Chapter 13 Flashcards
Define: Pseudoplasticity
The property of thermoplastic polymers when melted that indicates that the fluid becomes thinner at higher rates of shear. Thus, as temperature rises, viscosity decreases.
Define: Viscoelasticity
Another property of thermoplastic polymer melts that when extruded through a die opening, the extruded material attempt to return to its form shape after leaving the die orfice.
Define: Die Swell Ratio
When extrusion occurs, the amount at which the polymer returns to its former shape. Is termed as:
Diameter (extruded)/Diameter (die Orifice)
Define: Feed
In plastic extrusion, the feed section is the first portion of the extruder in which the stock is moved from the hopper port and preheated (rotating metal bit)
Define: Compression
Where the polymer is transformed into liquid consistency, air entrapped the pellets is extracted from the melt, and the material is compressed. (Still in drill bit of extruder)
Define: Metering Sections of Extrusion Barrel
The section where the melt is homogenized and sufficient pressure is developed to pump it through the die opening.
Define: Melt Fracture
A defect in extrusion in which the stresses acting onto he melt immediately before and during its flow through the die are so high as to cause failure, manifesting in the form of a highly irregular surface on the extrudate. (Laminar to turbulent flow)
Define: Shark Skin/ Bambooing
Common defect in extrusion in which the surface of the product becomes roughened upon exiting the die. As the melt flows through the die opening, friction at the interface results in a velocity profile across the cross section.
Define: Sheet vs. Film
Thermoplastic polymer form refering to stock with a thickness ranging from .5mm to about 12.5 mm. used for products such as flat window glazing and stock for thermoforming.
Define: Blown-film Process
Refers to thicknesses below .5mm; Thin films are used for packaging (grocery bags) while thicker films are sued as covers and liners (pools)
Define: Spinning
The process of producing fiber and filaments for textiles. Is a holdover for the methods used to draw and twist natural fibers into yarn or thread. In the production of synthetic fibers, the term refers to the process of extruding a polymer melt or solution through a spinneret.
Define: Injection Molding
Similar to an extruding, but where melted plastic is rammed into a mold, hardening the plastic into predetermined shapes.
Define: Reciprocating Screws
A name that also identifies the machine type. Older machines used a simple ram but the superiority of the reciprocating screw design has led to widespread adoption.
Define: Shot
Refers to the amount of polymer melt that is injected under high pressure into the mold cavity per run of the injection molding machine.
Define: Ejection System
System in which molded parts from the molding machine are ejected form the cavity at the end of the modeling cycle. Uses Ejector pins built into the moving half to push the part out of the mold cavity.
Define: Short Shots
A common defect in injection molded parts where a molding has solidified before completely filling the mold cavity. The defect can be corrected by increasing temperature and/or pressure.
Define: Sink Marks
Defect in injection molding that occurs when the outer surface on the modeling solidifies, but contraction of the internal material causes the skin to be depressed below its intended profile. Similar to “voids”
Define: Weld Lines
Defect in injection molding when polymer melt flows around a core or other convex detail in the mold cavity and meets from opposite directions; the boundary thus formed is called a weld line
Define: Reaction Injection Molding
Involves the mixing of two highly reactive liquid ingredients and immediately injecting the mixture into a mold cavity, where chemical reactions leading to solidification occur.
Define: Compression vs. Transfer Molding
Compression: process consists of loading a precise amount of molding compound, called charge, into the bottom half of a heated mold. Bringing the mold halves together to compress the charge, forcing it to flow and conform to the shape of the cavity, heating the charge by means of hot mold to polymerize, then opening the mold halves and and removing the part from the cavity.
Transfer Molding: a thermosetting charge is loaded into a chamber immediately ahead of the mold cavity, where it is heated; pressure is then applied to force the softened polymer to flow into the heated mold where curing occurs.
Define: Blow Molding
Molding process in which air pressur is used to inflate soft plastic inside a mold cavity. It is an important industrial process for making one piece hollow plastic parts with thin walls, such as bottles and similar containers.
Define: Parison
Involved in the first stage in blow molding, in which fabrication of a starting tube of molten plastic, called a parison.
Define: Rotomolding
Rotational molding uses gravity inside a rotating mold to achieve a hollow form. Also called rotomolding, it is an alternative to blow molding for making large, hollow shapes. used principally for thermoplastic polymers, but applications for thermosets and elastomers are becoming more common.
Define: Thermoforming
A process in which a flat thermoplastic sheet is heated and deformeed into the desired shape. The process is widely used to package consumer products and fabricate large items such as bathtubs.
