posted October 27, 2004 04:09 PM               

We found when we run rigid PVC part with rigid PVC screw, there were always many black lines on the surface. But when we switched the screw to ABS screw under the same process conditions, black lines had gone. I did tests and am sure the black lines were not from the die and screens. The only thing we are focus on our new rigid PVC screw. Could the screw cause this problem?

I contacted with the screw supplier. He said the black lines are from the barrel or the white masterbatch. We have three different 2" extruders and three different 2" matching rigid PVC screws (all from the same supplier).
Nomatter on which extuder and which rigid PVC screw, black lines were always there, just some was worse and some is light. Can the barrel cause it? Is the length of screw will cause this problem? I found the
contamination may from the very front of the barrel & screw (the area connect with the extruder head).

We are using 2" L/D 24:1 single screw. Our operation temp. is 340 F. The information about the screw that supplier gave to me: OD 1.996, Root diameter/melting+1.625, Root diameter/feed+1.175, pitch+2.000, width+.200.

One more question here: Is it normal the screw supplier won't provide the screw parameters about the screw? Ours won't give me.

Thank you very much!

posted October 27, 2004 06:42 PM               

Black lines on the extrudate surface would imply degradation of the PVC after the screw. If degradation was occuring in the screw, the degraded material would be more uniformly spread out on the profile.

If the PVC screw is shorter than the ABS screw this might be the source of the problem. Even if the flight of the PVC screw ends sooner than the ABS screw it still qualifies as shorter.

Before running the extruder check to see where the flight ends for both screws. After running some PVC pull off the die head and look for burnt material after the screw.

Many PVC screws will have a pointed end with flights in order to eliminate any dead spots.

Do the stripes occur on stsrtup, or after running for a while?

It costs screw manufacturers to design screws. The value of the design is not normally included in the quoted price. Next time ask for a quote for a screw, ask for a drawing of the screw. I can also provide computer simulations of the expected performance of the screw if asked.

Tom Cunningham www.ExtrusionTech.com

posted October 27, 2004 07:08 PM               

I did check the flights. They are not pointed end. We have all the same problems with this supplier's screws. We just ordered another two new screws from the supplier. I guess it is hard to cancel the order. Will keep reducing the process temperature help? Do you think it can be rebuilt??

I am not good at the screw termknowledge. What do you mean"Even if the flight of the PVC screw ends sooner than the ABS screw it still qualifies as shorter". PVC screw ends sooner?

Thank you

posted October 27, 2004 08:36 PM               

Sounds like it would be best for myself, or an partner of mine in Canada, to eyeball your situation. Likely any screw can be modified easily to resolve the problem. Please email me if you are interested in pursuing this further.

posted November 03, 2004 03:32 AM               

How streamlined is the breaker plate supporting your screen (pack?), posibily the outer circle of holes needs counter boring (both upstream and down) to eliminate a hang up point or stagnant zone (downstream).

Welcome to the single screw forum too

posted November 12, 2004 07:48 AM               

The chamfered breaker plate is different from the mixing invert plate (different disign, this is going to hiding the black (pink) lines, not get rid of the lines).

Our screw design is not proper, since the rigid PVC screw looks more aggresive than the ABS screw. The other thing is with the same flight number (26) the process length of rigid PVC screw is 15mm shorter than that of ABS screw, should that be?

posted November 12, 2004 02:14 PM               

quote:

---

The chamfered breaker plate is different from the mixing invert plate (different disign, this is going to hiding the black (pink) lines, not get rid of the lines).

---

Oopps, should be a new paragraph. What I inteded to ask, was, is the breaker plate shown in the link similar to the one recommended by your supplier.

Cooling a screw will reduce output slightly at any given screw speed, with PVC the idea of cooling the tip is to try and prevent material degrading at the end of it.

How many L/D's are the metering, transition and feed zones of your PVC screw and the one you use for ABS.

I am still inclined to think that material is hanging up around the outer edge of your breaker plate.