Orientation tenter frame and roll

In this Fig. 8-41 view (a) the feeder-roll speed to puller-roll speed ratio can be set, such as 1 4, and simultaneously the ratio of width can be set as 1 4. The machined direction ratio is usually accomplished prior to the plastic s entering the temperature controlled oven that contains the tenter frame, by having it move around heat-controlled rolls where the rotational speed of the rolls increases from one roll to the next. View (b) is a schematic of the drawdown phenomenon with swell to produce orientation in the machined (longitudinal) direction. 

Heat-shrinkable sheets (thickness 0.040 to 0.120 inch, or 1 to 3 mm) and films (thickness 0.001 to 0.020 inch or 0. 025 to 0.5 mm) are fabricated from many of the same materials as shrinkable tubing.91 They are produced by extrusion as a tube, sheet or blown film. Irradiation is done by the equipment shown in Figure 8.10. Orientation (stretching) after irradiation can be done by several methods, namely by differentially heated and driven rolls (in the machine direction) or by a tenter frame (see Figure 8.11) in the transverse direction. If desired, biaxial stretching can be done. 

The optimum stretching heat for amorphous plastics (PVC, etc.) is usually just above its glass transition temperature (Tg Chapter 1). Generally the orientation temperature is 60 to 75% between the Tg and Tm (melt temperature). For crystalline plastics (PE, PET, etc.) generally it is below the Tg. Stretching can take place in-line or off-line with or without tenter frames using the appropriate temperature-pull rates as the plastic travels first through a series of heated rolls. For unidirectional orientation just the rolls are used. 

Figure 11.1 A typical OPS (oriented polystyrene sheet) tenter frame process with stretching in the machine (MD) and transverse (TD) directions. Rolls 1-8 are heated and driven. Sheet is heated above and below in the ovens. Sheet geometry as a function of position in process ...

The difficulty with these tenter frames is that mechanically they are very complicated and other systems have been proposed in patent literature, for instance the manufacture of films with beaded edges. The beads slide in diverging slots or are gripped by rubber strips longitudinal orientation is realised by a difference in speed between feeding and take-off rolls. 

Another commercial application for flat-die coextrusion is biaxially oriented multilayer films (11) made with the tentering process to improve mechanical properties. Tentered film is biaxially oriented by stretching in the longitudinal and transverse direction, either sequentially or simultaneously, at uniform optimum temperature. In sequential stretching, the multilayer extrudate is cooled to a suitable orientation temperature on a first set of rolls and then stretched in the machine direction between a second set of rolls which is driven faster than the first set. The uniaxially stretched film then enters a tentering frame, which has traveling clips that clamp the edge of the film. The clips are mounted on two... 

Orientation is a continuous operation and occurs in the direction of the film motion or the machine direction (MD). Usually, a cast sheet is transported on heated rollers (Fig. 9) to permit the sheet to reach a uniform temperature at which the polymer molecules are sufficiently mobile. Having reached this temperature, the sheet is abruptly accelerated between two rollers of different speeds. This point is defined as the draw point. In some instances, additional heat may be required to boost the temperature, or nip rolls are needed to regulate tension and to prevent film slippage (Fig. 9). Uniaxial film can be obtained by orienting perpendicular (transverse) to the MD using tenter frames or melt inflation, where the melt removal rate is equal to the melt velocity at the die. However, these two methods are seldom employed. The properties required by the user determine the orientation required to develop the desired physical properties. 

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