Pipe dies | machinery
Demand for polyolefin pipe tools is healthy, thanks to growing sophistication of pipe systems, says Josef Dobrowsky of Conextru
Innovations in polyolefin pipe dies
In recent years, there have been a number of significant trends in pipe applications, including: higher demand on larger diameters; a shift from monolayer to multilayer; cross heads for pipe coating; an increase in capacity and throughput; and, performance increase of polymers. These trends have led to continuous adaptation, changes in design and new solutions for pipe heads. Today, there are many expectations of a pipe tool, and these include: l High throughput at low pressure: this has an influence on aspects such as the mechanical design, energy consumption and on melt temperature. l Optimised flow: carefully designed pipe tools help to reduce overweight, and save raw material. l Pipe inner cooling: higher output rates require longer cooling time. Modern tools should offer internal pipe cooling – which cuts cooling length and also improves factors such as roundness, eccentricity and internal stress. It can also exhaust some evaporated waxes of PE which would condense on the inner surface. Internal pipe cooling reduces the thermal load on the material, which can be seen on the OIT value. l Wide dimension range: nowadays, a basic pipe head is used to make pipe sizes that are bigger than its fixed melt outlet diameter. This is possible by changes in the mechanical design. Furthermore, a wider range of pipe sizes can be produced by draw down method for most polymers. Sometimes, three or four pipe sizes can be produced with a single tool set.
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l Easy handling: this means that low weight of die parts and horizontal die mounting allows changes to be made more quickly and easily. A low number of screws and easy cleaning are also important design criteria. l Low volume and small surfaces: this means short residence time – which leads to a short heat history for the material and short self-cleaning time. l Internal head cooling: this is needed for large size tools in order to keep temperatures stable over long term production. Air cooling systems are efficient and low in operational cost – and are preferred to oil or water cooling systems. Due to these criteria – and ongoing changes of
requirement – there is a growing market for polyolefin pipe tools. In my estimation, the demand for new tools each year is 1000-1500 units. The most common size is 250-630mm. These pipe heads are mainly for use in new extrusion lines, but also include changes from monolayer to multilayer, and investments in tools with internal pipe cooling, for instance.
Distribution points If we look more closely at how to design a new tool, we need to decide which distribution system to use. The most common is the helical spiral system, which is also found in blown film lines and blow moulding machines. The spiral distribution system requires a calculation
of the flow channel. No other distribution system is so dependant on calculation. This is quite complicated and
May 2017 | PIPE & PROFILE EXTRUSION 13
Main image: Conextru has designed a number of pipe heads,
including this 630mm model
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