Film gauging

Optical Properties The optical values of Equistar Chemicals NA 345 actually improve with decreases in film gauge and are maintained at wide die-gap settings, leading to cost savings. Film can be drawn down to a minimum gauge consistent with required physical properties. With wider die-gaps, back pressures are reduced, as are extrusion costs. 

Film Gauge 2.8 mil LLDPE-rich commercial film Figure 25-02. Hot-tack strength of octene vs hexene LLDPE resins. ... 

The next process variable to be controlled is film thickness (gauge). Either on-line or off-line measurements allow us to monitor film gauge. On-line devices usually employ a radiation source, such as a gamma backscatter system. These devices measure thickness by emitting radiation that reflects back to the sensor from both the near and far surfaces of the film. They can be mounted to measure a fixed location on the bubble or on a carriage to traverse around the bubble (Fig. 3.21). Also, a unit traversing back and forth across the flattened web can be used, but this measures two-layer thickness. Off-line devices typically operate on a capacitance principle, so they don t require the safety practices employed with radiation sources. 

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. 

Wet film gauge Device for measuring wet film thickness of coatings. 

A film or sheet extrusion line consists of an extruder, film or sheet die, cooling system, take-off system, wind-up system, and auxiliary equipment such as film gauging systems, surface treatment, and slitters. Single-screw extruders with relatively long barrels (L/D = 27 to 33 1) are used for most resins, but polyvinyl chloride powders are often processed with twin-screw extruders. As shown in Table 5.10, extruder size is matched to the die width. 

Wet film gauging Thickness of a wet film can be determined by any of several gauges which consist of a series of calibrated indentations along the edge of a handheld ruler (Fig. 14.25). Probable thickness of the dried film can be calculated if solids content by volume is known. 

Dry film gauging Electrical and magnetic nondestructive and visual destructive equipment for determining thickness are described in several places (Figs. 14.26 and 14.27). 

Film Description coc (%) Film Gauge micron (mil) Material Cost /m ( /MSI) Haze (%) Gloss 60 lastic Modulus MD MPa (ksl) Elastic Modulus TD MPa (ksl) Break Stress MD MPa (ksl) Break Stress TD MPa (ksi) Puncture Newton (Ibf)... 

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