The blow-up ratio is, in blown film production, the ratio of the diameter the bubble reaches to the diameter of the annular die. The molten tube leaving the die expands with the air blown into it; the extent of this expansion is expressed by the blow-up ratio.
As well as determining the width of the film, the ratio also affects the balance of the film’s strength in its two directions. The film is stretched both in the haul-off direction and across its width; the blow-up ratio governs the transverse stretching, while the haul-off speed governs the longitudinal stretching. The balance between the two directions is reflected in properties such as the film’s tear strength and impact strength. The appropriate ratio is chosen according to the raw material, the film thickness and the line’s equipment.