Heat Soak Oven 

Nickel Sulfide(NiS) has been known to be the cause of spontaneous burst in tempered glass. During tempering the nickel sulfide is transformed to the high temperature α-phase(T>380^oC) and has no time to return to the β-phase on quenching. With the passage of time NiS slowly inverts to the low temperature β-phase with an increase in volume of about 4%. This volume expansion may cause glass breakage.

The Heat Soak  is a destructive treatment which heats the glass for several hours at about 280 °C to speed up the α-NiS to β-NiS transformation and consequently forces the contaminated glasses broken within the oven.

Standard HST requires more than ten hours thermal treatment including 1 to 6 hours to heat large quantities of glass up to 260-300°C followed by a temperature plateau at these temperatures for 3 to 12 hours before cooling down slowly to ambient temperature. The time necessary for 99.9 % α to β-NiS transformation is less than 1 hour at 280 °C. For comparison, more than one hundred hours become necessary to complete the same transformations at 250 °C, i.e. only 30 degrees below!

The average temperature difference is about 30 degrees between the hottest and the coldest glass during Heat Soaking within most ovens in use. 

JGE heat soak ovens use the most advanced technology. Optimal pattern of  hot air circulation  insures homogeneous temperature distribution in the oven. Temperature difference is less than 10 degrees.

Heat Soak Process System

The heat soak process system consists of:

• The oven

• Glass support

• Separation system

The Oven

The oven shall be designed and constructed in such a way that it shall allow an unhindered air circulation around each glass pane. Even in the event of glass breakage the airflow shall not be hindered. This will require a careful design of the openings for the air ingress/egress so that fragments of broken glass will not cause blockages.

Glass support

It is very important to lay down exactly the way the glass is put up inside the HST oven. The glass stack shall be aligned with the heating air flow direction.

Separation system

The distance between the single glass panes in the stack combined with the alignment of the glass stack are the most important parameters decisively determining the quality of the test.  

The separators shall assure optimised airflow between the glass panes without hindering the airflow themselves. A minimum separation of 20mm is recommended. It goes without saying that glass to glass contact is not allowed! Distances between the glass panes of less than 20mm will not only badly influence the heat exchange but also increase the risk of breakage.

The latter will certainly be the case when the oven is fully loaded as huge temperature differences can occur between 2 points in the same glass pane.

Heat Soak Process System Calibration

The calibration test determines the method of operation of the heat soak process system during manufacture of heat soaked thermally toughened soda lime silicate safety glass. In other words, the calibration shall determine the heating phase of the process, glass separation distance, the positioning, material and shape of separators, the type and positioning of stillage(s) and define the operating conditions for use during manufacture and this for both 100% and 10% load. Thus the calibration test has to be done twice in order to check whether the heat soak process system is capable of meeting the time / temperature regime at both 100% and 10% load.