cooling after blocking

[essegn]

By express jobs is often used the cooling of the lens on an external cooling plate because of saving time.
Could be the "faster cooling method" used for all material indices (1.498 , 1.60 etc ..) or the only for index 1.498 ?

[MikeAurelius]

Remember that my company works exclusively in glass, we use cooling rings on all metal blocking machines. It is a pretty simple setup: 5 gallon bucket, small water pump (from a water fountain), some vinyl tubing, and most importantly: two one liter soda bottles filled with water, then frozen.

Cooling plates work also with the same type of setup, no need for expensive "air conditioning" compressors.

The reason for cooling is not only for rapid production in the following steps, but also to stabilize the blocking metal (or wax) at a proper temperature so it does not deform in the generator chuck.

[essegn]

Yea, i forgot to mention that my question had been related to the plastic lenses.
Do you use any cooling time for glass lenses after blocking ? I am aware that it could vary from alloy to alloy (or wax)..

[MikeAurelius]

Yes we do. With a water jacket around the lens block (either the sintered metal block or the formed metal block). With the ice bucket setup, cooling generally takes about 1 minute. The lens then sits to air cool before production usually at least 10 minutes, and this is enough time to reach near ambient air temperature.

[Tito]

Another trick if you don't want to spend any time waiting for your alloy blocked lenses to cool off before generating , is to have some semifinished lenses in the most common base curves previosly blocked (ie 4 & 6 base) ,then emmediatley after calculation you use a pair of this previously blocked lenses and run them through the generator . Do not forget to block and stock back again a new pair of semifinished lenses,then you will always have cooled blocked lenses for every new job .

[lens_man]

I wouldnt keep my semi-finished lenses blocked for too long. Creates stress on the lenses b y being blocked for a long time

[MikeAurelius]

I wouldnt keep my semi-finished lenses blocked for too long. Creates stress on the lenses b y being blocked for a long time

Stress? Not so much. Even in plastic materials, there is very little stress imparted by long term blocking. What does happen though, is the blocking tape adhesion both to the block and the lens tends to degrade with time. This will occur especially with plastic lenses which have a very high coefficient of expansion, whereas the metal block is very low once chilled. The temperature of the plastic lens changes with room temperature, and over the course of a week the expansion/contraction can and does lead to a failure of the blocking tape adhesion.

[lens_man]

Hi
We have tried this with a vft and we do a callibration procedure every morning. We would block lenses the previous day so that when we get to work in the morning we can start the procedure without having to wait for the alloy to heat and stuff

We then cut a lens after callibration and test it with a master piece and see if it passes. Almost all the time with pre blocked lenses the test lens fails to pass

I thought this was due to stress. We block cool and then do the test it passes

[MikeAurelius]

Hi
We have tried this with a vft and we do a callibration procedure every morning. We would block lenses the previous day so that when we get to work in the morning we can start the procedure without having to wait for the alloy to heat and stuff

We then cut a lens after callibration and test it with a master piece and see if it passes. Almost all the time with pre blocked lenses the test lens fails to pass

I thought this was due to stress. We block cool and then do the test it passes

No, it is the difference in coefficient of expansion between the materials. I've currently got a glass calibration lens I use every couple of months to check thickness on my generator. It's been blocked now for almost 9 months and still solidly attached to the blocking tape and the tape to the metal (117 F).

I used to use CR-39 lenses, and suffered catastrophic deblocking during generation after just a couple of days. The only difference was the lens material. Tape the same, blocking metal the same, cast block the same.

In a manner of speaking, there is "stress", caused by the expansion and contraction of the lens material. It is localized only to the point of contact between the lens and the tape and the block. If you crib the lens down to just larger than your block, you will probably have longer lasting blocking for calibration lenses.

[Don Gilman]

We do, and have done for over 25 years sold pre blocked lenses with our proprietary disposable blocking compound. On AR lenses we use tape and non AR the old fashioned blue coating. I have had lenses on the block for over 5 years with no problem and no distortion. Minimal heat involved may be the key.