PC chips are like "tough guys" in plastic, they are not easy to break when smashed; Resin sheets (such as the common CR-39) are like "soft girls", lightweight but not resistant to drops. The "personality" of these two materials directly determines whether they can handle the work of multi-point defocusing lenses.
2、 Why is it necessary to use PC lenses for multi-point defocusing lenses?
Wear it for children, safety comes first
Multi point defocusing lenses are mainly used for teenagers, who are prone to bumping into their glasses while running and jumping. The anti drop ability of PC chips is 10 times that of resin chips - imagine if the same thing falls off a table, the resin chips may crack and the fragments may scratch the eyes; At most, the PC chip has some scratches and is unlikely to break.
Just like choosing a water cup for a child, parents will choose a plastic cup that won't break, rather than a glass cup. The safety standards for children's lenses are very strict in various countries, and PC lenses are the only material that can steadily pass the "drop ball test" (a 16 gram steel ball falling from 1.2 meters high), while resin lenses often fail.
The 'small bumps' on the lenses can only be done well with PC lenses
There are hundreds of tiny "micro lenses" on the surface of multi-point defocusing lenses (used to control the development of myopia), and the curvature error of these small bumps cannot exceed 0.01 millimeters (thinner than a human hair).
When processing these small lumps, the high-speed rotating blade of the machine will generate heat through friction. PC chips are resistant to high temperatures (only deform at 120 ℃), like heat-resistant iron pots, and are not easily "burned out"; The resin sheet softens at 60 ℃ and is easily scratched by the blade, resulting in small bumps that are crooked and affect the effect.
Moreover, PC chips have smooth skin and are less likely to leave rough edges when the blade is cut; Resin chips are too "brittle" and easy to break when cut, just like cutting potatoes with a knife. If the potatoes are too hard, they will fall off.
Thin and light to wear, children won't resist
Multi point defocusing lenses have small bumps and their edges are slightly thicker than regular lenses. The "refractive index" of PC sheets is higher than that of resin sheets (1.59 vs 1.50), just like glasses of the same degree, PC sheets can be made thinner.
For example, for children with 600 degree myopia, the edge of the PC sheet is about 6.5 millimeters thick, and the resin sheet is 8 millimeters thick. Wearing it will make the nose bridge even more painful. Children do not love glasses in the first place, and if they are too heavy, they are more likely to take them off, which affects the effectiveness of myopia prevention and control.
3、 Why is the stress on PC chips greater than on resin chips?
Stress "can be understood as the" pent up force "of a material. The PC chip itself is relatively "hard" (with a high elastic modulus), like a spring that is pinched tightly, and has a strong rebound force when released; Resin sheets are relatively "soft" and have less strength.
More importantly, the small bumps on the surface of the lens make the PC chip "unevenly stressed" - the flat and raised areas "compete", just like a patch sewn on a piece of clothing, and the edges of the patch are always slightly wrinkled. The surface of the resin sheet is flat and uniformly stressed, so the stress is small.
But don't be afraid of this "force". The factory will use methods such as "slowly cooling down" and "rounding corners" to make it "breathe a sigh of relief". In the end, the stress value is far below the safety standard, and it is completely safe to wear.
4、 Why can't we use resin sheets to make multi-point defocusing lenses?
1. Too easy to break: When children wear resin sheets and hit them while playing or running, the lenses may crack into spider webs, and the fragments may harm the eyes.
2. Unable to produce qualified "small bumps": either deformed by machine grinding, or the edges cracked, causing the lens to deform when looking at things, which in turn affects vision.
3. Wearing it for a long time can lead to "weight gain": Resin tablets tend to absorb water (like sponges), causing sweat in summer and shortness of breath in winter. Moisture can seep in and deform small bumps on the surface, thus reducing the effectiveness of myopia prevention and control. PC chips hardly absorb water and remain unchanged even after wearing them for several years.
Of course, not all resin sheets are not suitable. For example, the newly released TPX resin sheet has performance similar to PC, but its price is three times that of PC sheets. A pair of glasses may cost thousands of yuan, which is difficult for ordinary people to accept.
Choosing PC lenses for multi-point defocusing lenses is like choosing professional hiking shoes for mountaineers - they should be drop resistant, comfortable, and able to handle complex road conditions. Although PC chips may be a bit stubborn (with high stress), they can be solved through technological means; Although resin sheets are gentle, they cannot do this job. That's why PC lenses have become the "best partner" for multi-point defocusing lenses.
