Extrusion. Key Issues to Address. Lecture 2. Process. Process Variants. Process Analysis. Problem Solving

Transcription

1 Extrusion Lecture 2 Chapter 4 Key Issues to Address Process Process Variants Process Analysis Problem Solving S.V. Atre 1

2 Extrusion Material is forced to flow through a die orifice to provide long continuous product whose cross-sectional shape is determined by the shape of the orifice Widely used for thermoplastics and elastomers to produce tubing, pipes, hose, structural shapes, sheet and film, continuous filaments, and insulated wire Overall Process S.V. Atre 2

3 Polymer Processing Lecture 2 Extrusion Summer 2007 Equiment: Extruder Components and features of a single-screw extruder for plastics and elastomers Extruder Barrel Internal diameter: 25 to 150 mm L/D ratio: 10 and 30 S.V. Atre lower ratios for semi-crystalline, higher ratios for amorphous lower ratios for elastomers, higher ratios for thermoplastic Feedstock fed by gravity onto screw, then rotation moves material through the barrel Electric heaters melt feedstock; subsequent mechanical work adds heat 3

4 Extruder Screw Divided into sections to serve several functions: Feed section - feedstock is moved from hopper and preheated Compression section - polymer is transformed into fluid, air mixed with pellets is extracted from melt, and material is compressed Metering section - melt is homogenized and sufficient pressure developed to pump it through die opening Extruder Screw S.V. Atre 4

5 Die End of Extruder Before reaching die, the melt passes through a screen pack - series of wire meshes supported by a stiff plate containing small axial holes Functions of the screen pack: Filter contaminants and hard lumps from melt Build pressure in metering section Straighten flow of polymer melt and remove its "memory" of circular motion imposed by screw Melt Flow in Extruder As screw rotates inside barrel, polymer melt is forced to move forward Principal transport mechanism is drag flow, Q d, resulting from friction between the viscous liquid and the rotating screw Compressing the polymer melt through the die creates a back pressure that reduces drag flow transport (back pressure flow, Q b ) Resulting flow in extruder is Q x = Q d Q b S.V. Atre 5

6 Dies and Extruded Products Shape of the die orifice determines the cross-sectional shape of the extrudate Common shapes: Solid profiles Hollow profiles, such as tubes Wire and cable coating Sheet and film Filaments Solid Profiles Regular shapes such as Rounds Squares Irregular cross-sections such as Structural shapes Door and window moldings Automobile trim House siding S.V. Atre 6

7 Polymer Processing Lecture 2 Extrusion Summer 2007 Solid Profiles Extrusion of Solid Profiles S.V. Atre 7

8 Polymer Processing Lecture 2 Extrusion Summer 2007 Hollow Profiles Examples: tubes, pipes, hoses, and other cross-sections containing holes Hollow profiles require a mandrel to form the shape The mandrel held in place using a spider Polymer melt flows around legs supporting the mandrel to reunite into a monolithic tube wall Extrusion of Hollow Profiles S.V. Atre The mandrel often includes an air channel to maintain hollow form of extrudate during hardening 8

9 Polymer Processing Lecture 2 Extrusion Summer 2007 Wire and Cable Coating Polymer melt is applied to bare wire as it is pulled at high speed through a die A slight vacuum promotes adhesion Wire provides rigidity during cooling - usually aided by passing coated wire through a water trough Extruding a Coated Wire Product is wound onto large spools at speeds up to 50 m/s S.V. Atre 9

10 Polymer Processing Lecture 2 Extrusion Summer 2007 Polymer Sheet and Film Sheet Thickness: 0.5 mm mm Used for flat window glazing and thermoforming stock Film Thickness < 0.5 mm Thin films used for packaging Thicker film applications include pool covers and liners for irrigation ditches Extruding a Sheet or Film Coat hanger die S.V. Atre 10

11 Extrusion of Ceramics Compression of clay through a die orifice to produce long sections of uniform cross-section, which are then cut to required piece length Products: hollow bricks, shaped tiles, drain pipes, tubes, and insulators Ceramic Extrusion: Examples cordierite catalytic converter 50 cells/cm 2 S.V. Atre 11

12 Ceramics Processing powder final debind flow shape sinter Fused Deposition Modeling (FDM) RP process in which a long filament of wax or polymer is extruded onto the existing part surface from a workhead to complete each new layer E.g. Stratasys ( The workhead is controlled in the x-y plane during each layer and then moves up by a distance equal to one layer in the z-direction The extrudate is solidified and cold welded to the cooler part surface in about 0.1 s Part is fabricated from the base up, using a layerby-layer procedure S.V. Atre 12

13 Sample Parts Design for Manufacturing (DFM) An approach to product design which systematically includes considerations of manufacturability in the design 26 S.V. Atre 13

14 Rapid Prototyping (RP) A family of unique fabrication processes developed to make engineering prototypes in minimum lead time based on a CAD model of the item The traditional method is machining Machining can require significant lead-times several weeks, depending on part complexity and difficulty in ordering materials 27 RP allows a part to be made in hours or days rather than weeks, given that a computer model of the part has been generated on a CAD system Why Rapid Prototyping? Because product designers would like to have a physical model of a new part or product design rather than just a computer model or line drawing Creating a prototype is an integral step in design A virtual prototype (a computer model of the part design on a CAD system) may not be sufficient for the designer to visualize the part adequately 28 Using RP to make the prototype, the designer can better assess its merits and shortcomings S.V. Atre 14

15 Layer Conversion Conversion of a solid model of an object into layers (only one layer is shown) Extrusion Freeforming Prototyping Reduce time to produce valid prototypes Stratasys Machine Extrusion Head Ceramic prototype S.V. Atre 15

16 Extrusion Freeforming Prototyping 3D CAD model (Pro-E, Solid Works).stl file Create 0.25 mm slices through model Create tool paths, mm Send to machine Part Build Time in STL Time to complete a single layer : Ai T i = + Td vd where T i = time to complete layer i; A i = area of layer i; v = average scanning speed of the extruder; D = diameter of the extrudate and T d = delay time between layers S.V. Atre 16

17 Part Build Time in STL - continued Once the T i values have been determined for all layers, then the build cycle time is: n = i T c T i i = 1 where T c = STL build cycle time; and n l = number of layers used to approximate the part Time to build a part ranges from one hour for small parts of simple geometry up to several dozen hours for complex parts Example A prototype of a tube with a square cross-section is to be fabricated using stereolithography. The outside dimension of the square = 100 mm and the inside dimension = 90 mm (wall thickness = 5 mm except at corners). The height of the tube (z-direction) = 80 mm. Layer thickness = 0.20 mm. The diameter of the extrudate = 1.25 mm, and the extruder head is moved across in the x-y plane at a velocity of 150 mm/s. Compute an estimate for the time required to build the part, if 10 s are lost each layer to lower the height of the platform that holds the part. Neglect the time for postcuring. S.V. Atre 17

18 Example: Solution Layer area A i same for all layers. A i = = 1900 mm 2 Time to complete one layer Ti same for all layers. T i = (1900 mm 2 )/(1.25 mm)(150 mm/s)+ 10 s = = 20.1 s Number of layers n l = (80 mm)/(0.20 mm/layer) = 400 layers T c = 400(20.1) = 8053 s = min = 2.2 hr Product Design Guidelines for Plastics General - I Strength and stiffness Plastics are not as strong or stiff as metals Creep resistance is also a limitation Strength-to-weight ratios for some plastics are competitive with metals in certain applications S.V. Atre 18

19 Product Design Guidelines for Plastics General - II Impact Resistance Capacity of plastics to absorb impact is generally good; plastics compare favorably with most metals Service temperatures Plastics are limited relative to engineering metals and ceramics Thermal expansion Dimensional changes due to temperature changes much more significant than for metals Product Design Guidelines for Plastics General - III Many plastics are subject to degradation Plastics are soluble in many common solvents Plastics are resistant to conventional corrosion mechanisms that afflict metals S.V. Atre 19

20 Product Design Guidelines Extruded Plastics - I Wall thickness Uniform wall thickness is desirable in an extruded cross-section Variations in wall thickness result in non-uniform plastic flow and uneven cooling which tend to warp extrudate Product Design Guidelines Extruded Plastics - II Hollow sections Desirable to use extruded cross-sections that are not hollow yet satisfy functional requirements Hollow sections complicate die design and plastic flow S.V. Atre 20

21 Product Design Guidelines Extruded Plastics III Corners Sharp corners, inside and outside, should be avoided in extruded cross-sections They result in uneven flow during processing and stress concentrations in the final product Process Analysis: Viscosity Fluid property that relates shear stress to shear rate during flow Due to its high molecular weight, a polymer melt has a high viscosity Important because most polymer processes involve flow through small channels Flow rates are often large, leading to high shear rates and shear stresses, so significant pressures are required S.V. Atre 21

22 Viscosity Viscosity relationships for Newtonian fluid and typical polymer melt Viscoelasticity Combination of viscosity and elasticity Possessed by both polymer solids and polymer melts Example: die swell in extrusion, in which the hot plastic expands when exiting the die opening S.V. Atre 22

23 Polymer Processing Lecture 2 Extrusion Summer 2007 Die Swell Extruded material "remembers" its former shape when in the larger cross-section of the extruder and attempts to return to it Polymer Viscosity Behavior S.V. Atre 23

24 Polymer Processing Shear stress in a cylindrical channel τ = Pr 2l Shear rate in a cylindrical channel γ& = 4Q πr 3 yields η = Pπr 8lQ P = pressure, Pa Q = volumetric flow rate, m 3 /s r = radius of channel, m l = length of channel, m 4 Example Methyl methacrylate monomer has a viscosity of 0.01 Pa-s at room temperature. Compare the pressure required to cause the monomer to flow along a circular channel with that of poly(methyl methacrylate) PMMA? Assume flow occurs in a circular channel of length 50 mm and radius 2.5 mm at a rate of 2.5 x 10-4 m 3 /s. Pπr η = 4 4Q γ& = 8lQ πr 3 S.V. Atre 24

25 Solution Monomer: P η 8lQ = πr 4 P = 0.01x 8 x x 0.05 π (2.5 x 10-3 ) 4 = 8 kpa PMMA: = 24 MPa Effects of Geometry on Flow S.V. Atre 25

26 Polymer Processing Lecture 2 Extrusion Summer 2007 Screw & Die Characteristics Geometry Effects on Throughput Process Design S.V. Atre 26

27 Analyzing the Metering Section HW 2 Problems: 4.6, 4.7, 4.8 Due: Wednesday morning Hint: Follow Solved Example 4.2 S.V. Atre 27

28 You should have learnt Extrusion process Extruding different geometries Process variants Some design principles Next Class Mixing (Chapter 5) S.V. Atre 28