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processing | Moulding masterclass


The challenge of determining


optimal hotrunner settings is simplifi ed if


broken down into distinct sections, according to


moulding expert John Goff


Hotrunner optimisation


One question regularly asked is: How do I obtain the optimum temperatures for a hot runner system in order to achieve effective mould tool performance? A simple protocol has been derived at G&A Moulding Technology that divides the hot runner into distinct sections. Most hot runner systems, either single or multi-level


confi guration, can be typically broken down into the sprue or entry bushing, the distribution manifold, and the feed bushings (commonly known as drops, thermal tips or probes). When selecting the temperatures for each section of the hot runner system, reference to the actual melt temperature value achieved upon exit from the screw and barrel assembly must be considered. It is common with most types of hot runner system


for the entry section to be independently controlled with its own heater and thermocouple combination. When this is not the case, in general, the dimensions of the bushing should allow unrestricted fl ow of the molten material into the distribution manifold and the skin temperature of the entry channel should be as close to the derived melt temperature as possible. For this reason, it is imperative that the actual melt


temperature is measured so that an accurate correla- tion can be obtained. Often the temperature value selected for the entry bushing is either identical or similar to that used for the distribution manifold unless


38 INJECTION WORLD | October 2013


certain criteria dictate otherwise – for example, the dimensions (length/surface profi le) of a snorkel incorporated with a stack mould tool. When comparing the difference in the mass of metal in both the entry bushing and distribution manifold, they are usually very different. Furthermore, the thermocou- ple is often located within the wall of the bushing itself, enabling the heat energy dissipated by the electrical resistance element and directed to the entrance channel to be effectively monitored, resulting in a usual skin temperature of 2 to 10°C of the set temperature. Correlation of this value to the derived melt temperature needs to take place to ensure good control. Some processors confi rm comparability by manually measur- ing the polymer temperature within the bushing. In comparison, the skin temperature of the inner


surface of the manifold channels can be up to 20 to 25°C lower than the derived melt value for the same temperature setting as the entry bushing. This ΔT can be due to various reasons: positioning of the thermo- couple, the geometry and layout of resistance elements, and heating capacity. Regardless of whether the skin temperature is 5 to 25°C lower, the set temperature value needs to be established to compensate for this temperature disparity. Hot runner manufacturers often quote temperature


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