posted June 05, 2001 05:41 PM               

In this context, melting energy is exponentially related to screw speed predicated on the power law value of the fluid in question. As such, small changes in screw speed result in significant changes in melting energy and subsequent alterations to the location of melt initiation. In contrast, a barrier screw profile has a fixed position for the introduction of the secondary flight required to develop the discrete solid and melt channels defined by barrier transition screws. This leads to a fairly narrow processing window and relies on greater operator skill level to operate successfully. A few of my concerns are as follows;

* loss of operator flexibility with barrel zone settings and screw speed settings due to their affect on the location of melt initiation
* surging discharge pressures and extradite gage variability as longer transition lengths are required to facilitate the introduction of the secondary screw flight and resultant screw channel

* these longer transition lengths tend to "overstuff" the metering section with material not adequately prepared for the screw pumping section
* the additional unwiped surface perimeter created by the leading and trailing flight flanks may lead to a greater potential for material degradation and longer, more arduous material and color changes

In light of all this, and as mentioned above, many of these barrier screw profiles succeed in spite of themselves. In this scenario, they are typically operating in an environment where there is a line dedicated to a specific material and set of operating conditions. It is by virtue of the longer than conventional transition length that they may develop a significant pumping capacity while avoiding the process anomaly of poor melt quality by virtue of the continuous leakage flow over the undercut secondary screw flight.

posted June 07, 2001 04:42 AM            

The problem is that, unless the volume of the melt channel increases at the same rate at which solid is being converted to melt, the melt channel will either run partially filled or solid material will be forced over the barrier flight into the melt channel.

The rate of conversion to melt is highly dependent on the thermal properties of the material and varies quite considerably. Even if you design a barrier screw specifically for a material it will only work at maximum efficiency for that material at one temperature and at one screw speed.

Barrier screws are frequently used in conjunction with grooved feed sections and, if the grooved feed is properly configured, they work very well.

Regards,

Peter P.
pepe@polytech.co.uk
www.polytechconsultants.com

posted June 11, 2001 11:28 PM               

I have seen conservative barrier screw designs function very well over a wide range of materials in a vented 2 stage extruder. Extrusion was generally stable until some high RPM.

The interaction between the die and the screw in a single stage extruder makes the situation much more complex.

If I were to design for a single resin, I would chose the best design for the task at hand which may one of many design types. If I was expecting to run a wide variety of resins, I would chose a barrier design for the increased versatility.