Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 110

A basic feature of the forming process is that it is highly non‐isothermal. On the one hand, temperature differences over the dimensions of the glass component are present and on the other, the glass experiences a great change in temperature and thus in glass properties. As a result, the forming process has of course to be designed to cope with these changes, which are the largest for viscosity.

When the gob enters the mold in the first forming step, it 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 T_C, 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 C_p, 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, C_p ≈ 870 J/kg⋅K, and ρ ≈ 2500 kg/m³. For laminar cast iron, appropriate parameters are λ ≈ 55 W/m⋅K, C_p ≈ 500 J/kg⋅K, and ρ ≈ 7300 kg/m³. From these values, one can estimate T_C 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 Q_{0 − 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.