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#1474
Author: Charles A. Bishop, C.A.Bishop Consulting Ltd.
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Subject: Cast versus blown film
When a polymer is melted and either poured or extruded through a die onto a cold surface, it is a cast film. In general terms the polymer chain molecules will be in random orientation and the properties of the film will be more or less the same in all directions.
If we take the same ribbon of cast film and re-heat it and draw it forward, we can start to align some of the chain molecules in the draw direction. This will increase the tensile performance in that direction, but may reduce performance in the other two directions. This film is now call uniaxially oriented.
If we take the uniaxially oriented film and heat it while holding it in tension in the direction of the first draw and draw it at 90 degrees to the first draw, we can do the same thing again in terms of aligning the chain molecules. We now have a film that has been biaxially drawn, and we have improved its mechanical properties. This can be done mechanically by using rollers that nip and are running at two different speeds for the forward draw and by the gripping the edges of the film and drawing it sideways on a tenter frame.
The alternative is to extrude a tube of polymer, nip the end and blow air through the center of the extruder. This inflates the tube into a larger diameter bubble and automatically creates biaxial orientation. A simple way of checking this out is to draw a square onto a deflated balloon and then inflate it and watch the square grow. Allowing for the shape of the balloon, it should remain a square but grow bigger.
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#1526
Author: Eldridge M. Mount III, EMMOUNT Technologies
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Subject: Cast versus blown film
The primary difference will be in the balance in properties in the machine direction (MD) and transvere direction (TD) in the film. Blown film also will exhibit a different tracking behavior due to the way gauge is randomized (rotation) versus a cast film where the gauge will be in straight lanes or oscillated several inches
Resin selection and the relative MD/TD melt orientation will impact tear and puncture properties.
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