 UBBFriend: Email This Page to Someone! |
 Plastic processing forums for extrusion information and advice and for other feed screw applications.
 Extrusion, Single Screw
 Surface Finish for Melt Pipes and Dies
|
| next newest topic | next oldest topic |
| Author | Topic: Surface Finish for Melt Pipes and Dies |
|
Bob Cunningham Senior Member Posts: 115 |
posted November 30, 2004 12:42 PM
   
Has anyone conducted (or know of any reports published regarding) research about the surface finish in melt pipes and dies, and their impact on polymer quality and time between equipment cleanings? Obviously "the smoother the better", but I would like to know how much a smoother surface *really* helps the extrusion process. -Bob C. IP: Logged |
|
louis33 Senior Member Posts: 139 |
posted November 30, 2004 01:12 PM
Bob You raise a great point Not sure if there are any studies on the subject as I have never run across any. However in my experience smooth wasn't always better. Just as sharp edges at the point of discharge wasn't always best. It would be interesting to apply some of the techniques used in sail boat racing where they reduce the surface friction of the hull by texturing the surface. I wonder how that might apply to the flow of plastics . . . IP: Logged |
|
Tom C Moderator Posts: 651 |
posted November 30, 2004 07:21 PM
Bob, The surface finish and/or surface treatment will affect the slip between the polymer and the metal surface. Some polymers will not slip under any conditions. Additionally the surface it will affect the point where the slip-stick effect will start, which causes shark-skin and other extrusion instabilities. The interaction at the interface can actually be characterized in the more recent rheological equipment. So if you thought you had a great idea for a surface you could test it. Once you get the information you could compare different samples to see how it performs. You could (*promotional warning*) put the information into a flow simulation program and see how your design performs using that surface. Presently Compuplast's program store emperical values which represent shear stress ranges to keep the process within. Below the minimum shear stress there is a warning that material may stick. Above the maximum shear stress there is a warning that extrusion instability may occur. Within these ranges the flow channel should flow cleanly and smoothly. I even use these values when designing screws in order to have them keep themselves clean. Sorry, small number required the big picture; Tom Cunningham www.ExtrusionTech.com [This message has been edited by Tom C (edited November 30, 2004).] IP: Logged |
|
Bob Cunningham Senior Member Posts: 115 |
posted December 03, 2004 05:19 PM
Pardon my ignorance- I can't tell how the picture demonstrates anything to do with surface finish. Would those numbers vary with surface finish in the program? Can you plug in a 32, 16, and 8 RMS to get different pressure gradients? Thanks... -Bob C. IP: Logged |
|
Tom C Moderator Posts: 651 |
posted December 04, 2004 06:58 AM
Bob, Sorry if I was not to clear. For each surface finish of interest, you would need to make a small die with that surface finish to use in the rheometer. The results of the rheological testing are then added to the program where a model is created that describes the interaction between the resin and surface of interest. The model is then used to check the design of flow channels. The RMS of the finish would only be desribed when adding a material to the database, such as: PVC w/32 RMS and so on. I'm not so sure surface finsish will make a big difference. Proper flow channel design is a proven method of reducing sticking and burning in die and adaptors without concern for surface finish. If the design is correct, an extrusion line should be able to run forever without needing to be stopped to clean out.
IP: Logged |
|
Bob Cunningham Senior Member Posts: 115 |
posted December 21, 2004 12:56 PM
Thanks for the thoughts. I'm gonna think about this some more, and possibly get some actual research going. -Bob C. IP: Logged |
|
Tom C Moderator Posts: 651 |
posted December 21, 2004 01:51 PM
Bob, I would be happy to suggest some things to look at, and to run some samples through the software. Tom C www. ExtrusionTech.com IP: Logged |
|
mademan5284 Senior Member Posts: 22 |
posted November 27, 2005 08:47 AM
I know this topic is old, but currently there is a battle taking place where I work. For about 12 years we have glass beaded dies tips and head pieces the grit is very small. A newly appointed supervisor is pushing very hard to have everything polished to a mirror like sheen or slightly less. It is very hard to tell the difference. Say there is a surface scratch not a gouge but a scratch. I do not think plastic will stick in that area but the opposition makes a point to come up with a point. If a customer has a (perfect) surface spec. ex. under small magnification I might go for it. For the most part in the past we may have polished a die and tip here or there. Ok for hdpe I think 3000psi or more is too much, because the extruder has a high pressure cut out around 4400, the extruder is a 1.5" old d.s. olympia. Anyway they think surface imperfections are from glass beaded head tooling, i think it is melt fracture from small die to tip gap and 40/60/80/100/200 screen pack. With 20% baso4 added and being passed twice beforehand, we should need minimal screening to catch agglomerates right. IP: Logged |
|
Tom C Moderator Posts: 651 |
posted November 27, 2005 09:37 AM
200 Mesh can pass a 0.003" particle. That is huge by microscopic standards. Typically all extruders will make agglomerates during processing. Aglgomerates are generated by pressure. The extruders also break down the agglomerates after they are generated. As the particles are broken down in size, it takes more and more stress to break the agglomerates down further. After a certain point shear stress just spins the agglomerate rather than break it. This is why twin screws sometimes makes better dispersions, because they employ more elongational stress which has a better chance of breaking the agglomerates. Even so, twin screws are not perfect because they can only stress particles down to certain size. You might be interested in testing a new single screw mixer design. It works without generating pressure. No pressure = no agglomerates. No agglomerates mean no stress required to break them down. Results have been amazing and it even mixes nano materials. Be forwarned that I represent this mixer. There are enough good results from this mixer that it would be worth your time to trial the mixer at Randcastles lab. Let me know if you are interested. ------------------
Tom Cunningham
Extrusion Technical Services www.ExtrusionTechnicalServices.com IP: Logged |
All times are ET (US) | next newest topic | next oldest topic |
  |
|
Contact Us | Feed Screw Designs
Powered by Infopop www.infopop.com © 2000
Ultimate Bulletin Board 5.45a
