posted April 03, 2007 08:26 AM            

We are trying to make thick HIPS sheet from recycled HIPS repro and regrind. The thicker we make the sheet the more problems we are getting with streaks caused by moisture or volatiles??

We can make sheet up to 0.080 with out much problem but it gets progressively worse on sheet up to 0.250 inch.

We have a 3.5 inch two stage with a gear pump. Our extruder is vented with a properly funcitoning vacuum pump.

We can also predry the material but I did not think that was necessary with HIPS if our extruder was vented.

I have tried different temperature profiles, checked to see if any heaters are overheating,different sources of regrind, repro etc. etc. etc.

Is this likely to be a moisture problem, vapor from degraded or material or am I just dreaming that we can make 1/4 inch HIPS sheet from recycled material???

posted April 03, 2007 08:48 AM            

What is the head pressure for the different thicknesses?

What is the vacuum level you are getting?

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Best Regards,

Tom Cunningham

posted April 03, 2007 11:46 AM            

We have tried the same conditions as with the thin sheet but we have also tried many other temp profiles and head pressures.

We have tried different head pressures both by using different screen packs and by adjusting the suction pressure to the gear pump.

Notes:
1. the moisture marks are across the whole width of the sheet. These "streaks" resemble an elongated bubble but it is superficial ie. not a big bubble from air entrapment.
2. I was thinking of drying our feedstock excessively in a dessicant drier as a trial. If the problem goes away, I could then conclude the problem is surface moisture vs. volatiles from degraded material. Then I could reduce drying time to a reasonable levle????

Thanks a lot for your help.

posted April 03, 2007 03:08 PM            

Welcome to the single screw forum.

Is the streaking present at all when you are running thinner gauge sheet? While HIPS isn't hydroscopic, surface moisture on granulated scrap will give the problem you describe, so a drying trial is worth a try.

Dust on/in regrind/repro, gives sheet a cratered appearance.

Do you know the history and grades of regrind/repro you are running, is there any chance of it including Flame Redardent HIPS.

Steve

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Steve Hodgson

posted April 03, 2007 07:39 PM            

Can you see the defects exiting the die, or only afterwards?

Translation - Could your downstream process be casuing the issue?

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Best Regards,

Tom Cunningham

posted April 03, 2007 10:08 PM            

A greater vacuum may help. Also, better pre-drying may help, as suggested in a previous note.

Also, be sure there is enough back pressure to properly keep the second stage filled so that it seals the degassing stage from the exit of the extruder.

Conversely, if the back pressure on the extruder is too high, then the second stage will be quite full (if not flooded.) This will minimize the area for degassing and impede the release of all of the troubling vapor/moisture. A good math model of your extruder can help you set the pressure for your thicker product.

Summary: At the higher rate, you may have reduced area and reduced time for gases to diffuse in your degassing stage of your extruder. This causes imcomplete degassing.

posted April 04, 2007 06:15 PM            

Replies to some of the questions:
1. History of the regrind... I know some of the raw material came from prime sheet that was ground up but the repro is hard to say where they get their feedstock. I think there is probably some fairly overprocessed stuff in there that is doctored up with rubber to improve impact.

2. Defects are coming out of the die. It is quite visible. It is definitely not a downstream issue.

3. Head Pressure, rates etc.
We are not really running it very quickly ...about 250 lbs per hour on the thick sheet. This is the same rate as for the thin sheet that we run. Therefore different shear rate before the die would not be an issue. Of course residence time in die is much longerwhen making thick sheet. As well most of our thin gauge is made on a 40 inch die vs . a 54 inch die for this thick sheet.

4.Streaking present on thin sheet... Its quite interesting but no it is not at all an issue running sheet in the 0.025 range. It is quite noticable in 1/8 inch and worse in 1/4 inch.

5. Dust etc. causing craters. We are actually getting a little of this but it is a different and tolerable problem.

OK I know you are all dieing to know what happened today after we dried the HIPS in a dessicant drier for 10 hrs. before we ran.....SAME SURFACE APPEARANCE!!!

What "idiot boy"( my new nickname) didn't do was verify that the vacuum was in fact getting to the vacuum port. There was a hunk of plastic lodged in a valve blocking the vacuum. I made the mistake of assuming we had vacuum to the port just because there was vacuum at the gauge in the pipe. (Of course a couple of our experienced operators didn't check either)

Restoring the vacuum cleared up the problem within 20 minutes.

Another interesting thing was that although predrying did not appreciably improve the appearance of the sheet it did dramatically stabilize the extruder.

Also interesting is that it was not in fact surface moisture or the drier would have corrected it. This means we must have an awful lot of residual monomer or degraded plastic in the feed stock.

Thanks again Tom , Steve, Stephen.

Regards,

Jamie

posted April 04, 2007 06:26 PM            

I don't want to pick on you too much, but usually all that vapor leaking out around the sides of the vent cap gives away the problem.

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Best Regards,

Tom Cunningham

posted April 04, 2007 07:37 PM            

I think the there may be a couple of things happening here- certainly some moisture, maybe some air entrapment issues. Could there be some "plate out" on the internal flow surfaces of your die- if the lines appear then fade out= moiture/volatiles, if they appear and stay/get worse= plateout on internal/die lips.

What are the mechanical propeties of your sheet like, colour HIPS gets a real yellow cast if it's "cooked" by agressive processing parameters/repeated passes through an extruder.

Steve

posted April 05, 2007 09:26 AM            

I know in a perfect world I shouldn't change more than one thing at once but I had to get the order out in a hurry.

Steve, I can't tell if we are cooking it terms of yellowing as it is black utility grade HIPS.

The streaks are oval shaped and look like a bubble getting flattened as it goes through the chill rolls. It is all over the surface of the sheet and it does not come and go. The streaks are approximately 1/8 inch wide and around 1/2 inch long typically.

What do you mean by "plate out"? I've heard the term but can't recall what it means.

Thanks again,

posted April 05, 2007 05:12 PM            

Plate out is a buildup of additive ingredient and/or degraded polymer on the screw, adaptors, die, chill rolls or sizing equipment.

Once a build up starts, it gets worse, the longer you run. The line/marks these buildups cause, remain at fixed points across the sheet.

Sometimes this will occur right on the lip of a sheet die, and you can scrape it off with a soft brass scraper (with care) and continue without shutting the line down. If the buildup is on an internal flow surface, shutting down, stripping everything down and cleaning is generally the best option. I have had some good results with chemical purges, but these usually only reduce the effect, and don't eliminate it.