Converting Magazine

Eldridge M. Mount

Reply to Medhat’s draw down question
May 25, 2008

Medhat asked the question "How does the draw down ratio affect on the film properties?" from the March 4th posting on film manufacturing methods, and because the posting is focused on cast films, this is how I will approach the answer. However, I will comment on blown films as well. Draw down ratio for cast films is generally considered to be the ratio of the die gap divided by the cast film final thickness while for blown films it is generally the ratio of the final film speed to the exit velocity of the melt in the die land and is line speed dependent.  This line speed impact on draw down is generally overlooked in cast films as the die gap to film thickness ratio is independent of line speed in cast films (so line speed needs to be considered separately) while it is encompassed in the blown film calculation method.

However, much of the final effect of draw down will be depend on the melt temperature at which the film is extruded and the molecular weight of the polymer. That is because the melt temperature and molecular weight affect the relaxation time of the polymer or the time which it takes the molten polymer to “forget” the stress which was placed upon it during the extrusion and draw down from the die lips. In general the shorter the relaxation time (higher melt temperature and lower molecular weight) the less memory the polymer will have of the draw down and the longer the relaxation time (colder melt temperature, higher molecular weight) the more memory and residual orientation will remain in the film. If the relaxation times are much shorter than the time the melt is drawn from the die to the point it contacts the chill roll surface then the polymer orientation will be greatly relaxed. But if the relaxation times are longer or the same order of magnitude as the time the polymer is drawn to the cast roll surface, then the melt orientation can be frozen into the film. A dimensionless group which is oftentimes used to compare relaxation time to process time is the Deborah number. It is used for correlating die swell and elastic instabilities in fiber spinning and can also be used for casting film, but more of that later. Having described the important caveats relating the effect of draw down to processing conditions, melt temperatures and polymer molecular weight, we can get to Medhat’s  question as to the impact of draw down on properties.

As the melt is forced to the die exit in the final land the shear rates can be high and the polymer molecules in the melt will disentangle to some extent and orient in the flow direction. This orientation in the melt can then be enhanced further as the molten polymer is quickly drawn away from the die lips. As the melt is drawn away from the die, the polymer can be oriented further due to the pulling stress on the melt curtain.  This induces a predominately machine direction (MD) elongation and orientation.   In general this will increase the MD strength as measured by the modulus and ultimate strength relative to the transverse (TD) direction in the film. The higher MD orientation will reduce the MD tear strength making the film easier to “split” or tear along the MD direction making it weaker in the TD as measured by the TD elongation to break and the modulus. Because the film is more oriented in the MD it will also show a higher MD thermal shrinkage.  In general the film will show a non uniform properties as a function of direction in the film (anisotropic properties) with the MD being generally stronger and the TD direction being generally weaker. 

The effect of the draw down can also be impacted by the draw distance. Longer draw paths tend to be more uniaxial and therefore will enhance the MD melt orientation relative to a shorter draw gap.    Of course the impact of various resins cannot be overlooked and this is generally related to the relaxation time spectrum for various polymer types in large part to the “standard” molecular weight for that type of polymer.

There are some notable differences in the impact of the draw down between blown and cast films as well. Cast films will be placed onto a solid support and quenched so cast films can tolerate lower melt strength when forming the film. In contrast, melt strength needs to be relatively high in blown films to permit the blowing of the film. Therefore melt temperatures in blown films tend to be lower than cast films with the same polymer. This impacts the relative e relaxation times in the two processes and therefore the residual orientation, which can remain in the film. An added advantage to the blown film process is that the TD properties can also be improved or managed by the blow up or TD orientation possible in the process. There being no real TD orientation in the cast film process it is not possible to improve TD properties during the casting step.  

Posted by Eldridge M. Mount on May 25, 2008 | Comments (0)