Glass Sheet Edge Stress Control via Annealing Ring Dwell
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Solution Overview
Problem
Glass sheet forming and annealing processes face challenges in controlling edge stresses due to uneven heating and cooling, particularly at the peripheral edges, which can result in uncontrolled tensile and compressive stresses that are difficult to manage within specified manufacturing ranges.
Innovation Solution
A system comprising a furnace, forming station, conveyor, and annealing ring with controlled heating and cooling mechanisms, including a preforming end section with inclined rolls, a forming station with upper and lower molds, and an annealing ring with baffles and insulation, where the annealing ring is maintained under the heated upper forming mold for a dwell time to slow cool the glass sheet, controlling edge stresses through precise temperature management.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional heating and cooling methods are used for glass sheets, then the glass sheet can be processed through forming and annealing, but the edge portions experience uncontrolled tensile and compressive stresses that fall outside manufacturing specifications
Solution Approach 1:
The patent applies different cooling rates to different regions of the glass sheet. The central region cools at a faster rate while the edge portions cool at a slower, controlled rate. This is achieved through the annealing mechanism that maintains the glass sheet at the strain point temperature for a specific dwell time, allowing differential cooling that puts the peripheral edges into compression and controls tensile stresses in the central region, thereby resolving the edge stress control problem.
Solution Approach 2:
The patent implements preliminary heating of the glass sheet to a temperature above the strain point temperature before the annealing process. This preliminary heating ensures uniform temperature distribution throughout the glass sheet, particularly at the edge portions, before controlled cooling begins. This preparatory step is crucial for achieving the desired stress distribution and meeting manufacturing specifications.
2Productivity
If the glass sheet is cooled rapidly to increase productivity, then the manufacturing cycle time is reduced, but the edge portions develop uncontrolled stresses that exceed specification limits
Solution Approach 1:
The patent employs a dynamic cooling process where the cooling rate is not constant but varies with time and position. The glass sheet is held at the strain point temperature for a controlled dwell time (at least 3 seconds) before final cooling. This dynamic approach allows the central region to cool faster while maintaining controlled cooling at the edges, achieving both productivity improvement and stress control within specification limits.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The system effectively controls edge stresses by maintaining the glass sheet at the strain point temperature for a controlled cooling period, achieving compression in the range of 8 to 33 megapascals and tension no greater than 7 megapascals in a 20 mm peripheral band, ensuring consistent and high-quality glass sheet formation.
Implementation Method 1
an annealing ring of the system is movable under the heated upper forming mold and the formed glass sheet supported thereon to receive the formed glass sheet upon termination of the vacuum drawn at the upper forming mold
Implementation Method 2
the controller is configured to maintain the annealing ring and the formed glass sheet thereon in the forming station below the heated upper forming mold for at least three seconds prior to movement from the forming station so as to provide slow cooling of the formed glass sheet toward the strain point temperature and thereby control edge stresses upon final cooling to ambient temperature
Implementation Method 3
a source of vacuum for drawing a vacuum at the upper forming mold to maintain the formed glass sheet on the upper forming mold as the lower mold is moved downwardly after the press forming
Implementation Method 4
each side of the lower forming mold has a pair of electrical heating elements at lower locations below its crossbar where it is supported on its vertical stem, and the forming face of the crossbar of each side has three grooves and three electrical heating elements respectively received within the three grooves
Data Source
AI summary
A glass sheet forming and annealing system disclosed provides control of edge stresses by maintaining a press formed glass sheet on an annealing ring (72) below a heated upper forming mold (58) within a forming station (12) for slow cooling toward the glass strain point temperature.