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Screw design – part 1 | moulding masterclass


Screw selection has an infl uence on moulding performance, but before you can select an appropriate design you need to understand the basics. In Part One of this series, John Goff talks through some of the critical terminology


Understanding screw designs: Mastering the terminology


It is well-known in the injection moulding industry that poor conversion of the solid granular plastic resin to a liquid melt during the plasticising process can greatly affect production of consistent mouldings from a capable injection moulding process. And many believe the geometry of the reciprocat- ing plasticising screw plays a major part in achieving the level of homoge- neity required in the molten polymer. Contrary to what many in the injection moulding industry believe, the design and geometry employed in a reciprocating screw to produce injection moulded plastic components is a non-standard entity. However, it is important. The continuous helical channel machined into the cylindri- cal length of steel provides the baseline within which the solid granular polymeric material is converted to a molten liquid, with the varying depth of the helical channel in defi ned sections along the length of the screw having a signifi cant infl uence on processing outcomes. Most global injection moulding machinery and raw material manufacturers employ either specialist engineers or external consultants to develop injection machine screw designs that will facilitate the conversion of the solid granular polymeric material into a homogenous molten state charac- terised by uniformity in both


temperature and viscosity (fl owability). The outcome of this previous and on-going research is that each moulding machine or screw manufacturer has created its own design for effective performance and this has led to varying performance and conversion rates. Without wishing to offend the more learned technical


staff working within the injection moulding industry, before embarking into a discussion of the types of screw designs available to moulders it makes sound sense to consider and explain the terminology used. Terms such as fl ighted length, overall screw length, effective working length (distance from the feed pocket to the end of the fl ighted section), screw diameter, compres- sion ratio, feed section length, transition section length, metering section length, fl ight depth, land length, fl ight/ helix angle, shank diameter, pitch, L/D ratio (the fl ighted length divided by the screw diameter), feed channel depth, metering channel depth, compression ratio (feed depth divided by metering depth), spline attachment and confi guration all have specifi c mean- ings. However, as in many other industries, terminology varies around the globe so to minimise any misunder- standings in later discussion the key terms are explained in Figure 1. All moulders will be familiar with the term “General Purpose” (GP), which is used to describe an Archime- dian (helical) type of reciprocating screw that is capable of being used to process a wide range of thermoplastics materials effectively. The GP screw is often regarded as the workhorse within the moulding industry. It is widely used and is also often considered as the standard for comparison with other types or makes. The big problem is that the range of reciprocating screws used within the moulding industry and given the GP title is quite varied. This variation and its potential impact on the moulding process will be discussed in the next


June 2014 | INJECTION WORLD 41


PHOTO: SUMITOMO (SHI) DEMAG


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