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 Extrusion, Single Screw
 Injection screw design for PC low grade
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| Author | Topic: Injection screw design for PC low grade |
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P.C.Cas Member Posts: 3 |
posted October 25, 2006 06:54 AM
   
Hello, Do you have experience on screw profile design 60 mm diameter (2.4") for polycarbonate transparent (optical)low grade (MFI 2)in order to reduce yellow index and contamination ? Many Thanks ------------------ IP: Logged |
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Tom C Moderator Posts: 781 |
posted October 25, 2006 12:10 PM
What is the screw L :D? What is the injection stroke length? What is the cycle time? What are the RPMs you are currently using? What is the shot weight? What is the recovery time? Low MI PC can be difficult. Some of these items asked above can help develop a picture of your process and lead to good process design choices. ------------------ Tom Cunningham [This message has been edited by Tom C (edited October 25, 2006).] IP: Logged |
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P.C.Cas Member Posts: 3 |
posted October 26, 2006 05:12 AM
Hello Tom, Yhanks for reply L/D ratio is 3.2 - Injection stroke is 210 mm - Cycle time is 6 minutes ( due to the thickness of parts) - RPMs are 15 due to low MFI - Shot weight is 550 grams - Recovery time is 2 mn Best regards ------------------ IP: Logged |
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Tom C Moderator Posts: 781 |
posted October 26, 2006 09:58 PM
The problem is oxidative degradation due to the long residence time the material experiences in the screw. One source of help would be to have more anitoxidant put into the resin. Putting nitrogen into the feed throat might help to displace oxygen in the screw. A shallower screw may be helpful in minimizing the residence time and increasing the scrubbing forces on the screw surface. Much of the resin will melt through conduction because of the long hold times, so the screw design can be modified to compensate for that. The long feed is not helpful, as the pellets will melt down as they sit, leaving large air gaps with oxygen. The screw could be designed to develop pressure progressivly after all the pellets were melted so that air is squeezed out, reducing oxygen content. For that matter a barrier screw provides a "solids channel" for air to escape from. The melt side runs in an oxygen free enviroment. Barrier screws are rare, but effective in injection. Lower melt temperatures are helpful, but PC viscosity increases greatly at low temperatures using much motor power and ultimatly heating the polymer. Given the long hold times, temperatures as low as possible would be helpful. Unfortunatly I can't hand you a specific screw design without doing a lot of work. I would, however, minimize the feed length since that holds the most material and oxygen. I guess 3 diameters of feed into the barrel in the screws most retracted position. Because of the high viscosity, and long hold the transition can be short, 4 Diameters. The rest can be metering to keep the material volume low and the screw clean. Unfortunalty this type of design can lead to unacceptably high barrel pressures in the transition. Ultimalty I use a conputer simulation to help balance all the competing issues in a design requirement like this. You can contact me directly if you are insterested in a custom engineered screw for your application.
------------------ Tom Cunningham www.ExtrusionTechnicalServices.com IP: Logged |
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P.C.Cas Member Posts: 3 |
posted October 27, 2006 06:24 AM
Hello Tom, That is a great answer which will be a lot helpfull for me. Many Many Thanks ... I have one more question : I will contact you in the next months, when I get all the informations for a custom engineered screw for my application. Thanks again ------------------ IP: Logged |
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Tom C Moderator Posts: 781 |
posted October 27, 2006 07:13 PM
As far as a check valve or not, everything depends. PC does not shear thin so that helps somewhat. If the PC can get through a tight clearance while plasticating without overheating too much, and that same leaky geometry can hold the pressure as long as needed during injection-pack-hold, then it would work. Since your plastication rate is low, a short tight clearance should not be that much of a problem during recovery. A lot then depends on the injection paramaeters. Certainly creative thinking! ------------------ Tom Cunningham www.ExtrusionTechnicalServices.com IP: Logged |
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