moulding masterclass | Screw design – part 2
Figure 1: Some basic injection screw terminology
Shank styles will vary
Bearing Pitch ➤ Feed depth
Land length ➤
Metering depth
valve threads Screw diameter (D)
Flight depth
Feed Section Flight or helix angle
Shank length ➤ L/D ratio = ➤ Overall screw length
Compression ratio = C.R. = Flighted length (L)
length of 600mm. This does not mean that the total flight length is 600mm. More often than not, a screw designated with a 20:1 LD ratio will have a 23:1 total flight length to diameter ratio to enable the plastic material to fall into the designated area of the screw under gravity with the minimum of difficulty. The distance between the overall flight diameter and
the core (root) diameter of the screw is known as the flight depth and is usually larger than the size of the granular material being processed. For screw diam- eters of 18 mm and below, therefore, granule size and shape becomes critical as the flight depth in the feed section can be less than the standard granule diameter. When the granule size is greater than the flight depth, shot weight inconsistency can occur as the scraping action of the screw flight propelling the material forward causes the oversized granules to be sliced or cut to varying sizes. This ultimately affects the compaction of the granules between the flights, causing variation in the shot volume of material in front of the screw tip assembly and also in screw recovery time. To avoid this variation, where small screw diameters are used then specially-dimensioned granules are often required rather than standard granular materials. Applied conductive heat energy from the barrel
Figure 2: The onset of melting can be clearly seen in the dark coloured regions on this screw. The feed zone is at the bottom
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heater bands is transmitted into the polymeric material contained in each of the zonal lengths of the reciprocat- ing screw. However, it can be seen from the screw shown in Figure 2 that a large portion of the feed system sees minimal heat transfer into the core diameter of the screw. In this example, the colour on the screw surface shows where commencement of melting takes place and, interestingly, confirms the widespread belief that the plastic material remains in a solid to semi-solid state from the beginning of the feed section to just before the start of the transition section.
INJECTION WORLD | November/December 2014
Screw diameter (D) Feed channel depth
Metering channel depth
The length of the feed section is usually between 8-10D, where D is the diameter of the screw, or in percentage terms is 40-50% of the length. Considering the transition section, the typical length is
between four and six times the diameter (4D-6D). This length provides the basis for effective conversion of the semi-solid plastic material entering the zone into a molten liquid on exit. In real terms, the total energy created by both frictional and conductive means accumu- lates in the solid or particulate-free molten material. The flight depth decreases along the entire length of
the transition zone of the screw. This results in a corresponding change in the bulk density of the granular material, such that the molten material exiting the tranisition zone is of a sufficient consistency (viscosity) and free of solid particles. As discussed in previous articles, conversion of the
solid material to a liquid melt is imperative for consist- ent component manufacture. For this reason the transition/compression melting zone is hugely influen- tial in achieving good melt homogeneity. The third and final section of the screw is the
metering or delivery zone. This will be discussed in more detail in the third instalment in this series.
About the author: Moulding Masterclass series author John Goff is a chartered engineer (CEng), Fellow of the Institute of Materials, Mining and Metallurgy (FIMMM), and CEO of injection moulding process consultancy and G&A Moulding Technology (
www.gandamoulding.co.uk), which provides consultancy services on all aspects of process setting, optimisation and control, including hot runner technology, and developed and markets its own Pro-Op process optimisation software tool. You can read the most recent instalments in this series here, here and here.
www.injectionworld.com
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