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MD/TD/MD tensilizing
August 21, 2008
Extending my promise to Daniel from an earlier posting, to compare the differences between MD and TD orientation in various situations, I'm upping the ante a little and adding MD/TD/MD orientation to the mix.
MD/TD/MD orientation when applied to film products is typically called tesilizing and produces tensilized films. It has the effect of increasing the MD tensile strength above that typical of biaxially oriented films while maintaining the majority of the TD strength from the biax process. Generally the film will be stronger in the MD with a lower MD elongation to break but not as prone to TD splitting as an MD-only oriented film. It does require the addition of a separate, full width MD stretching unit (the tensilizer) after the TD oven, or it can be practiced out of line on a slitter.
There have been two primary applications for tensilized films, which I am most familiar with, downgauged magnetic recording tape and shrink labels. Original magnetic recording tapes for cassette players were produced with 50-gauge (12 micron) PET films. Once the dimensions of the cassette were determined, this limited playing time to the length of the 50-gauge film that could be wound into the cassette. Simple film downgauging left a film which was too weak for the tape player, and the film would stretch, ruining the recording if not breaking the film.
Simply increasing the MD-stretch ratio before TD orientation was not possible as the higher MD stretching was too difficult and decreased line productivity due to increased TD splitting and poor productivity in the orientation process. So, to increase the play time and have a film with sufficient MD strength (so as not to redesign all the tape players in the world), the film was re-stretched after the TD-orientation step. This allowed the use of 35-gauge (9 micron) tensilized PET film in the cassette players, with sufficient strength not to stretch or break in the players, giving the desired longer playback times.
Next time, I will discuss the use of tensilizing on the production of shrink labels.
Posted by Eldridge M. Mount on August 21, 2008 | Comments (0)