Moulding masterclass | processing
pressure experienced when forcing the anticipated volume of molten polymer at a particular velocity through and within the channels and ultimately the gate orifi ce. Such pressure losses are predicted using rheological (fl ow) equations and material fl ow data, but it is not until the mould tool and the hot runner system is sampled that the actual resistance pressure is known. Pressure is created as a result of the extent of
resistance the molten polymer encounters as it fl ows along the hot runner channels and the selection of the correct temperature values for each element of the hot runner system becomes very important, not only for the manifold but also the entry bushing and tips (drops) integral within the gate. The consideration and selection of temperature
accordance with the system confi guration, therefore, both temperature uniformity within each of the elements and the ability to maintain a constant temperature is essential, particularly within the manifold and tips (drops). Temperature uniformity is infl uenced by the
placement and heating capacity of the electrical resistance heating elements, these being confi gured either in a zoned or standard layout, whereas sustain- ment is signifi cantly affected by the placement of the thermocouple and the manner in which the power is supplied to the resistance elements and its subsequent control and monitoring. It is a known fact that uniform distribution of the polymer melt throughout the resident system can be greatly affected by the temperature values chosen on the temperature controller. This distribution of the shot volume, whether equal or
disproportionate, has been a subject of major develop- ment by hot runner system manufacturers. Quite often guidelines are given for which temperature values are to be used when processing particular types of polymeric materials. As the words state, these are guidelines – the most effective temperature value for each segment of the hot runner system still needs to be determined. Changes of 10°C to 20°C, particularly to the manifold segment, can greatly affect the overall running perfor- mance and output of the mould tool, as well as the extent of volume imbalance between the impressions within a multi-cavity mould. The design of any bespoke hot runner system is created on important factors such as the proposed melt and mould temperatures, predicted cycle time, rheological behaviour of the moulding material, the volume of material to be used, and the rate of displacement through the proposed system.
Such channel sizes, lengths and confi guration are then proposed with the predicted loss of injection
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values varies widely throughout the moulding industry. On many occasions, the temperatures set for the tips/ gates are up to 50°C in excess of the melt temperature value used for the screw and barrel assembly, whereas the manifold and entry bushing temperatures are equal to, or lower than, the melt temperature. Furthermore, the values set for each tip/gate in a multi-impression mould tool again can differ by as much as 40-50°C as the result of attempts to overcome volume imbalance between the impressions. Equally, there are many processes where the
temperatures set for all elements of the hot runner system are lower than either the actual melt tempera- ture or those used for the screw and barrel assembly. It is quite common to see moulding processes that
have been set where imbalance from impression to impression is chased around the mould tool by altering the tip/gate temperatures. The resulting tip/gate temperatures can be widely different from each other. Scenarios that result from such an exercise include the manufacture of defective, over-packed or fl ashed mouldings in the remaining impressions, slight shorting of components due to the presence of gas, burn marks on mouldings, and high gates. To be continued.
About the author: John Goff is a chartered engineer (CEng), a Fellow of the Institute of Materials, Mining and Metallurgy (FIMMM), and CEO of injection moulding process consultancy and moulding process optimisation software developer 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. This is the 28th instalment in his Moulding Masterclass series. You can read the most recent instalments in this series here, here and here.
July/August 2013 | INJECTION WORLD 19
Left: Nozzle and tip type and design is important in any hot runner but it is vital not to overlook system design and setting
PHOTO: HASCO
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