Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 111

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 T_C, 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 10^8.8 Pa⋅s [4, 5]. For an average container‐glass composition that means sticking begins to take place when the interface temperature T_C 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].