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Generally pinch-off for Polypropylene is sharper than that used for HDPE or PVC. A
pinch-off land of about 1.2 mm is recommended for PP and 2.4 mm for PE & PVC.
Swell Ratio: the parison coming out of the die is of bigger diameter than the actual
diameter of die. The ratio of diameter of tube and the die is called SWELL RATIO.
The swelling of parison varies with effect of temperature, pressure and material
used. It varies from 1:1.2 to 1.8. There are no thumbs rules for it as many factors are
involved, however general indicative tendencies are:
The lower the melt temperature, the greater the swell
The higher the melt strength, the greater the swell
The higher the extrusion speed, the greater the swell
The gentler and slower the melt is formed into the required diameter, the less the
swelling.
Blow Ratio: the ratio of diameter of parision and that of blown article is called
BLOW RATIO. When air is blown into the parision it expands in ratio of 2 to 3 times
depending on shape of article and it also determines the pinch off size which in
normal is 60 to 90% of article diameter. The blow-up ratio has considerable
influence on the mechanical properties and shrinkage, with an increasing blow-up
ratio the orientation of the molecules increases in radial direction. Generally BUR
value is between 1.5 and 3.
BUR= largest diameter inside blow mould / parison outer diameter
The amount of stretching a parison is subjected to, is a function of the part size and
con?guration in relation to the parison size and orientation. In general, this can be
expressed as follows:
Average part thickness = parison surface area x parison thickness / product
surface area
Die Dimensions: The outer dimension of the extrusion die is obtained by following
formula:

OD of extrusion die = OD of parison / 1 + die swell
100
Air Pressure:
Blow pressure greater than 60 psi, is necessary to achieve a good surface finish on
the product, to pick up the mold detail, and to ensure the material is against the
blow-mold surface to be cooled.
Low blow pressure will increase the shrinkage of the product and also the cycle time
since the mold cooling is not being utilized effectively
In PET stretch blow molding, single-stage machines use a maximum pressure of
300 psi.
In the two-stage method for PET, two-stage blow air is used. The low blow pressure
would be approximately 220-250 psi and high blow pressure as high as 650 psi is
used.

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