Gypsum products such as wallboards, partitions, tiles, ceiling panels and the like are widely used in building construction. These products are prepared from slurries in which calcined gypsum (calcium sulfate hemihydrate) is mixed with water and other ingredients. A wallboard is a gypsum product widely used in building construction and it is typically manufactured by formulating a gypsum slurry and sandwiching it between two sheets of paper on a wallboard production line.
A slurry mixer is typically used for supplying agitated gypsum slurry to the wallboard production line. Several types of gypsum wallboard manufacture are described in co-assigned U.S. Pat. Nos. 6,494,609 and 6,986,812; both of which are incorporated by reference.
A mixer is provided for uniformly dispersing calcined gypsum into water to form a slurry and to mix the slurry with other ingredients such as for example foam, and then the slurry is cast into a desired shaped mold or onto a paper surface or some mat to allow the slurry to set and form hardened gypsum by chemical reaction of the calcined gypsum with water. A lightweight gypsum product is produced by uniformly mixing aqueous foam into the slurry to generate air bubbles. This results in a uniform distribution of voids in the set gypsum product when the bubbles are confined in the slurry before the gypsum hardens.
During production on a wallboard production line, gypsum slurry is distributed from a mixer onto a first sheet of paper, called the facer, which is fed from a roll onto a conveyer belt. The slurry is spread over the facer and a second sheet of paper, called the backer, is fed from a roll on the gypsum slurry, and the sandwiched gypsum product continues to move on the conveyer belt. The sandwiched gypsum product is formed into a panel which then continues to move on the conveyer belt until gypsum is set enough for the panel to be transferred into a drying oven. The panel is then cut to size into a wallboard. When slurry is positioned on the facer, it should be distributed as evenly as possible between the central portion of the facer and its edges. The width of the gypsum panel should be also either constant or have only negligently minor deviations from the preset value. Many factors affect the quality of gypsum slurry distribution, including such as a temperature at which the production line operates, a composition of the gypsum slurry, the source of water and other ingredients in the slurry, paper used as a facer, paper used as a backer, and humidity. In some production methods, gypsum panels are trimmed on each side to produce a wallboard with perfectly straight edges. However, this method is very costly and generates a lot of waste.
PCT publication WO 2000/012963 describes a method and apparatus for detecting the edge angle of a gypsum board. The system comprises a light source and camera and requires pictures to be taken for obtaining information about slurry distribution in a gypsum board. However, this method requires several steps to be performed. For example, a picture should be taken, before the edge angle can be analyzed. Thus, there remains the need to constantly measure and monitor the edges and width of a gypsum panel during production.