Encyclopedia of Glass Science, Technology, History, and Culture. Группа авторов
rel="nofollow" href="#ulink_59644068-62a6-5f58-a7cd-6fb9fa17fe95">Figure 5 (a–d) Press & blow process, blank‐side.
Restrictions in usage of the NNPB process are due to the plunger dimensions and finish openings and the corresponding cavities pressed into the parison. The parisons are usually shorter for NNPB than for BB if the same final container shape is to be produced (e.g. a 0.33 l beverage bottle). Another significant difference between NNPB and BB is that the required gob temperature is from around 20 to 50°C higher in NNPB because of difficult pressing conditions. This difference in consequence leads to different thermal requirements during the process in terms of mold‐cooling and reheat‐timing.
4 Making of the Gob: Forehearth, Feeder, and Shears
Most forming processes take place at a viscosity of 102–104 Pa⋅s. Hence, for soda‐lime‐silica containers, the glass needs to be cooled from melting and fining at ca. 1500°C and a viscosity of 10 Pa⋅s down to ca. 1050°C and a viscosity of 103 Pa⋅s. This quite demanding task is accomplished in the forehearth. The forehearth is directly connected to the working‐end and ensures the required homogeneity of the glass while bringing it to the desired temperature and viscosity.
After the forehearth, a feeder enables glass‐portioning and gob pre‐shaping (Figure 7). It consists of a refractory tube and one or more plunger(s) that are moving periodically up and down. The tube is rotating to homogenize the melt in this final stage. With each upward stroke of the plunger, the glass stream is released from the shear blades in order to cut a gob without having a glass stream loaded on top of these shears. For a single‐, double‐, triple‐, or quad‐gob setup, the respective number of plungers operates simultaneously in the feeder, hence as many openings in the orifice ring are required. The final gob shape is influenced by the sizes of the orifice ring and plunger, and by the shape, height, and motion profile of the plunger.
Figure 6 (a–d) Narrow‐neck press & blow process, blank‐side.
Figure 7 Cross section of a modern feeder (double‐gob setup).
Source: Courtesy Bucher Emhart Glass.
The originally continuous glass stream is cut by the shears right after it has been “pre‐shaped” by the feeder and plunger and has passed though the openings of the orifice ring. The gob needs to be completely separated from the glass stream by the shears to prevent any glass fibers from being attached to it. Any misaligned or poorly operating shear will result in shear marks and, consequently, in defects in the final container. For shears, the materials most commonly used are steel (cheap, but short‐lived) and hard alloys such as WC (more expensive, but long‐lived). In all cases, the shears are cooled by a shear‐spray, a mixture of water and cooling fluids.
5 IS‐Forming Machine
5.1 General Principles
Rotational forming machines are nowadays used only in some rare cases. The principles of glass‐container forming will thus be described for IS‐machines, with which almost glass containers are made. Derivatives of the IS‐machine such as the Emhart RIS and Heye H 1–2 machines have been developed in the past but are hardly in use any longer [9]. They work with two molds on the blow‐side forming, which are loaded alternately. This approach is advantageous in terms of longer reheat and more homogeneous glass thickness distribution but is much more complicated, expensive, and prone to jamming.
In a narrow sense, IS‐machines consist of a gob‐distributor and delivery equipment, blank‐side forming, invert, blow‐side forming, and take‐out and have several identical sections aligned in a row (Figure 1). The only differences between sections are the individual delivery (as different distances from gob‐cut to mold need to be overcome) and the distance of the section to the annealing lehr. The differences in delivery distances cause different gob speeds and different gob arrival‐times at loading and thus require different section‐timings. The differences in distance to the annealing lehr may cause different containers temperatures at the hot‐end coating and at lehr entrance. When entering the lehr, there is, for example, a difference of 50 K or more in surface temperature between containers from section 1 and from section 12, which are the farthest from the annealing zone.
The IS‐machines in principle can be adapted to all three forming processes that have been mentioned earlier. To a certain extent the machines can be converted between a triple‐gob setup to a quad‐gob setup or, given another machine construction, from a triple‐gob setup into a double‐gob setup. How widely a machine can be adapted depends on different parameters, especially on the inner‐section distance, which describes the possible center distances of the molds to each other within one section. The type of setup to be used depends on different parameters such as the size and weight of the container to be produced, desired machine speed, and portfolio of the respective glass‐manufacturing plant.
5.2 The IS‐Machine Families
The IS‐machines can be separated into three groups:
1 Pneumatic‐controlled IS‐machines with angular mold‐opening.
2 Pneumatic‐controlled IS‐machines with parallel mold‐opening.
3 Servo‐electric‐controlled IS‐machines with parallel mold‐opening.
In the earliest types of IS‐machines, all movements are controlled by pneumatic valves. The mold opening and closing is in an angular motion, which means that in a multi‐gob setup at the blank‐mold‐side, the inner blanks are more widely opened than the outer blanks, causing difference in radiation between the glass and the open blanks. At the blow‐side, the inner molds are not opened as wide as the outer molds, which may lead to difficulties in machine accuracy and forming.
A significant step forward, therefore, was the introduction of pneumatic‐controlled IS‐machines with parallel mold‐opening and closing. Here the mold‐halves from the inner, middle, and outer cavity open in a parallel motion to each other. This leads to more comparable conditions between the molds of a given section. Furthermore, the parallel closing and opening is more precise, leading to a more reliable forming. In the color section of this Encyclopedia, a picture of a modern pneumatic‐controlled IS‐machine is shown.
The next logical improvement was to exchange the pneumatic‐controlled movement for a servo‐electric‐controlled motion to take advantage of the enhanced stability, reliability, and precision of servo‐electric drives. In this way, motions are much more easily cushioned and are gentler for the hinges, molds, and also for the glass itself. In the latest generation of IS‐machines, mold opening and closing, plunger motion, invert, blow‐head, take‐out, pusher, and other parts are thus servo controlled.
The machine speed is a general parameter to describe the production performance for a given container. It is expressed as the cavity rate (C), namely the number of containers produced per minute (cpm) for each cavity considering the total numbers of cavities (NS) of the IS‐machine:
(5)
For a 12‐section machine with a triple‐gob setup