Chapter 9: Shaping Processes for Plastics

Question 1

What are thermoplastics

Answer 1

Chemical structure remains unchanged - 70% of total plastics

Question 2

What are thermosets

Answer 2

Undergo a curing process resulting in a permanent change in molecular structure called crosslinking. Once cured, they cannot be remelted

Question 3

What is a polymer melt and its important properties

Answer 3

Plastic heated to the consistency of a liquid - viscosity and viscoelasticity

Question 4

What is the viscosity of polymer melts

Answer 4

Fluid property that relates shear stress to shear rate

Question 5

Most polymer shaping processes inolve what

Answer 5

Flow through small channels or die openings

Question 6

Viscosity and temperature

Answer 6

Viscosity decreases with temperature increase thus the fluid becomes thinner at higher temps

Question 7

What is viscoelasticity and an example

Answer 7

Combination of viscosity and elasticity

Ex: die swell in extrusion - hot plastic expands after exiting a die opening (memory)

Question 8

What is Extrusion

Answer 8

Material is forced to flow through a die orifice to provide a long continuous product whose cross-section is determined by the shape of the orifice

Question 9

What are the two main extruder components

Answer 9

1. Barrel
2. Screw

Question 10

What determines the cross sectional shape of an extrusion

Answer 10

The die orifice

Question 11

Components of extrusion to create hollow profiles

Answer 11

requires mandrel to form shape
- often includes an air channel blowing air to maintain hollow form for extrudate
- Mandrel help by spider legs

Question 12

How is wire and cable coating made

Answer 12

Polymer melt is applies to bare wire as it is pulled at high speed through die
- a slight vacuum is drawn for adhesion

Question 13

What is the thickness for polymer sheet and film

Answer 13

Film - below 0.5 mm
Sheet - from 0.5 mm to 12.5 mm

Question 14

What are the processes for sheet and film production

Answer 14

• Slit-Die Extrusion of sheet and film
• Blown-Film Extrusion Process
• Calendering

Question 15

What is slit Die Extrusion of sheet and film

Answer 15

Production of sheet and film by conventional extrusion using a narrow slit as the die opening

Question 16

Problem in slit die extrusion of sheet and film

Answer 16

Thickness uniformity throughout width of stock due to drastic shape change

Question 17

What is Calendering

Answer 17

Feedstock is passed through a series of rolls to reduce thickness to the desired gage

Question 18

What is Calendering noted for and typical material

Answer 18

• good surface finish and high gage accuracy
• typically rubber or rubbery thermoplastics

Question 19

What is injection molding

Answer 19

polymer is heated to a highly plastic state and forced to flow under high pressure into a mold cavity where it solidifies and the molding is then removed from the cavity

Question 20

Key Characteristics of injection molding

Answer 20

• Produces discrete components to net shape
• mold may contain multiple cavities so multiple moldings can be produced each cycle

Question 21

What are the shape limitations of injection molded parts

Answer 21

-Capacity to fabricate a mold whose cavity is the same geometry as part
- shape must allow for part removal from mold

Question 22

Injection molding is economical when

Answer 22

only for large production quantities due to high cost of mold

Question 22

Injection molding is most widely used molding process for

Answer 22

thermoplastics

Question 23

Two principle components of injection molding

Answer 23

1. Injection Unit - melts and delivers polymer melt and operates like an extruder
2. Clamping Unit - opens and closes mold each injection cycle