Between the Lines      
  April 2008 a newsletter from BE      
         
 

BE continues to see potential in China

      
 

Chinaplas is the only annual international plastics show that now draws exhibitors and attendees comparable to NPE and K show. BE has exhibited at Chinaplas every year since the early 1990s and in 2000 set up a sales and service office in Suzhou. The market in Asia is important to our success and Chinaplas plays a significant role in our sales cycle. This year Bud Smith, president and CEO of BE expects to attend the show along with Philip Kwok, vice president of worldwide sales.

Kwok says “customers come to BE for the most efficient systems to manufacture blown film. With the costs of resin rising, efficiencies in reducing scrap are becoming critical in decisions to buy equipment. With our long history of producing equipment for the manufacture of multilayer film we can show our customers the efficiencies that our equipment brings to the market, for example our SCD®-3 die. The way it is designed allows for fast changeovers. The residence time of the polymer in each layer is very short and so when changeovers are required, our die uses much less resin than other dies. This is a significant saving for customers who are operating short runs of different structures.”

Interest in AquaFrost continues to strengthen. Kwok says “cast film producers are coming to us for AquaFrost lines instead of another cast line. The output rates of our AquaFrost line are similar to cast lines. But mostly it is because AquaFrost can run without taking trim. These are real measurable savings.” AquaFrost is a downward blown water-quenched process that produces clear film at high output rates. 

See BE at Chinaplas 2008 at the Shanghai New International Expo Centre, Pudong. Our booth number is W1J45.

 

 

AquaFrost film

     
 
     
         
  Troubleshooting blown film processing      
 

 

Relying on 35 years of experience BE helps to provide methods to quickly identify the most common problems and ways to systematically tackle them.

BE has identified some common issues that blown film processors routinely need to oververcome. In this issue we troubleshoot haze lines and melt instability. We will provide some suggestions on how to identify the causes and correct these problems so the you can reduce downtime and eliminate scrap. In the next issue of Between the Lines we will discuss gels, interfacial instability and poor gauge.

 

1. Haze lines on the film in machine direction – mean poor film quality and lots of rolls destined for the scrap bins. The visible lines on the film are usually hazier than the rest of the film.

Contributing factors for haze lines include:

Processing temperatures

Set the correct processing temperatures from the extruder barrel zones all the way to the exit of the die. Be sure that you know the processing temperatures of each component of blends. A variable depth thermocouple installed at the extruder adapter prior to the entry to the die so that the melt temperature of the polymer can be recorded. The metal temperature of the die should be similar or slightly hotter than the recorded melt temperature. Too cold a metal temperature of the die can cause haze lines as well, since it forces a melt temperature gradient in the melt stream in the die.

Temperature gradient

It is important to have a uniform melt temperature gradient throughout the extrusion process, from the screws to the die. The easiest way to check for melt temperature gradient is to use a variable depth thermocouple. A melt stream with a large temperature gradient as measured in the adapter prior to the die introduces a material with very different viscosities. The temperature variance contributes to different flow behaviour in the spirals. Use a variable depth melt temperature probe to help optimize extruder melt temperature profiles for improved melt temperature control and mixing.

                                                                                   

Screw and barrel conditions

Check into the screw design, as well as the screw and barrel conditions. Worn barrel and screws contribute to poor mixing of the different materials if a blend is used. This will also result in large temperature gradients. It is a good idea to establish a program that tracks screw, barrel wear via physical inspection and/or pumping rate, melt temperature measurements. BE recommends replacing them before they are worn beyond acceptable tolerances. This practice will significantly reduce gel issues and melt temperature related quality issues.

Die design

If the processing conditions have very little effect on eliminating or reducing the haze lines, and the haze lines correspond to the poor gauge uniformity coming out of the die, then it ‘s time consider a different die design.  

For troubleshooting haze lines look at processing temperatures, temperature gradient, screw and barrel conditions and die design.

BE Film Technology Centre

 

      
 

 

      
 

BE software tool predicts film quality before running the line

Experiment with what-if scenarios to determine the effect of introducing a new resin, change a layer thickness, or predict the performance of a new structure with BE Prophecy 2.0. Easy to use, powerful and packed with useful information, BE’s Prophecy 2.0 allows our customers to take the maximum advantage of the capabilities of our SCD-3 coextrusion dies.

Supplied with every SCD-3 coextrusion die, this BE software analyzes the polymer flow through the die. It is based on theoretical rheological models, which have proved their validity in industrial practice.

Prophecy 2.0 comes with a pre-loaded Polymer Property Database that is expandable by adding your proprietary polymer formulations or new polymers.

To generate the visual representation of the multilayer polymer flow results in the sample table shown, simply input the total mass flow rate through the die (either in lbs/hr or kg/h), select the individual layers from the drop down list and input the resin choice, the desired layer percentage and the process temperature. Press Next and Calculate to complete the table.

Prophecy gives the minimum and maximum shear rates at the spirals for each polymer layer. For example, if you test a new structure and the shear rate for a certain layer falls below a critical value, then the program will warn you by showing the shear rate value in bold (highlighted here in yellow). In this case, if you run this new structure, gels could be generated due to extended residence time.

You can also analyze the total pressure drop, and minimum and maximum shear rates at the spirals of any individual layer by selecting it from the dropdown list. With a click of the mouse, you can convert to or from the metric or imperial system of units.

Prophecy calculates the layer throughput based on the total die throughput and the individual layer pressure and shear rates. These values help predict the die performance (and hence, the total line performance) when changing resins and structures or changing process conditions. Different scenarios can be tested until the desired results are achieved to minimize production of scrap.

Prophecy can also be used as a tool for optimizing the polymer layer ratio in a coextruded structure or to troubleshoot coextrusion problems by doing a cause and effect analysis.

For the best film quality, use BE Prophecy

Easy to use, powerful and packed with useful information, BE’s Prophecy 2.0 allows our customers to take the maximum advantage of the capabilities of our SCD-3 coextrusion dies.

 

      
         


 

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