Color Forehearth Design
The primary function of the forehearth is to thermally condition the glass as it flows from the refiner/distributor to the forming machine. Typically, there are two more sections to the forehearth referred to as the cooling and conditioning (equalizing) sections. Today, typical forehearth channel widths vary from 16-48 inches; glass depths may vary from 6-12 inches.
In a color forehearth, an additional length of forehearth is added to allow feeding, melting and blending of the color, prior to the cooling and conditioning of the glass. This additional length is typically 12- 20 feet. The coloring section has three zones:
- Preheat zone: refiner to feed tube.
- Melt zone: feed tube to first bank of stirrers.
- Stirring zone: first bank of stirrers to last bank of stirrers.
Several parameters must be addressed to ensure proper conditions are achieved in each zone.
The preheat zone must have sufficient firing capacity and control to achieve and maintain proper glass temperatures to allow proper melting of the color concentrate. This often requires the use of double stack burners.
The melt zone must be long enough to allow sufficient time for the color to melt and de-gas on the glass surface prior to stirring. Methods for directing and exhausting combustion products from the melt zone (such as vents) must be evaluated. Sufficient firing capacity and precise control of combustion ratio is necessary to maintain excess oxygen in firing atmosphere as the color is melting.
The stirrer zone must be long enough to ensure that the melted color is properly blended into the molten glass without creating color streaks or seeds/blisters. Referred to as “stirrer capacity,” it is controlled by glass depth and the total number of stirrer banks and stirrers in each bank.
Other considerations include the placement of temperature control points (thermocouple or optical) and the location of "peep" or view holes in the forehearth wall that allow inspection and evaluation of the color melting characteristics during production.
The exact design and layout of a color forehearth will vary depending upon a number of design variables. These include:
- Desired glass pull
- Desired colors
- Glass depth limitations
- Space/layout limitations