moulding masterclass | Screw design – part 4
Barrier screws such as Xaloy’s Quantum provide very high plasticising capacities
fragments, causing the black specks in the mouldings during either the same or a subsequent production run. It is, therefore, very important when using such screws not to leave the screw and barrel assembly full of polymers such as PC on completion of a production run. In such situations, purging of the screw and barrel with either a proprietary purging agent or with GPPS or PP is essential as the bond strength of polymers such as PC to the nitride surface is suffi cient to severely damage the working surface, with complete sections, or lumps, literally being torn away from the screw surface. A screw produced from a bi-metallic Cr-alloyed steel does not experience the destructive (damage) issue and the problem of black specks is diminished to a great extent.
What is particularly important, irrespective of the design of screw employed, is its ability to effectively convert the solid granule, or particle, into a uniform liquid melt without generating too much heat energy and to plasticise the polymeric material at the necessary output rate for effective productivity. This and the previous three articles
relating to screw geometry have covered the fundamentals and generic issues with respect to screw design criteria and relevant processing performance. Of course, many manuscripts have been written and seminars presented on the subject of screw design/geometry, and many hours have been taken up discussing the pros and cons relating to the selection and performance of one design compared to another. The purpose of this series was not to replicate such data. Those wishing to explore this area in greater depth will fi nd plenty of technical publications within AMI’s Plastics Information Direct bookstore.
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.
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