PETpla.net Insider 03 / 2015

PREFORM PRODUCTION 44 PET planet insider Vol. 16 No. 03/15 www.petpla.net Next the speeds have to be cal- culated by using the wall thickness of the different parts. Let us assume that the preform in our example has the following wall thicknesses:  End cap: 2.8 mm  Body: 3.4mm  Transition: 2.5mm  Neck finish: 1.5 mm Taking the body as 100%, i.e., the highest fill speed, it is now easy to calculate the other wall thicknesses as percentage of the maximum:  End cap: 2.8/3.4 = 82.3%  Transition: 2.5/3.4 =73.5%  Neck finish: 1.5/3.4 =44.1% The maximum speed has to be determined by trial and error to get the desired injection time as outlined earlier. If it is 50 mm/s, the relevant values would be:  End cap: 50 × 82.3% = 41  Transition: 50 × 73.5% = 37  Neck finish: 50 × 44.1% = 22  Speed: 22 37 50 41 mm/s  Position: 23 32 40 101 120 mm 3.4 Cycle Time Calculations Preforms are molded with various machines and molds and of course they differ in weight, geometry, and wall thickness. Required cycle time for a particular preform and at a given machine is most dependent on wall thickness, not weight. It is easy to understand why this is so. At a recom- mended injection rate of 11g/s, the difference in injection time between 15- and 30-g preforms is only 1.4 s. How- ever, the difference in process time between a preform with a wall thick- ness of 2.3 mm and one with a 3.3-mm wall is about 5–6 s. This is because it takes that much longer to cool down the center of the wall before the pre- form can be demolded. This in turn can be understood because PET, like most other plastics, is a good insulator, slow- ing down heat transfer from the center to the cold mold wall. Machinery also plays an important role. A 20-year old machine without postmold cooling (in what we call “free- drop”) may process a given pre-form in 24 s, whereas the same preform could be made on the latest high-speed machine with four postmold cooling stations in 10 seconds or less. 3.5 Challenges in Thin-wall Molding As preforms get lighter and lighter, their wall thickness becomes smaller as well. This leads to considerable challenges in the molding process as higher pressure is needed to push the material through thinner mold chan¬nels. This leads to increased shear, and because PET is shear sen- sitive, great care must be taken not to overshear the material with subse- quent deteriora¬tion of the properties or the occurrence of defects such as yellowing. Thin walls are mostly used in the production of lightweight water bot- tles and feature body walls 2.2–2.4 mm thick. They are necessary to have adequate stretch ratios in these high- performance containers. As a windfall of this arrangement, extreme short cycle times as low as 6 s at the time of this writing are possible because it is mainly the wall thickness that controls cycle time. Although there are some preforms in the market with even lower wall thickness in the body, 2 mm and less, these lead to less operational efficiencies and are not recommended. Neck finish walls that are only 1mm thick are possible though. To visualize what happens inside the preform, it may be helpful to recall a diagram from Chapter 2.4 (Fig. 3.17). Figure 3.17