Encyclopedia of Glass Science, Technology, History, and Culture. Группа авторов
has a bulk temperature of about 1050°C. The mold itself has a temperature of 450–520°C at the end of the parison forming cycle, depending on forming conditions and container type. The glass–metal interface temperature TC, which is a very important parameter for the forming, is almost constant (Figure 2) because of the short contact time t of only a few seconds between the gob and mold material. It depends on the temperature of the glass T1, on that of the contact material (mold) T2, and on the thermal conductivity λ, heat capacity Cp, and density ρ of both the glass and the mold material [1–3].
For soda‐lime‐silica glass, at the relevant temperatures these values can be taken as λ ≈ 10 W/m⋅K, Cp ≈ 870 J/kg⋅K, and ρ ≈ 2500 kg/m3. For laminar cast iron, appropriate parameters are λ ≈ 55 W/m⋅K, Cp ≈ 500 J/kg⋅K, and ρ ≈ 7300 kg/m3. From these values, one can estimate TC with:
Figure 2 Temperature gradients and interface temperature between contact‐material and glass over time.
(1)
One finds in this way that temperatures of 1050°C for the gob and 470°C for the blank mold yield an interface temperature of ca. 614°C if no oxide layer resulting from corrosion of the mold is present and if the heat balance of the blank mold is correctly managed.
A certain cooling of the glass during the forming process is mandatory to achieve a stable enough product that does not lose shape in subsequent processes (handling, coating, etc.). If cooling is not 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.
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