"Cold Stamp for a Hot Product"

By John Robertson CEO InfoSight Corporation , Chillicothe Ohio

InfoSight specializes in product identification where conventional marking techniques such as adhesive bar code labels just don't work. "We Barcode Difficult Stuff" is our charter. We are currently developing a method to mark high quality unique bar codes on red hot (1000 deg. F) picture tube glass. Once this automated marking technology is implemented in production , it will be possible to "track" and create a database for each individual piece of glass from the mold through testing and inspection all the way to the shipping pallet.

Meanwhile , our customer has an immediate need for an inspection stamp; a different one for each inspector. Such a stamp would be used to determine who has inspected what . "Surely ", we thought , "the customer could find a simple inspector's stamp-- this isn't our forte" . But we were told that the customer had unsuccessfully tried to locate a suitable stamp for over 20 years ! Helping our customers is of course in our best interest --and fun --so we took up the challenge.

The stamp had to meet the following specifications:

• White mark on dark (non-porous) glass.

• Several stampings must be made in sequence without re-inking (skid load)

• Stamp at ambient temperatures.

• Dry to handle within 1 minute.

• Mark must survive multiple lehr oven passes (at 900 deg. F)

• Mark must survive several acid washings

The ink was the easy part for us. Over the years we have developed many high temperature coatings for the ceramics , glass and steel industries . Many are water based and unsuitable for this application because they will not dry quickly ; especially on a humid summer day! Others are solvent based , dry quickly , and are capable of surviving more than 1800 deg. F.-- we chose to use those.

If one tries to use a standard (non-porous) rubber stamp to pick up solvent based , high temperature capable pigmented ink from a porous pad , you find that the stamp borne ink is depleted after the first stamp. In this application ,the inspector needs to mark up to a full skid of glass parts without re-inking. Our attempts to utilize a ink soaked pad within a standard self inking stamp failed because the standard pad was not tightly sealed ( dried out over a long weekend) . The sealing problem could ,of course, be solved but there were bigger problems: the special high temperature inks are sticky and adhere tightly to the pad but they are repellent to the rubber stamp. Not a good combination for a stamp!


Our solution was to retain the ink within a soft , saturated open pore sponge material. The sponge was first laser imaged using a CO2 laser to remove and seal off the non marking areas of the individual inspector's stamp message. Once saturated , the sponge is capable of doing several hundred stamps before requiring the inspector to (squeeze bottle) refresh it's ink load.

When the inspector has completed his stamps , he or she must place the stamp in a cup which prevents the sponge from drying out .

Fig. 1- The retracted laser cut foam pad stamp.


The foam stamp is placed within a cylindrical PVC holder which has a button on top. When the button is depressed by the inspector's thumb, the foam pad advances to touch the glass surface and the desired stamp mark is produced. Of course the same device could mark a wide variety of non-absorbent surfaces.


Fig 2- Inspector's thumb extends the pad.


Fig 3- Resultant stamp mark on glass

We have tried this stamp with a wide variety of non-standard inks. For example the stamp has been used to apply a laser darkenable white patch , and ceramic pastes.


The inspection stamp problem is solved -- now back to barcodes on red hot glass!