Mini II NV coater problem

[DragonLensmanWV]

Every now and then, with no discernible reason (low curves/high curves) our coater will produce a roughly circular pattern in the center of poly lenses. Other lens materials don't get this problem. The circular area is about 10 mm in diameter and looks like the lens was not fined properly, leaving a dimple in the center, that was not there before coating. Additionally, if you clean the back of the lens, it appears the affected area is not dulled by the IPA wash, that is the very center looks "more polished" than the rest of the lens.If this shows up, the lens is lost, nothing will remove it and nothing will coat over it.
Anyone run across this and have a fix?

[mshimp]

Seems that not all of your poly are fined and polished as well as they should be. The problem is not your coater. Check generator curves, lap curves, and type of pads used, and plenty of clean water put on fining lap before putting the lens on the tool.

[DragonLensmanWV]

Actually, this has been going on for a couple of months. Last week, I installed a new diamond cutter and recalibrated all the curves. And very close inspection of the lenses prior to coating reveals no imperfections. The part that throws me is that it favors no particular curves. I thought that maybe the lens was entering too deeply into the cleaning tub and was being rubbed against by the clean or air nozzle, but it does it on steep curves too.

[cmhargrove]

The next step would be to check the timing and stroke pattern on your cylinder machines. Also, Poly tends to "build up" on the pads during fining. Any chance that the pad still has significant central build up before polishing?

Have you made any other changes on the cylinder machines? Pressure, time, pads, polish?

[DragonLensmanWV]

Since I replaced the diamond, there's almost zero pad loading. We have tried using high speed fining, slow speed fining, high/low polishing. And I say again, there is no defect in the central area of the lens prior to coating.
Heck, we've even deliberately chucked the lens off center in the coater and the defect always shows up in the center of where it's chucked. We get a good pattern on the cylinder machines (5056 Coburn). The lenses that fail are of all thicknesses so there's little chance the lenses are being distorted by vacuum.
Also, this happens very rarely, but it's a big mystery to me as to why it happens.

[braheem24]

Check your rotation speed at the end of the cycle, it may not be rotating fast enough to build inertia to distribute the coating on the center evenly.

[DragonLensmanWV]

Hmmm. Might have to round up a laser tach. Is the speed adjustable or might I need a new spin motor?

[braheem24]

I would check with tech support, I dont know much about the machine that was a " I slept at a Holiday Inn Express answer.

[Groundrunner]

The spin speed is adjustable. In the upper left hand corner behind the control panel are the adjustment screws. I can't recall off the top of my head what the speeds are supposed to be. You can call ultra optics and get that from their tech support. Shannon or chad would be the techs I would recommend talking too. You can also send them in lenses that have the problems and they can run some test on them to help try and figure out the issue. Good luck getting it fixed, random Coater issues can be a big pain.

[Speed]

Ultra has great support. They loaned me a tach once when I was having a problem.

[James Handgrinder]

We had something similar before: good poly lens came out with un-polished marks all over the lens after hard coating. It has been a while, I forgot exactely how did we fix the problem, in my vague memory it was the coating solution/spin speed combination.

[PRECISIONLAB]

Have you checked the coating height? The stream of coat should be above the bowl. While checking have the machine cycle quickly to determine if the pump can maintain a steady stream. If not, there might be an issue with the consistency of the coating pump. Also keep in mind the viscosity can change if the coating has been exposed to moisture or the ambient air temp has change in the lab. I would avoid adjusting the spin speeds since this can change the final coat thickness.

[fjpod]

I presume you have done a 10 second test on your finer? Is the lens fining in the center last?

[mshimp]

We had a problem just like this. A power surge changed some of the curve configurations in our SGX. A technician from gerber, at that time, had to go to a hidden menu and re-input the values needed to cut properly again. Tell us the update in your case.

[braheem24]

from the dragon's mouth.

very close inspection of the lenses prior to coating reveals no imperfections.

[fjpod]

from the dragon's mouth.

mmmmm...that doesn't necessarily mean there aren't any.

[DragonLensmanWV]

Talked to Todd at Ultra. I bought a non-contact tachometer to check speed. Seems like ours was not spinning slow enough when the coating fountain was active, it just kept spinning at 2000 RPM. So I got that reduced,also had to adjust the angle of the clean/dry nozzles that were not moving far enough forward. Now (still) my problem is getting a coating fountain high enough. I thought the pump was too weak, but it appears my coating is thickening up sooner than it should. So I'm going to switch to the UO coats and hope I can deliver enough material to the lenses. I sent him up a few lenses so he can check the thickness of the coating.
Now I'll have to be out of the coating business for a couple of days.

[James Handgrinder]

Glad you got it!
Usally thicker coat can cover up fairly deep scratches. you don't want the coat be too thick , but if you are not a/r coating after, the tolerance is bigger I guess.