applied correctly, too low a viscosity will prevent the parison from maintaining its shape and, thus, correct dimensions from being achieved and the final container from conforming to its specifications. In glass‐container forming, this stabilization is realized, thanks to the surface layer of the parison that cools down through contact with the mold. Heat transfer thus is, in general, an important aspect in the forming of container glass. Not only large amounts of heat need to be removed from the glass, but heat transfer must be controlled locally to avoid internal tension that would build up if the shrinking rate were variable throughout the glass. The average heat transfer Q0 − t during the short contact period t between the glass and mold can be calculated according to
(2)
where m designates the mold and g denotes glass properties. With the aforementioned parameters, one, for instance, finds a very large average heat transfer of 647 kW/m2 for a typical forming cycle for which t = 6 seconds.
2.2 Interface Interactions in Glass‐Container Forming
Whenever a glass container is formed through contact with a solid material, such as a mold or roller, the interface between the two bodies is a crucial point. From the preceding presentation, it appears that problematic situations occur when the interface temperature is either too high or too low.
If cooling of the mold is not rapid enough in relation to the gob temperature, the interface temperature between the gob and mold increases and at a certain point the glass begins to stick to the mold. The sticking temperature has a lower bound at which the glass still can be separated from the mold without significant damage. Nevertheless, reaching this lower bound leads to process failure because sticking of the glass causes a bad loading and a inhomogeneous temperature distribution, which themselves give rise to defects in the final container. At the upper sticking temperature, removing the glass from the contact material inevitably leads to damages of the glass, like checks or torn‐out pieces.
The glass sticking temperature is widely independent of the type of the contact material. It is basically a function of the interface temperature TC, but surface conditioning of the contact material may play a role as well. Sticking appears when the viscosity of the glass at the interface becomes lower than 108.8 Pa⋅s [4, 5]. For an average container‐glass composition that means sticking begins to take place when the interface temperature TC between the gob and mold becomes higher than ~ 645°C.
The interface temperature of about 614°C calculated above is lower than the sticking temperature. However, it can easily happen in production that this temperature increases locally such that sticking of the glass does occur. This may happen because of changed cooling conditions, cooling failure, or the growth of an oxide layer on the molds, which significantly decreases thermal conductivity.
The friction coefficient μ between the glass and contact material plays a crucial role during forming. A low dynamic friction between the contact material and the glass favors a good gob‐loading and glass‐forming. Often the molds and the finish equipment are coated with a lubricant that decreases the friction. This so‐called swabbing process is widely used in glass‐container manufacturing. The swabbing lubricant mainly consists of graphite with various additives. Periodically, the molds are swabbed automatically by robot or by human hand to allow precise and stable forming. The swabbing intervals depend on the respective machine setup and container produced and can range between 15 minutes and several hours. The effect of swabbing on the friction coefficient is a temporary decrease in friction as well as a change in the heat‐transfer characteristics between the glass and mold.
There are also different permanent and nonpermanent coatings available that can be applied to the mold and forming equipment to extend or even avoid the swabbing. Important physical aspects of gob‐loading, including models of the mechanics of gob/blank‐mold interaction and of dynamical friction, have been extensively discussed in papers to which we refer for further details [e.g. 6, 7].
2.3 Deformation Rates in Glass‐Container Forming
As already pointed out, in container‐glass manufacturing, the gob is first formed into the parison, and then the parison into the final container. The (de‐)formation of both the gob and parison depends on the actual viscosity of the glass. A low forming‐ or interface‐temperature leads to a high viscosity at the glass surface. Hence the glass surface starts to get “brittle.” If such a glass is then subjected to high deformation rates, as it happens not only upon pressing and blowing but also earlier in the process upon gob‐cutting, it can experience too high tensile or shear stresses. The critical tensile stress σc (in MPa) that a hot soda‐lime‐silica glass can sustain at a given temperature T may be estimated from an empirically derived correlation [8]:
(3)
Hot fracture occurs if the tensile stresses exceed this critical value. The maximum velocity vmax at which a glass container with a thickness d can be formed at viscosity η without experiencing hot fracture can be approximated by:
(4)
One thus concludes that at temperatures of about 1000°C, deformation velocities of ca. 500 m/s are, for instance, possible without hot fracture for a 2 cm‐thick soda‐lime‐silica glass layer. At 900°C, the maximum allowed velocity is already down to 100 m/s and is lower than 10 m/s at 800°C. Below 700°C the risk of defects caused by hot fracture becomes significant. Because usually such defects cannot be inverted (“healed”) in later forming steps, care must be taken to prevent them from appearing.
3 Glass‐Container Forming Processes
3.1 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
