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machinery | Pipe dies


e.g. a protection layer to a PE pressure pipe or multilayer and there are a lot of different versions existing. The rules for designing the cross head are the same


– only the size will vary, because we need to have an internal passage that is around 10% larger than the pipe to be coated. There also needs to be some space for internal heaters. The process is called tubing coating – which means that the additional layer is applied under pressure inside the head. We use low capacity vacuum pumps or blowers for building up the internal pressure, and a high vacuum is not required. In one recent example, a large diameter cross head


Above: Fuel pipe within a three-layer protective pipe


version. If the output for the outer layer is a maximum of 10% then the best solution is to add a co-extrusion unit – based on radial spiral design – to the main head.


Pipe coating An increasingly important tool segment is cross heads, which are used to coat a plastic or steel pipe in line with a single or multilayer structure. A further application is in bundling small pipes that are used for optical fibre protection: three, five or more pipes are arranged into a round, flat or rectangular shape, followed by a coating process to keep the shape. A two-step coating method using a cross head is very


simple and offers some advantages compared to single-step multilayer extrusion process. These points are summarised in Table 1. The polymer structure of the coating can be monolayer


Table 1: Advantages of two-step pipe coating with cross head 1 STEP PROCESS


With multilayer die Advantages


Good adhesion of additional outer layer


Conventional downstream process


Existing lines can be converted to co-ex lines by modifying existing heads


Simple process Calibrated outer surface Disadvantages


For an extra outer layer, new calibrators and seals are needed


Monolayer production is possible if outer layer extruder is not in use


Keeping the outer diameter of inner layer round and stable is difficult


Difficult to peel outer layer in the case of electrofusion welding


up to 180mm was designed to cover a water pressure pipe with three or four layers, to produce a barrier pipe according EN 12 201. The head offers a quick, simple change from three to four layers if needed. For quick cleaning, the distributor for EVOH can be pulled out from the back side. This principle has also been used for: a large


diameter single-layer cross head up to 630mm, to cover a PE100 water pressure pipe with a 2mm PP coating; and three-layer (PE/adhesive/EVOH) protection pipe over a four-layer PE pipe, for fuel transportation on petrol stations.


In summary, demand for multilayer pipes will


continue – as new polymers and applications are developed. Meeting this demand will require flexible engineering. The polyolefin tools currently being designed and developed will play an important role in the development of these pipes. ❙ www.conextru.eu


2 STEP PROCESS


With single or multilayer die and single or multi-layer cross head Advantages


Disadvantages


Adhesion is adjustable by temperature – which is


important if the layer must be peeled off


Quick change from mono-layer to multi-layer


Cross head dies have very wide outer diameter range


Only one solution for pipe in pipe extrusion


Good solution for more than one co-extruded layer


Easy to insert wire for detection Printing on outer surface of basic pipe is possible


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


Pre-heating of pipe is needed for good adhesion


Downstream equipment is more complicated


Tools and ancillaries are more expensive


Extrusion line is longer Not a calibrated surface


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