TV Panel Glass Forming and Annealing Process Model

by John H. Chumley, Techneglas, Inc.

Introduction
This article describes the modeling technique used in forming a TV glass panel from the formation of the droplet of molten glass (gob) delivered to the forming machine (ELP) through the pressing and cooling operations plus the annealing oven (lehr). To assure the continuity of the solution, the model also tracks the return of the empty mold parts to their initial position on the ELP. The complexity of the problem and variety of solution domains involved requires multiple solution techniques. The basic strategy is the sequential solution of individual finite element models to determine the temperature history. That thermal history is then used as input for the structural solution of the glass including residual stress calculations. The intent of the model is to be of sufficient detail and robust enough to accurately predict the condition of the actual product during and after this complex operation, using only readily observable forming parameters as input. One way coupling is achieved is by taking the ending temperature of each component from the previous time-step it participated in and using that as the starting temperature for the current time-step. Since the process is cyclic, the process is allowed to loop back upon it until a quasi-steady state temperature is achieved. New heat energy is input to each cycle as a fresh gob. Film coefficients, which are only weakly coupled to the temperature of the mold equipment, have been calculated offline using the ANSYS/FLOTRAN CFD code. Since the actual filling of the mold takes only a small percentage of the process time (occurs above the glass temperature), it was also modeled offline using the DeForm finite element code. The pressing model results are only used to map the temperature drop during the pressing operation from the gob to the fully formed panel. The gob’s initial temperature distribution is taken as a given for this analysis.

PROCESS and MODEL DESCRIPTION

The forming process is carried out on an 11-station carousel that indexes two stations with each gob drop. (See Figure 1).

Figure 1 - Panel Forming Machine (ELP)

After two complete revolutions of the carousel, each mold returns to its original position, the mold that started at station one is ready for the next gob. At station one the gob is dropped into the mold; at station three the panel is pressed out. At stations 5, 7, and 9 the formed panel is cooled by air blown over its inside surface. The shell ring restrains the panel during this time. At station 11 the shell is removed. The mold coasts through stations 2 and 4 without any action. At station 6 cooling air is again blown over the inside surface of the panel; this time the sides of the panel are no longer restrained by the shell ring. At station 8 the panel is removed from the mold and carried toward the annealing lehr on a conveyer belt. At station 10 the shell is returned to the empty mold before it indexes back to station 1. For the purpose of modeling, the process was divided into nine sub-models (Process 0 through Process 8), one for each combination of glass and mold equipment. They were used to calculate the temperature history. A tenth sub-model (Process 9) of the panel only was run using temperature data from Process 1 through Process 4 as input for a structural analysis of the panel using the ANSYS VISCO89 element type.

As stated above, the film coefficients for air-cooling are calculated in a supplemental CFD code. The pattern of the film coefficients is determined by the geometry and flow rate of the air but is not strongly influenced by the actual panel temperature. Therefore, for a given experimental setup the film coefficients at each location can be represented as a polynomial in T and v0 and calculated in real time for each node as required. There are three separate CFD models used to calculate film coefficients for the forming operation. The film coefficient for the plunger has been estimated from process data. Shown below are the some figures, which illustrate the CFD calculations.

CFD Calculations - CFD mesh, 2-Vsum Plot, 3-Film coefficients map, 4-Flow Vectors

Likewise the mold filling during pressing was calculated using a supplemental program DeForm. Since the pressing operation is only a small part of the total cycle and occurs above the glass transition temperature, it was only necessary to extract the temperature difference from the gob to the finished panel to link the process to the overall model. This task was accomplished by doing a particle trace (backwards in time) in the DeForm code from the panel node positions and calculating the delta temperatures for later use in the ANSYS process model. See Figure 4 below.

Figure 4 - Deform Model of Panel Filling

RESULTS

Some results of the model are shown in figures 5 through 14 (see below), which display the ending temperatures for selected temperature calculation processes and the one residual stress calculation plot.

Figure 5 - Process 0	Figure 6 - Process 1	Figure 7 - Process 2
Figure 8 - Process 3	Figure 9 - Process 4	Figure 10 - Process 5
Figure 11 - Process 6	Figure 12 - Process 7	Figure 13 - Process 8

LIMITATIONS AND FUTURE WORK

Future work will include the use of Discrete Ordinate Method (DOM) radiation analysis capability for calculating heat transfer by radiation within the glass at elevated temperatures. Also, a model of the gob formation process including initial shape and temperature distribution will be added. In the current model radiant heat transfer within the glass is approximated using the Rosseland formulation, which is known to overestimate the effect near the surface of a semi-transparent medium, such as glass. For this model an idealized gob shape with a uniform (center to edge) temperature was assumed. An improved gob model will improve the initial temperature distribution for the overall model as well. Work has begun, and will continue, using the results of the model to refine the forming process to produce desirable changes in the product, especially as they relate to controlling residual stress in the finished panel.