Threading melt process

It is possible to more effectively deliver small-molecule additives to polymers by employing either crystalline additive-CD-ICs or additives that are permanently complexed/rotaxanated with CDs and that remain soluble. Additive-CD-IC complexes may be delivered by melt-processing up to temperatures of 300°C, while the permanently threaded and soluble additive-CD-rotaxanes can be delivered from their solutions. 

Because of the lower ductility of the fluorocarhon materials (e.g., TFE, CTFE, FEP), snap-fit and press-fit joints are seldom nsed. Rivets or studs can be used in forming permanent mechanical joints. These can be provided with thermal techniques on the melt-processable grades. Self-tapping screws and threaded inserts are used for many mechanical joining operations. In holted connections, some stress relaxation may occur the first day after installation. In such cases, mechanical fasteners should he tightened thereafter, stress relaxation is negligible. 

Prokunin, Sevruk, and Fridman 37,68,69) have suggested an additional characteristic of rheological and technological properties of thermoplastics used as raw materials in such processes where an important stage is the longitudinal deformation of melts, for example, to produce films, fibers, flat threads, thermal drawing of sheet blanks, etc. 

With the exception of silk, which the silkworm or spider extrudes as a continuous filament, natural fibers are of finite length. For textile use, these need to be cleaned and then spun into threads or yams. Synthetic fibers, on the other hand, are continuous filaments produced from a solution or melt. The term spinning is used to describe the formation of synthetic fibers, but in this sense it has no relation to the process for combining fibers into threads. 

Extrusion—The processing step in which a threaded shaft (a screw) melts a polymer sample and forces it through a die that produces the desired shape. 

If the orientation process in semi-crystalline fibres is carried out well below the melting point (Tm), the thread does not become thinner gradually, but rather suddenly, over a short distance the neck. The so-called draw ratio (A) is the ratio of the length of the drawn to that of the undrawn filament it is about 4-5 for many polymers, but may be as high as 40 for linear polyolefins and as low as 2 in the case of regenerated cellulose. 

Criteria of extrudability, mouldability, spinnability and stretchability as used in practice are described (melt index, spiral length, mouldability index, melt strength, ultimate thread length, etc.). These criteria are based on the dimensionless parameters (numerics) of the processes. 

Spinnability is the ability of a polymer to be transformed into long continuous solid threads by a melt-spinning process. Therefore a spinnable polymer must conform to three requirements ... 

Startup operations are made at rates people can handle. The process is very slow compared to standard operating speeds. The puller starts its movement at just about the same speed the person has been pulling or there may be a pile-up or tear-off of melt at the die. That will usually mean threading up again. 

The flat dies, or slot dies as they are sometimes called, are used to produce webs in a variety of processes. They all have an interior manifold for distributing the plastic and lips for adjusting the final profile of the web (extrudate). Some dies have movable restrictor bars for changing the manifold for proper melt distribution (Figure 17.10). All flat dies have flexible lips that can be adjusted by bolts to remove humps or bumps in the web s profile. Die lips can have their adjustment bolts push only, where internal plastic melt pressures are adequate to keep the lips positioned against the bolts, or can be push/pull for low pressure applications. Direct acting or differential thread designs (for minute adjustments) are available. Profile variations of at least 3% or less can be achieved with flat dies. 

---