Topic 5 - Polymer Processing - Extrusion

processing plastics

pellets --> melt

extruder

could be a part of a blow molder, a part of injection molding machine or any other process.

uses heat, pressure, and shear to transform solid pellets to a melt

hopper

loads the pellets

barrel

main body of extruder, externally heated

screw

housed in barrel, helical channels which melt and convey the plastic.

extruder screw

conveys plastic forward, compresses/melts, meters plastic

profile extrusion

uses a die to create desired shape

melt filters

screens for filtering impurities out of the melt

Drive mechanism and screw speeds

The speed of rotation of screw is generally controlled by a variable speed motor. A speed reducer is also needed between motor and screw.

cast film

downward extrusion onto a roll

orientation

tends to increase crystallinity, barrier properties and strength in the stretched direction (s).

biaxial orientation

involves stretching the film in both the machine and crossmachine directions.
increased crystallinity

tentering process (tenter frame)

biaxial orientation, stretches out film

coat hanger die

most common cast film die

blown film extrusion

extruded upward, forms a bubble, then is cooled and put onto a roll (polyolefins)
stretched biaxially

blow-up ratio

The ratio between the diameter of the final tube of film to the Die diameter

frost line

The film turns from molten to semi-solid

sizing cage

is used to stabilize the bubble for uniform cooling and stretching

blown film applications

bags
construction film
agricultural film
coextruded film
Food packaging

blown film transparency

blown film is lower quality bc of slower cooling and increased crystallinity

blown film barrier properties

better barrier properties than cast.

calendaring

critical finishing technique that enhances the physical and aesthetic qualities of materials, making them suitable for specific applications.

cast film characteristics

1.) Relatively unoriented
2.) Can be oriented uniaxially or biaxially by adding a step known as tentering prior to winding
3.) Cast film generally has uniform film thickness and good clarity
4.) Quick cooling caused by chill rolls or quench tank prevents rearrangement of the chains in an ordered fashion

blown film characteristics

1.) Width of the end roll and the thickness if the plastic are controlled by the volume of air, rate of extension and the temperature of the plastic
2.) Blown film are hazier than cast film as they have longer time to crystallize and are inherently biaxially oriented
3.) Less expensive for long runs
4.) Polymers with low melt strength cannot be run e.g. PET
5.) Bubble usually vertically upward ; can be downward

coextrusion

Multilayer structures are used in packaging in order to take advantage of the properties of several polymers at once

coextrusion advantages

1.) Uses thinner layers than lamination
2.) Multiple properties obtained with a single operation
3.) No solvents required (compared to lamination)
4.)Pigmented layers can be buried (heat seal layer)

coextrusion disadvantages

Machinery is expensive, Initial set up of coextrusion line is expensive

single screw extruder

Has one screw in the barrel. They generate higher pressure and output than twin screw extruder. They are used for converting plastic into final product such as film, bottle etc.

twin screw extruder

Such extruder have two screws in the barrel. They generate high shear force in extruder which is ideal for better mixing, blending etc.