Page 49 - Glass Machinery Plants & Accessories no. 3-2019

and distortions that disappear annealing point. The amount of ment of the glass-making pro-
slowly. However, unless cooling time glass is maintained at this cess – more complex than it may
is controlled, unbalanced stress temperature is determined by first appear. Consequently, thor-
levels could lead to breakages. time-thickness calculations. ough knowledge of the process
Of course, controlled and strong Precisely controlled cooling is is becoming increasingly neces-
cooling to generate high stresses required during the annealing sary. A number of scientists have
makes toughening desirable in phase until the strain point is developed theories and estab-
some instances. Increasing the reached. The cooling gradient lished rules that are relatively
compression stress on the sur- is given by calculations depend- easy to use, including Adams and
face gives the glass high mechan- ing on glass thickness and type. Williams (widely used), Shand
ical and thermal resistance. Last but not least, controlled, (Corning Glass); Owens (the
fast cooling is necessary to bring most widely used and easy-to-
ANNEALING CURVE the glass to ambient temperature. use curves); and Redstone and
Passing through the tunnel This process must also be main- Stanworth (mainly used for opti-
of a continuous annealing lehr, tained within calculated toleranc- cal glass).
glass follows a fixed temperature es to avoid breakages resulting Neutral annealing, avoiding the
gradient that is necessary to pro- from temporary stresses. creation of stresses, is not neces-
duce the desired level of stress. sarily the ultimate goal to attain.
Glass must be reheated or cooled CONCLUSIONS In certain instances, the intro-
to assure homogeneity above the Annealing is an important ele- duction of controlled stresses to

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