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| Author | Topic: Black Speck on pellet |
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Woo Meng Wai Member Posts: 3 |
posted September 28, 2005 09:46 AM
   
Hi, I am producing acetal (engineering plastics) pellets using a twin screw extruder. It is a single stage extruder and has a vent port in the middle section. We also use a screen pack (150 mesh) which we changes regularly. Currently, there is a high occurrence of black specks on the extruded pellets (tiny black dots on the surface). Property of this black speck: An investigation on this revealed that once the polymer is melted away, the black spec can be isolated and it does not melt even at 300 degree C. We have weekly cleaning work on the screw, die and barrel. Any idea what is the root cause of this problem? Well, this happened to both of our extruders, so I am not sure if it is a machinery problem. However, it has never happened in the past before. IP: Logged |
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Tom C Moderator Posts: 622 |
posted September 28, 2005 01:05 PM
Woo, Given that the specs are on the outside of the pellets the source of the contamination is likely to be found in the are past the screws. POM requires lots of stabilization. Are you having more formadehyde smell than in the past? This could inducate poor stabilization. Are you using a screen pack? Do the screens seal well so there is no leakage around the edges? The fact that the black does not melt indicates a carbon deposit. What is your melt temperature? What are the temperature settings in the adaptor and die area? Have you changed the resin supplier or melt flow rate of the resin recently? Are you using powder feedstock or pellet? What style of twin screw?
Tom Cunningham
Extrusion Technical Services [This message has been edited by Tom C (edited September 28, 2005).] IP: Logged |
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Woo Meng Wai Member Posts: 3 |
posted October 16, 2005 09:25 AM
Currently, I am using powdered feed and the melt temperature is approximately 180 - 200 degree celcius. Temperature at the die adaptor is approimately 180 degree celcius. As for the type of extruder, I am not very sure, but they do rotate in the same direction. Hope this info helps. IP: Logged |
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