Take-off speed

The film thickness and the degree of uniaxial and biaxial orientation of the material are controlled by the blow-up ratio and the haul-off rate. The haul-off rate controls the film orientation in the machine direction, while the blow-up ratio controls the orientation in the transverse direction. Common values for the blow-up ratio are in the range of 1.5-4, depending on the material and desired film thickness. Take-off speeds are usually around 10 to 50 m/min. 

Cast film is generally produced by downward extrusion of the melt onto chilled chrome rollers, which are highly polished to impart good surface characteristics to the film, as shown in Fig. 7.4. The extrudate contacts the first chill roll tangentially, and then typically travels in an S-pattern around two or more chill rolls. The first chill roll typically operates at a temperature of at least 40°C (104°F), with subsequent rolls operating at successively lower temperatures to cool the film enough that it can be trimmed and wound. An air knife is typically used to pin the plastic against the first chill roll. The film dimensions are controlled primarily by the die dimensions, extrusion rate, and take-off speed. Film produced in this manner is... 

The methods of producing GF-reinforced profiles made of UP and EP resins known under the collective term pultrusion result in molding material bending strengths up to 700 N/mm. Take-off speeds exceeding 1 m/min are achieved. 

Surging] screw speed too high/take off speed too high/back pressure too low/ compound temperature too high. 

The principal controls for a blown film line are barrel (cylinder) and die temperatures, die gap, extrusion rate, internal air pressure, bubble diameter, cooling air flow or cooling rate, and line speed (take-off speed). These controls influence the film dimensions and properties. The frost (freeze) line height, which is a ring-shaped zone where the bubble frequently begins to appear frost because the film temperature falls below the softening range of the resin and crystallization occurs, is an indicator for many of these variables. The frost line may not be visible at times. When it is not, the zone where the bubble reaches its final diameter is considered to be the frost line. 

Increasing the take-off speed increases film length and uniaxial orientation but reduces film thickness. The machine-direction stretching of the film is gauged by the drawn-down ratio (DDR)... 

Initial die gaps are set to about 20 percent greater than the final film thickness, and then adjusted to accommodate changes in polymer flow which are resin and rate sensitive. Higher screw speeds increase extruder output, overall film thickness, the tendency toward melt fracture, and may alter the flow pattern. Thus, extruder speed is not a recommended control. In contrast, increased chill-roll speeds decrease film thickness, reduce film width due to increased neck in, increase uniaxial orientation, and alter the optimum air gap or drawdown distance. The optimum air gap, which produces the best orientation, crystallization, and surface properties, depends on the material and chill-roll speed. At 23 to 30 m/min (75 to 100 ft/min), the air gap for low-density polyethylene is about 100 mm (4 in), but when the line speed increases, the air gap is found by trial and error, Since the chill-roll speed controls film stretching, the take-off speed has little effect on the film dimensions. 

While barrel, die, and water-bath temperatures and extrusion rate can be varied in pipe extrusion, the primary controls are the die gap, calibration rings or extended mandrel, and the take-off speed. Die gap and centering of the mandrel provide the initial wall thickness and thickness uniformity, whereas take-off speed determines the final thickness. While extended mandrels fix the inner diameter, calibration rings establish the outer diameter in vacuum and pressure cahbration systems. The other diameter varies with the take-off rate. Water-bath temperatures and temperature profiles control the cooling rate for the pipe or tubing. This alters the shrinkage, crystallinity, and retained stresses in the products. 

The polymorphic composition of extruded p-nucleated iPP products depends significantly on the cooling conditions and on the take-off speed. Sheets of almost pure p-iPP were successfully produced by means of a laboratory extruder (Viskosystem) with wide-gap die and equipped with a calendering unit composed of three superimposed, thermo-regulated compression rollers. The temperature of the first two compression rollers should be set between 105-110°C, i.e. in the temperature range between T( p) and T(pa) where the formation of pure p-iPP is... 

It has four jet engines that can each produce 50,000 lb of thrust at a take-off speed of 150 miles per hour. Therefore its dynamic performance parameter is... 

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