesion (Figure 7-7, C).

If the lens is on the correct base block, the next most likely problem is incomplete cleaning. The lens and the block should be cleaned thoroughly before blocking occurs. A dirty or oily surface prevents tight adhesion. Extreme torque is on the lens during edging. Without maximum adhesion, the lens may twist or slip. Then, even if the shape is right, the prescription will be incorrect.

Methods of cleaning vary. Some people find that only minimal cleaning of the blocks is necessary; others soak recycled metal blocks in solvent or place blocks in an ultrasonic cleaner to remove all grease and oils before reusing.

If more thorough cleaning does not alleviate the situation, it may indicate that the roll of adhesive pads has been exposed to excess heat or humidity. Excess humidity decreases the adhesion of the pads and also can result in slippage (Box 7-1).

C H A P T E R 7 B L O C K I N G O F L E N S E S

BOX 7-1

Possible Causes of Lenses Slippage on the Block

If lenses slip on adhesive pad blocks, check the following:

1.Is the block correct for the base curve of the lens?

2.Are the lenses clean?

3.Is each block free from foreign material?

4.Have the pads been exposed to humidity or high temperatures?

5.Are the edger wheels worn or in need of dressing?

6.If the edger is a dry edger, are the cutter blades dull?

7.Is the coolant flow sufficient? Insufficient coolant flow will cause the lens to heat and the adhesive on the blocking pad to soften.

FIGURE 7-8 The lens block is placed on the adhesive pad.

ADHESIVE PAD BLOCKING PROCESS

In preparation for blocking, a pad is peeled from the roll and placed against the lens block (Figure 7-8). The fingers should not touch the surface of the adhesive. This introduces oil onto the surface and keeps the pad from sticking to the block as tightly as it should.

Maintaining consistency in how the tab is placed on the block may be helpful. This way when picking up the block to mount it in the blocking device, no uncertainty exists regarding whether the block is right side up or upside-down. The protective paper is not peeled from the pad.

The pad is pressed firmly against the block.

143

FIGURE 7-9 Some efficiency may be gained by placing a number of lens blocks on a strip of adhesive pads ahead of time.

Alternatively the block may be mounted on a strip of pads. The backing is peeled from the pad while it is still on the roll. The block is placed on the pad. In this way a whole strip of pads is neatly prepared and ready (Figure 7-9). When removing the pad and block from the roll, the practitioner should be careful to avoid touching the exposed adhesive.

Next the block is placed in the centration device in preparation for blocking. It is mounted on a movable arm that may be manually or automatically swung into place over the lens and pressed onto the lens surface. The protective paper may be removed from the block.

The lens is now placed on the centration device and decentered as required. This was previously described in earlier chapters on centration of lenses. Once the operator is assured that the alignment is correct, the block is swung into place and pressed against the lens (Figure 7-10).

The blocked lens is removed from the instrument and the block pressed against the lens to ensure maximum contact between the adhesive pad, lens, and block (Figures 7-11 and 7-12).

High Adds and Wide Segments

Plastic flat-top bifocal lenses have their segment area on the front surface of the lens. The segment gets its additional plus power because it has a steeper curvature than the rest of the lens (Figure 7-13). With regular plus lenses, the larger the size of the lens, the thicker the lens will be. And again, with regular plus lenses, the higher the power of the lens, the thicker the lens will be. The same is true for the segment area of a plastic

FIGURE 7-13 Two flat-top bifocal lenses with the same bifocal segment size. One has a low add power, the other a high add power. When the two lenses are edged, the higher add power lens is more likely to crack along the segment line or have the segment line indented or chipped because of chucking pressure unless precautions are taken.

bifocal lens. The wider the bifocal segment, the thicker the ledge will be on the flat-top bifocal. Flat-top 35 segment ledges will stick out farther from the lens surface than will flat-top 28s of the same power.

A flat-top 28 with a +1.25 add has a much thinner ledge than a flat-top 28 with a +3.50 add.

If a thick bifocal ledge is left unprotected during edging, the lens can be damaged because of the pressure exerted when the lens is chucked for edging. The following may happen:

•A thin lens can split along the bifocal line.

•The segment line can be slightly indented or can chip.

To prevent such problems, an adhesive pad is cut into two. The half pad is placed against the lens just above the segment as shown in Figure 7-14. The half pad should be just far enough above the line to make the segment line visible during lay out. The hole in the pad should be placed so that it allows the center lensmeter dot to show clearly.

Next the lens is blocked normally. (The back must be peeled off the half-pad first.) The half pad completes the circle of the bifocal segment and allows the block to hold the lens along the segment line evenly.

Protective Tape

All currently used blocking systems adhere well when directly mounted on the surface of the lens. (A notable exception is the seldom-used metal blocking system, which requires that the lens be spray-coated ahead of time.) However, in some instances using protective tape on the lens may be advisable.

Formerly the primary concern was to protect a plastic lens surface from scratching. With most plastic lenses coming with scratch-resistant coatings, concern about scratching the lens itself may be reduced. However, concern remains for protection of the coating.

A large variety of lens coatings are possible: scratchresistant, antireflective, and mirror, for example. When the edger holds the lens tightly in the chuck during the edging process, excess pressure on small areas of the lens can damage the coating, which causes it to crack or peel. To reduce the likelihood of this happening, many use a protective tape disc, such as the 3M Blue Chip lens protector, on the back surface of the lens (Figure 7-15). These discs come precut in 35-mm circles on a large dispensing roll with nonadhesive tab areas for easy application and removal.

These tape discs protect the surface and are also said to help in reducing excessive, localized pressure during edging. That pressure may be the result of too many pounds per square inch of chucking pressure. Reducing the chuck pressure somewhat may help. But excess localized pressure results from use of the wrong base block for the lens being edged. Selection of the correct base block is important.

For extra protection from pressure on thin, antireflection coated lenses, it may be helpful to use the tape disc and an extra adhesive pad. First the tape disc is placed on the back side of the lens. Next an adhesive blocking pad is placed on the back side of the lens on top of the tape disc. The protective paper backing from the adhesive pad should not be removed. When the lens is placed in the edger, the pad further cushions the lens against points of localized pressure.

Lensmeter/Blockers

To increase efficiency, a natural evolution of any process is to combine steps. Blocking may be done as a step