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Extrusion Process
Extrusion Equipment
Extrusion • Basics • Blown • Sheet
Orientation • Basics • Process
– Tenter Frame
Basic Extrusion
Resin
Screw
Barrel To Die
Basic Extrusion
– Resin is fed into rotating screw
– Heat (external & shear) and pressure melt the resin
– Molten polymer is forced through a screen pack to remove foreign matter, create back pressure, and aid in mixing
– Melt from extruder flows into die to provide the desired and a uniform thickness of film
– Flat die or annular (circular) die
Basic Extrusion
– Critical region in extrusion is in the transition of solid to melt phase
– Gels (unmelted or crosslinked polymer) can develop randomly due to:
• Incorrect temperatures, poor screw design
• Poor incoming resin, polymer buildup
– In regions where polymer flow is slow, oxidized polymer can exist (carbon)
– Carbon can randomly discharge into melt stream
Blown Film
Primary Nip Roll
Resin Blenders
Hopper
Extruder
Guide Rolls
Blown Tube
Windup Rolls
Air Ring
Treater
Gauge Bands/Baggy Film
Blown Film Extrusion - Blenders
Blown Film Extrusion
Blown Film Extrusion
Blown Film Extrusion – Gauge Control
Blown Film Extrusion – Gauge Control (IGC)
Non-contact (air cushion) Nuclear Gauge - Measures thickness around the bubble
Vertically Sliding Teeth - Adjusts airflow through ¼” gap
Sliding Teeth Actuator - Raises or lowers teeth
Live Ring – Oscillates the thickness sensor around the bubble
Bubble
Blown Film Extrusion – Gauge Control (IGC)
Profile Control System Off
Profile Control System On
Blown Films - Process Basics
Blown Film Extrusion – Top Nip
Blown Film Coextrusion
• Up to 9 independent extruders feed the die
• Each discrete melt stream can be made up of a blend of polymers and additives
• Allows for optimization of performance properties vs. cost
Die
Molten Resin
Top View
Cross Section
Blown Film Extrusion
• Monolayer die has a single melt stream
• Coextrusion die combines multiple melt streams to create a single film with multiple layers
• Melt exits die at 350-550ºF (177-288ºC)
• Bubble immediately cooled with chilled air
• Bubble oscillates to distribute caliper variation
• Bubble is collapsed and surface treated
• Thickness is controlled by extruder speed, line speed, die gap
Corona Treatment of Film
• Film is corona treated to enhance surface energy • Generates an electrical discharge which oxidizes the
polymer surface • Treatment improves adhesion properties required for
subsequent laminating or printing processes • Base polymers have varying initial levels of surface
tension:
Material Surface Tension ( dynes/ cm 2) Polypropylene (PP) 29Polyethylene (PE) 31Polyester (PET) 43Polytetrafluorethylene ( “Teflon”) 19
Corona Treament
Primary Nip Roll
Resin Blenders
Hopper
Extruder
Guide Rolls
Blown Tube
Windup Rolls
Air Ring
Corona Treater
Corona Treatment
Treatment Level Determination
• Dyne pens or dyne solutions are utilized to determine treatment level
Sheet Extrusion
Extruder
Resin Additives
Blender / Mixer
Die Windup
Cooling Roll
Cooling Roll
Cooling Roll
Produces materials from 10-60 mil thick Typically used for thermoforming applications (trays) Can be mono-layer or co-extrusion
Orientation Basics
Orientation enhances several key properties: – Tensile strength – Impact strength – Heat resistance – Oxygen and water barrier – Abrasion resistance – Gloss and clarity
M.D. Orientation Annealing
T.D. Orientation Extrusion
OVEN
Tenter Frame Process
Orientation Basics
• Oriented films are highly stretched and heat stabilized
• Polymer is initially extruded into a thick sheet
• Thick sheet is re-heated and stretched in both machine and transverse directions:
– Orientation stretch ratios from 12X - 40X
• After orienting, the film travels through an annealing oven to relieve internal stresses and heat stabilize
Orientation Property Comparison
Polypropylene
Property Non-Oriented PP Tenter Biax PP MD Tensile-psi 3,000-6,000 17,000TD Tensile-psi 3,000-6,000 30,000MD Elongation-% 400 150TD Elongation-% 400 50MDTear- gram/mil 100 5TD Tear- gram/mil 100 3MVTR- at 1 mil 1.0 0.35(gr./ 100 sq.in./24 hrs)
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