Screw design – part 3 | moulding masterclass
The metering zone of the screw plays a critical part in the melt preparation process. Moulding expert John Goff explains how to steer clear of problems
Understanding melt metering
The main function of the metering (or melt delivery zone) of the plasticising screw is to ensure a uniform tempera- ture is established throughout the mass of molten polymer material being delivered from the compression zone (discussed in the previous instalment in this series). Temperature uniformity of the polymer melt is of prime importance for consistent moulding as variation in temperature often results in changes in its melt density value, causing inconsistency in the shot volume con- tained in front of the screw tip assembly. As this volume of polymer is what is ultimately injected into the cavity within the mould, any variation from cycle to cycle will inevitably be refl ected in changes in part quality. The fl ight depth in the metering section of the screw
is usually the shallowest of the three sections. The reason for its shallow depth is to allow the applied heat energy to permeate through the molten polymer contained between the fl ights and so create the basis for achieving a homogeneous thermal distribution through the melt.
The actual depth will be related to the diameter of
the screw and the designated compression ratio selected for the type of generic screw design. However, in general, the depth of fl ight in the metering section (zone) ranges from 2mm to 6mm for plasticising screws of 20 to 160mm in diameter. The length of the metering section varies from between 3D to 6D, where such a selection of length is used for the processing of a wide range of thermoplastics materials.
www.injectionworld.com Most General Purpose or GP screws will have a
compression ratio (ratio of feed channel depth to metering channel depth) of between 2.1 to 2.5:1. This means the performance of conversion of the solid granule to liquid melt will vary considerably across the range of GP screws available. An area regularly discussed when melt preparation
problems are encountered is the amount of screw stroke (or shot volume) that should be utilised for effective component manufacture. Many process specialists advocate that the most profi cient screw stroke should be between 1D to 3D, with the proviso of increasing to 4D in particular circumstances. Following this guideline means that for a screw of 30mm diameter (D), the recommended screw stroke to be used will range from 30mm to 90mm. Similarly, for a screw of 110mm diameter (D) the recommended screw stroke to be used is between 110mm and 330mm. We can correlate this 1D to 3D recommendation to
the more common approach of shot capacity (or percentage) utilisation. For the example of the 30mm diameter plasticising screw specifi ed in Figure1, the optimum screw stroke to be used when calculated in terms of shot capacity percentage is as follows: The maximum available screw stroke for the 150
tonne (1,500kN) moulding machine equipped with a 320 size injection unit and 30mm screw is 175mm and the total shot volume (cylinder head volume) is 124cm3 Therefore, for every 1mm of screw stroke an equivalent
. January/February 2015 | INJECTION WORLD 53
Main image: Consistent moulding begins in the plasticising screw
PHOTO: SUMITOMO SHI DEMAG
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