Encyclopedia of Glass Science, Technology, History, and Culture. Группа авторов
breakage. Containers are thus reheated in the glass transition range in a continuous‐annealing lehr long enough to ensure complete stress relaxation. The annealing times depend on the size of the containers, but are typically between 45 and 60 minutes. The containers can then cool down to room temperature homogeneously.
7 Cold‐End Handling and Inspection
When they leave the annealing lehr, containers have a temperature between 80 and 120°C. They are coated a second time in a spray process. The main purpose of this cold‐end coating is to protect the container against scratches upon further handling and later at the filling line. Usually a polyethylene wax is used to decrease surface friction, hence making the containers less susceptible to scratches when being handled or touching one another. The cold‐end coating also serves to provide a surface suitable for good adhesion of the label, which is usually fixed at the filler.
After application of the cold‐end coating, another inspection takes place in a highly automated way such that additional inspection by human eye is applied only in rare cases. Most inspection systems are based on various sophisticated optical systems, each checking for a certain type or group of potential defects that include flaws, scratches, cracks, blisters, seeds, loose or stuck glass particles, or dimensional errors of the container. Because special attention is paid to the finish of the container, mechanical inspection systems are, for instance, applied for testing that it is free of obstacles. Finally, random checks are made offline, e.g. to check that the strength exceeds definite values that depend on the kind of container. For impact strength, usually a minimum lot‐size of 30 containers is tested on a shift, daily or some other regular basis, depending on the container made and plant procedures. The container is hit by a pendulum of a certain weight – depending on the chosen specification and domain of the container – in a well‐defined matter until it breaks. As for the burst pressure strength, which is especially relevant for carbonated liquids such as sparkling wine, water, and champagne, the container is filled with water and its pressure is increased until breakage. Also here, due to the Weibull‐characteristics of brittle materials, a minimum of at least 30 containers should be tested. After inspection, the containers are pelletized, wrapped, and prepared for shipment.
8 Perspectives
With the introduction of PET and other plastic containers, the glass industry has experienced a severe loss in market share for certain types of products such as juice, water, and milk. After years now the market seems balanced and the container‐glass industry could maintain a quite stable production level of ca. 65 million tons worldwide, which amounts to ca. 200 billion containers in 2012.
Recently, however, disadvantages of plastic containers have been recognized. Because of the leaching of endocrine‐active substances, phthalates and other substances into the container content [10, 11] and the dispersion of plastic waste on land and especially in the oceans [12], glass might even be able to gain more acceptance from the consumer. Here, glass can play out it strengths as, among other positive aspects, it is fully inert and can be 100% recycled. Furthermore, glass in a landfill does not pose any threat to the environment as it degrades to the components it was made of.
Nevertheless, glass has also its well‐known drawbacks and a positive future of container glass strongly depends on the capabilities to overcome them. First, there is the fact that glass containers are energy‐intensive to produce. Emission control, CO2‐trading, rising energy costs, and other environmental regulation force the glass‐industry to push limits farther (Chapter 9.7). As processes are already highly optimized, there are no more “low‐hanging fruits.” Although the effort that has to be made financially and risk‐wise increases exponentially with the possible benefit that can be gained, the container‐glass industry is aware of the need of innovations. The increasing number of batch and cullet preheaters or the latest concepts to allow heat recovery for oxy‐fuel fired furnaces illustrate the kind of efforts made to keep shifting the limits.
Other weak points are the fact that glass containers are brittle and mostly regarded as heavy. In specific instances, however, weight is considered a positive aspect of glass packaging. For example, think of the smooth texture, the reassuring heft, and the feel of value when lifting a glass bottle of wine or perfume. Nevertheless, many investigations have been carried out to decrease the weight of glass containers while increasing their strength. Coatings based on silica‐sol‐gels (Chapter 8.2) can, for instance, increase the strength of glass containers by 20% or more.
Besides, most impressive are current developments which aim at thermal strengthening of containers. After forming, the containers are reheated and then cooled from both inside and outside much faster and in a much more controlled way than in an annealing lehr. In this manner, compressive stresses at the inner and outer surface are introduced, which significantly increase the strength of the glass so that containers can be light‐weighted further or be reused. The key to this development is to control the cooling very precisely and to adjust the balance between compressive stresses at the surface and tensile stresses in the bulk of the container (Chapters 3.7 and 3.12).
Innovations are also targeted in the field of glass‐contact materials. Coatings for enabling a full non‐swabbing production of all types of containers are investigated, but this task is still unsolved. Furthermore, it is highly desirable to avoid all lubricants that are currently used in shear‐, trough‐, and mold‐lubrication. This would allow a full “dry‐gob” delivery that would give considerable advantages over current process.
Figure 9 A modern pneumatic‐controlled 12‐section double‐gob individual‐section machine; on the left, the conveyor belt evacuating the newly blown bottles (courtesy Bucher Emhart Glass).
Figure 10 Parison just before blow‐mold closing and final container on the blow‐mold‐side.
Source: Courtesy Bucher Emhart Glass.
Concerning the IS‐machine itself (Figures 9 and 10), a significant improvement would be to ensure a more precise and controlled forming process and, hence, a more homogeneous wall‐thickness distribution in the final container. This could, for instance, be achieved by a direct gob‐loading without the need of a delivery or by a forming process, which would allow a more precise reheat to have a more homogeneous final container forming. Because cost competition with alternative packaging is one of the biggest drivers and deciders for innovations, however, all the aforementioned approaches and concepts will have to distinguish themselves not only by their technical feasibility but also by their economic efficiency.
References
1 1 Carslaw, H.S. and Jaeger, J.C. (1986). Conduction of Heat in Solids, 2e. Oxford, UK: Oxford University Press.
2 2 Rieser, D., Manns, P., Spieß, G., and Kleer, G. (2004). Investigations on sticking temperature and wear of mold materials and wear of coatings. In: Advances in Fusion and Processing of Glass, Part IV (eds. J.R. Varner, T.P. Seward and H.A.