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Fiber Drawing (Polymers)

Fiber Drawing is the process in which polymers are lengthened via mechanical or gravitational means after initial extrusion, such as spinning (link). This process not only reduces the cross section of the polymer fiber, but also alters its mechanical properties.

Process

Most fiber drawing begins with melt spinning. In this process, solid polymer pellets or powder are melted in an extruder, and pushed through a die with a large number of small holes, known as a spinneret. To prepare fibers that are unable to be melt spun, dry or wet spinning is used. In wet spinning, the polymer is dissolved and extruded through a spinneret into a chemical bath. In dry spinning, the same general process is used, but the fibers are not extruded into a bath; instead, the solvent is allowed to evaporate from the fibers.

Typically, fiber drawing occurs after spinning without any sort of uptaking process, in which the fibers are wound around a spool. Immediately past the exit orifice of the spinneret, the polymer melt undergoes die swell (link) despite the application of a drawing force. Once die swell is complete, the drawing force causes the fibers to contract laterally and lengthen. The radius of the fibers have been shown to decrease hyperbolically as they lengthen. Once the fibers solidify, they may begin to crystallize, with each grain initially randomly oriented. Further drawing will cause the crystal grains to elongate and reorient in the direction of the tensile axis.

Spinning Stability

In practice, not all polymers are suitable for fiber spinning or drawing. This is particularly an issue in extensional-thinning polymers, where capillary failure or necking can cause separation of the melt before solidification.

Draw resonance is the most common issue that can occur during drawing of the polymer melt, regardless of polymer suitability. Resonance occurs when the rate of mass flow is not constant between the spinneret and fiber take up roller, despite being constant at each of those individual components. When the mass flow rate is not constant, the diameter of the fiber will vary to accommodate the variation. Once started, this resonance may not correct itself, requiring a complete shutdown of the extrusion line.

It has been shown that draw resonance occurs once a critical drawdown ratio is exceeded; this ratio is dependent on the flow behavior (i.e. Newtonian, shear thinning) and viscoelastic behavior of the fluid. Draw resonance has not been found to be a function of the flow rate, however. A polymer melt approaching a Newtonian fluid such as PET can have a drawdown ratio of around 20, whereas highly shear thinning and viscoelastic polymer melts such as polyethylene, polystyrene, and polypropylene may have critical drawdown ratios as low as 3.

Sheet Extrusion

Sheet extrusion

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For products such as plastic sheet or film, the cooling is achieved by pulling through a set of cooling rolls (calender or "chill" rolls), usually 3 or 4 in number. Running too fast creates an undesirable condition called "nerve"- basically, inadequate contact time is allowed to dissipate the heat present in the extruded plastic. In sheet extrusion, these rolls not only deliver the necessary cooling but also determine sheet thickness and surface texture (in case of structured rolls; i.e. smooth, levant, haircell, etc.).

Often co-extrusion is used to apply one or more layers on top of a base material to obtain specific properties such as UV-absorption, soft touch or "grip", matte surface, or energy reflection, where it is needed on the surface.

A common post-extrusion process for plastic sheet stock is thermoforming, where the sheet is heated until soft (plastic), and formed via a mould into a new shape. When vacuum is used, this is often described as vacuum forming. Orientation (i.e. ability/ available density of the sheet to be drawn to the mould which can vary in depths from 1 to 36 inches typically) is highly important and greatly affects forming cycle times for most plastics.

Thermoforming can go from line bended pieces (e.g. displays) to complex shapes (computer housings), which often look like they have been injection moulded, thanks to the various possibilities in thermoforming, such as inserts, undercuts, divided moulds.

Plastic extrusion onto paper is the basis of the liquid packaging industry (juice cartons, wine boxes...); usually an aluminium layer is present as well. In food packaging plastic film is sometimes metallised, see metallised film.

Film Blowing

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Tube Extrusion

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Coextrusion

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