Glass-Forming Machine

any of the machines used in fashioning (forming) glass items through such processes as casting, stamping, pressing, rolling, drawing, and blowing. Forming is carried out in a viscosity range of 10²–4 ×10⁷ newtons-sec/m²(1 newton-sec/m² = 10 poises), which corresponds to a temperature range of 700°-1000°C.

To obtain sheet glass (window glass, glass used in display cases), glass-forming machines of the vertical-drawing type are used. These machines effect a continuous upward drawing of the molten glass, which is either drawn through the slot of a debiteuse (Figure 1) or drawn directly from the free surface of the melt. Drawing produces a glass sheet 2.5–3 m in width with a thickness of 2–6 mm. The sheet is transported upward by asbestos rollers through the machine’s annealing lehr. At the top of the lehr, the glass is cut and packaged. Glass with a 2-mm thickness can be drawn at a rate of approximately 120 m/hr.

Sheet glass is also formed by horizontal drawing and rolling. Plate glass is formed in a float process that uses molten tin. Here, molten glass is poured onto the surface of molten tin, where, through forces of gravity, surface tension, and drawing, the glass acquires plane-parallel surfaces. Glass with widths of 3–4 m can be formed at rates of up to 1,000 m/hr.

For the production of such items as glass bricks, structural shapes, water gauges, jars, bottles, and bulbs for lamps, pressing, blowing, and a process combining the two are used.

Figure 1. Glass forming using a debiteuse made of grog; (1) debiteuse, (2) melt

For example, widemouthed glass containers are formed in press-and-blow machines. Items are produced by these machines in two stages. In the first stage, a preliminary shape is pressed from a gob of molten glass, and the neck is formed. The preliminary shape is then blown with compressed air in the blow mold to the dimensions of the finished product (Figure 2).

Figure 2. Forming of a glass article in a press-and-blow machine; (1) deposit of the gob, (2) pressing of the gob into a preliminary shape, (3) and (4) transfer of the preliminary shape to the blow mold, (5) blowing the item into its final shape, (6) removal of the item to a conveyor

Narrow-mouthed containers are produced in glassblowing machines in which both the preliminary and final shapes are blown using compressed air; the forming of the neck is sometimes carried out in a vacuum.