SHAPING PROCESSES FOR PLASTICS Flashcards
What are some of the reasons why plastic shaping processes are important?
The reasons include (1) many of the processes are net shape processes; (2) in general, less energy is employed than in metalworking processes; (3) lower temperatures are required to process plastics than metals or ceramics; (4) there is great flexibility in geometry; and (5) painting and other finishing processes are generally not required.
Identify the main categories of plastics shaping processes, as classified by the resulting product geometry.
The categories are (1) extrusion, (2) molding, (3) forming of continuous sheets and films, (4) fibers, (5) foamed products, and (6) discrete formed sheets and films.
Viscosity is an important property of a polymer melt in plastics shaping processes. Upon what parameters does viscosity depend?
Viscosity of a polymer melt depends on (1) temperature and (2) shear rate. Also, (3) the molecular weight of the polymer affects viscosity.
How does the viscosity of a polymer melt differ from most fluids that are Newtonian.
A polymer melt exhibits pseudoplasticity, which means that its value decreases with increasing shear rate.
What does viscoelasticity mean, when applied to a polymer melt?
Viscoelasticity is a combination of viscous and elastic properties which cause the melt to exhibit memory - the tendency to return to its previous shape, as exhibited by die swell in extrusion.
Define die swell in extrusion.
Die swell is the tendency of the extrudate to expand in cross-sectional dimensions immediately on exiting the die orifice. It results from the viscoelastic properties of the polymer melt.
Briefly describe the plastic extrusion process.
In plastic extrusion, a polymer melt is compressed to flow through a die orifice and thus the continuous length of the plastic assumes a cross-sectional shape that is approximately the same as that of the orifice.
The barrel and screw of an extruder are generally divided into three sections; identify the sections.
The sections are (1) the feed section, in which the feed stock is fed from the hopper and heated; (2) the compression section, in which the polymer changes to a viscous fluid; and (3) the metering section, in which pressure is developed to pump the plastic through the die orifice.
What are the functions of the screen pack and breaker plate at the die end of the extruder barrel?
The functions are to (1) filter dirt and lumps, (2) build pressure, (3) straighten the flow and remove memory of the polymer melt.
What are the various forms of extruded shapes and corresponding dies?
The shapes are (1) solid profiles, such as rounds and L-shapes; (2) hollow profiles, such as tubes; (3) wire and cable coating; (4) sheet and film; and (5) filaments (continuous fibers).
What is the distinction between plastic sheet and film?
The distinction is based on thickness. Sheet stock has a thickness greater than 0.5 mm (0.020 in), while film stock is less than 0.5 mm (0.020 in) thick.
What is the blown-film process for producing film stock?
The blown-film process is a widely used process for making thin polyethylene film for packaging. It combines extrusion and blowing to produce a tube of thin film. The process begins with the extrusion of a tube that is immediately drawn upward while still molten and simultaneously expanded in size by air inflated into it through the die mandrel.
Describe the calendering process.
Calendering is a process for producing sheet and film stock out of rubber or rubbery thermoplastics such as plasticized PVC. In the process, the initial feedstock is passed through a series of rolls to work the material and reduce its thickness to the desired gage.
Polymer fibers and filaments are used in several applications; what is the most important commercial application?
Textiles.
What is the technical difference between a fiber and a filament?
A fiber is a long, thin strand of material whose length is at least 100 times its diameter; a filament is a fiber of continuous length.
