Shaping Processes for Plastics Flashcards
Is polymer shaping a bulk process or net shape process?
Net shape
What is a polymer melt?
A fun sandwich.
Or a polymer heated to the consistency of a liquid.
You choose.
What are the important properties of polymer melts?
Viscosity
Viscoelasticity
How does viscosity change with shear rate? How about temperature?
Polymer melts become thinner with higher shear rates and higher temps
What is die swell?
An extruded polymer ‘remembers’ its previous shape after it goes through a die
Due to viscoelasticity
What are the two main components of an extruder?
Barrel
Screw
(The die is not an extruder component)
Describe the extruder barrel
Internal diameter ranges from 25 to 150 mm
L/D from 10 to 30
Feedstock fed by gravity onto screw, and rotated through the barrel
Electric heaters melt feedstock, mixing and working adds extra heat to maintain the melt.
Describe the extruder screw
Has 3 sections:
Feed - feedstock is moved from hopper and preheated
Compression - Polymer is transformed into fluid, air mixed with pellets is extracted and material is compressed
Metering - melt is homogenized and pressurized to pump through die opening
Describe the screen pack of extruder
Melt first passes through a screen pack before die to filter out contaminants and lumps, builds pressure, and straighten flow of melt to remove ‘memory’ of circular motion
What are the different types of flow in the extruder?
Drag flow Qd is the friction of the polymer and screw that pulls the polymer forward
Back pressure Qb is the compression force that reduces drag flow transport
Resulting flow in extruder is Qx = Qd-Qb
What is a mandrel?
A tool to create a hollow profile using air pressure
What type of polymers are best for films and sheets?
Thermoplastics
Describe slit die extrusion
A narrow but wide slit: has problems with uniform thickness across sheet, and thickening at edges
Describe blown film extrusion
A tube is extruded, then drawn upward while still molten and expanded by air inflating it through the die
How are fibers produced?
Spinning
What are the three types of spinning?
Melt spinning - polymer is pumped through spinneret while molten then cooled and spooled onto bobbin\
Dry spinning - starting polymer is in solution, then solvent is evaporated out after being forced through spinneret
Wet spinning - polymer is in a solution that must be chemically coagulated
Describe injection molding
Polymer is melted to highly plastic state and forced under pressure into mold cavity. Can create complex or even multiple parts.
Mold are expensive.
What are the two parts of an injection molding machine?
The injection unit (like an extruder) Clamping unit (opens and closes mold)
How can you compensate for shrinkage in injection molding?
Dc = Dp + DpS + DpS^2
Where Dc is dimension of cavity
Dp is mold part dimension
S is shrinkage value
How can we reduce shrinkage?
Add fillers to the plastic
Higher pressure (to force more material in)
Longer compaction time (to force more material in)
Higher molding temp (decrease viscosity)
When are ribs used for plastic products?
To add stiffness without increasing wall thickness. Ribs should be thinner than walls to minimize sink marks.
