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Moulding masterclass | processing


variances across a distribution manifold to be within 5°C or lower. This variance is the surface temperature and is not to be confused with the inner channel skin temperature or to the set temperatures measured by the thermocouple. Most direct or externally heated manifold hot runner systems encounter this ΔT issue and some systems will have a greater ΔT than others. If one considers the temperature requirements of the tips (drops), then the set temperatures should refl ect the actual temperature of the melt passing through and around the gate entry. In order to control the temperature at the gate, cooling channels are strategically positioned either within the impression plate or in and around the gate bushing. Effective cooling is essential to ensure minimal gate vestige, the overcoming of poor gate scars, differences in surface gloss in and around the gate area, and the presence of fi bres attached to the moulding upon separating the mould halves. Poor gate cooling can lead to increased cycle times and lower performance capability. The values selected at the gate are often much higher than the measured melt temperature as the increase in the set temperature value is essential to prevent premature freeze-off taking place behind the gate aperture, which will cause inconsistency in component manufacture and/or process cycle repeat- ability. It is often the case that the temperature of the coolant passing in and around the gate plate/bushing is set at too low a value because it is the temperature that is being used for the remainder of the mould tool. Interestingly, the lower the coolant temperature, the


more signifi cant is the extent of imbalance between impressions within a multi-impression mould tool. Good moulding practice recommends the gate/feed plate assembly should be separately controlled at a temperature value which allows comparable tempera- tures to the measured melt value to be selected. In fact, if the plate temperature is correct, then slightly lower tip values can be used without moulding quality issues or output issues being encountered. More importantly, correct temperature selection is


critical for the static thermal probe type hot runner system where the detachable tip assembly, in its expanded position, must be accurately positioned to ensure the concentric conically-shaped gap in the gate entry allows the correct volume of molten polymer to fl ow unrestricted into the cavity. The higher the selected tip temperature, the greater the tendency for the tip to expand further into the gate orifi ce. In practice, when increasing the tip temperature,


greater fl owability occurs due mainly to the increasing shear being applied to the molten material as a consequence of the smaller available gap. Such


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fl owability is achieved without any temperature control and often leads to the production of mouldings with quality issues due to the lack of control. Furthermore, the use of high tip temperatures induces increased wear to the external profi le of the detachable tip, leading to a reduction in the overall tip height and position. This often results in poor gate cosmetics. In terms of importance, the distribution manifold


temperature is the highest priority. A key factor in selecting the optimum value is that, at each incremen- tal temperature setting, reference is made to a measurable process parameter or moulding attribute. As part of the selection process for a designated hot runner system, the use of CAE to predict the loss in injection pressure at a given temperature and fl ow rate provides the basis for the channel geometries. By determining the extent of pressure loss and part volume, as well as the variability incurred over a number of consecutive shots for each incremental setting, invaluable information for achieving the optimum value can be derived. Upon determining the optimum manifold temperature value, refi nement of the tip/probe values can then follow if required.


About the author:


John Goff is CEO of UK-based injection moulding process consultancy G&A Moulding Technology (www.gandamoulding.co.uk), which provides consul- tancy services on all aspects of process setting, optimisation and control, including hot runner technol- ogy, and developed and markets the Pro-Op process optimisation software tool. This is the 28th instalment in his Moulding Master-


class series. You can read the most recent instalments in this series here, here, and here.


All of the articles published in the Moulding Masterclass series by John Goff between 2009 and August 2013 have now been collected into one convenient volume. You can now benefi t from all his experience and practical advice by keeping a copy on your desk or the shop fl oor. Re-read early articles on the


infl uence of screw design, and the choice of injection time, holding pressure, gate sizes and much more.


This essential guide to process optimisation and cost effective injection moulding is priced at


€60 (£50 or $75). To fi nd out more


follow the link: http://bit.ly/1dM2Yhx


October 2013 | INJECTION WORLD 39


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