Polymer Processing Module 3b

Introduction Processing Methods and Operations Choice is dictated by the product desired and the quantity desired. Fiber, film, sheet, tube Cup, bucket, car bumper, chair. Fiber manufacture is different, it is continuous. Large quantities usually use extrusion or injection molding Smaller quantities use compression molding or transfer molding Spring 2001 ISAT 430 Dr. Ken Lewis

Extrusion This process is fundamental to both metals and ceramics as well as polymers. Definition Extrusion is a compression process Material is forced to flow through a die orifice Cross-sectional shape determined by the shape of the orifice Product is long and continuous Spring 2001 ISAT 430 Dr. Ken Lewis

Extrusion2 Rarely used for thermosetting polymers Products Tubing, pipes, and hose Window and door moldings Sheet and film Continuous filaments (as we saw in module 3A) Coated electrical wire and cable. Spring 2001 ISAT 430 Dr. Ken Lewis

Extrusion3 The extruder consists basically of a hopper and a barrel and a screw. Spring 2001 ISAT 430 Dr. Ken Lewis

Extruder Usually ~ 1 – 6 in. dia. Up to 60 rpm The die is not part of the extruder Flight clearance of only 0.002 in. Spring 2001 ISAT 430 Dr. Ken Lewis

Extruder Feed section Compression or plastication section Compact to a solid mass Pre heat Compression or plastication section Melting progresses, degassing occurs Metering section Internal heating from viscous flow Pressure is developed to extrude the material through the die Spring 2001 ISAT 430 Dr. Ken Lewis

Extruder2 The screw is a tight fit in the barrel. Note how the channel depth. changes in the plastication section. Is constant in the metering section. Channel depth These section lengths will change depending on the polymer being processed. Compression section Short for materials that melt suddenly (nylon) Long for gradually softening materials (polyvinyl chloride) Spring 2001 ISAT 430 Dr. Ken Lewis

Channel depth Pressure applied to polymer melt is a function of the channel depth, dc. Feed section dc is relatively large Allows lots of granular polymer to be added to barrel Compression section dc gets smaller Applies additional pressure to metering section Metering section dc is smallest Can be carefully designed, but… In general, industry uses general kind of “off the shelf” extruders. Spring 2001 ISAT 430 Dr. Ken Lewis

Screw details Helical flights with space between them Carries the polymer. Flight land is hardened and barely clears the barrel. The Pitch (distance the flight travels in one complete rotation) is usually about equal to the diameter. Spring 2001 ISAT 430 Dr. Ken Lewis

Melt Flow in the Extruder OK, the screw turns, the flights advance, WHY DOES THE POLYMER ADVANCE? Why doesn’t it just slip and slide back? DRAG FLOW Friction between the fluid and the two opposing surfaces The stationary barrel The moving channel of the turning screw RECALL… Spring 2001 ISAT 430 Dr. Ken Lewis

Melt Flow in the Extruder Qdr, the volumetric drag flow rate. If we assume that the velocity v is ½ the flight velocity (the moving plate velocity) Where: v = velocity of the plate (m/s) D = distance between the plates (m) W = width of the plates(m) Spring 2001 ISAT 430 Dr. Ken Lewis

Melt Flow in the Extruder Most analyses of extruders unroll the helical shaped channel Leads to a rectangular channel covered by an infinite plate moving at constant velocity The fluid motion (or flow) in the channel can be decomposed A cross flow in the x – y plan An axial flow in the z direction. Spring 2001 ISAT 430 Dr. Ken Lewis

Melt Flow in the Extruder The axial flow in the z direction is responsible for the pumping The cross flow does most of the mixing. Spring 2001 ISAT 430 Dr. Ken Lewis

Extruder Mixing & Melting Direction of travel. The happenings in the channel are complex. Near the leading edge the polymer has experienced the longest residence time. Mixing is poor Flow is laminar Zero turbulence Polymer cooking can be a problem. Spring 2001 ISAT 430 Dr. Ken Lewis

Extruder transport Using the unrolled screw model, we can show that: Where: v = velocity of the plate (m/s) D = distance between the plates (m) W = width of the plates(m) Extruder transport Using the unrolled screw model, we can show that: We have assumed: wf is negligible Note: or Spring 2001 ISAT 430 Dr. Ken Lewis

Extruder transport – back pressure. This is the maximum possible output for an extruder. Conveyance of the polymer through Smaller and smaller cross sections the screen pack and die… Creates a back pressure, Qbp. Spring 2001 ISAT 430 Dr. Ken Lewis

Extruder transport – back pressure The back pressure is a function of Barrel dimensions The polymer viscosity The flight angle The pressure gradient dp/dl… The pressure gradient dp/dl Is a function of the screw shape, the barrel size, the flight angel. If we assume the pressure profile is linear along the barrel, then dp/dl becomes p/L Spring 2001 ISAT 430 Dr. Ken Lewis

Extruder transport Then: Where: p = the head pressure (Mpa) L = length of the barrel (m) Spring 2001 ISAT 430 Dr. Ken Lewis

Back Pressure Flow A misnomer So what is the net flow? It is not back pressure flow It is resistance to forward flow So what is the net flow? Qnet is what finally comes out of the die! Spring 2001 ISAT 430 Dr. Ken Lewis

Back Pressure And the maximum pressure becomes: There is some (hopefully) negligible slippage of fluid between the flight and the barrel wall. Back pressure reduces flow but causes plastication. In the limit, the back pressure can stop the flow And the maximum pressure becomes: Spring 2001 ISAT 430 Dr. Ken Lewis

The Net Flow There are a lot of parameters in the above equation (relation) They are of two types Those we control (design parameters) Those we don’t control (operating parameters) Spring 2001 ISAT 430 Dr. Ken Lewis

Design Parameters These we control at conception time and are fixed thereafter. Barrel diameter Flight or Helix angle Channel depth dc Barrel length L Spring 2001 ISAT 430 Dr. Ken Lewis

Operating Parameters These we can fiddle with to optimize the process. Rotational speed, N The head pressure (change the die, slow the screw, change the temperature) The hidden variable … TEMPERATURE. The viscosity But only to the extent that the shear rate and temperature will allow! Spring 2001 ISAT 430 Dr. Ken Lewis

Extruder Characteristics For a given extruder: or Spring 2001 ISAT 430 Dr. Ken Lewis

Extruder Characteristics Flow up with Increasing N Decreasing p Increasing  Ignores non-Newtonian flow behavior Ignores friction Spring 2001 ISAT 430 Dr. Ken Lewis

Extruder Characteristics A useful estimate of extruder capacity with a L/D = 24 is: Usual Recommended Ce scr Output in Kg/h 0.006 2.2 2.3 Output in lb/h 16 20 2.35 Actual output may ± 20% (good for back of envelope calculations) Spring 2001 ISAT 430 Dr. Ken Lewis

What is the output for zero back pressure? A screw extruder has D = 75 mm, dc = 5 mm, A = 17.5°. It rotates at 100 rpm. The plastic has a density of 1 gm/cc. What is the output for zero back pressure? What is the output expected for normal conditions? Spring 2001 ISAT 430 Dr. Ken Lewis

Die Characteristics Flow through a die generates back pressure For a simple cylindrical flow channel the flow rate is given by the famous Hagen – Poiseuille equation: D = diameter  = melt viscosity [=] Spring 2001 ISAT 430 Dr. Ken Lewis

Die characteristics So flow increases with p Look at the power of the die diameter! This gives the linear die characteristic curve. Note: some people write the above equation as: Spring 2001 ISAT 430 Dr. Ken Lewis

Die characteristics Where Ks is called the die shape factor Still just equation for laminar flow through a pipe. Spring 2001 ISAT 430 Dr. Ken Lewis

Extrusion Curve Spring 2001 ISAT 430 Dr. Ken Lewis

Operating Point The values of Q and p where the curves intersect is the extruder operating point. Note the shape factor Ks is the slope of the die characteristic curve. Spring 2001 ISAT 430 Dr. Ken Lewis

example Consider an extruder with the following properties: D = 3.0 in L = 75 in N = 1 rev/sec dc = 0.25 in A = 20° Let the melt have a shear viscosity of  = 125 lb-sec/in2 = 103.4 Pa sec Spring 2001 ISAT 430 Dr. Ken Lewis

example2 Knowing the above characteristics, calculate Qmax and pmax. Spring 2001 ISAT 430 Dr. Ken Lewis

example3 Knowing the above characteristics, calculate Qmax and pmax. Spring 2001 ISAT 430 Dr. Ken Lewis

example4 These two values define the abscissa and the ordinate for the extruder characteristic. If we have a circular die with a diameter Dd = 0.25 in, and a length Ld = 1.0 in What’s the shape factor for the die? Spring 2001 ISAT 430 Dr. Ken Lewis

If we have a circular die with a diameter Dd = 0 If we have a circular die with a diameter Dd = 0.25 in, and a length Ld = 1.0 in What’s the shape factor for the die? remember Spring 2001 ISAT 430 Dr. Ken Lewis

example5 And from the die equation Now we can find the operating point for the extruder. We can express the extruder characteristic as the straight line between Qmax and pmax. And from the die equation Setting these equal provides the operating point Spring 2001 ISAT 430 Dr. Ken Lewis

Spring 2001 ISAT 430 Dr. Ken Lewis

Dies The polymer is extruded past the breaker plate into the die. Our previous example assumed a cylindrical die Dies come in many flavors. The die must take into account several factors Die swell bambooing Spring 2001 ISAT 430 Dr. Ken Lewis

Die Swell On the left is a cylindrical die and on the right is an annular die. Note the Barus bulge Due to release of stored elastic energy obtained in the die and the radical change in velocity of material close to the die walls. Spring 2001 ISAT 430 Dr. Ken Lewis

Die Swell Note that as soon as the polymer has left the die, its surface is free Stress free Polymer will relax unless it is kept under tension. Spring 2001 ISAT 430 Dr. Ken Lewis

Surface Fracture At high shear rates The polymer in the middle of the round channel is quiescent while the material near the walls is in high shear. The energy stored is high enough that upon emerging from the die, the polymer fractures in trying the equilibrate the stresses. Spring 2001 ISAT 430 Dr. Ken Lewis

Effect of Die Swell Knowing that die swell will occur is important After the polymer leaves the die it is rapidly cooling and becoming fixed in shape For each polymer, if we know Viscosity Temperature Shear rate We can account for the die swell in the shape of our die Spring 2001 ISAT 430 Dr. Ken Lewis

Die shapes The dies The finished shapes Spring 2001 ISAT 430 Dr. Ken Lewis

Pipe extrusion The central mandrel is supported by spider legs These disrupt the flow of polymer The polymer rejoins itself because the flow rate is low The conditions haven’t changed (temperature) To minimize the effect of the spiders, the mandrel is tapered. Spring 2001 ISAT 430 Dr. Ken Lewis

Internal sizing mandrel Pipe extrusion To control the pipe size, other means are used. Internal sizing mandrel External sizing using air pressure External sizing using vacuum Spring 2001 ISAT 430 Dr. Ken Lewis

Tubing Die Note the expansion to the spider legs and the reduction afterwards. If the extrusion is too rapid, the spider leg openings will not heal. Spring 2001 ISAT 430 Dr. Ken Lewis

Wire Coating Die The wire runs straight through Polymer comes in vertically into a distribution cavity Used for wire diameters of 1 mm up to submarine cables with diameters of 150 mm. Spring 2001 ISAT 430 Dr. Ken Lewis

Wire Coating Die2 Note here the wire is helping draw the polymer from the die! The taught wire provides rigidity during cooling The product is usually cooled by passing it through a liquid bath These system roll, making coated wire at speeds up to10,000 ft/min. Spring 2001 ISAT 430 Dr. Ken Lewis

Injection Molding

Injection Molding Polymer is heated, mixed, the then forced to flow into a mold cavity Similar to extrusion Hopper, barrel, screw Screw rotation is the principal motion only in one part of the cycle Mixes, compacts, plasticizes, and heats Pressures may reach 10 – 20 MPa (1450 – 2900 psi) Spring 2001 ISAT 430 Dr. Ken Lewis

Injection Molding2 In the injecting stage, the screw is driven axially by a piston to generate the working pressure 150 – 250 MPa (21,756 – 36,260 psi) Spring 2001 ISAT 430 Dr. Ken Lewis

Spring 2001 ISAT 430 Dr. Ken Lewis

Injection Molding Sequences (1) Close the mold (2) Inject the melt (3) Retract the screw (4) Open mold – eject part Spring 2001 ISAT 430 Dr. Ken Lewis

Two Plate Mold The mold here is closed The mold is position between two platens One stationary One moveable Note the water channels for quickly cooling the mold and its polymer load. Sprue: channel from die nozzle Into the mold Gates: restrict the polymer Flow into the cavity Runner: channel from Sprue Into the cavity Spring 2001 ISAT 430 Dr. Ken Lewis

Two Plate Mold2 The mold here is open The ejector pins push the rather fragile plastic from the mold cavity The sprue and runners are waste.. Spring 2001 ISAT 430 Dr. Ken Lewis

Two Plate Mold3 Cooling system Gas vents Usually water passages in the mold itself Gas vents Usually about 0.001 in deep and 0.5 wide. Allows the air to escape when the cavity is filling Too small to let the viscous polymer follow. Spring 2001 ISAT 430 Dr. Ken Lewis

Two Plate Mold - Parts Cavities (shape the part) Distribution channels (get the polymer to the cavity) Ejection system (safely remove the part) Cooling system (change the polymer from soup to part) Gas venting facility ( allow the cavity to fill) Spring 2001 ISAT 430 Dr. Ken Lewis

Thermoforming

Thermoforming A flat thermoplastic sheet is softened and deformed into the desired shape. Used for large items Bathtubs Skylights Freezer interior walls Bumpers Two steps Heating Deforming / forming Spring 2001 ISAT 430 Dr. Ken Lewis

Thermoforming Three major types of thermoforming Vacuum Pressure Pressure limit of 1 atmosphere Pressure Higher allowable pressures Mechanical Spring 2001 ISAT 430 Dr. Ken Lewis

Vacuum Thermoforming Spring 2001 ISAT 430 Dr. Ken Lewis

Pressure Thermoforming Spring 2001 ISAT 430 Dr. Ken Lewis

Mechanical Thermoforming In (1) the polymer is pre stretched In (2) the polymer is draped over the positive mold and pressure applied to force it in place Spring 2001 ISAT 430 Dr. Ken Lewis

Mechanical Thermoforming2 In (1) the polymer is pre heated In (2) the polymer is forced into place in the negative mold. Spring 2001 ISAT 430 Dr. Ken Lewis

Product design Considerations In general Strength Plastics are not metals Should not be used in strength or creep critical applications. Impact resistance Good, better than many ceramics Service temperature Much less than metals or ceramics Degradation Radiation Oxygen or ozone Solvents Corrosion resistance Better than metals Spring 2001 ISAT 430 Dr. Ken Lewis

Extrusion Considerations Desirable product traits Wall thickness should be uniform Hollow sections seriously complicate the extrusion process Corners Avoid as they cause uneven polymer flow and are stress concentrators Spring 2001 ISAT 430 Dr. Ken Lewis

Molded Part Considerations Economic production Injection molding minimum ~10,000 parts Vacuum etc. usually around ~1,000 parts. Part complexity Possible, just makes the mold more complicated Wall thickness Wasteful and can warp during shrinkage Use ribs for stiffness Spring 2001 ISAT 430 Dr. Ken Lewis

Molded Part Considerations2 Corner radii and/or fillets Sharp corners are stress concentrators, bad Holes OK but complicate the mold Draft (the taper of the cavity) Should be there to allow easy mold removal Recommended drafts Thermosets: ½° - 1° Thermoplastics: 1/8° - ½° Spring 2001 ISAT 430 Dr. Ken Lewis

Molded Part Considerations3 Tolerances Shrinkage will occur but is predictable The more generous the tolerances the easier the manufacture. Typical dimension tolerances are: +/- 0.006 – 0.010 inches Typical hole tolerances are: +/- 0.003 – 0.005 inches Spring 2001 ISAT 430 Dr. Ken Lewis