1. MFGT 142 Extrusion
Professor Joe Greene
CSU, CHICO

2. Chapter 11: Extrusion Overview
Purpose, advantages, disadvantages, and cost elements
Equipment: extruder, die, cooling, puller, removal, special equipment, plant layout and capacity
Normal operation and process control: start-up, part dimensional control, maintenance, safety
Extrusion problems and trouble shooting
Material and product considerations
Post-extrusion forming
Coextrusion

3. Introduction Extrusion: (Greek) push out
Pump supplies a continuous stream of material to a shaping tool or to some other subsequent shaping process.
Materials
plastics
metals (Al window frames
caulking with a caulk gun
spaghetti and other noodles
cookie dough
hamburger
toothpaste
hot melt glue gun
Extrusion useful for shaping parts with extruder dies
Extrusion is basis for injection molding and blow molding
Melting device to add fillers, colorants, fibers in compounding

4. Extrusion Principle Equipment
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)

5. 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

6. Extruder Equipment Exit zone- die Cooling zone Auxiliary equipment
die imparts shape on the material, e.g., rod, tube, sheet, channel
exit material is called extrudate
extrudate swells at end of die due to normal forces from the polymer flow, called die swell
Cooling zone
water bath or air cooled to lower the temperature below Tg
Auxiliary equipment
puller
rollers for proper thickness
Wind-up or cut off
Die Swell

7. Extrusion Equipment Tooling (Shaping tool associated with extrusion)
filaments: multiple orifices with tapered entrances
profile products: without hollow sections with plate dies
tubes or pipes: hollow extrudate with use of mandrel
films or sheets:several slit die configurations are used
T-shaped, coat hanger, fishtail,
Difficult to get uniform product thickness across entire width
remedy is to use restricter bars and flexible lips
Coextrusion of 2 or more plastic materials through a single die
Auxiliaries
Cooling (water bath), sizing (forced against external sleeve or against a mandrel with vacuum), post forming (jigs, fingers, shoes, rollers), cutting (fly cutters), winding devices

8. Extrusion Materials
Materials (high MW, high viscosity, and melt strength)
Polyethylene used for films and pipes, either as LDPE (flexible) or HDPE (rigid)
Polypropylene used as oriented film
PVC used in rigid form and plasticized form with twin-screw
PTFE or UHMWPE processed by screwless (ram) extrusion
Products
Fibers and monofilaments: PA, acrylics, polyesters ( ft/min)
Profiles (flat strips, simple channels, hollow sections): vinylic, styrenic, and olefinic resins.
Profiles with cellular (foam) core and a solid skin: exterior wall siding, window tracks, door tracks, handrail covers, weatherstripping
Tubular products: tiny tubing (artificial kidney or drinking straws) to pipes with 63” diameter

9. Extrusion Products Specialty tubes Flat products
Pipes made with coextrusion foam core
Very long tubes made with helically winding and then bonding a suitable extruded profile
Flexible reinforced tubing made by extruding an inner liner, braiding around it a fiber reinforcement, and then extruding over it a cover (jacket)
Materials are PVC, LDPE or HDPE
Flat products
Films (<0.010in or 0.25mm)
Sheets (above that)
Widths as large as 13 ft and thickness as high as 0.5 in.

10. Extrusion Covering
Principle- Fully surround a continuous substrate with a cover of polymeric material
Equipment
Substrate pay-off (input) system, substrate preheater, extruder, cooling device, take-up system
Tooling
Wire coating: extrusion covering die mounted with extrusion axis perpendicular to the extruder axis.
Materials
PVC and PE are most widely used
PA, fluoropolymers, and cellulosics are used for wire insulation or where transparency is desirable

11. Extrusion Covering Products
Wire and cable coating, sheathing, and jacketing
Single conductor (wire) or multiple conductor (cable) covering
Flat ribbon multi-conductor cables for appliance cords, TV antenna cables.
Decorative trim elements with polished brass, Al, or chrome plated substrates in a protective, transparent cover. (plastic bag ties)

12. Advantages & Disadvantages Extrusion
Continuous
High production volumes
Low cost per pound
Efficient melting
Many types of raw materials
Good mixing (compounding)
Disadvantages
Limited complexity of parts
Uniform cross-sectional shape only

13. Extrusion Equipment- Extruder
Extruder base- used to isolate extruder and to reduce vibrations
Drive motor- turns the screw and provides power for operation.
Extruder power capacity- key characteristic on extruder
Power increases as
polymer output increases
barrel diameter increases
screw length increases
requirements for high output at high polymer temperature
Thrust bearing is mounted on the screw near where the screw attaches to the linkage. It prevents the screw from moving backwards and absorbs the thrust of the screw as it tutns against the resistance of the resin.

14. Extrusion Equipment- Extruder
Barrel
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
Plastic pellets flow form hopper through a feed throat
Screw
root is screw diameter
flight depth
Pitch is angle

15. 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. 16:1

16. Extrusion Equipment- Screw
Screw Zones
Feed section- material enters screw, usually first turns of the flights
Feed section has small. Constant root diameter that results in large, constant depth (deep) flights that accommodates the bulky solid resin
Bridging occurs when material is heated too quickly and is caused by the resin sticking to the root and thus not moving forward.
Compression section- also melting or transition section. Identified with a gradual increase in the diameter of the root along the length of the section.
The flight depth gradually decreases throughout the section, compressing the resin and forcing the air (volatiles) out of the resin through a vent port in the barrel
Compression can be increased by decreasing the pitch of the flights.
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)

17. Extrusion Equipment- Head Zone
Follows the end of the screw.
Melt flows from end of extruder to screen pack and through breaker plate allowing for additional mixing and homogenizing
Screen pack is a collection of wire screens to filter out contaminants. It needs to be changed if it gets clogged, or blinded causing high back pressure.
Breaker plate is a disk of sturdy metal with many holes drilled in it., e.g., a coarse screen

18. 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.

19. 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

20. Extrusion Equipment- Auxiliary
Removal
Material is removed from the line and prepared for shipment
Rigid parts are cut in standard lengths, e.g., PVC pipe (20 ft)
The bell at the end of PVC pipe is thermoformed by heating the pipe in an oven and then pushing a metal plug in the end to form the bell shape.
Flexible product is rolled onto reels or into coils with a cut-off
Special Equipment
Twin screw extruders is valuable for certain low shear resins and for compounding additives, colorants, and fibers.
Twin screw operates side by side in tandem requiring different linkages and barrels than for a single screw. (Can’t covert 1 to 2)
The screws intermesh so that the relative motion of the flight of one screw inside the channel of the other acts as a paddle and pushes material from screw to screw and from flight to flight.

21. Extrusion Equipment- Twin Screw
Twin screws have more positive pumping action than single screws and can be used for higher output.
Types
Co-rotating: popular for compounding
Counter-rotating

22. Extrusion Equipment- Twin Screw
Co-rotating: popular for compounding
Higher heating efficiency, equal shear on the material, better material mixing and flow with less stagnation
Counter-rotating
Material bank: junction of the two screws build up material
High shear at material bank, low everywhere else
venting is necessary through port in barrel by suddenly increasing flight depth or reducing screw diameter
Additives can be added through vent port
Static mixers added at end of screw to improve mixing
The screw is considerable shorter to allow for 8-12 inches mixer

23. Plant concepts (Layout and Controllers)
Extrusion lines are long, typically 45 feet.
Line should be straight
Material fed from vacuum fed gaylords or from mezzanine above
Controllers
Feedback or automatic controllers and monitors are used extensively for monitoring portion of line including heat zones