machinery | Pipe dies Right:


A multi-layer die head, to produce


three-layer PE or PP-R pipe of 75-630mm diameter


requirements. For co-extrusion, the solution is a combination of one of these distributors depending on polymer, pipe diameter, output and application. Once the distributor or distributors have been


selected, it is necessary to define the pre-distribution of the melt stream, because each channel – not related to the spiral system – requires the same amount of material. Symmetrically dividing the main inlet into two, four or eight melt inlets is quite simple, though splitting it into uneven numbers is more complicated. In these cases, channel width/depth may be different. After this evaluation follows the definition of the melt


feed points which is based on the required extruder position. The next step in the design is to look to the melt merging point. It should be designed in a way that a wide range of layer distribution (5/90/5 to 45/5/45) is possible. In the past, small channels were used – which create high flow speed, high shear rate with tempera- ture increase and high back pressure. After melt merging, the tool should have the shortest


possible melt path to the final die sets. Long dies create pressure, shear stress, and long residence time – and the trend is to shorten the dies as much as possible. The latest designs of die heads follow melt merging with a centring unit, in which a master die is mounted. This die covers a certain range. Further die- and mandrel-rings are mounted onto the master die. These die set rings can be bigger or smaller in diameter than the melt merging area. (We call this system ‘add-on dies’.)


Below: A four-layer, 180mm cross-head in action – cover- ing a water pressure pipe to produce a barrier


New tools For new tools it is necessary to make a pressure calculation based on the required output and specific polymer. For this calculation, Conextru uses three main geometries: round channel, rectangular channel and ring channel. Using these, it is possible to make a precise calculation of pressure. This calculation is not based on flow simulation, but is a simple numeric method. It is used to get information about the pressure for


the areas of the adapter, pre-melt distribution, and the area after melt merging or the end of the spiral. The spiral itself is not re-calculated, as the pressure build-up is known – and it is not normally a critical part. Finally the total pressure of the entire system – start-


ing from the extruder flans to the end of the die – is calculated. The target is to be below 250bar. Adapters play an important part because in co-extrusion based on the space, adapters are sometimes long and can create high pressure if the cross section is too small. Low pressure has an influence on energy (pumping


energy) and also on the mechanical design – especially the number and size of screws. This in turn influences the time required for maintenance and mounting, as well as affecting the melt temperature. This is why it is important to calculate pressure build-up. The formulas can be found in literature but a viscosity curve needs to be supplied by the raw material supplier or – if that is not accurate – from a testing institute. These tools can be used for single- and multi-layer heads, as well cross-heads for pipe coating.


Case studies Conextru recently used these principles to update the design of a single-layer pipe head, which was then final engineered and manufactured by Rollepaal India. The head has low volume short pre-melt distribution, in a helical spiral design. It has a throughput of 1200kg/h, a centring unit with 400mm master die, die set for 630 mm, internal head cooling by air, plus internal pipe cooling. The same is true for multilayer heads, including a


recent one that was designed to produce three-layer PE or PP-R pipe from 75 to 630 mm. The head was designed by Conextru and final engineered and manufactured by Mikrosan. The head is designed for a two-extruder version according to customer require- ments, and is in operation in Serbia. More common is to work with three extruders. Another example is a simple two-layer co-extrusion


16 PIPE & PROFILE EXTRUSION | May 2017 www.pipeandprofile.com


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