Chemical Strengthening Unit

[Jim M.]

I am looking for input on how to trouble shoot issues such as scratches, hairline cracks and pitting, which manifest during glass lens chemical hardening.

Thanks,

-Jim

[PRECISIONLAB]

When was the last time salts were changed..there might be contaminants causing the pitting and cracks. Verify unit temps as well.

[Jim M.]

Thank you,

Have you ever heard of issues with the basket’s springs causing scratches?

-Jim

[Rafael]

Check the baskets and talk with the operators. I have the same issue several times when my main operator was on holidays.
They must work more careful and you will have no scratches. Check also if the basket on loading has no vibrations, and if there is no hard salt on the edges inside the machine.
Issues with cracks i never had. I´m changing the bath once year, only refilling so the level is constant.Which temperatures are you running and how long are the lenses in the machine, and is your machine 24/7 turned on, and how many lenses/day, recepture?

[MikeAurelius]

If you are getting hairlines and/or cracking, there are two things going on here:

1. QC issues with the lenses going into the chemicals
2. Bad/old/contaminated salt in the bath.

Double check to see if the lenses going into the basket are absolutely free from defects. Use an inspection lamp in a dark box to look at the lenses for hairlines or generator pits. The liquid salt in the bath is much like water, it will seep into surface defects, and when removed from the bath, as it cools, they expand and crack the lens.

If you run hardening cycles every day, do not turn the unit off - leave it running 24/7/365. It's far cheaper to keep the unit running than it is to turn it on/off because of the time required to reheat the bath to melting point.

You can get new salts from Salem or (I think) DAC. To remove the old salts, use an aluminum or stainless steel dipper and pour the liquid salts into a metal (dry) bucket. Be sure to use high temperature long gloves, at 650 F, it's an immediate 3rd degree burn. Watch out for spatters as well. You need to get most of the old salts out of the tank, leaving no more than about an 1" at the bottom. Be sure to stir up the bath as you do this so that you get the scum off the bottom.

New salts are in powder form, and can be poured directly into the tank. You will most likely need to top off the tank with more salts once the original charge melts. Let the salts sit overnight to allow the bath to "settle", then it can be used the following day.

If there is a build up of cooled salts along the sides of the tank, use a long 18" or longer, rod and break the build up and allow it to fall back into the tank. The salts do "crawl" up the sides of the tank, and need to be chipped back probably at least once a month as they start to interfere with the free travel of the basket.

[Jim M.]

The hardening unit is on 24/7, its set to 450°Cfor crown and 400°C for photo; we run one batch of each (approximately twentylenses per a side) over night for sixteen hours. Handling could be a big culprit

[Jim M.]

The hardening unit is on 24/7, its set to 450°C for crown and 400°C for photo; we run one batch of each (approximately twenty lenses per a side) over night for sixteen hours. Handling could be a big culprit.