Converting Magazine

Hello, I have recently returned from a technical conference put on by the Society of Plastics Engineers (SPE) and it was a worthwhile trip. In addition to the many fine technical papers it is a fine place to meet peers in our industry and to renew friendships. There are far to many papers to attend all of them (a good thing) but you can visit the SPE web site (www.4spe.org) and review the papers given and if you are interested you can obtain a copy of the conference CD with all of the papers delivered. This is a great way to keep up with technology even if you cannot travel to the conference. Next year it will be held in June in conjunction with NPE in Chicago.  Here are a few paper titles which were presented in the Extrusion Division Films session and the Fle...Read More

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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

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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

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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

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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

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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

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The attached graphs came from US Patent 6,680,097 B1 which is titled "Easily removable label for reusable containers” and uses film shrinkage to help in recycling containers with labels and makes use of film shrinkage in a very unusual manner, to my thinking. The curves show TD (transverse) and MD (longitudinal) shrinkage for both PVC and PET based films. The heat shrinkage of the film is coupled with an engineered failure in the adhesive so that when the container is placed in a hot bath for washing the adhesive force is decreased and the film shrinks and pulls itself off the container.

 However, unlike the TD growth (negative shrinkage) behavior we saw in the MD stretched polyethylene film in the earlier posting on heat shrinkage, these films both show positive TD and MD shrinkage across ...Read More

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When designing a modern coextruded film you are combining polymers to obtain a set of properties that you cannot obtain from a single polymer. That of course is because no single polymer has all of the desirable properties necessary for a modern film. Several polymers are fairly capable and supply a lot of properties, such as Polyvinylidene chloride (PVDC) which has fairly good mechanical properties and the rare combination of good moisture and oxygen barrier, a rare combination. However, it suffers from a very high density, about 1.6 gm/cc, one of the highest densities for a polymer. What is the problem with this? Well it means that the film yield (area/weight) is low, i.e. you get fewer square inches of film for a pound of polymer and while film is sold by the pound it is usually used by the square inch or coverage area!

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Today, there are many coextruded films containing a wide range of polymers combined together. In many cases the film layers are symmetrically placed and the stresses are balanced about the center line of the film. In this case the films will generally remain flat and curl free. However, there are many instances where an asymmetric structure will be better in terms of film properties.

In many instances the asymmetric structures will be prone to curl if the stresses are imbalanced. The tendency to curl will be a function of the relative thickness of the layers and which layer has the mechanical strength to control the curling tendency. For instance in an oriented four layer film of nylon/tie/PP/CoPP, if the nylon layer is less than about 5% of the structure the film will lie flat. As the...Read More

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While looking through my reference materials this weekend for heat shrinkage data for films, I used a fine reference which is focused heavily on oriented films and a special focus on shrink and stretch films. The book is “Plastic Films For Packaging”, Calvin J. Benning, Technomic Publishing Co., Inc., Lancaster, (1983).

Dr. Benning worked, among other places, at W.R. Grace, where he focused on shrink films and foams. The book has several good chapters on the stretching process as well as the properties which are developed, and whi...Read More

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In my earlier answer to Juan, I discussed the shrinkage characteristics of various films relative to a specification. During this, I said that the best dimensional stability, aka shrinkage, data was data as a function of temperature. That is primarily because a single temperature does not fully describe the behavior of the film itself. As an example of this is figure 1 which shows the thermal shrinkage for a polyethylene blown film which has been solid state stretch oriented approximately 200% in the MD. In this case the orientation erases the shrinkage behavior of the blown film itself and has created an MD shrink film....Read More

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