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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