ChETE 3 - Shaping Plastics (Profile, Cast Film, and Blown Film)

Profile

The polymer is forced through a die with a shape of constant cross section and a continuous length called a

Profile extrusion

Creation of products of two dimensional shapes with a continuous length

Puck

For filling, the tubes are inserted inside a _____ or tube holder with the bottom up.

Polyethylene

Usual material for tubes

PP or EVOH

If additional properties are needed for tubes PE is coextruded with

Cellulose-based plastics

For novelty markets, these plastics are used

Friction fit plug

Most common closure in tubes

True

TRUE or FALSE

Sheet and film extrusion are types of profile extrusions

Cast film

Sheets and film made through profile extrusion are called

Below 10 mils

Thickness of films

10 mils or higher

Thickness of sheets

• Die

• Cooling System

• Traction/Calendering

• Winding

Processes in Cast Film Extrusion

Inlet channel

This delivers the plastic from the extruder to the die

Manifold

Distributes the melt evenly within the die

Die approach or die land

Carries the melt from the manifold to the die opening

Die lip

Perform the final shaping of the melt as it exits the die

Die opening

Slit-shaped and produces a thin wide stream of plastic film

T-slot or coat hangers

Most flat dies are of this design

• Cold roast or chill roll cast

• Quench tank or water bath

Cooling mechanisms in Cast film extrusion

Chill roll

Downward extrusion onto chilled chrome rollers in order to cool the film

Air knife

This is used to pin the plastic to the first chill roll

First - 40C

Succeeding - lower

The first chill rolls operate at this temperature while succeeding rolls operate at what temperature?

Highly polished

Chill rolls have this property to impart good surface characteristics

Water bath

Once the extrudate leaves the die, it is immersed in a water tank

Gap between rolls

This help control the thickness in a cast film extruder

Biaxial orientation

Done in a process called “tentering“

balanced

If orientation is equal in both directions, the film is referred to as _________

± 3%

Gauge variations of ______is common, although this can already lead to problems

1. Oscillating the film as it is wound

2. Use sensors/scanners to monitor the gauge. More modern systems automatically feed the information to the die so the die can adjust

In gauge control, how to minimize gauge inconsistencies?

Blown film extrusion

A continuous process in which the polymer is melted, the melt is forced through an annular die, and the resulting tube is inflated with air into a bubble and cooled

Polyolefins

Most common polymer used in blown film extrusion

mono-axially

A blown film is always oriented in what way?

Longitudinal stretching

How can you turn a blown film biaxially oriented?

1. Blow-up ratio

2. Speed or drawdown

What are the properties of a final blown film?

Blow up ratio

The ratio of the final diameter of the tube and that of the die

diameter of tube/diameter of die

BUR formula

• Extrusion

• Bubble

• Collapsing

• Winding

Processes of extrusion

melt phase

Below the frost line

solid phase

Above the frost line

Annular

What is the shape of a blown film die?

Spiral channel

Most common shape for blown film die

Die gap

Diameter at the die land opening

speed at which extrudate is pulled from the die / speed at which the melt emerges from the die

Drawdown ratio

16:1

DDR of amorphous polymers

160:1

DDR of crystalline polymers

± 10%

What is the gauge variation for blown film dies?

Made from steel but coated with copper/nickel

Materials of blown film die

Two air rings

Dual lip design makes use of

1st: provides bubble shape and stability

2nd: provides actual cooling

Purpose of the 2 air rings

Internal bubble cooling (IBC)

This further enhances cooling by providing cooling inside the inner surface of the bubble

Collapsing frame

Its function is to convert the bubble tube into a flat film

Maple slats

Oldest material used as collapsing frames but are still in use

Adding rollers and use higher towers

How to reduce wrinkling after collapsing frame?

Nips

Pair of rollers (one metal, one rubber) which moves the film and stretches it, can be oscillating

• Center drive

• Surface drive

• Surface/center drive

3 kinds of winders

Too little = telescoping

Too much = Crushed core

What happens if there is too much or too little tension in winding?

Blown film requires lower

CAST vs BLOWN
Required melt flowrate

Blown film operates lower

CAST vs BLOWN
Process temperature

Blown film is more crystalline due to slower cooling

CAST vs BLOWN
Crystalline

Blown film is hazy and not glossy

CAST vs BLOWN
Opacity

Blown film has lower

CAST vs BLOWN
Output

Blown film has adjustable size while cast film need more rework

CAST vs BLOWN
Web Control

Blown film is less flat

CAST vs BLOWN
Flatness

Double bubble process

The tube is extruded downward, cooled using a water bath, then reheated, reinflated and annealed

Stretch wrap

The molecules in the film attempt to return to their original conformation after stretching

Stretch wrap

Holding force increases with tension

Tackifiers

How to impart cling on a stretch wrap?

Co-extrusion

Used to combine different polymers

• Flame treatment

• Corona discharge

• Ozone Treatment

Surface treatments