Effect of Blow Up Ratio on Film Properties:
Higher Blow Up Ratio
- Increase Bubble diameter
- Thinner film
- Higher tensile strength in the transverse direction
- Improve optical properties (clarity, gloss)
- Reduced stiffness
Lower Blow Up Ratio
- Smaller bubble diameter
- Thinner film
- Higher tensile strength in the machine direction
- Reduced optical properties
- Increased stifness
Draw Drawn Ratio (DDR):
The draw ratio in film extrusion refers to the ratio between the speed at which a polymer melt is extruded through the die and the speed at which the resulting film is pulled or stretched in the machine direction (MD) or transverse direction (TD)
Draw Ratio = Speed of Film Extrusion (meters per minute) / Speed of Film Take-Up (meters per minute)
High Draw Down Ratio (greater than 1):
When the take-up speed is significantly higher than the extrusion speed, the film undergoes stretching, which orients the polymer molecules in the MD. This results in a thinner film with increased tensile strength, improved optical properties (such as clarity and transparency), and reduced elongation.
Low Draw Down Ratio (less than 1):
When the extrusion speed is higher than the take-up speed, the film may become thicker and less oriented. This may result in diminished tensile strength, decreased clarity, and increased ductility.
Balanced Draw Down Ratio (Equal to 1):
When the extrusion and take-up speeds are equal, there is minimal stretching or orientation of the polymer molecules. The resulting film will have properties that are relatively balanced between MD and TD, but it may not exhibit the extreme characteristics seen in films produced with high or low draw ratios.
Draw Drawn Ratio (DDR) = 1000 × die gap (mm) / Thickness (micron) × BUR
Multiple choice question, answer and Explanation
Que: What is the blow-up ratio (BUR)?
a) The ratio of die diameter to bubble diameter
b) The ratio of bubble diameter to die diameter
c) The ratio of die gap to film thickness
d) The ratio of film thickness to die gap
Ans: b) The ratio of bubble diameter to die diameter
Explanation: BUR is a critical parameter in blown film extrusion. A higher BUR generally leads to improved TD strength and reduced MD strength.
Que: How does an increase in BUR typically affect film properties?
a) Increase MD strength, decrease TD strength
b) Dcreased MD strength, increased TD strength
c) No significant effect on film properties
d) Increase MD and TD strength
Ans: b) Decreased MD strength, increased TD strength
Explanation: A higher BUR results in greater orientation in the TD, leading to enhanced TD strength at the expense of MD strength.
Que: What is the draw-down ratio (DDR)?
a) The ratio of die gap to (film thickness x BUR)
b) The ratio of film thickness to die gap
c) The ratio of bubble diameter to die diameter
d) The ratio of die diameter to bubble diameter
Ans: a) The ratio of die gap to (film thickness x BUR)
Explanation: DDR measures the extent of stretching and orientation in the machine direction (MD).
It is influenced by factors like die gap, film thickness, and BUR.
Que: How does an increase in DDR typically affect film properties?
a) Increase MD strength, decrease TD strength
b) Dcreased MD strength, increased TD strength
c) No significant effect on film properties
d) Increase MD and TD strength
Ans: a) Increase MD strength, decrease TD strength
Explanation: A higher DDR indicates greater stretching in the MD, resulting in improved MD strength and potentially reduced TD strength.
Que: Which of the following factors does NOT directly influence BUR?
a) Air ring pressure
b) Die gap
c) Melt temperature
d) Screw speed
Ans: b) Die gap
Explanation: While the die gap affects DDR, it does not have a direct impact on BUR. BUR is primarily controlled by air ring pressure and other factors influencing bubble stability.
Que: How can you increase BUR in a blown film process?
a) Dcrease air ring pressure
b) Increase the die gap
c) Decrease melt temperature
d) Increase air ring pressure
Ans: d) Increase air ring pressure
Explanation: Higher air pressure inflates the bubble to a larger diameter, leading to a higher BUR.
Que: How can you increase DDR in the blown film extrusion process?
a) Increase die gap
b) Dcrease the die gap
c) Increase air ring pressure
d) Decrease melt temperature
Ans: a) Increase the die gap
Explanation: A larger die gap provides more material, allowing for greater stretching in the MD and, accordingly, a higher DDR.
Que: What is the relationship between BUR and film thickness?
a) Directly proportional
b) Inversely proportional
c) No significant relationship
d) Depends on the specific polymer
Ans: b) Inversely proportional
Explation: For a given die diameter and air ring pressure, a thicker film will generally result in lower BUR.
Que: How do BUR and DDR interact to influence film properties?
a) They have no significant interaction
b) They act independently to determine film properties
c) They can have synergistic or antagonistic effects
d) DDR always dominates over BUR in determining film properties
Ans: c) They can have synergistic or antagonistic effects
Explanation: The relationship between BUR and DDR is intricate (complicated).
In some case, they may work together to enhance certain properties,
while in others, they can counteract each other.
Understanding this interaction is crucial for optimizing film properties.
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