Frost-line height

See references (11,48,50, and 51) for the effect of blowing conditions on film properties of LLDPEs. The result in this table was obtained for the following blown film conditions blow-up ratio = 2.5 1 die gap = 2.54 mm output = 32 kgh film thickness — 25.4 mm die size = 102 mm frost line height = 230 mm melt temperature = 483... 

Approximate melt index of Ig(lOmin)" and density 0.920gcm". Extrusion conditions blow-up ratio = 2 1 die gap = 2.03 mm output = 32kgh film thickness = 31.75 pm die size = 63.5mm frost line height — unknown melt temperature = 466 K. 

The strong interdependence of process variables is another aspect of the process that requires a high level of operator skill and has led to extensive advancements in measurement and control techniques. There are many process variables - screw speed, nip speed, internal bubble air volume, and cooling rate (frost-line height) - that influence bubble geometry and, as a result, film properties. An adjustment to any one... 

Ambient air temperature around the extrusion line also has a large effect on bubble cooling, even when chilled air is used in the process. This is why frost-line height may change significantly from day shift to night shift in plants that are not air-conditioned. 

The fixed-position roll is motor-driven to establish the line speed. Since the line speed (nip speed) is a primary control determining film thickness, bubble diameter, and frost-line height, fluctuations in the motor speed should be minimized, generally less than 1% full scale. 

The final control variable is frost-line height. The position of the frost line is very sensitive to any changes in the process. Therefore, this is an excellent parameter to track as a measure of stability during a run. Any changes in ambient or process temperatures, line or screw speed, or material feed conditions will affect the frost-line height. This, of course, will lead to variations in film gauge and layflat as well. 

Table 4.1 defines the response that increasing each of the four main process variables (nip speed, screw speed, cooling speed, and bubble volume) has on each of the three main bubble geometric variables (film thickness, bubble diameter, and frost-line height). An asterisk identifies the primary response to each increase. 

Variable to increase Film thickness Bubble diameter Frost-line height... 

When more air is inserted into the bubble, bubble volume increases, primarily the diameter increases by stretching more in TD. The increased TD stretching results in thinner film. Thinner film cools more quickly, consequently lowering the frost-line height. 

Record the film thickness, bubble diameter, layflat width, and frost-line height. Explain why each geometric variable changed from step 9 in the direction (increased or decreased) that it did. 

Change any controls to produce film with the following specifications film thickness = 1 ( 0.1) mil, bubble diameter = 14 ( 0.1) in, frost-line height = 7 ( 0.1) in. For these conditions, record the nip speed, screw speed, and blower motor speed. 

Record the film thickness, bubble diameter, layflat width, and frost-line height. 

The polyethylene samples obtained from Catalysts 3 and 4 were compoimded on a Brabury mixer with a suitable additive package and then the samples were fabricated into 1.0 Mil LLDPE film using a Gloucester extruder (430°F) operating with a 100 mil die-gap 2 1 blow-up ratio and a 25-inch frost line height and operating at a fabrication rate of 250 lbs PE/hr. The film mechanical properties dart... 

FLH frost line height UHMWHDPE ultrahigh molecular weight... 

Figure 6.6 Bubble conditions at various drawdown ratios (DDR) and blow-up ratios (BUR) with frost-line height of 0.2 m for LDPE ( stable metastable x unstable), LLDPE (A stable A metastable, x imstable), and mPE(l) ( stable metastable x unstable)...

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