posted August 06, 2003 08:10 AM               

Look closely at conventional throats and barrels. The design is an evalutionary thing and it simply is easier to unbolt the barrel based on current designs.

Zone 1 is restricted to zone one. It would take a lot of redesigning for the water jacket too. However, your idea would be most interesting to see once it's operational. It does make sense though to eliminate one variable, but because the first section is merely the feed section, rarely does any true melting occur there. I would think this is why the conventional system evolved. Even with a slight dead spot there, little if any degredation would occur there unless it is affuly mis-aligned.

posted August 06, 2003 05:27 PM               

The minimum clearance can be calculated given the materials of construction and their thermal expansion. (Plus an aligmnment factor, and other fudge factors if needed.)

Calculate with the throat at 60F and the screw at 600F, or 750F for flouropolymers.

posted August 07, 2003 11:50 AM               

However because most wear occurs in the center of the barrel, and usually clearances aren't so critical in the feed section anyhow, when it comes time to replace the barrel usually only the forward section requires replacement. Since the forward section is much cheaper than a one-piece feed/forward, most (all?) OEMs use a 2-piece barrel now and end users only have to replace the forward section.

posted August 08, 2003 02:54 PM               

quote:

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Originally posted by samin:
Thank you Tom C. By undercutting screw in feed area we get more clearance but would not it decrease frictional forces depending on pallet sizes. What is your recommendation for 1" extruder in a normal application running nylon 12? Our experience shows that when we change clearance on each side to .004" material stop feeding.

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Over the pat 20+ years, I have specialized in medical applications and in particular have used .5"-1.25" machines quite a bit.

I have been using screws with .001"-.0015" clearance, no undercut, and feed throats with xalloy liners for ages and I have never had a misalignment issue.

I find that when running these small machines with undercut screws at low RPM can cause problems with pellets getting caught on top of flights (resins like 40A PUR, chronoflex, etc. can be maddening at times).

Of course, running at higher speeds would help, but when using a specific tool for specific properties of the tubing, we are forced to rely on specific designs for our applications (we like to keep the head pressure <5kpsi).

An .008" OD tube with a .0005" wall while minimizing orientation in the extrusion direction will do that to ya!

On that note, I never met a <1.25" Davis Standard feed throat that I liked......I've remanufactured all of them I have used. Poor cooling is their biggest issue, as is the casting opening.