moulding masterclass | Screw design – part 4
In the fi nal instalment of this four part review of screw
selection, moulding expert John Goff discusses
material residence time and selection of special screw profi les and materials
Improving your profi le
So far in this series we have looked at the three different sections of the plasticising screw and their role in the feeding and preparation of a consistent and reproducible melt. In the previous instalment, we discussed how use of too high a shot capacity value – 70% and beyond – can result in reduced shot-to shot consistency and part qual- ity due to insuffi cient residence time of the polymer melt in the plasticising system. Of course, the converse can apply when injection moulding components using screw and barrel shot capacities of 20% or less and when lengthy cycle times are needed to achieve the required quality standards. In such cases, the residence (or dwell) time that the
molten material remains in the screw and barrel assembly can signifi cantly infl uence the overall proper- ties of the polymeric material being processed due to thermal degradation. Such degradation affects the molecular structure of the polymer, causing a reduction in the length of the molecule which is refl ected in the resultant mouldings displaying issues, including reduced mechanical strength, brittleness, poor surface fi nish and colour inconsistency due to degradation of pigments. For this reason, it is crucial to ensure that shot
capacity and cycle time are optimally matched to prevent excessive residence time impacting on the in-service performance expectation of the moulded component. This is especially critical when working with temperature sensitive materials such as saturated polyesters (PBTP and PETP), acetals (POM homopoly- mer and POM copolymer), or any UL94 V-0 (fl ame
36 INJECTION WORLD | March 2015
retardant) grade of thermoplastic material. It is recognised that, for commercial reasons, the
common approach within the injection moulding industry is to use a particular designated screw and barrel assembly of a given diameter for a wide range of moulded components of differing mass and type of thermoplastic. Unfortunately, in certain situations this principle will result in defective components being produced when attempting to achieve the required moulding quality standard at the budgeted cycle time – correct end service requirements and overall consistency of performance will not be maintained.
Alternative screw designs As stated previously in this series, the ‘General Purpose’ design of screw is often classifi ed as a good workhorse but it can lack the fi nesse required to achieve ultimate melting performance across the entire range of thermoplastics materials. For this reason many other designs of screws are commercially available where changes in the geometry concerning the length of the feed, compression and metering zones, the pitch between fl ights, and/or the compres- sion ratio are made to accommodate particular types of thermoplastic material. Changes in the length of the transition (compression) zone of the screw have a marked effect on both output rate and the melt quality that is achieved at the front of the screw. When processing highly viscous materials such as acrylics, SAN, PES/PSU, PEI and PCs, use of
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