PETpla.net Insider 12 / 2013

BOTTLE MAKING 29 PET planet insider Vol. 14 No. 12/13 www.petpla.net preform moulding gate and the bottom surface of the bottle is measured. This value must not be zero or a negative figure because otherwise the bottle would rock. All measurements are first taken on empty bottles and then, in a next step, on bottles filled with the intended contents or with carbon- ated water. These values are first determined immediately after filling at ambient temperature. The bottle is then stored for 24h at ambient tem- perature, and the measurements are repeated. Depending on the specifica- tion, this may be followed by a further thermal test measurement taken after the bottle has been stored for 24h at 38°C in an environmental chamber. In special cases, e.g., for certain coun- tries, other storage times at different temperatures may be required. Hot-fill bottles are measured after the hot filling and again after they have been emptied and cooled down. For the determination of wall thick- ness distribution, non-destructive measurement techniques are increas- ingly replacing the simple microm- eter test instrument for cut bottles. In measurements with the MagnaMike, for example, a small steel ball is placed in the bottle. Then a sensor is attached that will determine the wall thickness with an accuracy of 1μm on the basis of the magnetic field, which will vary as a function of the distance. This measurement system utilises the physical concept of the Hall Effect. Another very accurate method uses infrared absorption. Here, infrared radiation is directed through the wall of the bottle and is analysed in vari- ous wavelength ranges in which the absorption rate of PET is well defined and independent of the type of PET used in the actual case. The wall thickness is determined at fixed loca- tions across the circumference and the height of the bottle, as well as at prominent locations in the feet, at the shoulder, at the recessed handle, etc. Capacity As a consequence of shrinkage, the dimensions and thus also the capacity of the bottles will decrease after blowing, compared with the cavity dimensions. For bottles that are filled in line, i.e., that are conveyed from the blow moulding machine directly to the filling line by means of, for instance, air conveyors, the capac- Please order your copy at the PETplanet insider book shop: https://www.petpla.net/books Bottles, Preforms and Closures A Design Guide for PET Packaging Second Edition by Ottmar Brandau € 115,00 180 pages © Copyright Elsevier 2012 ity is determined on freshly blown bottles. If the bottles are first packed and then delivered to the line at the bottlers, the capacity is only meas- ured after a dwell time of, say, 7h. Usually two capacity measurements are taken: level-mark capacity (as indicated in the bottle drawing) and brim-full capacity. For the measure- ment itself, the bottle is filled with degassed water at a known tempera- ture and then weighed. The volume is then determined using a density table or is calculated from the weight. Top load For all bottles, the top load is also measured. For this purpose, an empty sample bottle is compressed by a plunger, usually at a set deformation speed and path. Because the force required to compress the bottle will reduce after the bottle has started to fail (as a result of collapsing, buckling, or denting), a load cell can be used to measure the maximum force applied in this test. Normally, the measurement is performed on empty bottles. Espe- cially for noncarbonated contents or to determine the load of the bottles when several pallets are stored on top of one another, a measurement with filled bottles may be useful as well. For special applications such as vending machine bottles, a similar instrument is used to measure the lateral side load or panel load. Comparable tests are carried out to determine the handling stability of, for example, large lightweight bottles. Burst pressure (or internal pressure) tests The burst pressure test is particu- larly important for carbonated drinks. As has already been mentioned, such bottles are prestressed at 4–6bar in the filler to prevent release of the carbon dioxide during the filling pro- cess. For the burst test, a bottle filled with still water is pressurised relatively rapidly until an internal pressure of 7bar has been attained and the pres- sure is maintained for 12s. (These specifications may vary with beverage fillers.) The pressure is then increased until the bottle bursts. The test log not only records the pressure at which the bottle bursts but also records where it bursts because it is important that the bottle fails at the sidewall and not at the base. For hot-fill bottles, the reverse situation must be tested, i.e., the resistance of the bottles to vacuum during cooling. For this pur- pose, sample bottles are evacuated with, for instance, a manual vacuum pump until they deform beyond a given measure, collapse, buckle, or dent in an undefined manner. At this point, the extracted volume and vacuum are recorded. * This article was published in Bottles, Preforms and Closures, Ottmar Brandau, Chapter 1.8.3.1. to 1.8.3.4., Copyright Elsevier 2012