Raffia Tape Line Process

2.   Stretched Tapes (Raffia)  are uniaxial oriented thermoplastic semi finished products with a high width to thickness ratio. These tapes can be converted into twines, ropes, woven and knitted fabrics. A range of applications for stretched tapes have expanded considerably from woven sacks to tarpaulins, primary carpet backing, industrial fabric, carpet yarn, ropes, geotextile fabrics, concrete reinforcement etc. Manufacture of Tapes. The principal stages involved in tape manufacture are Extrusion of film, Quenching of film, Slitting of film into tapes, Orientation of tapes, Annealing of tapes and Winding. Formulas
4.   The equipment and processing requirements for conversion of PP into tapes are quite similar to those for HDPE. Although the blown film process has not phased out completely, more and more processors are shifting to the cast film process owing to its better gauge control and higher outputs. Equipment for Cast Film Extrusion Extruder PP can be processed on conventional extruders with three zone screws viz. feed, compression and metering. The output obtained from PP depends upon the L/D ratio(Length to Diameter Ratio). Higher the L/D ratio, higher will be the output per revolution of screw. For good melt homogeneity and optimum output, extruders with L/D ratios of 24:1 up to 30:1 are preferred, whilst compression ratios between 3.0 - 3.5 should be used. Screw diameters range from 65 upwards. 1.Die The type of die used is referred to as a coat hanger die / T-die which provides a good streamlined flow. Quenching of the film The polymer melt is partly oriented during extrusion through a die. To prevent melt relaxation the melt is quenched rapidly after exiting from the die. Fast cooling promotes finer crystalline structure of polymer in the film which in turn improves the performance of film during the stretching operation, apart from rendering better physicals. In cast film, the cooling is done by quenching the film in a water tank. The film from the die is directly taken into the tank filled with water. Film quality and performance of the resulting tapes mainly depend on the quenching conditions. During quenching, the significant parameters which control the physicals of the tapes are die-water distance (air gap) and quench water temperature. Operation with a lower air gap will reduce the time for melt relaxation and result in films, with higher strength. A very fast rate of quench will result in a very fine crystal structure in the film, which will give higher clarity and strength, than a film which is quenched at slower rates. To achieve optimum strength and elongation, air gap of 20-35 mm and quench water temperature of 30-40°C is recommended.
6.    Slitting of film into tapes The flat film after quenching is slit into tapes of specific width according to the end use requirements. The slitting tools generally used are industrial or surgical blades with sharp edges. Blunt blades produce poor cuts, which lead to problems in drawing, winding and weaving of tapes. The blades are equally placed on a bar using spacers and are set at an angle of 30 ° and 60 ° with the film. Initial tape width is adjusted by selecting appropriate spacer. Orientation of tapes Orientation is accomplished by stretching the tapes while passing them through a hot air oven or over a hot plate, maintained at a temperature just below the melting temperature of PP. PP has a higher melting point (160-165 °C) than HDPE. Hence, it needs to be oriented at a higher temperatures than that for HDPE to fully develop the mechanical properties in stretched products. Stretching of tapes is done by passing them over two sets of rollers, called godet rollers, placed on either side of the hot air oven / hot plate and operating at different speeds. Ratio of speed of second set of rollers, operating at higher speed, to that of first set is termed as stretch ratio. Stretching of tapes in presence of heat media results in molecular / chain orientation and thus greatly increases the mechanical strength of tapes. PP tapes have to be oriented at a stretch ratio of 5-6 and temperature of 135-155°C in hot air oven or at 125-160 °C on a hot plate. Hot Air Oven Tapes from the first set of godet are taken through an oven on to the second godet rollers. Hot air is blown in the oven, counter current to the movement of tapes, and recirculated through a blower - heater system at linear flow rate of 10-30 mtrs/sec. Higher flow rate of circulating air is preferred as it enables faster and more uniform heating of the tapes and lower the risk of leaving some areas in tapes undrawn or underdrawn. Once the required tape properties are obtained, the orientation temperature and the stretch ratio are kept constant and checked randomly throughout the process. In hot air oven system, uniform heating of the tapes takes place, which ensures better tape properties compared to the Hot plate system. The length of this unit is around 3 meters and its width is slightly more than that of the godet rolls, for all the tapes to traverse freely. A good temperature control system with an accuracy of at least ± 5 °C is required. Hot air circulation in the oven should be adequately controlled to avoid excessive turbulence.
8.    Hot plate is heated electrically or by circulating hot oil. In this system, tapes are in close contact with the hot metal surface while they are oriented. The upper and lower hot plate design is preferred as it provides enough contact for uniform distribution of the tapes, since half the tapes can be stretched over top surface while the other half are stretched over lower surface, giving better heat transfer to all tapes and less variations in mechanical properties. Although hot plate offers 25% saving in terms of energy requirement, the hot air circulating ovens provide uniform heating to the tapes, thereby enabling the converter to exploit the superior mechanicals of PP. Annealing of tapes Drawn tapes are "annealed" immediately after stretching operation. This helps to minimize tape shrinkage which may occur as a result of residual stresses in the oriented tapes. Annealing is done by heating the stretched tapes while they are passing over from second goddet rollers to third godet rollers; the latter being maintained at a slightly lower speed (5% less) than the former. The annealing ratio is a function of second and third goddet rollers. The annealing temperature is slightly lower (5-10°C) than the orientation temperature. For PP, it is between 125-145 °C, while for HDPE it is between 100-110°c. Alternatively, tapes can be annealed by using hot goddet rolls, which are usually heated electrically or by circulating hot oil. Relaxation takes place over a short gap (the distance between the rolls).Winding After the final goddet stand, the tape proceeds to a winder stand and is taken up by the bobbins. For good winding, controlled tension is essential.