PETpla.net Insider 05 / 2015

PREFORM PRODUCTION 38 PET planet insider Vol. 16 No. 05/15 www.petpla.net 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 * This article was published in Bottles, Preforms and Closures, Ottmar Brandau, Chapter 3.8. Copyright Elsevier 2012 bottom cavities are colder than they need to be. Furthermore, preform parameters, especially in the gate area, will be different and may cause some preforms to be rejected. In order to balance the water flow to cavities or cores for that matter the supply and return path of the water should be equal. This can be achieved by the arrangement shown in Fig. 3.26. Although the bottom cavities receive water first and thus at the highest pressure and flow, the return flow path is longest, whereas it is the opposite for the top cavities, and so the total path of coolant is the same for each cavity. The way it is shown here would mean that water enters from the bottom and leaves from the top. This is often not practical because electrical connectors may be mounted on top of some tools. In this case, a third water line can be drilled that travels behind the two shown here, taking the coolant down to the bottom level. 3.9 Tool wear issues Preform tools have a number of wear parts such as heater bands or valve gate cylinders. These will be replaced during run times by mainte- nance personnel and are no reason to replace the tool altogether. How- ever, when the taper locks wear out, it is usually time to either replace or at least refurbish the tool. Taper locks are the parts of the neck inserts (male side) and cavities (female side) that engage during closing and clamping and transmit the clamping force onto the tool. As these parts start to wear, they do not close as tightly and flash develops. When the flash exceeds a certain value [different for differ- ent geographical regions and brand owners but around 0.13 mm (0.005 in) per side as per the neck drawing], the tool is usually finished. There are several reasons why taper locks wear out even when they are properly hard- ened, assembled, and run. Tolerances in the manufacture of the injection machine platen, the tool plates, and the indi-vidual taper lock parts lead to slight misalignments, which do not allow all tapers in a multicavity tool to match perfectly. Clamping force is also not perfectly balanced (a difference in machine construction users should be aware of as there are differences between manufacturers), leading to some tapers receiving more clamping pressure than the others. Accumula- tion of dirt is another factor that forces tapers together unevenly. On horizon- tal machines, gravity also plays a role. Ninety degrees opposed to the closing action gravity leads to a slight sagging of the moving tool side after the bush- ings or linear bearings holding them have worn to some degree. Vertical machines do not suffer from this par- ticular issue and feature indeed very impressive tool lives but have not been able to capture a lot of market share as they are more difficult to work on and require more energy to move the tools. With today’s fast-running machin- ery, tool wear has become a hot topic for users. Imagine a machine running a 7-s cycle time for a lightweight water bottle preform. If used 350 days/year and 24h/day, this machine will wrap up over 4 million cycles per year. Tools on horizontal machines are meant to last about 8 million cycles, which translates into a 2-year life for this tool. Some mould manufacturers now guarantee this number of cycles using the afore- mentioned flash value as benchmark. Figure 3.25 Top cavities receive less cooling than bottom cavities. Figure 3.26 The path of the coolant to and from each cavity is the same.