Screw design – part 4 | moulding masterclass
of the screw. These overrides are observed where the set temperature is lower than the actual temperature being recorded by the thermocouple positioned in the wall of the barrel assembly and can arise when processing stiff fl owing (highly viscous) materials such as UMWHDPE, UPVC, PC/PBT blends, PC/ABS blends, PC/PETG blends, PEI or PES/PSU. For this reason it is important that the correct compression ratio - typically of 2.0-2.3:1 - and zonal lengths of 3.0-4.5D are selected. Again, it is the melt temperature and its variability that is affected and this is often the root cause of moulding quality issues and process inconsistency. To ensure effective conversion of the polymeric
too short a length can cause semi-solid plastic material to be forced from the end of the feed section into and along the inclined section of the compression zone. As a result of the progressive reduction in fl ight depth along the length of the compression zone, a breakdown in the melt column takes place leading to poor and/or inconsistent melting of the semi-solid material, as well as a failure to transfer the same shot volume each moulding cycle into the metering section and subse- quently into the front of the screw tip assembly. Such a problem can be experienced in practice
where, upon rotation of the screw, it either remains in same forward screw stroke position without picking up any new material in the feed section, or it begins to move backwards and then stops with the screw continuing to rotate. This behaviour is often termed as ‘screw slippage’ and will continue to prevail until the melt column reforms to allow the polymer melt to be transported forward. In so doing, it provides suffi cient pressure at the front of the screw to move it backwards to the screw stop position. Due to the slippage issue, large differences in the dosing (screw recovery) time can occur causing a corresponding variation in the shot volume and ultimately leading to short and/or discoloured mould- ings being intermittently produced. In certain cases, the extent of screw slippage can be so excessive that it prevents the thermoplastic material being readily processed on a particular moulding machine. The use of too long a compression zone, however,
induces a high level of shear on the material contained within the screw fl ights, resulting in an excess of adiabatic heat energy. The outcome of this is noted by temperature override, particularly in the zones encompassing the compression and metering sections
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material takes place, particularly for the semi-crystal- line engineering type materials, a different geometrical design of screw is needed to achieve melt homogeneity that includes changes in the length of the compression zone and compression ratio, typically of 5-6D and 2.6 to 2.8:1 respectively. By contrast, for polyolefi n type materials the use of multi-fl ight or barrier type designed screws is common in order to achieve the melt homogeneity required when moulding large thick walled components, fast cycling thin walled containers, or mouldings associated with the medical, pharmaceu- tical and optical industries.
Surface considerations A development seen for quite a few years now is to surface coat the entire working length of the screw with a pre-determined thin layer of chromium, titanium nitride, tungsten carbide or chromium carbide to prevent the degraded thermoplastic material from permanently adhering or attacking the uncoated hardened steel surface causing pitting through corrosion. Such adhesion or corrosion often leads to black specks being present in clear or light coloured mouldings. With regard to adhesion of polymer melt to the
surface of the screw, a further issue is often encoun- tered when processing PC and PC-based blends, PSU/ PES, PEI and certain grades of acrylic where a standard non-alloy carbon-based steel with an ionised nitride surface is used for the screw. This nitride skin provides an ideal surface for the molten material to adhere to, causing it to remain on the screw surface and subsequently degrade over a period of time as a result of the prolonged heating and running in production. When the screw and barrel
assembly is allowed to cool down this solidifi ed degraded skin breaks off into minute brittle
March 2015 | INJECTION WORLD 37
General
purpose screw profi les are not suitable for every polymer processing task
PHOTO: KRAUSSMAFFEI
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