Polymer Processing Lab 7:
Film Blowing
Objective: To make blow films and to investigate the processing parameters in this polymer processing operation.
Background: Web notes and
Middleman:
Chapter 10 Film Blowing
Chapter 3 Elongational Flows p. 42-48
Chapter 3 Viscoelastic Flow p. 54
Chapter 14 Elastic Flows
Chapter 15 Flow Stability
Chapter 13 Heat and Mass Transfer
Terms You Should Know: Air Ring, Air Duct, Pull Rolls, Collapse Frame (we don't have this), Nip Rolls, Blow-up ratio, Rate of Haul-Off (like Draw Ratio), Flim Thickness, Biaxial Orientation, Uniaxial Orientation, The Tent, Frost-line, Feedback Gauge Control, Wrapping Step, Slitting, Roll Casting, Coat Hanger Die.
Material: Either HDPE or LDPE.
Procedure: We will blow a number of polyethylene films under different processing conditions. The extrusion rate, FAST or SLOW, will be varied for a single barrel and die temperature. We will also vary the air flow rate into the bubble to two settings fast and slow. This results in 4 runs on which we will measure:
1) The bubble diameter as a function of "z", distance down the bubble, using a ruler.
2) The frost line position, Zf.
3) The film thickness as a function of "z".
4) The Die Exit thickness, d0.
5) XRD 2-d patterns will be collected for the films produced. You will need to provide a stack of about 20 films all oriented in the same way with respect to the extrusion direction, about 1 cm square. These can be taped together with one layer of scotch tape but the machine direction has to be clearly marked. (A volunteer will be needed for this part).
6) A single sheet from each of the 4 processing conditions will also be examined between crossed polars to estimate the crystalline orientation visually. You should sketch what you observe and include copies of these sketches in the report. Make sure to note the MD in your sketches.
Control Parameters: Air Flow Rate, Take-up rate (We can't vary this for now),Extruder rate (Fast: Slow), Barrel Temperature, Die Temperature
Measurable Parameters: Die Diameter, Die exit thickness, d0, Frost line position, Zf, Film Thickness, Film Orientation, Bubble Diameter as a function of "z", as in Tadmor figure 15.7 pp. 641 (Below) and Middleman figures 10-3 and 10-4 where BUR is the blow-up ratio (use theory on Tadmore pp. 639-640 or Middleman Chapter 10 to predict. Does it agree?), XRD on Films use Phillips and Polaroid Film, Orientation by Polarizers.
Your Results Should Include
1) A plot of Bubble diameter as a function of z for all conditions with calculations based on Middleman Chapter 10 or Tadmor chapter 15 (figure 15.7 (above)). The plot should clearly mark where the frost line forms.
2) Film thickness as a function of z for the 4 processing conditions. This plot should clearly give the exit thickness, d0, for each of the 4 conditions.
3) Descriptions of differences in the films under crossed polars.
4) Discussion and copies of the XRD patterns from the films.
Problems:
1) Define each term in the "Terms You Should Know" list above.
2) Middleman Problem 3-23
3) Middleman Problem 10-2
4) Middleman Problem 10-6
5) What is the limiting factor for a film blowing operation? Discuss. (Note that heat transfer is generally the limiting factor in polymer processing operations.)