PETpla.net Insider 10 / 2016

CAPPING / CLOSURES 48 PET planet Insider Vol. 17 No. 10/16 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 4.5 and 4.6 Copyright Elsevier 2012 4.9 General IM Process Parameters for CSD Closures The process steps for injection- moulded caps for CSD applications are much alike. However, there are minor differences to be found depend- ing on cap design, gating solutions, and resin as well as machine capabili- ties. The following information repre- sents a typical cycle time breakdown for a 28mm CSD cap (Figs 4.38–4.40, 4.47, and 4.48). Figure 4.38 Typical cycle time break- down for injection moulding of CSD caps (picture courtesy of Corvaglia Ltd.) The cap’s physical, as well as opti- cal, performance is quite often related to certain process steps as follows: 4.9.1 Injection Time The injection time depends on three factors: the machine, the mould design, and the plastic resin. 4.9.1.1 Machine Depending on the IM machine capability, the injection speed as well as the resulting injection pressure is subject to optimisation. 4.9.1.2 Resin Some resins cannot be injected with high speed, as degrading or burning effects would occur. The higher the melt flow rate of the resin, the faster the injection can take place. 4.9.1.3 Cap and Mould Design 4.9.1.3.1 Number of Cavities The more cavities a mould has, the longer it takes to fill. With cold runner systems, the length of the run- ners requires additional resins, shot for shot. In hot runner moulds, the pressure drop will increase by every additional cavity installed. The smaller the pres- sure drop, the faster the mould will be filled. 4.9.1.3.2 Product Shape A product having a short flow path and a heavy wall thickness offers the best conditions to achieve the shortest possible fill times. 4.9.1.3.3 L over t Ratio The ratio between flow path (L) and wall thickness (t) influences the fill speed drastically. The higher the ratio, the more injection pressure will be required to fill the product. This high filling pressure requires much more clamping force to counteract. In addi- tion, the mould is under higher load. 4.9.2 Hold Time Cap designs with heavy wall cross sections shrink considerably after the injection process step and may show sink marks because the resin solidi- fies and continues to do so although in contact with intensely cooled mould surfaces. The purpose of the hold time is to maintain pressure on the melt in order to finalise filling while the product shrinks in the cavities. The quality of the products gating area is also influenced by the injection–hold process step. 4.9.3 Cooling Time The cooling time parameters used depend on two major factors: cap and mould design as well as cooling water supply of the moulding plant. The cool- ing water temperature should be within a range of 8-14°C. In order to achieve the best possible cooling time, the cooling water should be delivered at a system pressure between 4 and 6 bar measured at the mould. A proper control can be achieved if the system water pressure is moni- tored at the inlet as well as the outlet of the mould cooling. A maximum pressure difference between inlet and outlet is required to receive maximal water flow. All connections must be of adequate cross section, installed and laid out correctly, avoiding kinks that cause flow restrictions. Preferably, clean, filtered, and treated water which will not clog or corrode the cooling channels should be used. The mould design also has an impact on the ideal cooling time. By optimising the wall thickness of the part forming components as well as the cooling water flow rate, the cooling time can be tuned to a minimum.