Page 48 - Glass Machinery Plants & Accessories no. 3-2019

ANNEALING
Annealing

ery solutions for many appli- geous in some instances, it is an There is a necessary balance
cations of the glass industry, inconvenience in annealing. to maintain between compressive
equipment and systems from To explain the origins of stress, and tensile stresses. Imbalances
Vidromecanica come into oper- consider that glass is made up of create varying degrees of
ation everywhere – worldwide. a series of distinct layers, lying destruction in the glass thick-
Whether in the glass contain- parallel to the surface. Heat loss ness but if cooling is achieved
ers or tableware the decision from the glass depends on the sufficiently slowly, the layers will
for glass engineering ‘Made by transfer of heat from layers in not be submitted to disturbance
Vidromecanica’ is synonymous the body of the glass to those and stress generation will be
with safety for function and at the surface. These layers are avoided. All stresses created at
investment. interdependent and in different this point remain permanent, and
states of expansion. As the exter- annealing is important because
RESIDUAL STRESS nal surface cools faster, it reaches mechanical and thermal resist-
Permanent stresses its final shape while the centre is ance are conditioned by it.
There are two types of residual still in expansion. All the mol-
stress: permanent and temporary. ecules are intimately bound and Temporary stresses
Annealing is, in fact, a cooling their reciprocal action will cre- When being cooled below
process. Glass has poor tempera- ate compression stresses once strain point, glass can still be
ture conductivity and although the inner layers have cooled and submitted to thermal influenc-
this property can be advanta- contracted. es creating temporary stresses

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