Chapter 13

Question 1

Define: Pseudoplasticity

Answer 1

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.

Question 2

Define: Viscoelasticity

Answer 2

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.

Question 3

Define: Die Swell Ratio

Answer 3

When extrusion occurs, the amount at which the polymer returns to its former shape. Is termed as:
Diameter (extruded)/Diameter (die Orifice)

Question 4

Define: Feed

Answer 4

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)

Question 5

Define: Compression

Answer 5

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)

Question 6

Define: Metering Sections of Extrusion Barrel

Answer 6

The section where the melt is homogenized and sufficient pressure is developed to pump it through the die opening.

Question 7

Define: Melt Fracture

Answer 7

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)

Question 8

Define: Shark Skin/ Bambooing

Answer 8

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.

Question 9

Define: Sheet vs. Film

Answer 9

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.

Question 10

Define: Blown-film Process

Answer 10

Refers to thicknesses below .5mm; Thin films are used for packaging (grocery bags) while thicker films are sued as covers and liners (pools)

Question 11

Define: Spinning

Answer 11

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.

Question 12

Define: Injection Molding

Answer 12

Similar to an extruding, but where melted plastic is rammed into a mold, hardening the plastic into predetermined shapes.

Question 13

Define: Reciprocating Screws

Answer 13

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.

Question 14

Define: Shot

Answer 14

Refers to the amount of polymer melt that is injected under high pressure into the mold cavity per run of the injection molding machine.

Question 15

Define: Ejection System

Answer 15

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.

Question 16

Define: Short Shots

Answer 16

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.

Question 17

Define: Sink Marks

Answer 17

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”

Question 18

Define: Weld Lines

Answer 18

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

Question 19

Define: Reaction Injection Molding

Answer 19

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.

Question 20

Define: Compression vs. Transfer Molding

Answer 20

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.

Question 21

Define: Blow Molding

Answer 21

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.

Question 22

Define: Parison

Answer 22

Involved in the first stage in blow molding, in which fabrication of a starting tube of molten plastic, called a parison.

Question 23

Define: Rotomolding

Answer 23

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.

Question 24

Define: Thermoforming

Answer 24

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.