Encyclopedia of Glass Science, Technology, History, and Culture. Группа авторов
a 12‐section IS‐machine with a triple‐gob setup producing 240 containers per minute is running a lower cavity rate than a 10‐section IS‐machine with the same triple‐gob setup producing the same number of containers per minute. Highly efficient IS‐machines can go up to cavity rates of 25 for small container sizes. This rate translates to production speeds of more than 700 containers per minute. In general, one can state that the higher the gob weight and the larger the container size, the lower is the corresponding cavity rate.
As illustrated in Figure 8 for 0.3‐l beverage bottles, the performance of IS forming machines has steadily improved since their inception in the 1920s. In 90 years, one forming line has been producing 26 times more containers per minute. And in the same period the weight of such containers could be decreased from more than 300 to less than 170 g. These figures show vividly the very strong potential that this forming process had when it was invented.
Figure 8 Performance increase of IS forming machines over the years in containers per minute.
5.3 Delivery Equipment
The delivery equipment consists of the gob‐distributor (also called scoop), the trough, and the deflector. After the gob has been cut, it falls into the scoop which distributes the gobs to the different sections in the forming machine. The gobs slide through the respective troughs and then are redirected by the deflectors into the blank‐molds. Although the delivery section looks like a simple part of the IS‐machine, it bears considerable neuralgic points. The gob temperature decreases during the delivery but the upward side of the gob loses less heat than the side in contact with the metal delivery, which in some cases is in addition cooled and lubricated. Forming problems can thus happen if the gob acquires a nonuniform temperature profile.
The speed of the gob when it leaves the deflector is also an important parameter. When leaving the deflector, the gob is loaded into the mold. The higher the speed of the gob, the more beneficial it is for a good loading. Too slow a gob speed may lead to incorrect loading and hence to problems in the forming process or defects in the final container. In extreme cases, the gob is not fully loaded into the mold and the upper end of the gob is caught by the baffle. This leads to immediate failure of the respective section. The average gob speed at loading is between 6.5 and 7.5 m/s.
5.4 Blank‐Side Forming
At the blank‐side the three different forming processes come to play as explained in Section 3. Molds at the blank‐side are usually made of laminar cast‐iron. The glass gob is loaded into these molds and is formed into the parison. Because of process‐sequence, the parison is formed upside down, the finish facing downward and the bottom of the container upward before the invert leads to the proper upright formed final container. Upon forming of the parison, the mold is always cooled by air to allow fast heat extraction from the glass. As already explained, this is necessary to have a stable parison with a high enough viscosity after the mold has opened. If the parison is too hot and hence has a too low viscosity, it may collapse after opening the mold, causing section failure.
Different mold‐cooling techniques are available. Most dominant are inside‐mold cooling systems, where air is lead through channels in the mold either from below or from above and an older technique called stack‐cooling. Here, fins are attached to the outside of the mold and the mold is streamed by air. This version is less efficient than inside mold cooling so that it usually leads to lower machine speeds.
5.5 Invert and Reheat
After forming of the parison, the blank‐mold opens and the parison is transferred via an invert to the blow‐side. The invert consists of an invert‐arm in which the finish equipment with neck‐ring and guide‐ring is fixed. As soon as the blank‐mold opens and invert takes place, the so‐called reheat starts. During the blank‐side process, the glass‐surface has been cooled down, especially through the contact with the molds and blowing air. The glass viscosity rises in this way, which is necessary to give the parison a certain rigidity to move it without deformation. After having been released from the blank‐mold, the outer parts of the parison get reheated from the hotter inner part through thermal conduction, which is in fact needed to lower again surface viscosity before the last forming step that will give the container its final shape.
5.6 Blow‐Mold Forming
The forming of the final container in the blow‐mold is from a mechanical point of view identical for all three forming processes (BB, PB, and NNPB) as explained in Section 3.
As with parison forming at the blank‐side, final blowing at the blow‐side can also be aided by vacuum. A main task for the blow‐mold is to extract as much heat from the container as fast as possible to increase its viscosity, stabilize its shape, and avoid deformations during take‐out and transport of the container after it has been released from the blow‐molds. Because of this need to extract large amounts of heat in a short time, blow‐molds often are made out of aluminum‐bronze, which has much higher heat conductivity than cast‐iron, hence allowing faster heat removal from the container.
A picture of a triple‐gob setup in the color‐section of this Encyclopedia shows the parisons just having arrived at the blow‐side and the final containers just having been removed from the blow‐mold and placed over the dead‐plate by the take‐out.
6 Hot‐End Handling, Hot‐End Coating, and Annealing
The conveyor belt transports the container to the annealing lehr for stress‐relaxation. Once the container has left the IS‐machine, a first inspection often takes place with cameras that record the infrared images of the hot containers. From these images, defects can be identified and immediate corrections of the process can be initiated. This is very beneficial as the feedback between defects and applied corrections is direct and without the time delay that would occur if the container were first annealed and then inspected.
Before entering the annealing lehr, the outside body (not the finish) of the containers receives a hot‐end coating of a 5–15 nm thickness. Even so thin, the hot‐end coating serves different important functions. It first saturates the highly reactive surface bonds that are present at the surface of the new glass container. It also provides a surface suitable for good adhesion of the cold‐end coating, which is applied later. Furthermore, it may slightly increase the strength of the container by disabling surface flaws that have been introduced during the forming process.
As precursor for hot‐end coating most frequently used is monobutyltin‐trichloride, C4H9SnCl3 (MBTC) or tin‐tetrachloride, which both gives rise to a SnO2 coating on the container. The process is chemical vapor deposition under air at atmospheric pressure (atmospheric CVD) and is supported by the moisture of the air. The reactions that take place are:
(6)
(7)
Other precursors based on titanium or titanium‐silicium are also in development to yield a TiO2 or TiO2‐SiO2 coating. As explained, after forming and hot‐end transport, the containers from different sections experience different cooling whereas the surfaces of a given container cool faster than its bulk, the rate being higher for the outer than for the inner