10_polymer processing.pdf

7/29/2019 10_Polymer Processing.pdf

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SHAPING PROCESSES FOR PLASTICSy Properties of Polymer Melts

y

y Molding Processes

y

y Polymer Foam Processing and Forming

y Product Desi n Considerations


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y Plastics can be shaped into a wide variety of products:

y

Continuousy Extruded sections

y Films

y Sheets

y

Insulation coatings on electrical wires

y Discrete

y Molded arts

y Injected parts


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Wh Plastic Sha in Processes are Im ortant

y Almost unlimited variety of part geometries

y

Plastic molding is a net shape process; further shaping is notneeded

y ess energy is require t an or meta s ecause processing

temperatures are much lower

y

lower temperatures

y Strength to weight ratio is good.


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y To shape a thermoplastic polymer it must be heated so that it

softens to the consistency of a liquid

y In this form, it is called apolymer melt

y mportant properties:

y Viscosity

y


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Viscosit of Pol mer MeltsFluid property that relates shear stress to shear rate during flowy Due to its high molecular weight, a polymer melt is a thick

fluid with high viscosityy Flow rates are often large, leading to high shear rates and

s ear stresses, so signi icant pressures are require to

accomplish the processes


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Combination of viscosity and elasticity

y

Possessed by both polymer solids and polymer meltsy This property determines the strain experienced by a

materia w en su jecte to a com ination o stress an

temperature over time.

y

gradually on removal

y Example: die swell in extrusion, in which the hot plastic

expands when exiting the die opening


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e weExtruded material "remembers" its former shape when in the

lar er cross-section of the extruder and attem ts to return to

it after leaving the die orifice


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Compression process in which 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

y Widely used for thermoplastics and elastomers to mass

roduce items such as tubin , i es, hose, structural sha es,sheet and film, continuous filaments, and coated electrical

wire

y Carrie out as a continuous process; extrudate is t en cut intodesired lengths


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Components and features of a (single-screw) extruder for plastics and elastomers

L/d ratio of barrel for TP is high compared to elastomers


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Two Main Components of an Extruder

1. Barrel

2. Screw

y

Die - not an extruder componenty Feedstock fed by gravity onto screw whose rotation moves

materia t roug arre

y

Electric heaters melt feedstock; subsequent mixing and

y Clearance between barrel and screw to limit leakage of the

melt backward to the trailin channel

y The dc changes from entry to exit


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Extruder Screwy Divided into sections to serve several functions:

y Feed section - feedstock is moved from ho er and reheated

y

Compression section - polymer is transformed into fluid, airmixed with pellets is extracted from melt, and material is

compresse , crys a n y p ays a ro e n e eng .

y Metering section - melt is homogenized and sufficient pressure

developed to pump it through die opening

Tan A = P/D


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y Progress of polymer melt through barrel leads ultimately to

the die zone

y Before reaching die, the melt passes through a screen pack -

small axial holes

y Functions of screen ack:

y Filter contaminants and hard lumps from melt

y Build pressure in metering section

y Straighten flow of polymer melt and remove its "memory" ofcircular motion imposed by screw


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y As screw rotates inside barrel, polymer melt is forced to move forward

toward die; as in an Archimedian screw

y Principal transport mechanism is drag flow, Qd, resulting from friction

between the viscous liquid and the rotating screwy Qd= volume drag flow rate = 0.5vdw

y V= cos ; = c; w= wc= tan wf cos ; wf s smay Qd= 0.5

2D2Ndccos A sinA

yCompressing the polymer melt through the die creates a back pressure, b

Qb = [(DNdc3 sin 2A)/12]*dp/dl

dp/dl pressure gradient, we can assume to be constant, p= head

y Resulting flow in extruder is Qx = QdQby We have design parameter and operational parameters in the formula


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y Regular shapes such as

y Rounds

y Squares

rregu ar cross-sec ons suc as

y Structural shapes

y Door and window moldin s

y Automobile trim

y House siding


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Cross-section of an extruder die


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y Examples: tubes, pipes, hoses, and other cross-sections

containing holes

y Hollow profiles require mandrel to form the shape

y an re e in p ace using a spi er

y Polymer melt flows around legs supporting the mandrel to

reunite into a monolithic tube wall

y Mandrel often includes an air channel through which air is

blown to maintain hollow form of extrudate during

hardening


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Wire Coatingy Polymer melt is applied to bare wire as it is pulled at high

speed through a die

y A s ig t vacuum is rawn etween wire an po ymer to

promote adhesion of coating

y -

passing coated wire through a water trough


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Pol mer Sheet and Filmy Sheet Thickness from 0.5 mm - 12.5 mmy Film Thickness below 0.5 mm (0.020 in.)

y

Processes include:y Slit-Die Extrusion of Sheet and Film

y B own-Fi m Extrusion Process

y Calendering


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Slit-Die Extrusion of Sheet and FilmPro uction o s eet an i m y conventiona extrusion, using a

narrow slit as the die opening

.

y A problem in this method is uniformity of thicknessthrou hout width of stock due to drastic sha e chan e of

polymer melt during its flow through die

y Edges of film usually must be trimmed because of thickeningat edges


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Blown-Film Extrusion ProcessCom ines extrusion an owing to pro uce a tu e o t in i m

y Process begins with extrusion of tube that is drawn upward

inflated into it through die mandrely Air is blown into tube to maintain uniform film thickness and

tube diameter


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CalenderingFee stoc is passe t roug a series o ro s to re uce t ic ness

to desired gage

,

accuracyy T ical materials: rubber or rubber thermo lastics

y Products: PVC floor covering, shower curtains, vinyl table

cloths, pool liners, and inflatable boats and toys


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Fiber and Filament Products

:y Fiber- a long, thin strand whose length is at least 100 times its

cross-section

y

Filament - a fiber of continuous length


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Fiber and Filament Production - Spinningy e term spinning s a o over rom met o s use to raw an tw st natura

fibers into yarn or thread

For synthetic fibers, spinning = extruding a polymer melt or solution through a

spinneret a e w t mu t p e sma o es , t en raw ng an w n ng t e

product onto a bobbiny Three variations, depending on polymer processed:

1. Melt spinning :Starting polymer is heated to molten state and pumped

through spinneret, similar to conventional extrusion

2. Dry spinning :Similar to melt spinning, but starting polymer is insolution and solvent can be separated by evaporation

3. Wet spinning :Polymer is also in solution, only solvent is non-volatile , To

separate polymer, extrudate is passed through a liquid chemical that

coagulates or precipitates the polymer into coherent strands which arethen collected onto bobbins

Filaments produced by any of the three processes are usually subjected to

further cold drawing to align crystal structure along direction of filament

axis


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Injection Molding discrete productsy 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

en remove rom cav y. , an as omers are use as

working material.

y

y Typical cycle time 10 to 30 sec., but cycles of one minute or

more are not uncommon. Mold ma contain multi le cavities

y Two principal components:

y Injection unit melts and delivers polymer melt, operates much like an

extrudery Clamping unit opens and closes mold each injection cycle


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Diagram of an injection molding machine, reciprocating screw type (some mechanical

details are simplified)

Inside the barrel is a screw which has two functions:. o a es or m x ng an ea ng e po ymer

2. Acts as a ram to inject molten plastic into mold

Non-return valve near tip of screw prevents melt flowing backward along

y Functions of clamping unit

1. Holds two halves of mold in proper alignment with each other

.

resist injection force

3. Opens and closes the mold at the appropriate times in molding cycle


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:is retracted (4) mold opens and part is ejected


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The Moldy e specia too in injection mo ing

y Various types of mold for injection molding:

-

y Three-plate moldy Hot-runner mold

(a) closed (b) opening


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Two-Plate Mold Featuresy Cavity has geometry of part but slightly oversized to allow for

shrinkage

y Created by machining of the mating surfaces of two mold halves

mold cavityy Sprue - leads from nozzle into mold

y Runners - lead from sprue to cavity (or cavities)

y Gates - constrict flow of plastic into cavity

y

molding cycle

y Ejector pinsbuilt into moving half of mold usually accomplish this

y Cooling system - consists of external pump connected to passageways in

mold, through which water is circulated to remove heat from hot plastic

y Air vents to permit evacuation of air from cavity as polymer melt

rushes in


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Three-Plate Mold

mold opens

y Advanta es over two- late mold:

y

Allows automatic operation of molding machiney As mold opens, runner and parts disconnect & drop by gravity into two

conta ners un er mo

y Flow of molten plastic is through a gate at the base of part

rather than side, allowing more even distribution of plastic meltinto sides of cup


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Hot-Runner Moldy m nates so cat on o sprue an runner y ocat ng eaters aroun t e

corresponding runner channels

y While plastic in mold cavity solidifies, material in sprue and runner channels

rema ns mo ten, rea y to e n ecte nto cav ty n next cyc e

y This saves material that otherwise would be scrap in the unit operation

Shrinkage Factorsy Fillers in the lastic tend to reduce shrinka e

y Injection pressure as pressure is increased, forcing more material into mold

cavity, shrinkage is reduced

ompac on me - - v y

shrinkage

y Molding temperature - higher temperatures lower polymer melt viscosity,

a ow ng more ma er a o e pac e n o mo an re uc ng s r n age


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Blow Molding

Molding process in which air pressure is used to inflate soft

plastic into a mold cavity

y Important for making one-piece hollow plastic parts with

thin walls, such as bottles

nce ese ems are use or consumer everages n mass

markets, production is typically organized for very high

uantities


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y Accomplished in two steps:

1. Fabrication of a starting tube, called aparison

2. Inflation of the tube to desired final shape

orm ng e par son s accomp s e y e er

y Extrusion or

y In ection moldin


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Extrusion blow molding: (1) extrusion of parison; (2) parison is pinched at the

w w v

mold come together; (3) the tube is inflated so that it takes the shape of the

mold cavity; and (4) mold is opened to remove the solidified part


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Injection blow molding: (1) parison is injected molded around a blowing rod; (2)

injection mold is opened and parison is transferred to a blow mold; (3) soft

polymer is inflated to conform to the blow mold; and (4) blow mold is openedand blown product is removed


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Stretch blow molding:(1) injection molding of parison; (2) stretching; and (3) blowing

Variation of injection blow molding in which blowing rod extends downward into,

than conventional blow molding Resulting structure is more rigid, with higher transparency and better impactresistance


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Rotational molding Alternative to


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ThermoformingFlat thermo lastic sheet or film is heated and deformed into desired sha e usin a

mold

y Heating usually accomplished by radiant electric heaters located on one or both

sides of startin lastic sheet or film

y Widely used in packaging of products and to fabricate large items such as

bathtubs, contoured skylights, and internal door liners for refrigerators

Negative mold concave cavity

Positive mold- convex shape

Th l ti F I j ti


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Thermoplastic Foam Injection

surrounding lightweight foam centery Part has hi h stiffness-to-wei ht ratio suited to structural

applications

y

Produced either by introducing a gas into molten plastic ininjection unit or by mixing a gas-producing ingredient with

starting pellets

,

cavity, where it expands to fill cavity

y Foam in contact with cold mold surface colla ses to form

dense skin, while core retains cellular structure

C i M ldi


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Compression Moldingy An old and widel used moldin rocess for thermosettin

plastics

y Applications also include rubber tires and polymer matrix

composite parts

y Molding compound available in several forms: powders or

pe e s, qu , or pre orm

y Amount ofcharge must be precisely controlled to obtain

Compression molding for thermosetting plastics:

(1) charge is loaded, (2) and (3) charge is compressed and cured, and (4) part isejected and removed

T f M ldi


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Transfer Molding

cavity, where it is heated; pressure is then applied to force

soft ol mer to flow into heated mold where it cures

y

Two variants:y Pot transfer molding - charge is injected from a "pot" through a

vertical sprue channel into cavity

y Plunger transfer molding plunger injects charge from a heated


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(a) Pot transfer molding

c arge s oa e n o po ,

(2) softened polymer is pressed into mold cavity and cured, and

(3) part is ejected


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plunger transfer molding

(1) char e is loaded into ot,

(2) softened polymer is pressed into mold cavity and cured, and(3) part is ejected

Compression and Transfer Molding


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Compression and Transfer Molding

y In both processes, scrap is produced each cycle as leftover

material, called the cull

y The TS scrap cannot be recovered

y rans er mo ing is capa e o mo ing more intricate part

shapes than compression molding but not as intricate as

in ection moldin

y Transfer molding lends itself to molding with inserts, in

which a metal or ceramic insert is placed into cavity prior to

injection, and the plastic bonds to insert during molding

P l F


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Polymer Foam

- -cellular structure

y Most common ol mer foams: ol st rene St rofoam a

trademark), polyurethaney Other polymers: natural rubber ("foamed rubber") and

polyvinylchloride (PVC)

Characteristic Properties of a Foamed Polymer

y Low density

y High strength per unit weight

y

Good thermal insulationy Good energy absorbing qualities

T l f


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Two polymer foam structures

(a) c ose ce an ( ) open ce

10_polymer processing.pdf

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