May 7, 2008 | Link This | Email this | Comments (0)

The use of polymer processing simulation packages for complex die and extrusion flow problems as well as the calculation of melt pipe pressure drops requires the use of accurate, corrected viscosity data to insure the best results. However, the viscosity data is often times the weakest link in the simulation because the effective use of polymer melt viscosity data is a much more difficult procedure than would be expected. First it is difficult to obtain high quality melt viscosity data covering a desired melt temperature and shear rate range in a timely fashion. Then you may only receive a graph with no tabulated data or easy way to represent the data in the manner which the simulation package requires. In other cases you may want to substitute one resin for another in an existing manufacturing process and it be...Read More

May 1, 2008 | Link This | Email this | Comments (0)

Hello, I am a little behind in blogging this week so hope you don’t mind a little less technical one tonight. 

Continuing along the recent viscosity focus, I measured my first flow curve around 1975 to start my study of polymer melting behavior in an extruder.  That is where I first started using the 6 constant equation from an earlier posting. It was also the first time I used an Instron table top rheometer. They are in my mind the best mechanical rheometer due to the gimbaled polymer chamber, but I have not used all the various types of capillary rheometer available. Anyway I like the Instron so much I have two which I use for viscosity measurements. Well one of the reasons I am a little behind in blog input is that this week I measured the most difficult samples I have every m...Read More

April 21, 2008 | Link This | Email this | Comments (0)

One of the best things you can do to improve the productivity of a film (or any line) is to increase the output of a product. For film products this means increasing the line speed. Several things happen which affects the potential for a melt disturbance of the third kind when we do this. First and foremost the melt temperatures are increasing with increased line speed as the screw speeds are increased. Depending on the polymers you are using and the temperature sensitivity of the polymer this can impact the relative melt viscosity of the skin and core. 

A second change that occurs is that the shear rates increase as the output increases. This is easily seen from the shear rate expression for a power law fluid in a slit

Shear rate = [(2n+1)/n]*(2Q/WH...Read More

April 17, 2008 | Link This | Email this | Comments (0)

In my last post, I spoke of making sure the outer layer was lower viscosity than the inner layer in a two- or three-layer coextrusion. How do I know this to be true? Well, let’s look at what is happening in the flow of a polymer melt. There is a phenomenon called encapsulation, which happens in the flow of two melts in a pipe which demonstrates the viscosity behavior or misbehavior if we don’t set about things right. 

If I take two polymer streams and coextrude them with the lower viscosity material in the center of the flow (Figure 1), this is what happens. First, the center flow moves to the wall and then it begins to coat the wall until it completely surrounds the higher viscosity polymer. When this happens the pressure drop decreases, and it takes less energy to pump th...Read More

April 14, 2008 | Link This | Email this | Comments (0)

When we coextrude films, it is necessary to have careful control of the melt rheology of each layer. If we don’t, then we are prone to get a melt disturbance. Now melt disturbance is a rich word and will mean many things to many people so it will be important to decide what we mean when we say melt disturbance. Personally, I recognize three melt disturbances which I call a melt disturbance of the first kind, the second kind and the third kind. So far, I have not found a melt disturbance of the fourth kind. I thought I was close once, but I am still at only three kinds. My nomenclature comes from the three principal process sources I have found to date. For indeed, there are many things which can distort a film or sheet from pinner’s bubbles to die lines to insects, etc. But here I will focus on the three p...Read More