Advances in mould and temperature control technology have resulted in a continuous reduction in cooling times and significantly higher throughput. The required melting capacities are often not achieved with conventional three-zone screws. A double flighted screw newly developed by Engel Austria achieves this performance boost.With its flat cut feed zone the double flighted screw ensures a high input of shearing energy, so that a high degree of melting has already occurred prior to compression.
This prevents the formation of a "solid wedge" which would result in deflection of the screw and increased wear. In addition, a second flight reduces deflection from the centre of the intake zone. This is situated directly opposite the first and ensures a symmetrical pressure profile during plasticising. To avoid feeding problems, the material is initially delivered through a single flight. In this way, significantly higher throughputs can be achieved compared to the standard screw, particularly in applications which were hitherto considered problematic.
This includes, for example:
Technical articles made of PP: As an easy-flowing plastic with poor thermal conductivity, PP is difficult to melt. In order to nevertheless achieve a good melt quality with standard cycle times of 10 to 20 seconds for technical injection moulded parts, barrier screws are normally employed far below their performance limit.
Thin-walled technical articles made of PA: Similar to PP, PA requires high melting energy, but due to its hard granulate structure and high melting temperatures it is not suitable for processing on a barrier screw. For fast cycles substantially larger screw diameters or injection units must therefore be used.
Components made of POM: POM already starts to degrade just above its processing temperature of approx. 200 to 220°C. At excessively high speeds or throughput rates solids are pushed into the compression zone and the screw is pressed against the barrel, which may soon result in wear-related failure of the unit. For this reason large screws must usually be employed and additionally the speed limited. However, the use of larger screws incurs higher costs, particularly when a larger injection unit is required.
Compared to conventional plasticising screws, significantly more cost-effective and technically optimised solutions and possible throughputs can be realised with double flighted screws in the processing of easy-flowing plastics, particularly in the above-named applications. Depending on the plastic, average cycle time reductions of approx. 30 percent can be achieved with the same screw diameter.
The double flighted screw is available in standard lengths (L/D 20) with diameters of 35 to 80mm. Three materials designs can be supplied: nitrided screws for unreinforced plastics, through hardened screws for corrosive and reinforced plastics with a fibre content of up to 30 percent and through hardened materials with a modified CrN coating for medical applications and plastics with corrosive additives, such as flame inhibitors.
