Shaping Processes for Plastics

Question 1

Is polymer shaping a bulk process or net shape process?

Answer 1

Net shape

Question 2

What is a polymer melt?

Answer 2

A fun sandwich.
Or a polymer heated to the consistency of a liquid.
You choose.

Question 3

What are the important properties of polymer melts?

Answer 3

Viscosity

Viscoelasticity

Question 4

How does viscosity change with shear rate? How about temperature?

Answer 4

Polymer melts become thinner with higher shear rates and higher temps

Question 5

What is die swell?

Answer 5

An extruded polymer ‘remembers’ its previous shape after it goes through a die
Due to viscoelasticity

Question 6

What are the two main components of an extruder?

Answer 6

Barrel
Screw
(The die is not an extruder component)

Question 7

Describe the extruder barrel

Answer 7

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.

Question 8

Describe the extruder screw

Answer 8

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

Question 9

Describe the screen pack of extruder

Answer 9

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

Question 10

What are the different types of flow in the extruder?

Answer 10

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

Question 11

What is a mandrel?

Answer 11

A tool to create a hollow profile using air pressure

Question 12

What type of polymers are best for films and sheets?

Answer 12

Thermoplastics

Question 13

Describe slit die extrusion

Answer 13

A narrow but wide slit: has problems with uniform thickness across sheet, and thickening at edges

Question 14

Describe blown film extrusion

Answer 14

A tube is extruded, then drawn upward while still molten and expanded by air inflating it through the die

Question 15

How are fibers produced?

Answer 15

Spinning

Question 16

What are the three types of spinning?

Answer 16

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

Question 17

Describe injection molding

Answer 17

Polymer is melted to highly plastic state and forced under pressure into mold cavity. Can create complex or even multiple parts.
Mold are expensive.

Question 18

What are the two parts of an injection molding machine?

Answer 18

The injection unit (like an extruder)
Clamping unit (opens and closes mold)

Question 19

How can you compensate for shrinkage in injection molding?

Answer 19

Dc = Dp + DpS + DpS^2
Where Dc is dimension of cavity
Dp is mold part dimension
S is shrinkage value

Question 20

How can we reduce shrinkage?

Answer 20

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)

Question 21

When are ribs used for plastic products?

Answer 21

To add stiffness without increasing wall thickness. Ribs should be thinner than walls to minimize sink marks.