L15 Polymer Processes

Question 1

True or false, isthe processing of polymers similar to metals?

Answer 1

True

Question 2

Unlike metals, polymers require…? Their properties are highly influenced by what two things.

Answer 2

• Less energy to process

* Highly influenced by manufacturing processes and parameters

Question 3

Polymers typically start out in what form when being processed? And for reinforced plastic parts?

Answer 3

• Pellet form

* Liquids for reinforced plastic parts

Question 4

Briefly describe the4 stages of Extrusion in the Extruder(Refer to diagram on page 3 of L15)

Answer 4

• The raw thermoplastic materials in the form of pellets are fed into a hopper and then into a barrel
• Barrel houses a screw that conveys the pellets down the barrel
• Shearing action of the screw as well as external heaters cause the pellets to melt
• In the last section of the screw, pressure builds and the polymer is extruded through a die

Question 5

What happens at the end of the barrel in the Extruder?

Answer 5

Once the meltedplastic reaches the end of the barrel, its forced through a die and cooled

Question 6

How do you minimize distortion in theextrusion die?

Answer 6

Uniform Cooling

Question 7

Various die shapes can produce different…

Answer 7

• Profiles

* Even thin sheets and films

Question 8

What does the phenomena of Die Swell cause?Such ImportanceIf Alan Doesnt Ask Something About This, I will let Mike Punch MeIf He does, we all get to punch Mike

Answer 8

Causes the plastic to be shaped differently than the extrusion die (experience is key in designing extrusion dies)

Question 9

What is Coextrusion?

Answer 9

Involves simultaneous extrusion of multiple polymers/materials through a single die

Question 10

In Coextrusion, each material has its own characteristics and functions.Name 3 Characteristics

Answer 10

• Barrier to both oil and water
• Electrical insulation and conduction
• Sealing and labelling

Question 11

How are plastic coatedelectrical wires produced?Know this for test

Answer 11

Coextrusion

Question 12

What is the most common polymer process?

Answer 12

Injection Molding

Question 13

Injection Molding is similar to what metal process?

Answer 13

Hot Chamber Die Casting

Question 14

How is the “melt” created in Injection Molding? Then what happens to the melt to reach the mold?

Answer 14

• Polymer pellets are plasticized creating the melt

* The melt is forced through a nozzle into the mold

Question 15

Mmodern machines use this to carry and inject the plastic in Injection Molding?

Answer 15

Reciprocating-Screw Type Injector

Question 16

What is the difference betweenThermoplastics and Thermosets when used in Injection Moldings?What does each promote?

Answer 16

For thermoplastics the mold is usually cool

• To speed up the processing For thermosets the mold is held hot
• To promote polymerization and crosslinking

Question 17

What is a major concern in Injection Molding? What is done to compensate?

Answer 17

• Shrinkage (1-7%)

* Molds are made oversized to compensate

Question 18

Identify the Injection Mold Geometry

Answer 18

See page 5 of L15

Question 19

Name the 3 Types of molds that can be used in Injection Molding

Answer 19

• Cold Runner Two-Plate
• Cold Runner Three-Plate
• Hot Runner Mold

Question 20

Briefly describe the Cold Runner Two-Plate

Answer 20

• Simplest mold design

* Sprue and runners must be removed by hand

Question 21

Briefly describe the Cold Runner Three-Plate

Answer 21

• Runner system is removed from part via intermediate stripper plate
• Runners and parts are seperate and solid

Question 22

Briefly describe the Hot Runner Mold

Answer 22

Runner system is kept in melted state after parts have solidified

Question 23

What are 2 types ofBlow Molding?

Answer 23

• Extrusion Blow Molding

* Injection Blow Molding

Question 24

Briefly describe theprocess of Extrusion Blow Molding. What is the pressure range it operates in?

Answer 24

• A thin tube (Parison) is extruded and clamped into a mold
• High Pressure (350-700KPa) then forces parison outward to fill cavity
• Upon cooling the part is ejected

Question 25

Explain the difference between Extrusion Blow Molding and Injection Blow Molding. Name Applications of Injection Blow Molding

Answer 25

* Instead of extruded, the parison is injection molded then transferred to a blow mold and expanded using high pressure gas * Ex. Beverage Bottles, Hollow Containers

Question 26

Rotation Molding is similar to Extrusion and Blow Molding but?

Answer 26

At a lower cost

Question 27

Describe the process of Rotation Molding

Answer 27

* A thin walled metal mold is made and powdered plastic is placed inside the mold * The mold is heated and rotated about 2 perpendicular axis * The powdered plastic coats the mold and is fused from the heat

Question 28

Pros and Cons of Rotational Molding plus Applications

Answer 28

* Large thin walled parts can be made * Mold and equipment costs are much lower than blow and extrusion molding * Cycles times are much longer than blow and extrusion molding Applications * Trash Cans, Boat Hulls, Carrying Cases, Tanks

Question 29

What is Thermoforming?

Answer 29

A family of processes where thermoplastic sheet is formed over a mold via the application of heat and pressure (or vacuum)

Question 30

Briefly describe the process of Thermoforming

Answer 30

* A thin plastic sheet is heated in an oven until soft (the sag point) * Then placed over a mold and pressure is applied until the plastic solidifies

Question 31

How are hollow parts made in Thermoforming?

Answer 31

By forcing 2 thin plastic sheets together against molds and applying a combination of vacuum and pressure

Question 32

What is the major limitation of Thermoforming? Name Applications

Answer 32

* **************No Parts with holes**************** | * Signs, Packaging, DIY

Question 33

What is Compression Molding?

Answer 33

A preshaped charge (or powder or viscous liquid) is placed into a heated mold cavity and held under pressure until cured

Question 34

During Compression Molding, what is formed that must be removed?

Answer 34

Flash, must be removed in subsequent operations

Question 35

What polymer is usually chosen for Compression Molding and why?

Answer 35

* Thermosets | * Longer cycle times and heated die (Which allows crosslinking)

Question 36

Advantages of Compression Molding and Applications

Answer 36

* Molds generally less complex Undercuts are possible because molds can be made to open sideways Applications * Dishes, Handles, Pipe Fittings

Question 37

Which costs less Injection Molding or Compression Molding?

Answer 37

Compression

Question 38

In Polymer Casting, some thermoplastics and thermosets may be cast into...?

Answer 38

Rigid or Flexible Molds

Question 39

For Thermoplastics in Polymer Casting, the mixture poured into the mold is made of? When it is poured what happens?

Answer 39

* Mixture of Monomer and Catalyst is poured into mold | * Then allowed to polymerize at ambient pressure

Question 40

In Polymer Casting, what are examples of Flexible Molds and why is it used?

Answer 40

* Polyurethanes * Silicones * Allow complex, intricate shapes to be formed

Question 41

What makes Polymer Casting more versatile?

Answer 41

Inserts and other components may be cast into the part as well, allowing versatility

Question 42

Applications of Polymer Casting

Answer 42

* Form molding of cups * Food Containers * Packing Materials