Extrusion Machinery - Polymer Processing - Lecture Slides, Slides of Software Engineering

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MFGT 142

Extrusion Machinery

MFGT142 Docsity.com

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Chapter 1: Extrusion Machinery

• Overview

• What is an extruder?
• Different Types of Extruders
• Components of an Extruder

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Extrusion

• Principle
  • Continuously shaping a fluid polymer through an orifice of a suitable tool (die), and subsequently solidifying into a product.
• Equipment
  • Single screw extruder consists of
    • Screw, barrel, feed hopper, and die
  • Common extruders are rated by barrel bore diameter (0.75” to 6”)
  • Plastics extruders can be 24” diameter and 48 ft in length
  • Electric heaters for barrels and Air (or water bath) coolers for extrudate
  • Screw is matched to material produced
  • Simple screw has flights and decreasing gaps along channel
  • Screw Aspect (L/D) ratio = screw length to screw diameter (range 20-30)
  • Venting zone for gases that evolve during processing
  • Twin screw- used for shear sensitive materials (e.g., PVC)

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Extruder Components

• Hopper- Feed thermoplastic pellets

• Heating and shear zone- rotating single or twin screw

• conveys plastic to heat zone and applies shear (friction) to melt

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Extruder Types

• Single screw

• Most common
• A screw rotates in a cylinder and creates a pumping action.

• Twin Screw

• Twin screws have more positive pumping action than single screws and can be used for higher output.
• Co-Rotating Twin Screw: popular for compounding
• Counter- rotating Twin Screw Extruders

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Extrusion Equipment- Extruder

• Screw

• root is screw diameter
• flight depth
• Pitch is angle

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Extruder Types

• Ram Extruders

• Reciprocating Piston forces the material forward and through the die
• Line speeds are quite low, 25 to 75 cm /hour (10 to 30 in / hour)
• Advantages
  • Very good conveying characteristics
  • Can develop high pressures
• Disadvantages
  • Low melting capability
  • Used with intractable plastics that can not be extruded through normal extruders - PTFE, UHMWPE

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Extrusion Equipment- Screw

• Extruder screw

• Attached to the drive linkage through the thrust bearing and rotates inside the barrel. Made of chrome plated steel.
• Each turn of the helix is called a flight like thread of a screw
• Function
  • conveying resin through extruder
  • imparting mechanical energy to melt the polymer
  • mixing ingredients together
  • building pressure to push resin through die
• Type
  • Single screw (90%)
  • Twin-screw (10%)
• Length/diameter (L/D) ratio: Length of flighted portion of the screw to the inside diameter of the barrel. L/D is a measure of the capability of the screw to mix and melt materials. High L/D ratios indicate good mixing and melting capabilities, e.g. 16:1Docsity.com

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Extrusion Equipment- Screw

• Screw Zones

• Metering section- constant root diameter and very shallow flight depths. - The melted resin is further heated with shear resulting and higher pressure. The higher pressure allows the melt to be pushed out. - Gives final mixing and ensures uniform heating.
• Compression ratio is the ratio of the flight depth in the feed section to the flight depth in the metering section. Range from 1.1:1 to 5:1 (avg 2.25:1)

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Extrusion Equipment

• Feed Throat

• Connected to the barrel and contains the feed opening to extruder
• Feed throat is water cooled to prevent bridging.
• Opening can be offset or elongated with the length about 1. times the diameter of barrel and the width of 0.7 times diameter
• Fig 1.

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Extrusion Equipment

• Barrel

• Cylinder equipped with a bimetallic liner that is hard and wear resistant. Barrel has higher wear resistance than screw.
• Provides the rigid surface against which the thrust bearing seals
• Made from hardened steel that is lined with wear-resistant and corrosion -resistant metal.
• Sized by the inside diameter dimension
• Jacketed with electrical heating elements (3) and thermocouples
• Barrel is vented through which volatiles escape with two stage screw to prevent resin from flowing out vent.
• Fig 1.

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Extrusion Equipment

• Barrel Heating and Cooling

• Heating is done with electrical heater bands along length of extruder.
• Three temperature zones are common.
• Dies may have one or several heating bands.
• Barrel is cooled with the use of blowers or water jackets.
• Fig 1.

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Extrusion Equipment- Die

• Die Zone

• Gives shape to the melt.
• Mounted with an adapter ring.
• Made of tool steel and usually chrome plated (for production).
• Design of the die needs to ensure proper melt flow without stagnation regions, where material sits for long periods of time.
• Land : Short section at the exit orifice of the die where the walls are parallel. - This builds uniform pressure in the melt and stabilizes the flow. - Zone of maximum pressure in the extruder and influences back pressure. - If land is too long, higher back pressures are generated causing reduced output and internal wear on the thrust bearing. - If land is too short, the melt flow is erratic and the part will be less uniform.

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Extrusion Equipment- Auxiliary

• Cooling

• Water bath: most common method. A simple trough, tray, or box
• Air cooled if reduced throughput is acceptable.
• Sizing plates or rings can be used inside the water bath to ensure proper shape and dimensions on film or sheet.
• Vacuum is used with a sizing plate to assist in having the outside of a hollow material (tube) to give better control of the outside diameter.

• Puller

• After the part is sufficiently cooled to retain its shape, it is drawn away from the extruder, through the cooling, and then pushed into the removal equipment.
• Adjustable speed belt or caterpillar pullers are common