posted March 01, 2007 12:51 PM            

it's a while since I've been running my extrusion dept, but have always found your advice to be superb. Hopefully you can follow on from previous problems???

I am looking to run a 4mm HDPE round tube (more like a solid rod)with an aramid fibre (Kevlar) through the centre only for added strength.
The extruder is 60mm and I am using a landed die with a pressure set up. The tooling is similar to what we would normally run our extruded pipe with, but solid extrudate required. The extrudate is water cooled at exit point but the shape is deforming (flattening) further down the cooling tank. This is not because of heat retention as the product is cool at this point. The Kevlar is quite a fine Tex and will only measure in the region of 0.5mm. Has anyone any thoughts on how best to process this? I am using HDPE as this is readily available but would consider other materials with similar cost.
I have a large quantity to run, circa 600,000 metre so would dearly love a solution.

Many thanks for your anticipated help.

posted March 01, 2007 01:12 PM            

I have seen small round extrusions flatten when a slightly foamed center was present and the extrusion would collapse in on foamed center. Could you be dragging in a lot of air or moisture with the Kevlar to casue a similar effect? Or perhaps you are just not wetting out the Kevlar good enough. Some type of special die arrangement might be required.

I have some 2D flow modeling available which has proven useful in wire coating trouble shooting. Please email me if you are interested in some professional services.

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Best Regards,

Tom Cunningham

posted March 01, 2007 01:18 PM            

I think we may be dragging in some air, but not sure how to prevent this. My "gut feeling" is pointing to the tooling configuration, but not sure which is the best way to approach this.
Professional advice may be the solution.

posted March 01, 2007 02:03 PM            

The way I would approach this is to design a 2D cross section of your process and put it into a computer flow simulation. From the simulation I can see things like whether the HDPE is frezzing on the Kevlar before it can wet it, or if there is a pressure peak before the Kevlar is wetted. If the pressure peaks fists, then air can be trapped.

What might be required is a combination of pre-heating the Kevlar with IR lamps, and then configuring the die so that pressure builds continuously, and air has a chance to escape out the rear.

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Best Regards,

Tom Cunningham