Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 117

Most forming processes take place at a viscosity of 10²–10⁴ Pa⋅s. Hence, for soda‐lime‐silica containers, the glass needs to be cooled from melting and fining at ca. 1500°C and a viscosity of 10 Pa⋅s down to ca. 1050°C and a viscosity of 10³ Pa⋅s. This quite demanding task is accomplished in the forehearth. The forehearth is directly connected to the working‐end and ensures the required homogeneity of the glass while bringing it to the desired temperature and viscosity.

After the forehearth, a feeder enables glass‐portioning and gob pre‐shaping (Figure 7). It consists of a refractory tube and one or more plunger(s) that are moving periodically up and down. The tube is rotating to homogenize the melt in this final stage. With each upward stroke of the plunger, the glass stream is released from the shear blades in order to cut a gob without having a glass stream loaded on top of these shears. For a single‐, double‐, triple‐, or quad‐gob setup, the respective number of plungers operates simultaneously in the feeder, hence as many openings in the orifice ring are required. The final gob shape is influenced by the sizes of the orifice ring and plunger, and by the shape, height, and motion profile of the plunger.

Figure 6 (a–d) Narrow‐neck press & blow process, blank‐side.

Figure 7 Cross section of a modern feeder (double‐gob setup).

Source: Courtesy Bucher Emhart Glass.

The originally continuous glass stream is cut by the shears right after it has been “pre‐shaped” by the feeder and plunger and has passed though the openings of the orifice ring. The gob needs to be completely separated from the glass stream by the shears to prevent any glass fibers from being attached to it. Any misaligned or poorly operating shear will result in shear marks and, consequently, in defects in the final container. For shears, the materials most commonly used are steel (cheap, but short‐lived) and hard alloys such as WC (more expensive, but long‐lived). In all cases, the shears are cooled by a shear‐spray, a mixture of water and cooling fluids.