Die swell
Encyclopedia
Die swell, also known as extrudate swell, is a common phenomenon in polymer
processing. Die swell occurs in instances of polymer extrusion, in which a stream of polymeric material is forced through a die
, a specialized tool in manufacturing to shape or cut polymeric materials. Die swell is an instance where a polymer stream is compressed by entrance into a die, and is followed by a partial recovery or “swell” back to the former shape and volume of the polymer after exiting the die, hence the term die swell.
Die swell is a phenomenon directly related to entropy
and the relaxation of the polymer within the flow stream. Initially, a flow stream has a constant rate before entering the die, and the polymers within the stream occupy a roughly spherical conformation, minimizing entropy. Extrusion through the die causes an increase in flow rate through the polymer flow stream. As the polymer spends time inside the die and is subject to the much increased flow rate, the polymers lose the spherical shape, becoming longer due to the increased flow rate. Physical entanglements may relax, if the time scale of the polymer within the die is long enough. When the polymer stream leaves the die, the remaining physical entanglements cause the polymers in the die stream to regain a portion of its former shape and spherical volume, in order to return to the roughly spherical conformation that minimizes entropy.
The disentanglement of polymer chains is a kinetic
process, and so the longer the die is, the more time is given for the physical entanglements within the polymer stream to disentangle. With a longer die and a slower polymer flow stream, less pronounced die swell will be observed. This is due to the longer die providing a longer time period for polymer, when subject to the increase flow rate, to disentangle. This characteristic relaxation time determines the length of time the polymer must spend inside the die to minimize die swell.
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
processing. Die swell occurs in instances of polymer extrusion, in which a stream of polymeric material is forced through a die
Die (manufacturing)
A die is a specialized tool used in manufacturing industries to cut or shape material using a press. Like molds, dies are generally customized to the item they are used to create...
, a specialized tool in manufacturing to shape or cut polymeric materials. Die swell is an instance where a polymer stream is compressed by entrance into a die, and is followed by a partial recovery or “swell” back to the former shape and volume of the polymer after exiting the die, hence the term die swell.
Die swell is a phenomenon directly related to entropy
Entropy
Entropy is a thermodynamic property that can be used to determine the energy available for useful work in a thermodynamic process, such as in energy conversion devices, engines, or machines. Such devices can only be driven by convertible energy, and have a theoretical maximum efficiency when...
and the relaxation of the polymer within the flow stream. Initially, a flow stream has a constant rate before entering the die, and the polymers within the stream occupy a roughly spherical conformation, minimizing entropy. Extrusion through the die causes an increase in flow rate through the polymer flow stream. As the polymer spends time inside the die and is subject to the much increased flow rate, the polymers lose the spherical shape, becoming longer due to the increased flow rate. Physical entanglements may relax, if the time scale of the polymer within the die is long enough. When the polymer stream leaves the die, the remaining physical entanglements cause the polymers in the die stream to regain a portion of its former shape and spherical volume, in order to return to the roughly spherical conformation that minimizes entropy.
The disentanglement of polymer chains is a kinetic
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...
process, and so the longer the die is, the more time is given for the physical entanglements within the polymer stream to disentangle. With a longer die and a slower polymer flow stream, less pronounced die swell will be observed. This is due to the longer die providing a longer time period for polymer, when subject to the increase flow rate, to disentangle. This characteristic relaxation time determines the length of time the polymer must spend inside the die to minimize die swell.