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MMS330S, Spring 2001, Dr. S. Doroudiani 1

Forming Polymers

• Introduction

• Flow properties of polymers

• Solidification of polymers

• Extrusion

• Injection moulding

• Blow moulding

• Other moulding processes

MMS330S, Spring 2001, Dr. S.

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Introduction

Most polymers forming processes consist of the following

main steps and operations:

• Heating the polymer into the molten state (or dissolving

it in a solvent);

• pumping the melt (or solution) to the forming unit;

• forming the melt (or solution) into the required shape;

• fixing the shape and solidification by cooling the melt

(or evaporating the solvent)


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In polymers processing operations some polymer

properties are of interest and some vital points should

be examined:

• Flow properties and behaviour of the liquid polymer,

like: viscosity, strength and their dependence on

temperature, pressure and mechanical conditions;

• Factors controlling the cooling rate, which is often the

rate-determining step in polymers processing;

• Effects of processing parameters on the

microstructure of the final product: residual stress-

strain, crystallinity and morphology.


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Flow Properties of Polymer Melts

In polymers processing three different behaviours are

important:

• Bulk deformation;

• Elongational flow

• Shear flow

– Flow in a capilary

– Newtonian vs. non-Newtonian flow

– Melt flow index (melt flow rate)

– Parameters affecting viscosity

– The viscoelasticity effects


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Bulk deformation

Bulk modulus is inverse of compressibility:

K = (V/V) / P = compressibility-1

Bulk modulus of liquid polymers is about 1 GPa and

independent on RMM. It plays an important role in polymer

processing, particularly in moulding.

Example: change in volume of a polymer melt at ~1000 atm.:

V/V = P / K = (1000 * 1.01 * 105)/109 = 0.101 = 10%

means, at 1000 atm pressure the volume decreases about

10%, which raises shear viscosity and fills 10% more

material in the mould.


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Elongational flow • Elongational flow occurs when the fluid is

under tensile force. It is important in

polymer processing: film processing,

fibre spinning, blow moulding, vacuum

forming. The fluid resists against tensile

force, which results a kind of viscosity,

called “elongational viscosity”. By

balancing forces and applying continuity

equation on a fibre of liquid polymer

pulled continuously elongational

viscosity can be derived, as follows:


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Effect of Polymer Structure on

Elongational Viscosity ()

depends on stress, especially at higher stress: non-Newtonian.

Example:

– increases with stress for BPE (tension-stiffen)

– decreases with stress for LPE (tension-thinning)


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• Depending on the flow behaviour of the polymer fluid, it can be

drawn without necking:

– Tension-thinning liquids lead to unstable neck;

– Tension-stiffening liquids lead to stable neck, which is critical for a

successful processing (I.e., fibre spinning, film blowing).

• Consequences of elongational flow:

elongational stress in the liquid polymer increases as the

processing speed increases, so it is an important factor

controlling speed of processing. Elongational flow at high speed

spinning leads to failure of product or a distorted product.

• In reality, elongational effects usually present with shear effects,

so their effects should be considered together.


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Shear flow

Polymer liquids behave non-

Newtonian and have high shear

viscosity.

Pseudoplasticity (shear thinning):

decreases as shear stress

increases. This is an advantage

for processing.

• Measurement of shear viscosity:

by capillary viscometer

(rheometer).


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Flow in a Capillary


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Newtonian vs. non-Newtonian Flow

• In a Newtonian fluid, viscosity is

independent of the stress.

Velocity distribution in a laminar flow in a

circular pipe:

Volume flow through the whole cross section:

• In a non-Newtonian fluid, viscosity depends

on the stress.

Power law model:

for shear-thinning polymer fluids: n < 1


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Melt Flow Index (Melt Flow Rate)

(MFI or MFR)

MFI or MFR is often used to characterize a

polymer melt. The flow rate of polymer

melt is measured using an exrusion

plastomer, according to standard

procedure described in the ASTM D1238.

MFI is obtained by measuring the mass of

extrudates which has flowed through the

orifice in a certain time. Greater MFI is

obtained from polymers with lower

viscosity.

There are several (13) test conditions with T

in the range of 125-275°C and pressure in

the range of 0.045-3.0 MPa for different

polymers.


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Parameters Affecting Viscosity • Temperature: strongly influences

apparent viscosity. The shear thinning characteristics of the curves are the same and the curves are shifted vertically. The Arrhenius equation is a good fit: = A exp (E/RT)

where A is a constant and E is activation energy for viscous flow

• RMM: is the most effective parameter affecting viscosity. In log-log plot of vs. Mw, beyond a critical value (Mc) the viscosity rises more rapidly. 0= K MW

3.4-3.5

where 0 is the zero shear rate viscosity.

• Combined equation:


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Viscoelasticity Effects

• Die swell: occurs as a result of

elastic effect of polymer melts.

Dtotal = DHE + Dvisc

Dtotal, DHE and Dvisc are total, high elastic

and viscous flow deformations.

Preventing die swell: processing at

higher temperature, using lower

RMM polymer

• Orientation (in injection moulding)

• Anisotropy (in injection moulding)

• Warping (in injection moulding)


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Cooling and Solidification • Cooling and solidification are limiting the production rate in polymer

processing. Internal heat conduction is the limiting process.

• Polymers are poor conductors of heat:

Typical values of thermal conductivity (W / m K)

HDPE PA6.6 PMMA POM PP PTFE Copper

0.43 0.33 0.18 0.31 0.21 0.25 390

• Variation of temperature (T) within a polymer

with thermal concuctivity k, density and

specific heat capacity of cp:

T/ t =(1/ cp) (k T / z)/ z

k is relatively independent of temperature, so:

T/ t = 2T / z2 where = k / cp


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In polymers processing, solidification and

cooling is the rate determining step, so to

make the process competitive it is

necessary to evaluate the cooling time

and to make it shorter. Solving this

equation gives a good stimation of

cooling progress. Considering this, the

cooling time is found in the order of 10-

100 sec.

Thermal stresses are generated during

cooling, as a result of contraction. These

stresses may cause warp or even

formation of voids, so they should be

prevented as much as possible. This can

be achieved at the cost of longer

manufacturing time. A compromise

between these conflicting factors is a

must.


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Extrusion

Extrusion produces an endless product with constant cross section.

Products: fibres, monofilaments, pipe, tubing, film, sheet, …

• Main sections of an extruder:

– Feed hopper

– Barrel

– Screw

– Heater

– Perforated breaker plate and filter screen

– Die and calibration equipment


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Extruder Barrel

• Barrel must be extremely strong.

• Size: length-to-diameter ratio; L / D = 5 - 34

shorter extruders are used for elastomers and longer ones for

thermoplastics.

• Feeding: from “feed hopper” by gravity or a rotating helical shaft.

Feed throat is cooled by circulating water, to prevent blocking the

flow.

• Heating : by electrical heaters and controlled by thermocouples.

• Cooling: might be necessary because of additional heat generated

by working screw


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Extruder Screw

• Screw should:

– transport solid feedstock;

– compress and melt the solid polymer;

– homogenize, meter and pump the melt to the die.

• Aparent volume of the polymer decreases as it goes forward. The

screw channel cross sectional area must decreas gradually to keep

the mass flow along the screww constant.

• Compression ratio of screw: the ratio of the largest channel depth

(the first channel) to the smallest one (the last channel).


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Screw Design

Design of screw mainly depends on properties of the polymer

(morphology, …) and flow behaviour (rheology) of its melt.

Crystalline polymers (like, PA6.6) show sharp melting, while

amorphous polymers melt slowly.


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(A) a 3-zone screw, constant flight depth, a compression and a metering

zones;

(B) a 3-zone screw, with a venting section to evacuate gases;

(C) a typical screw design for amorphous polymers (PVC, PS);

(D) a typical screw design for crystalline polymers with sharp melting (nylon);


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Modification of Screw Design

• To improve the performance of the extruder and the homogenity

of the melt, screw design should be modified. This is particularly

important in filled polymers and polymer alloys.

• Addition of some mixing elements to screw enhances mixing. In

this way the melt is subjected to intensive shear stress, which

breaks down granules and agglomerates and improves melting

by distributive mixing.


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Twin Screw Extruders (TSE)

• The are two screws in TSE, which turn either in the same (co-

rotating) or opposite direction (counter-rotating). TSE is useful for

difficult mixing processes, like compounding and processing rigid

PVC.

• Co-rotating, intermeshing TSE is the most common system for

compounding polymers.


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Die and Calibration Equipment

• Die is shaping section at the end of the extruder. There are a

breaker plate and/or filter screen between mould and screw to

break the rotational flow and direct flow along the axis and clean

the melt.

• Die swell: due to viscoelastic character of the melt. The size is

corrected by pulling the extrudate.


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Injection Moulding

Reciprocating screw injection moulding (RSIM) is the most popular

IM system. The duties of screw in RSIM are:

– Plasticizing the polymer;

– forming a metered volume of homogeneous melt;

– acting as a ram and injecting the melt into the mould.


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RSIM Cycle

(a) Screw has been pushed forward injecting the

melt into the mould. The screw remains forward

to keep melt pressure on the moulding as it

starts to cool and shrink. When the gates have

frozen, the screw starts to rotate, which pushes

melt toward the front of screw but it cannot

leave the barrel. Back pressure pushes screw

to the right.

(b) When sufficient melt has been plasticized for

the next shot, the screw stops rotating. During

the screw-back period the moulding will have

been cooling in the mould. When it is solid, the

mould opens and the part is ejected.

(c) The mould then closes and the screw pushes

forward to inject melt into the mould.

(d) The screw maintains pressure until the gates

freeze and then screw-back starts. The cycle is

repeated.


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Mould

• Comprises two halves: fixed and moving.

• The impression (cavity) is formed between two halves.

• Single impression mould: polymer melt flows from nozzle, passing

through sprue, fills the mould.

• Multi-impression mould: polymer melt flows from nozzle, passing

through sprue, runner and gate, fills the mould


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Mould Details


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Mould Shrinkage (Contraction)

- Stage A-B: T constant, P increases; leads to a major decrease in V.

- Stage B-C: P constant (at high), T decreases; leads to a major

decrease in V.

- Stage C-D: V constant (no further filling at C due to freezing over the

gate), T decreases; leads to decrease in P to atmospheric pressure.

- Stage D-E: P constant, T decreases; leads to a minor decrease in V.

1-vE/vA ~ 10% 1-vE/vD ~ 1%


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Mould Design Considerations

• Hot-runner mould: is a kind of modified mould in which the polymer melt is heated up to the gate. In cooling the mould, the gate freezes but the melt in the runners does not freeze.

• Gate: is important in mould design. It increases T and decreases , so enhances mould filling; improves control on the melt flow; insulating the mould from the barrel by freezing.

• Weld line (a source of weakness in the product) is a result of incorrect design.

• Product design vs. mould design


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Blow Moulding (a) Extrusion-blow moulding

(b) Injection-blow moulding

(c) Stretch-blow moulding


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Thermoforming


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Compression and Transfer

Moulding


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Transfer Moulding

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