posted May 18, 2005 09:00 PM               

Typically the throttle valve will be at the end of a set of kneading blocks. Closing the valve will increase the pressure drop accross the valve and subsequently fill more of the screw length before the valve. The overall effect depends on what the screw design is before the valve.

The characteristics of fractional melt LLDPE are; high viscosity, little shear thinning, and likely some slip on the barrel. Closing the throttle down should result in more working of the LLDPE, which means higher temperatures and more mixing.

I ran some simulations on PolyTech's TXS twin screw process modeling program which seemed to indicate not that much more heating and mixing would occur. If the throttle were located after the first set of kneading blocks where the melting occured, closing the throttle could help improve melting, as LLDPE seems to be more difficult to melt in kneading blocks than some other resins.

TXS indicated very high pressure could be generated as the throttle is closed down becasue of the high viscosity and lack of shear thinning in LLDPE. Typically slip is a large consideration with fractional melt olefins and affects the process considerably. The combination of all the above effects need to be studied in order to determine the real impact of closing the throttle.

I would be careful.

------------------
Best Regards,

Tom Cunningham

Extrusion Technical Services
www.ExtrusionTechnicalServices.com

posted May 19, 2005 02:00 PM               

I have noticed that the MI of the powder decreases approximately 10 - 20 % when the valve is pinched.

I have started to run experiments to find out for sure if this MI drop correlates to the valve position.

posted May 19, 2005 02:42 PM               

You should track your specific energy and your melt temperature as you close the valve.

------------------
Best Regards,

Tom Cunningham

Extrusion Technical Services