Screw design – part 2 | moulding masterclass
Consistent preparation of a homogenous melt is essential in achieving a repeatable moulding process. Moulding expert John Goff discusses the basic elements and functioning of a plasticising screw
Screw tips: preparing the perfect melt
In the fi rst instalment in this series here we outlined the basics of plasticising screw terminology and discussed the critical role this key component plays in production of good quality parts. Very simply, poor conversion of solid granular polymer resin to homog- enous liquid melt during the plasticising process means production quality will suffer. The workhorse of the plastic moulding industry is
the General Purpose (GP) screw. It is widely used and often held up as the standard when comparing screw designs and makes. However, as was made clear in that introductory article, GP screws vary considerably from moulding machine manufacturer to moulding machine manufacturer. All plasticising screws are comprised of a series of functionally-designated sections (see Figure 1 for basic screw terminology). Generally, three sections are found along the length of a typical screw: feed; compression/ transition; and metering/delivery. Each section is designed to perform a specifi c task. The feed section facilitates the acceptance of the
granular, powder, solid polymer into the screw and barrel assembly. As the screw is rotated, the incoming polymer material is moved forward along the feed section in and around the continuous Archimedian spiral fl ight (most commonly designed with a 17.7o
www.injectionworld.com To ensure consistent moulding production, the
movement of compressed material along the gap created between the screw fl ight, the barrel wall and the constant core diameter of the screw needs to be consistent in quantity and repeatable from one moulding cycle to the next. For this reason, it is important that the temperature at the entrance port (throat/intake zone) where the material enters the screw is consistent and controlled. This will ensure that a molten fi lm of polymer is created at the point where the internal surface of the barrel is in contact with the granular material. Through the creation of this layer of molten fi lm, the
plastic granule will stick to the inner barrel surface and so, upon screw rotation, the leading edge of the fl ight will move the granular material along and around the continuous spiral. The extent of movement depends upon the timed rotation of the screw and the volume of material utilised to produce fully packed mouldings. Process consistency requires precise control of the
throat temperature and effective selection of process parameters. This will prevent screw slip occurring and also minimise the issue of inconsistency of the shot volume.
In general, the common effective working length to helix angle).
diameter ratio of a GP screw is 20:1. This means that a 30mm diameter screw will have an effective working
November/December 2014 | INJECTION WORLD 45
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