Excessive sparkouts

I run a high-speed THHN extrusion line (12/19 copper mainly). It's set up with 2 Davis-Standard extruders (they were meant to be nylon extruders, however, thus a very small barrel and screw) with a Microdia crosshead. The problem is this, the faster the line is ran the more sparkouts it generates. I'm currently running at 1000 FPM, on a line that will run 2500 FPM. This is a new set-up, and I personally think that the small nylon extruders can't provide a proper melt to the PVC at a high speed. Anyone have any thoughts on this?

Hello line108,

I guess the basic questions are:

- Do you have a pvc screw in the pvc designated extruder and are all the PID temperature controls on the whole line in good shape and calibrated?
- Is this screw in good shape and are the screw dimensions OK? (Especially the diameter over the screw flights.)
- Do you have indications of burning of the pvc embedded in the pvc extruder screen pack (This is created by excessive screw shear.) or on the screw itself?
- Are you near or over the Davis Standard projected output (Say in lbs per hour.) for (flexible?) pvc for that extruder/ screw combination?
- Do you have a nylon screw in the nylon extruder?
- Is the nylon extruder screw in good shape?

A dual pvc/nylon crosshead is quite tricky because of the different melt temperatures of the nylon and the pvc. As I recall, Microdia (Swiss) worked a long time developing such a crosshead. Be very sure that the pvc is not overheating and burning in the crosshead.

Presumably the nylon resin is not smoking when you are running at high speed.

Can you tell us the diameter and L/D ratio of the two extruders?

Kindest regards,
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com

Thank you for your reply. I didn't clearly explain the set-up of this line. The crosshead is a "quick color change" system. It has two extruders flowing PVC through it at all times. There is a seperate extruder coating the PVC with nylon. The two extruders that feed the crosshead with PVC were originally nylon extruders, with a small barrel and screw. As we all know, PVC extruders are significantly larger. Is it possible for PVC to flow through a small extruder made for nylon and get a good melt at a high speed ?

Hello again,

You are not giving us any help with your problem. We cannot see what you see or understand what you are not telling us.

Regardless of the configuration of the two crossheads (One single for the nylon and one dual for the pvc base and the pvc color coat.), the fundamental problems still remain.

With any pvc extruder, one of the the most important things is to make sure that you are not burning the plastic by excessive shear inside the extruder or by heating the plastic to too high a temperature and burning it that way. Likewise, dead zones or areas of long residence time inside the extruder and crosshead are to be avoided.

The faster you turn the screw, the more chance there is of burning the pvc by screw shear, expecially over the screw flights. Moreover it is a really good idea to have a regular clean out of the extruders and crossheads scheduled to ensure there is no burned plastic stuck to the screw.

You should also be using screws designed for pumping pvc.

The temperature and burning concerns as described in my first reply are still appropriate and to the point. They are the starting point in your investigation.

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You have to evaluate your process and determine if indeed pvc burning is the issue. Good luck with your investigation.

Kindest regards,
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com

Peter is correct in the main question of the screw design as you had mentioned that these were both designed as nylon extruders. A nylon screw will be significantly different from a PVC screw and would not process efficiently if running PVC. I would personally check the L/D of the PVC extruder and the screw configuration (compression ratio, # of flights of feed/transition/metering and actual flight depths) and then talk to a quality screw designer (check with your local screw manufacturer) to ensure you can process with that screw. I would say that if the 2 extruders are the same size and configuration, you will not be able to run very efficiently.

Your screen packs will be a good indicator of problems - if there are black specs there, you are making scrap. If there are unmelted pieces on the pack, likewise garbage.


Erik Macs
VP North American Machinery Sales
Fine International Corporation
148 Oak Street
Natick, MA 01760 USA
508-315-8200 fax: 508-315-3621
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