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| Author | Topic: Brown coloured pellets |
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Woo Meng Wai Senior Member Posts: 6 |
posted October 16, 2005 09:41 AM
   
Hi, Currently I am facing colour problems extruding acetal pellets. These pellets are meant to be white in colour but recently there were two cases of slightly brown pellets produced. The second case happened approximately 3 weeks after the first. In both cases, the problem stopped abruptly after a few contaminated lots. Initial investigation revealed that there were very frequent screen pack changes during the estimated time of contamination, every 3 to 4 hours (normally every 8 hours). Screen packs are changed when pressure before the screen pack reaches 12 MPa and drops to 8 MPa after the new screen is switched. during the changing of screen packs, the system will be switched to the rerun tanks to prevent any contaminated pellets from entering the product tank. Any idea why is this affecting the colour of the pellets? Furthermore, is there any solution to prevent this problem? Wai IP: Logged |
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Tom C Moderator Posts: 624 |
posted October 16, 2005 11:13 PM
Potential Sources: Wood or paper from packaging Back flow from vacuum vent lines Degraded raw materials ------------------
Tom Cunningham
Extrusion Technical Services www.ExtrusionTechnicalServices.com IP: Logged |
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Woo Meng Wai Senior Member Posts: 6 |
posted October 17, 2005 06:17 AM
Hi, Some thing that I left out is that currently, we are running two lines and this problem happens to one particular line only. After checking the records, there was really a drop in vacuum during the frequent screen pack changes. However, both lines also recorded this same drop in vacuum. Is it common under such condition, that only one line is affected? IP: Logged |
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Tom C Moderator Posts: 624 |
posted October 17, 2005 08:39 AM
It is not the vacuum level, but the volitile condensate which causes the problem. Depending on the vent stack design and the plumbing the condensate might end up in one line and not the other. Please contact me through email if you need professional assistance with your vacuum venting system. ------------------
Tom Cunningham
Extrusion Technical Services www.ExtrusionTechnicalServices.com IP: Logged |
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