Q. What is blow-up ratio and what effect does it have on films?
A. Blow-up ratio or BUR is a term unique to the blown film industry. It is a number that refers to the mathematical ratio of the diameter of a blown film bubble (at its largest point) to the diameter of the extrusion die it comes out of. The higher the BUR, the larger the bubble diameter is in relation to the die.
To explain, if the bubble were to come straight out of the die with no increase in size, the bubble diameter would be equivalent to the die diameter. In this case, the BUR, which is equal to the bubble diameter divided by the die diameter, would be 1:1 (or 1.0; for example 16”/16” = 1.0).
If the bubble were to neck-in so the bubble diameter was smaller than the die diameter, the BUR would be less than 1.0 (e.g. a 12” bubble from a 16” die; 12”/16” = 0.75 expressed as 0.75:1). If on the other hand, the bubble diameter was larger than the die diameter, the BUR would be greater than 1.0 (e.g. a 32” bubble from a 16” die: 32”/16” = 2.0 expressed as 2.0:1).
Blown film lines are set up to run within a certain BUR range. This means a line can basically run a fixed range of bubble sizes or a fixed range of finished film widths. Based on the line set up, there may be physical limitations that prevent running outside a certain BUR range, such as exceeding idler roll widths if the BUR is too high. At other times, a line may have the physical capabilities to run outside the specified BUR range, but this may not be optimal for the product performance.
So what effect does BUR have on processing and film properties? For a blown film manufacturer, films that are soft or thin are easier to produce using a high BUR because the bubble typically has better stability, while films that are stiff or thick are easier to produce using a low BUR. This often fits well with the property effect.
BUR affects the orientation of the polymer molecules. Running a high BUR, the film is stretched or pulled in the cross direction to reach the ultimate diameter of the bubble. At the same time, it is being pulled in the machine direction by the movement of the downstream equipment. By pulling the film in two directions, the molecules become oriented in both directions. This gives a better balance in properties from MD to TD so these films are typically tougher and have more balanced shrink properties.
Running films at low BURs gives very little cross-direction orientation. These films typically have higher properties in the machine direction, less TD shrinkage and low MD tear strength. Where there is flexibility in the width of the film, the BUR can sometimes be used as another control of the properties of the film to better utilize the natural orientation effects of the process. Film manufacturers will, whenever possible, use these orientation effects to better suit the needs of the application.