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IE 337: Materials & Manufacturing Processes

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1 IE 337: Materials & Manufacturing Processes
IE 337 Lecture 7: Polymer Processing IE 337: Materials & Manufacturing Processes Lecture 10: Polymer Processing Sections and Chapters 8, 13 S.V. Atre

2 This Time What are plastics and polymers? Polymer Rheology
Major Plastics Molding Processes Extrusion Injection Molding Thermoforming Compression Molding Molding Machine

3 IE 337 Lecture 7: Polymer Processing
Engineering Plastics Chain of organic molecules Properties: Lightweight Corrosion-resistant Low strength Low stiffness Relatively inexpensive Very formable Temperature concerns smaller M w larger M Giant molecules with repeating units (monomer) Mw is molecular weight S.V. Atre

4 IE 337 Lecture 7: Polymer Processing
What are polymers? polypropylene (PP) polyethylene (PE) Repeats of “mer” units polyvinyl chloride (PVC) polytetrafluoroethylene (PTFE) S.V. Atre

5 Classification: Chemistry
polyethylene (PE) polyvinyl chloride (PVC) polytetrafluoroethylene (PTFE) polypropylene (PP) polymethyl methacrylate (PMMA) polystyrene (PS)

6 Classification: Chemistry
polyhexamethylene adipamide (Nylon) polyethylene terephthalate (Polyester, PET) polycarbonate (PC)

7 Two Types of Plastics Thermoplastics Thermosets
Chemical structure remains unchanged during heating and shaping More important commercially, comprising more than 70% of total plastics tonnage Thermosets Undergo a curing process during heating and shaping, causing a permanent change (called cross‑linking) in molecular structure Once cured, they cannot be remelted

8 Families of Plastics Thermoplastics Thermosets Acetals Acrylic
Cellulose (Acetates) Fluorocarbons Teflon Nylon Polycarbonate Polyethelene Density Polystyrene Vinyl Thermosets Epoxies Bonding Melamines Resistant Phenolics Bakelite Polyesters Silicones Sealant Urea-formaldehyde Environmental concerns

9 Plastic Family Properties
Thermoplastics Reversible softening & hardening Softening range (not melting point) Weak bonds between molecules Properties inverse with temperature: Stiffness Hardness Ductility Solvent resistance Thermosets Irreversible hardening reaction Strong bonds between molecules (cross-linking) Compared with Thermoplastics: Stronger Rigid Heat resistant Brittle Low impact toughness Lower ductility

10 Classification: Structure
Linear thermoplastic Branched thermoplastic Crosslinked thermosetting Network thermosetting

11 Classification: Structure
random coil (amorphous) partially extended (semi-crystalline)

12 Elastomers Exceptional elastic deformation Chemical forms
Near-complete* recovery Viscous deformation is permanent Twisted/coiled molecular chains Can be cross-linked (vulcanization) Degradable Insulative Chemical forms Natural Rubber Synthetic Polyisoprene (Santoprene) Silicone rubber Urethanes

13 polydimethylsiloxane
Elastomers polychloroprene (Neoprene rubber) polyisoprene (natural rubber) polydimethylsiloxane (silicone rubber) polyisobutylene (butyl rubber)

14 Plastic Utility Degradable Modifiable Properties
UV Light Flammable, Oxidation Modifiable Properties Color Conductivity Adhesiveness Mechanical Additives Make Polymers into Plastics Stabilizers, Flame retardants Dyes (translucent), Coloring Agents (opaque) Anti-statics, Anti-microbials Plasticizers (improve flow), Lubricants (improve moldability) Reinforcements, Fillers

15 Classification: Structure
a) random b) alternating COPOLYMERS more than one “mer” c) block d) graft

16 Economics of Plastics Compared with Metals (+):
Lower fabrication tooling costs Higher production rate Greater DFA (Design For Assembly) potential Snap fits/fastener-less assembly Friction/ultrasonic/solvent welding Self-tapping fasteners Lower reuse cost (scrap)* Lower finishing costs Lower density Compared with Metals (-): Higher cost / weight Lower impact resistance Lower strength Lower stiffness Smaller operational temperature range Lower resistance to: Flame Solvents Light (UV)

17 Plastic Shaping Processes
Almost unlimited variety of part geometries Plastic molding is a net shape process; further shaping is not needed Less energy is required than for metals because processing temperatures are much lower Handling of product is simplified during production because of lower temperatures Painting or plating is usually not required

18 Viscosity of Polymer Melts
A fluid property that measures the resistance to flow – quotient of shear stress to shear rate within a fluid Due to its high molecular weight, a polymer melt is a thick fluid with high viscosity Important because most polymer shaping processes involve flow through small channels or die openings High flow rates lead to high shear rates and shear stresses, so significant pressures are required to process polymers

19 Viscosity Like liquid metals, polymer viscosity is dependent on temperature Unlike liquid metals, polymer viscosity depends on shear rate “Non-Newtonian fluid” “Shear thinning”

20 Viscoelasticity Viscous and elastic (pseudoplastic) properties
Possessed by both polymer solids and polymer melts Example: die swell in extrusion, in which the hot plastic expands when exiting the die opening Swell ratio, rs = Dx/Dd

21 Extruder Sectional View
Components and features of a (single‑screw) extruder for plastics and elastomers

22 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

23 Dies and Extruded Products
The shape of the die orifice determines the cross‑sectional shape of the extrudate Common die profiles and corresponding extruded shapes: Solid profiles Hollow profiles, such as tubes Wire and cable coating Sheet and film Filaments

24 Extruding a Coated Wire
Side view cross‑section of die for coating of wire by extrusion

25 Injection Molding Polymer is heated to a highly plastic state and forced to flow under high pressure into a mold cavity where it solidifies; molded part is then removed from cavity Produces discrete components almost always to net shape Typical cycle time 10 to 30 sec, but cycles of one minute or more are not uncommon Mold may contain multiple cavities, so multiple moldings are produced each cycle

26 Injection Molded Parts (Moldings)
Complex and intricate shapes are possible Shape limitations: Capability to fabricate a mold whose cavity is the same geometry as part Shape must allow for part removal from mold Part size from  50 g (2 oz) up to  25 kg (more than 50 lb), e.g., automobile bumpers Injection molding is economical only for large production quantities due to high cost of mold

27 Polymers for Injection Molding
Injection molding is the most widely used molding process for thermoplastics Some thermosets, elastomers, metals and ceramics are also injection molded Modifications in equipment and operating parameters must be made

28 Injection Molding Machine
Two principal components: Injection unit – melts and delivers polymer melt, operates much like an extruder Clamping unit – opens and closes mold each injection cycle

29 Injection Molding Machine
A large (3000 ton capacity) injection molding machine (courtesy Cincinnati Milacron)

30 Injection Molding Cycle: Stage 1
Typical molding cycle: (1) mold is closed

31 Injection Molding Cycle: Stage 2
Typical molding cycle: (2) melt is injected into cavity

32 Injection Molding Cycle: Stage 3
Typical molding cycle: (3) screw is retracted

33 Injection Molding Cycle: Stage 4
Typical molding cycle: (4) mold opens and part is ejected

34 The Mold Custom‑designed and fabricated for the part to be produced
Various types of mold for injection molding: Two-plate mold Three-plate mold Hot-runner mold Cavity Mold

35 Shrinkage Reduction in linear size during cooling from molding to room temperature Polymers have high thermal expansion coefficients, so significant shrinkage occurs during cooling in mold Typical shrinkage values for selected polymers: Plastic Shrinkage, mm/mm (in/in) Nylon‑6, Polyethylene Polystyrene PVC

36 Compensation for Shrinkage
Dimensions of mold cavity must be larger than specified part dimensions: Dc = Dp + DpS + DpS2 where Dc = dimension of cavity; Dp = molded part dimension, and S = shrinkage value

37 Shrinkage Compensation Factors
Fillers in the plastic tend to reduce shrinkage Injection pressure – as pressure is increased, it forces more material into the mold cavity, and shrinkage is reduced Compaction time - similar effect - forces more material into cavity during shrinkage Molding temperature - higher temperature lowers the polymer melt viscosity, allowing more material to be packed into mold and reducing shrinkage

38 Thermoforming Flat thermoplastic sheet or film is heated and deformed into desired shape using a mold Heating usually accomplished by radiant electric heaters located on one or both sides of starting plastic sheet or film Widely used in packaging of products and to fabricate large items such as bathtubs, contoured skylights, and internal door liners for refrigerators

39 Thermoforming Process - Step 1
Vacuum thermoforming: (1) a flat plastic sheet is softened

40 Thermoforming Process - Step 2
Vacuum thermoforming: (2) sheet is placed over mold cavity

41 Thermoforming Process - Step 3
Vacuum thermoforming: (3) vacuum draws sheet into the cavity

42 Compression Molding Thermosets with axisymmetric shapes

43 Blow Molding Hollow shapes

44 IE 337 Lecture 7: Polymer Processing
Stereolithography Additive Manufacturing “Rapid prototyping” S.V. Atre

45 You should have learned
The difference between plastics and polymers Viscoelastic properties of polymers Key plastics molding processes Extrusion Injection Molding Thermoforming Compression Molding

46 Next Week Mid-Term Exam (Tuesday) Forming (Thursday)

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