posted April 26, 2002 04:11 PM               

Can you see anything in the center of the fish eye? Do they occur at regular intervals?

posted April 26, 2002 06:07 PM               

quote:

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Originally posted by Steve H:
Hi Dave

Can you see anything in the center of the fish eye? Do they occur at regular intervals?

Steve H

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Steve,
Thanks for your consideration of my post. No, nothing evident at center of fisheye. Regularity is hard to gage at such low frequency, but there is no regularity with respect to time of day, day of week. All attempts to rectify have neither resulted in reducing frequency or (something maybe as significant) increasing the frequency. Such attempts have included:
1. Cycle time increase and decrease of 10% with corresponding screw RPM changes.
2. Reducing inner-body head gap to increase back pressure. This reduced marbelization, but had no effect on fisheyes.
3. Reversing extruder temperature profile dramatically.
4. Pulling out of gaylords vs. silo with different lots of resin.
5. Placing pressurized dry air diffuser in feed throat of resin hopper. (This has helped in the past when moisture in the resin was present.)

Unfortunately, I do not have melt pressure indication on this machine.
Any ideas you might have or subsequent questions would be welcome.

posted April 28, 2002 01:32 AM               

The fact that you haven't found any black specs at the center of your fish eyes suggests the cause is either Higher Molecular
Weight material, or cross linked material. Have you tried other suppliers equivalents to the grade you normally run?

Are you adding anything to the resin, colourants or demoulding aids- if these are incompatable with the parent resin, that could be a cause.

As far as processing issues go;

- Have you cleaned the barrel and screw, are there any marks/damage to the screw that
could be causing material to hang.

- Is the screw worn?

- Temperature of the feed throat?

- Can you measure melt temp? could you
retrofit a melt pressure transducer?

- Is the problem of recent origin, or has
it always been present. Is the screw design
appropriate for HDPE (what are the depths
of the feed and metering sections)

- Has the calibration/operation of temp
controllers been checked lately?

As far as the marbelization issue goes- could
a static mixing section be fitted. This would
improve mixing without raising back pressure too much.

posted April 29, 2002 05:13 PM               

2. No additives whatsoever, a natural product.

3. As far as wear or cleanliness level of screw, we've seen no output per RPM drop whatsoever and have used nothing but 100% natural virgin material on this machine. We have in the past used purging material to remove char buildup, the last time about a year ago.

4. Temperature of the feed throat varies wrt to the temperature profile we've programmed.

5. WRT measuring melt temperature, we use an optical pyrometer (Raytek) but do not use any type of immersion measuring.

6. WRT measuring melt pressure, I can't find an obvious penetration for a pressure transducer, but will check with barrel OEM for such

7. The problem emerged when we transitioned to a new material due to a HDPE reactor closing at our supplier. Our end product is an ultraclean container and we only have one qualified material.

8. Calibration of thermocouples is a good question.

9. Right now, would like to eliminate the problem with process controls rather than screw, tip or screen pack modifications.

Are there discrete charateristics differences in the void that would determine whether the cause is moisture or trapped air?

On a much happier subject, I am an avid hiker. Have you ever had the opportunity to visit or walk the Milford Track?

Thank for your thoughts.

posted April 30, 2002 04:09 AM               

No, haven't walked the Milford, sailing is more my bag- but I must do it before they screw down the lid :-) Where in the world are
you?

Getting back to your problem, the reactor that was producing your material has been shut down, and the material you're have problems with is from another one. Maybe thats the "change" thats causing your problem.

In any batch of material, there is going to be small pockets of different molecular weight material- perhaps there are some subtle differences between the screws on your
machines. On "A" the small amount of HMW material present in the normal "spread" is enough to start the formation of a gel off- B's different enough that this doesn't occur.

I'd be inclined to pull the screw and clean it by hand once in a while- and check for
screw wear/damage etc.

As far as a settings change (magic bullet), have you tried running a slower screw speed, and slightly cooler barrel profile. Does your
BM extruder have a water cooling jacket around the feed pocket. On a conventional single screw, these are cast iron- and condensation or a crack can be a source of moisture problems.

You might try posting this problem at www.plastics.com, there is a little more
traffic there and someone may have some cunning ideas to solve your problem. If you are an SPE member, you could try Ask PIP- www.4spe.org

posted April 30, 2002 08:45 PM               

Based on the fact you sometimes have poor dispersion, I would guess that your screw is not melting and mixing the material well. This could lead to unmelted material getting through the mixer even though it has tight clearances. The unmelt can melt later, but it will be at much higher viscosity, and cause flow problems. If you have a breaker plate and the ability to add filter screens I would try to add a lot of back pressure by putting in a thick pack. This should solve the fish eye problem, but may cause other problems. Thus is the nature of extrusion.

posted May 01, 2002 11:27 AM               

I have tried lower speed and temperatures to no avail. I do have a tech rep from the material supplier coming out week of May 13, will be interesting to hear what he has to say after seeing my extrusion conditions first hand.

posted May 01, 2002 05:23 PM               

Keep a good eye out for "rattlers"- Land yachting happens in Navada, so a man can always get a feel for speed under sail where ever theres wind (and some flat land).

Am I right in assuming that your gel problem is of recent origin- have you always had dispersion problems or did they start occuring about the same time as the gel issue?

We use both Maddocks and Pinned screws, I've never had any evidence of unmelt getting past a Maddocks section (HIPS,ABS,PP).

My bet is either a material, wear or calibration problem if the problem has just started to occur. If it's always been present, I think you have to look at your screw design (screen/breaker plate adaptors)
for either incorrect design or a hang up point.

If the "techie" from your material supplier can't help, you could try talking to Allan Griff-PH (301)654 1515. Allan is an extrusion
consultant, but he may be able to offer some insight on your problem. His charges are very reasonable and he is a very knowledgeable guy. Tell him I pointed you in his direction.

Keep us updated with what you find.