APPEARANCE OF BUBBLES UNDER THE BLOCKED BASE CURVE
I have received calls about bubbles under the blocked base curve. These appear after the front curve is turned to final thickness. I have been told that the bubbles are more common on thin minus lenses.These calls got me thinking about the blocking and lathing process; blocking compound, dispenser, temperature, surface contamination, and cutting loads.I am aware of two kinds of bubbles: air bubbles within the blocking compound, and “blisters”, a separation between the base curve and the blocking compound. The air bubbles appear as hemispherical cavities in the blocking compound. The blisters appear as a separation of the base curve from the blocking compound. The difference in the appearance between these two types of bubbles can be determined by the edge of the bubble, a sharp edge indicates a bubble in the blocking compound and shallow edge indicates a blister caused by separation.
If air bubbles are present, I first look to the dispenser to see if it is working properly. Is it leaking? Is the reservoir low so that the dispenser is sucking air? Dispensers pull in the next charge of blocking compound with each cycle and create a slight vacuum. If the seals are worn, air can pass by them and become mixed with the blocking compound. If the dispenser is leaking, it needs to be serviced.I talked to Mike Fischer at Misupco. He advised not to hand mix the blocking compound as that entrains air. When they make it, they vacuum evacuate it so that there are no bubbles.If there are blisters present, there is a problem with adheasion. A few factors affecting adhesion are: cleanliness of the base curve, lathe cutting loads, lens thickness, and relative temperature of the blocking compound to the lens.Contamination on the surface will compromise the bond.
Bonded joints fail in shear and peal, very rarely in compression. Lathe turning imparts shear and to a small degree and peal (pulling) the material way from the bonded joint.
There are two basic cutting modes for the front surface, traversing and step. Traversing mode (common on three axis lathes) advances the diamond from the outside to the center imparting a side and torque load on the lens. The step (pyramid) mode advances the diamond toward the spindle in diameter increments imparting an axial and a torque load on the lens. This mode keeps the lens on the block better than traversing. This mode reduces the shear on the blocking compound because there is less side load, but the diamond is still pulling material away from the lens surface which the material has to resist.Lens thickness is a factor in the blistering problem. Techicians have reported that thinner lenses have more blistering. This may be caused by diamond “pulling” the lens material away from the blocking compound. The thinner the material, the less stiff the material is and the less support there is to resist the cutting action of the diamond. A possible fix may be to make the last two diamond passes very light, say .02 mm.
Let’s look at the relative temperature of the blocking compound and the lens.Traditionally, metal blocks were heated to bond to the blocking compound, but with plastic blocks now common, the heat from the blocking compound warms the block.
This is how heat conduction works: When you hold a metal object, it feels cool. This is because the heat in your hand is being conducted (carried away) into the metal. The metal stays cool until it is uniformly heated up. When you hold a plastic object, the heat is not transferred into the plastic and the surface of the plastic becomes warm, thus feeling warm.
When blocking base curves, usually the blocking compound is dispensed in the base curve. The initial drop at around 50-70 C lands and some of that heat is transferred into the lens material (plastic). A second later, the block makes contact with the blocking compound (not the base curve) and spreads it out to the diameter of the lens. So, at that time, the blocking compound and the lens material are closer in temperature. So, it begs the question: Should the lens be warmed to improve the bond in the center? This could be tested using an infra-red temperature sensor to make sure that the base curve does not cool too much before it is blocked. Then cut the front curve and see if the blistering occurs.This test or making lighter finish cuts may eliminate the blisters.
We will be happy to work with anyone interested in studying the effects of warming the lens to learn how that affects the bond.
As usual, manufacturing contact lenses can be an ordeal that many of us thought would be a smoother process, but we have to be vigilant.