Blow Ratio

Determining blow ratio is one of the most important parts of designing a blow molded part. Blow ratio is a measure of the amount of stretch the parison will experience when it is blown into the part’s shape.

Imagine that the above part is half of a blow molded part, with the open surface being the parting line. The parison will mostly cover the center 1.5” x 4” surface, and must be stretched into the 1.25” depth. The theoretical area of the parison is 1.5” x 4” = 6 in². The total area of the part is (1.5” x 1.25”) * 2 + (4” x 1.25”) * 2 + (1.5” x 4”) = 3.75 in² + 10 in² + 6 in² = 19.75 in². If the original thickness of the parison is 0.1”, then the part thickness will be (6 in² / 19.75 in²) * 0.1” = 0.030”, assuming perfectly even stretching. The parison will not stretch uniformly, however, will have thin spots in the corners.

Next, lets determine the blow ratio for this part. . In the above example, the depth of the cavity is the 1.25” dimension, and the cavity opening is the 1.5” x 4” open surface. The blow ratio is, . In general, the maximum recommended blow ratio is 2. Blow ratios can be increased if generous radii (at least 2 times the wall thickness), and generous draft (at least 2 degrees) is used. The figures below show several different cases and the recommended maximum blow ratio for each configuration. In the figures, H is the cavity depth and D is the smallest opening dimension, so Blow Ratio = H / D.

Case 1: Circular projection (female mold). Maximum blow ratio = 0.33.