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Multilayer pipe heads are established in the market since many years. The number of layers are depending on product and application from 2 up to 7. Usually a multilayer head is designed for a certain pipe structure and is used just for this structure only. Quite often pipe head manufacturers are asked about flexibility in using the head for other layer structures and unfortunate the answer is in most of the cases that the particular head is not designed for this kind of flexibility.
CONEXTRU has developed a pipe head system which can be used for a quite big range of layer structures as well as with different materials. As an example here the range of structures: A/A/A, A/B/A, A/A/B; A/D/C; C/A/B The materials are: A - PE or PP; B - PE or PP Regrind; C – PA or PVDF or PET, D- Adhesive. Pipes for Monolayer PE for water and gas, 3 layer PP-H for sewer pipe, 3-layer PE with regrind, 2-layer PE with PP protection layer, 3 layer for fuel transportation and some more.
One of the important requirements for flexibility is that the position of each extruder must be same for all pipe structures. Only a short longitudinal movement of few centimetres are allowed – only for mounting to the head.
Of course the extruders must have this flexibility in terms of processing different polymers without changing screw. The extruders are optimised for that and instead of highest output – wide processing range is required. This can be achieved if extruder has corrosion resistance screw and barrel for processing corrosive polymers, have barrier screw design for and temperate grooved intake which can be operated at high temperature if needed. Gravimetric system for output control is also a must in case of co-extrusion at different output levels.
The multilayer head have adapter connection which allows that the extruder can keep the position as it is by extruding different layer structure. But how this is possible?
This is possible if in between the multi spiral channel head and the extruders is an adapter block which brings the melt of each extruder to the right distributor of the head.
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For each layer structure – as shown before – is a special adapterblock designed. The adapterblock works like a feedblock bringing the melt A from extruder A to the inner or middle or outer layer depending on the product. If the layer structure has to be changed because of a new product than the foreseen adapterblock has to be mounted in between extruder and head.
The multilayer head has 4 different spiral distributors. They are designed for best melt distribution for low viscosity polymers like PVDF, PA, EVOH or Adhesive as well as for high viscosity materials like PE 100 RC types or PP high modulus types.
The run different layer thickness and a wide range of layer ratios inside to outside or in the middle – the melt merging area has to be designed for. In our particular case – mentioned before – we have a wide range of viscosities. High shear stress as well as high flow speed differences at the border in between two layers must be avoided to prevent flow anomalies. Flow anomalies are visible as waves or shown by penetration of polymer B into Polymer A as an example CONEXTRU calculate based on rheological data's shear rate, viscosity, flow speed at the melt merging point for the required melt structure and design the gaps at merging point accordingly to avoid high shear stress.
For best tempering of the head outside ceramic heaters are installed – with isolation if needed and with higher installed power in case for running PET or PA at high temperatures.
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Inside the head is heated as well and also there is an option to cool them by air – if needed (IHC internal head cooling) can be used. The inner parts is sealed to all openings and the air is brought by tubes into this area and brought out from that area. The air flow is low and generated by small blower.
Not all multilayer pipes can be produced with same die design so it's sometime needed to have specific die design depending on the pipe product. However the system works with a master die set which is placed in the centring unit. On this master die set – so called add on die sets can be mounted. The horizontal die set change combined with segmented mandrels makes the system further flexible and user friendly.
If colour stripes are required than the best option is to apply them via a CSR colour stripe ring mounted at die end. For corrosive materials it's recommended to use stainless steel for all parts which may come in contact with corrosive polymers. A complete chromed version of the head and adapter blocks would be an alternative – which is in the same cost level as stainless combined with nitration steel.
Cleaning is a big issue in case of using different polymers. Extruders can be cleaned just by running the material thou the barrel – without connected to the adapter block. The adapter block is a small light weight part which can be cleaned by hand or by a burning process. The cleaning of the head can be done either just running the new polymer thru. This pre melt distribution as well the spirals have low volume and low flow channel surface and a good self-cleaning behaviour. This makes this method possible.
Another option would be to clean the head form polymer by dismantling and cleaning by hand. The head can be motorised turned into backside up and die set down position. At this position each distributor can be pulled out up wards by crane and then a good access for cleaning is given.
The author: Josef Dobrowsky, CONEXTRU GmbH
