Encyclopedia of Glass Science, Technology, History, and Culture. Группа авторов
Glass Composition
For cost reasons, glass containers are made out of soda‐lime‐silica glass whenever special constraints do not apply. An exception is, for instance, laboratory ware for which borosilicate glass is used instead (Chapter 7.7). Even though the forming process needs to be adapted to account for the specific properties of each glass type, the principles at work are similar for given kinds of containers. Three different forming processes are applied, namely BB, PB, and NNPB. Their main differences concern blank‐side forming where the parison is made, the subsequent forming steps to make the final container being identical. As a matter of fact, many containers can be produced with more than one process so that there is much overlap between them in terms of product range.
3.2 Blow & Blow Process
The BB process is the oldest and remains still widely used in manufacturing of large and heavy containers such as wine or sparkling wine bottles. The gob is loaded into the blank mold, often via a funnel (Figure 3a). After loading, the mold is closed with a baffle and a settle‐blow is applied from above through the baffle (Figure 3b). The settle‐blow presses the glass gob deeper into the mold and down to the finish equipment, which basically consists of a neck‐ring, a guide‐ring, and a short plunger. On loading, the plunger is in upper position. In this very first step, the opening, sealing surface, and thread (if present) are thus formed before the bulk of the container itself, which represents an important difference with respect to the other forming processes.
In the current IS‐machines, the loading speed of the gob is often so high that the finish is already formed at the gob‐loading step, which would make the settle‐blow unnecessary for the finish forming. This step is nonetheless maintained in the process to guarantee a constant heat transfer between the glass and the blank, from cycle to cycle, before counter‐blow. During settle‐blow, a vacuum can be applied through cavities in the molds to support the parison and finish forming. Some modern BB IS‐machines work without a funnel. They control the switch between settle‐blow and counter‐blow by a valve in the baffle to exhaust the compressed air that is used for settle‐blow.
After settle‐blow, the baffle is quickly lifted, the funnel is removed, and the baffle settles again and closes the blank‐mold completely. A counter‐blow is applied from the down side through the formed finish, blowing the glass fully into the mold shape and forming the parison (Figure 3c).
This two‐step blowing process with settle‐blow and counter‐blow on the blank‐side causes an inhomogeneity in the container because of different contact times between the glass and the mold above and below the loading line. Such an inhomogeneity can be seen as a horizontal, optical streak in the body of the containers. It is called settle‐ or feeder‐wave and can be reduced in several ways, but not fully avoided. When looking at a final container, the existence or nonexistence of a settle‐wave thus indicates whether or not the container has been produced with the BB process.
Figure 3 (a–d) Blow & blow process, blank‐side.
After the parison has been formed, the baffle is removed (Figure 3d), the mold opens, and the parison is transferred via the invert mechanism to the blow‐side.
The final forming of the container is in principle the same for all three forming processes. The following description will thus apply to all of them. When transfer to the blow‐side is over, the parison lengthens into the blow‐mold as determined by its viscosity and the machine parameters. This causes the outside of the parison to be reheated by the heat stored in the hot inside and temperature to homogenize within the parison (Figure 4a). This reheat is essential to ensure that the container is to be precisely formed to its intended shape. The blow‐mold is then closed, leaving the finish outside. The invert‐tongs open and the container is released from the invert. Directly after releasing, the blow‐head with a blowing‐tube is placed on top of the finish (Figure 4b). Through this blow‐head, the final‐blow is applied, giving the parison the final shape of the container (Figure 4c). Strictly speaking, the reheat ends when the final‐blow is triggered.
After the container has been released by the blow‐mold, a take‐out grips it by its finish (Figure 4d) and places it over a dead‐plate through which air is blown from below to cool it further. Finally the container is transferred via a pusher onto a conveyor belt.
3.3 Press & Blow Process
The PB process is used for wide‐mouth containers of all weights, such as jars and baby‐food containers. It differs significantly from the BB process at the blank‐side. After loading the gob (Figure 5), the blank‐mold is closed fully by the baffle and a plunger presses in an upward movement the glass from below into the mold (Figure 5b). The plungers usually are made out of tungsten carbide (WC) or another hard metal to ensure a long lifetime. In contrast to BB‐forming, the process causes the finish to be formed at the end of the blank‐mold process (Figure 5c) and no disturbance such as settle‐wave is introduced into the parison because it is formed by a single (smooth) motion at the blank‐side. As in BB, the baffle is removed after the parison has been formed, the mold opens (Figure 5d), and the parison is transferred to the blow‐side to get its final shape as described above.
Figure 4 (a–d) Forming of the final container at the blow‐side (same for all processes).
3.4 Narrow‐Neck & Blow Process
The NNPB process is mainly used for lighter containers such as beer bottles, small water and juice containers, and other lightweight containers. It is the most advanced forming process because it yields not only the highest machine speeds but also a homogeneous glass distribution in the final container. It is similar to the PB process in that the gob is not blown into the parison but pressed via a plunger. After loading the gob (Figure 6), the mold is closed fully by the baffle and a plunger smaller in diameter than in PB presses in an upward movement the glass into the mold (Figure 6b). The plungers are also made out of tungsten‐carbide. As with PB, the finish is formed at the end of the blank‐mold process (Figure 6c). Again, the baffle is removed after the parison has been formed, the mold opens (Figure 6d), and the parison is transferred to the blow‐side.