MACHINERY | SCREWS & BARRELS


Design matrix linking screw Design Parameters (DPs) and Functional Requirements (FRs)


DPs FRsS Flow rate (high)


Shear rate (medium-high) Residence time (low) Heat conduction (high) Leakage flow (low) Efficiency (high)


Melt pressure (high) DP1 = Screw diameter


DP1 DP2 DP3 DP4 DP5 ↑↑


↑↑


↓ ↓ ↓


DP2 = Screw length Source: David Kazmer/UMass Lowell He presented a fractal screw design that he said


would provide the same processing for each pellet “regardless of extruder scale”. The intent is to “maintain the same – or rationally adjusted – chan- nel height and width across different extruder sizes”. Fractals are endlessly repeating patterns that are


often found in nature – such as a line constantly splitting into new channels. Kazmer said that the principle can be applied to screw design. In his case, a large channel section is divided into many smaller downstream channels.


Second principle A second principle guiding the screw concept was that of axiomatic design – which says that each functional requirement (FR) is controlled by one design parameter (DP). FRs include flow rate and melt pressure, while DPs include screw diameter and flight widths. Kazmer notes that there is “significant coupling” between screw design parameters and extrusion functional requirements. “Changing most DPs has both beneficial and


negative impacts on the FRs,” he said. Kazmer has designed a 38mm diameter screw with one channel in the feed section, two in the transition section and four in the metering section. Each channel has been designed to achieve a specific objective. In the feed section, the screw uses a single channel to ensure efficient loading with pelletised and granulated feedstock from the feed throat. The lead, channel width and channel depth in this section are 100%, 90% and 20% of the screw diameter, respectively. After four turns of the screw, the feed channel splits into two transition channels – each with a flight thickness of 10% and new helix angle of 20°.


40 FILM & SHEET EXTRUSION | March 2018


↓↓ ↑↑ ↓


↑↑


↓ ↓ ↑ ↓ ↑


DP4 = Channel depth(s) DP5 = Flight width(s) Increasing the DP either IMPROVES (↑) or DEGRADES (↓) FR


↑↑ ↓↓ ↓


↓↓


↓ ↓


DP3 = Channel width(s)


↑↑ ↑ ↑


↑↓ ↑


There are two objectives: to physically break up the solidified bed and impart more work on the feedstock; and to provide more uniform – and greater – shear on the processed material. The metering section introduces an extra set of


flights at a helix angle of 24°. Each has a channel width of 25.1% of the screw diameter and final channel depth of 9%. An alternative design – suggested by other


researchers – was to have an intermediate mixing between the transition and metering zones. It can help to homogenise melt variations while ensuring uniform flow. According to Kazmer, the screw design is particu-


larly relevant to larger extrusion screws. The designs are currently being machined, and performance measures – including output volumetric flow rate, melt pressure, melt homogeneity, residence time and energy efficiency – will be compared to general purpose and barrier screws, he said. “In the long term, the research should lead to new methods for analysing and designing com- plete extrusion systems – including screws, dies and other control subsystems,” said Kazmer.


Screw and barrel research NewEx is a pan-European research project – in- volving six organisations – that aims to build and test an “innovative new extruder” that can process composite and nanocomposite materials. The main approach will be to develop a new plasticising system – with parts including an innovative active grooved feed section (IAGFS), original rotational barrel segment (ORBS) and special screw (SS). A key central part of the project relies on a screw designed jointly by some of the researcher partners – which is subject to a patent application. At a project meeting at machinery manufacturer


Zamak Mercator – one of the partners, which is based in Poland – participants discussed the feasibility of making the extruder’s rotating barrel segments. Several design concepts were present- ed, which variously addressed: how to heat the barrel; how to transfer the drive onto the rotating segment; and how to assemble – and especially seal – the rotating barrel segment with the station- ary elements of the barrel. An afternoon workshop included an extrusion demonstration, using polyethylene (PE) with MDF (medium density fibreboard) flour and compatibi- liser. The tests were conducted at 145˚C, at screw speeds of 10-200rpm, and filler content of 30-60%. Other partners include project coordinator Lublin University of Technology in Poland, and the


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