Understanding Extrusion Chapter 5 Professor Joe Greene CSU, CHICO MFGT 144

Chap 5: How an Extruder Works Solids Conveying Gravity Induced Conveying Drag Induced Conveying Starve Feeding Grooved Feed Extruders Melting Contiguous Solids Melting Dispersed Solids Melting Melt Conveying Melt Temperature Mixing Degassing Die Forming

Solids Conveying Gravity Induced Conveying: Material flows down the feed hopper into the feed throat and from there into the screw channel Important Bulk properties Bulk density- density of material including air voids Compressibility- change in bulk density when pressure is applied Internal coefficient of friction- between the plastic particles External coefficient of friction- between plastic and hopper Particle size and distribution- PCR material are difficult to handle due to large particle size distribution Design of feed hopper effects flow in hopper to prevent stagnation and bridging Circular hopper is better than square hopper (Fig 5.1) Crammer feeder can be used for PCR materials Diamondback feed hopper has a circular cross section to a oval cross section to a circular cross section. (Fig 5.4)

Solids Conveying Drag Induced Conveying Fig 5.5 Plastic moves forward from rotation of screw due to friction with the barrel wall and not the friction with the screw. Analogy is a nut on a screw. If the nut is free to rotate it will not move up the screw. If the nut is held the nut moves forward. Starve Feeding (Fig 6.5) Method of feeding the extruder where the plastic is metered into the extruder at a rate below the flood feed rate. The screw channel is partially empty in the first few diameters of the extruder. Results in very little pressure buildup in the plastic and as a result very little frictional heating and mixing. Effectively reduces the length of the extruder, e.g. a 25:1 L/D extruder may have an effective length of 21 L/D with the first 4 diameters partial Used on high speed twin screw extruders. Reduces motor load, melt temperature, and useful when adding several ingredients simultaneously through one feed port from several feeders.

Solids Conveying Grooved Feed Extruders (Fig 5.6) Driving force for the conveying process is the frictional force at the barrel surface. Grooves effectively increase the barrel temperature. Grooves typically run in the axial direction with the length of several screw diameters. Advantages of grooved feed extruders (Fig 5.7) Output is less dependent on pressure resulting in increased stability Output tends to be higher than that of smooth bore extruders Allows extrusion of very high molecular weight plastics, HMWPE Disadvantages Grooved barrel section has to be cooled well enough to avoid premature melting of plastic in the grooves reducing energy efficiency and adds to complexity of extruder Stresses can be high in groove causing them to wear Pressure required can be high, thus need strong barrel

How an Extruder Works Conventional extruders without grooved barrel sections can be modified to include grooves Modify the feed throat with a grooved liner (fig 5.6) Most extruder stability problems occur in solids conveying section Grooved feed throat can improve extruder performance Improve barrel friction or reduce screw friction Affected by screw design, screw temp, and screw material Screw design features that reduce screw friction (Fig. 5.8) Single flighted geometry (avoid multiple flights) Large flight flank radius Large Helix angle

How an Extruder Works Reduce screw friction Internal screw heating Coring the screw and circulating heat transfer fluid Cartridge heater inside the screw Apply a coating to the screw or a surface treatment. PTFE impregnated nickel plating PTFE/chrome plating Titanium-nitride Boron-nitride Tungsten-disulfide (WS2) Catalytic surface conversion (J-Tex) Advantage of a low friction coating Improves conveying along screw Reduces tendency of plastic to build up on screw surface, is easier to clean Coatings can be used for extrusion dies which reduce pressure drop Reduces tendency for material to build up at exit or dye drool

Melting Contiguous Solids Melting (Fig 5.9) (CSM) Solid particles are compacted and form a solid plug that spirals along the length of the screw channel. Thin film of plastic is located between solid bed and barrel Most of the melting occurs at the interface between two Newly melted material collects in the melt film then is dragged away Most often observed in single screw extruders Dispersed Solids Melting (Fig 5.10) (DSM) Solid particles are dispersed in a melt matrix, decrease in size till melted Observed in high-speed twin extruders and reciprocating single screw compounding extruders. Melting is more efficient than Contiguous solids melting (CSM) Length to achieve melting in the axial direction is 1 to 2 screw Diameter Versus single screw extruders length is 10 to 15 diameters Versus twin screw extruders length is 5 to 6 diameters Important when have a 25L/D extruder that won’t have length for melt conveying, mixing or degassing. Twin screw is more versatile than single screw

CSM Melting CSM Theory developed by Tadmor Determine how plastic properties, processing conditions and screw geometry affect melting Two sources of heat Barrel heat conducts from heaters through barrel and to melt Viscous heating caused by viscous movement of melt Drag induced melt removal Melted material is dragged away by rotation of screw Thin melt film is essential to proper melting Single screw extruders melt more efficiently than ram extruders Similar to a stick of butter melting in frying pan; best if moved around Thin melt film is essential parameter to high melting efficiency Melt thickness determined from flight clearance. Larger flight clearance results in thicker melt film. Important to keep flight clearance small Increase in barrel temperature causes increased heating of plastic causing viscosity to drop causing viscous dissipation to drop and less efficient melting. Thus, increase in barrel temp can reduce melting efficiency

CSM Melting Helix angle can have considerable effect on melting Helix angle increases, melting efficiency increases Fig 5.11 Highest melting efficiency (shorter length) is with 90º Such angle not good for conveying since 90º means that screw flight is parallel to the axis of screw and conveying capability is zero. Good range for helix angle is between 20º to 30º Multiple flights can also improve melting (Fig 5.12) Melt film is thinner than in a single flighted screw (Fig 5.13) Drawback is that it reduces solids conveying and melt conveying. Use multiple flights if extrusion is limited by melt capacity

Barrier Screw Barrier screw has two flights: main and barrier Main flight: is to separate the solid material from melted Barrier flight (Fig 5.14) separates solid bed from the melt pool. Barrier screw melting capability is same as a single flighted screw without a barrier flight.

Barrier Screw Advantages Disadvantages Achieves more stable extrusion than simple conveying No chance of unmelted material being beyond barrier section Certain amount of dispersive mixing occurs as plastic flows over barrier into melt channel. Disadvantages No better performers than screws with mixing sections More expensive than non-barrier screws More susceptible to plugging due to solid material restricted to the solids channel. Melting can not keep up with reduction in the size of the channel in compression section of the screw, resulting in the solid material getting stuck in the screw channel. Creates a momentary obstruction to flow and leads to surging or variation in extruder output

Melt Conveying Melt Conveying starts when melting is completed Melt conveying zone is region where all plastic is melted Mechanism for melt conveying is viscous drag Viscous force at the barrel is responsible for conveying Viscous force at the screw is responsible for retarding force Melt conveying is improved by reducing barrel temperature and increasing screw temperature Optimum screw geometry for conveying Optimum helix angle is dependent on degree of non_Newtonian behavior of plastic melt, n (power-law index, which is slope of log viscosity-log shear rate) Optimum helix angle for melt conveying decreases as power law index decreases; in other words, when the plastic is more shear thinning Optimum depth depends upon: Viscosity, pressure gradient, and power law index When plastic becomes more shear thinning, the channel depth should be reduced to obtain good melt conveying

Melt Temperature Temperatures vary considerably in melt conveying Due to low thermal conductivity Local temperatures are difficult to measure due to screw Numerical techniques can predict temperatures with finite element analysis with flow and pressures (Fig 5.15) Hottest near center of channel and coolest at screw Temperature distribution due to curling flow pattern (Fig 5.16) Fluid close to barrel surface flows in direction of channel Recircualting flow (Fig 5.17) causes inner layer is trapped Outer layer insulates inner hot layer Important to keep non-uniform heating layers away from end of screw with mixing sections in design of screw to achieve thermally homogeneous.

Distributed Mixing Takes place in melting and melt conveying zones Due to plug flow behavior, little mixing occurs in solids conveying. Waits until all plastic is melted Distributed Mixing Extent determined from total shear deformation of plastic melt Total shear deformation = shear rate · length of time exposed to shear rate . Where, shear rate is found from velocity/distance and length of time is Volume/flow rate Example, plastic melt is exposed to shear rate of 100 sec–1 for 15 seconds the resulting strain is 1,500. (Dimensionless) Mixing is determined by velocity in 2 directions Fig 5.18 Direction of channel (z-direction velocity is vz) Direction across channel (x-direction velocity is vx) The third direction, parallel to flight flank is usually small, vy.

Distributed Mixing Down channel velocities (vz) depend upon Pressure gradient Positive: pressure is increasing along melt conveying zone Negative: pressure is decreasing along melt conveying zone Fig 5.19 Cross Channel flow At top of channel, material flows to the left by drag flow At bottom of channel, material flows to the right by pressure flow Fig 5.20 Shear rate Determined from the slope of the velocity profile (velocity versus position) Slope of velocity profile is also called velocity gradient Fig 5.21 Pressure affects on shear rate Press gradient is positive then the shear rates increase toward the barrel surface Press gradient is zero then the shear rates is constant Press gradient is negative then the shear rates decrease toward the barrel surface Cross channel shear rates can be determined in the same way. Fig 5.22

Shear Rate Channel flow is helical (Fig 5.23) If unroll screw channel onto flat plane material follows helical path At top, the fluid element travels in the direction of the barrel At bottom, the fluid elements travel across the channel. Resident time is the length of time the material spends in the channel Dependent on velocities and geometry of channel (Volume of channel/ Volumetric flow rate) Resident time as function of distance: Fig 5.24 Fluid elements at center have shortest resident time Residence time increases toward the screw and barrel surfaces Residence time is very long at barrel and screw surfaces Outer Region A and inner region B (Fig 5.25)

Cross Channel Shear Strain Total cross channel shear strain can be determined adding the shear strain of the upper portion to that of the lower portion of the channel Fig 5.26: Total shear strain vs. normal distance (thickness) at several throttle ratios (pressure flow to drag flow: rd=0 pressure flow or rd = 0.333 then drag flow is 3 times pressure_ common) Elements close to barrel wall experience high strain and mixing Mixing in center increases with increasing rd by increasing resistance at end to flow by adding screen packs

Mixing Best way to improve mixing in single screw extruders is to incorporate mixing sections Desirable characteristics for mixing section Minimum pressure drop with forward pumping capability Streamlined flow and no deadspots Barrel surface wiped completely with no circumferential grooves. Operator friendly and easy to install, run, clean, etc. Easy to manufacture and reasonably priced.

Distributive Mixing Sections Specific characteristics for distributive mixing Plastic melt subjected to significant shear strain Flow should be split frequently with reorientation of melt Types Cavity mixers Pin mixers Slotted flight mixers Variable channel depth mixers Variable channel width mixers

Cavity Mixers Cavity transfer mixer (CTM) Advantages Disadvantages Consists of screw section and barrel section. Both containing hemi-spherical cavities (Fig 5.27) Advantages Good mixing capability Disadvantages No forward pumping capability and is pressure consuming Reduces extruder output and increases temperature buildup Streamlining is not very good, high cost, high installation $$ Barrel not completely wiped during processing Twente Mixing Ring is easier to install, clean, and operate. Barrel is wiped

Pin Mixers Pin mixers are common and come in many sizes and shapes Circular, square, rectangular, diamond-shaped. Fig 5.29 Pin barrel extruder common in rubber extrusion Advantages Good mixing capability Disadvantages Pins cause restriction and reduce extruder output Pins create regions of stagnation at the corner of pin and root of screw

Slotted Flight Mixers Common ones are Axon, Dulmage and Saxton Fig 5.31, 5.32, and 5.33 Advantages Good mixing capability and high output Disadvantages Barrel not completely wiped

Variable Depth Mixers Channel depth varies periodically in each channel One channel decreases in depth, the other increases Fig 5.34 Advantages Improved mixing Disadvantages No strong mechanism for flow splitting and reorientation Mixing capability is moderate

Summary of Distributive Mixers Table 5.1: Ranking of various Mixers Most important characteristic is Splitting and and reorientation For distributive mixing the following are desirable Mixing section should have high stress region, preferable elongational stresses High stress region designed for short times All fluid elements should pass through high stress many times All fluid elements should pass through high stress the same number of times

Blister Ring Circumferential shoulder on the screw with a small clearance between ring and the barrel (Fig 5.36) All material must flow through a small clearance between ring and barrel where it is exposed to high stress High pressure drop occur across the blister ring. Stresses and mixing are not uniform

Fluted Mixing Section Mixers have inlet and outlet flutes separated by barrier flights Material passes through a narrow gap of barrier flights where mixing takes place. Egan mixing section: flutes have helical orientation Fig 5.37 Poor Helix angle design of 30º. (Optimum is 50º) Fig 5.39 Leroy Union Carbide: has straight flights Fig 5.38 No forward pumping capability and thus high pressure drop Inefficient streamlining at entry and exits Most common for single screw extruders Poor Helix angle design of 90º

Planetary Gear Mixers Have six or more planetary screws that revolve around circumference of main screw. (Common in Europe) Barrel section must have helical grooves corresponding to the helical flights of planetary screws. (Fig 5.41) Benefits (Good for PVC, ABS, PU, acrylic, PE) Good homogeneity of the melt at low temperature Uniform shear exposure High output per screw revolution Low production cost per unit throughput Self-cleaning action for easy material change Good dispersive and distributive mixing of various additives

CRD Mixer Uses slanted pushing flight flank to create wedge shaped lobal region (Fig 5.42) Developed by Chris Rauwendaal to reduce problems Relying too mush on shear stresses to disperse materials rather than elongational stresses Material passes over high stress region only once.

Summary of Dispersive Mixers Comparison of Dispersive Mixers Table 5.2

Degassing Degassing is done on a vented extruder Extruder with vent port in barrel (Fig 5.46) Special design to insure there is zero pressure region under vent Vent is need to rid the extruder of volatiles (Table 5.3) Most common volatile is water. Plastics can tolerate about 0.1% moisture Some hygroscopic (Water seeking) plastics degrade when exposed to heat and moisture Polyester, Polycarbonate, nylon and polyurethane

Die Forming Shaping of molten plastic occurs in extrusion die As plastic flows in die it takes the shape of the die. Fig 5.48 Exit region of the die flow channel is called the “land area” Land region for pipe or tubing has an annular shape Land region for sheet has a shape of a slit Die Changes

Die Forming Shape of the die is changed due to Drawdown: thinning of part caused by pulling extrudate Extrudate swell: expansion of the dimensions of the plastic caused by normal stresses and viscoelastic nature. Cooling: causing shrinkage in plastic. Semi-crystalline materials shrink more than amorphous because of the higher density in semi-crystalline materials. Fig 5.51 and 5.52 Relaxation: gradual reduction of internal stresses causing the plastic to sag. Warping: Caused by non-uniform stresses during flow. Fig 5.53

Tubing and Pipe Dies Extrusion dies are categorized based upon the shape of the product produced Annular dies Tubing (< 1” OD), pipe (>1” OD), blown film (D can be > 30ft with thickness from 0.002” to 0.010”), wire coating Inline Fig 5.55 Melt enters from the left and flows around a torpedo Uniform stresses in extruded product

Tubing and Pipe Dies Extrusion dies are categorized based upon the shape of the product produced Annular dies: Crosshead: Fig 5.56 - wire coating Melt is split around the flow splitter, flows over a shoulder to tip and die Allows for axial adjustment allows for concentricity of extruded tube Spiral Mandrel die: Fig 5.57 - blown film Plastic flows through central channel to a series of helical channels Weld lines are eliminated, good flow distribution, hoop strength

Flat film and Sheet Dies Same general shape for flat film and sheet Sheet dies Fig 5.58 inlet channel manifold pre-land section relaxation section land section Manifold designs T-die: No well-streamlined flow Good for low viscosity plastics No clam shelling (Fig 5.60) coat hanger die: Fig 5.61 Complicated, but has streamlined flow horse shoe die: Fig 5.62

Profile dies Many different shapes and sizes Shapes other than rectangular, annular, or circular Plate die: Fig 5.63 uses a plate with a cavity shaped to produce the extruded product Easy to make, but is not streamlined and likely to have dead spots Fully streamlined die: Fig 5.64 Has a flow channel that gradually changes to the exit geometry Velocities slowly increase causing streamlined flow and no dead spots Appropriate for long runs and plastics with limited thermal stability More expensive to manufacture