Flow defects

11.2.6 Flow defects

The complex nature of possible flow defects under- lines the need for careful product design (sections, shapes, tools) and close control of raw materials and operational variables (temperatures, shear rates, cool- ing arrangements). The quality of processing makes a vital contribution to the engineering performance of a polymer.

Ideally, melt flow should be streamlined throughout Figure 11.9 Representation of stress relaxation under

the shaping process. If the entry angle of an extru- constant strain conditions (Maxwell model) .

sion die causes an abrupt change in flow direction, the melt assumes a natural angle as it converges upon the die entry and a relatively stagnant ‘dead zone’ is cre- ated at the back of the die. In this region, the melt

molecules to take place and stress will fall rapidly. will have a different thermal history. In addition to its dominant shear component, the convergent flow con- tains an extensional component that increases rapidly

rubbery behaviour, to zero at the start of viscous during convergence. If the extensional stress reaches a behaviour. A real polymer contains different lengths of

critical value, localized ‘melt fracture’ will occur at a molecules and therefore features a spectrum of relax-

frequency depending upon conditions. The fragments ation times. Nevertheless, although best suited to poly-

produced recover some of the extensional strain. The mers of low molecular mass, the Maxwell model offers

effect upon the emerging extrudate can range from a

a reasonable first approximation for melts. matt finish to gross helical distortions. The associated Let us now apply the relaxation time concept to

flow condition is often termed ‘non-laminar’ despite an injection-moulding process in which a thermo-

the fact that the calculated value of the dimensionless plastic acrylic at a temperature of 230 °

C is sheared

Reynolds number is very low. The choice of entry

angle for the die is crucial and depends partly upon (deformation) time is 2 s. For the shear rate given,

rapidly at a rate of 10 5 s 1 . Assume that the injection

the polymer.

Figure 11.8 indicates that the apparent shear viscosity As a melt passes through the die, velocity gradients is 9 Ns m 2 and the corresponding maximum shear

develop, with the melt near the die surface moving stress is 0.9 MN m 2 . At this shear stress, the elastic

slower than the central melt. Upon leaving the die,

the outer layers of extrudate accelerate, eliminating the µ s, which is very small compared

shear modulus for acrylic is 0.21 MN m 2 . The value

velocity gradient. Above a critical velocity, the resul- tant stresses rupture the surface to give a ‘sharkskin’

to the injection time of 2 s, hence viscous behaviour effect which can range in severity from a matt finish will predominate. A similar procedure can be applied

to regular ridging perpendicular to the extrusion direc- to deformation by extrusion. For instance, PP with a

tion. ‘Sharkskin’ is most likely when the polymer has relaxation time of 0.5 s might pass though the extru-

a high average molecular mass (i.e. highly viscous) sion die in 20 s. The time difference is smaller than

and a narrow molecular mass distribution (i.e. highly the previous example of injection-moulding, indicating

elastic); these factors cause surface stress to build up that although deformation is mainly viscous, elastic-

rapidly and to relax slowly. Fast extrusion at a low ity will play a greater part than in injection-moulding.

temperature favours this defect. Heating of the tip of The previously-mentioned phenomenon of die swell

the die lowers viscosity and reduces its likelihood. then becomes understandable. (Swelling is equivalent

An inhomogeneous melt will produce a non-uniform to the spring action in the Maxwell model.) Although

recovery of elastic strain at the cooling surface and the degree of elastic behaviour may be relatively

influence its final texture. Thorough mixing before small during injection-moulding and extrusion, it can,

shaping is essential. However, inhomogeneity may nevertheless, sometimes cause serious flow defects.

exist on a molecular scale. For instance, in both Relaxation times for extensional flow, as employed

injection-moulding and extrusion, a broad distribution in blow-moulding, can be derived from the ratio of

of molecular mass gives a more matt finish than a nar- apparent tensile viscosity to elastic tensile modulus

row distribution. Thus, extrusion of a polymer with R ⊳ . Suppose that a PP parison at a temper-

a narrow mass distribution at a rate slow enough to ature of 230 °

C hangs for 5 s before inflation with prevent the development of ‘sharkskin’ will favour a air. If the tensile viscosity and tensile modulus are

high-gloss finish.

Plastics and composites 361 Volatile constituents tend to vaporize, or ‘boil off’,

from the melt if processing temperatures are high. For instance, water vapour may derive from hygroscopic raw materials. Sometimes, a polymer degrades and releases a volatile monomer. Hydrostatic pressure usu- ally keeps the volatiles in solution; as the hot polymer leaves the die, this pressure is released and the escap- ing volatiles form internal bubbles and may pit the surface.

When a polymer is heated to the processing temper- ature, the weak intermolecular forces are readily over- come by thermal vibrations. Its density may decrease by as much as 25%. The subsequent cooling can produce shrinkage defects, particularly in crystalline polymers, which assume more closely packed con- formations than amorphous polymers, and in thick sections. Polymers have a low thermal conductivity and the hot core can contract to produce depressions or ‘sink marks’ in the surface. If the surface layer cools rapidly, its rigidity can encourage internal voids to form. Careful product design can minimize shrink- age problems. Cooling under pressure after injection- moulding is beneficial.

Orientation effects, which are common in shaped polymers, have a special significance when the poly- mer is ‘reinforced’ with short lengths of glass fibre. (It has been estimated that roughly half of the engineer- ing thermoplastics are fibre-filled.) In extruded pipes, fibres will tend to be aligned parallel to the extrusion direction and improve longitudinal strength. However, this orientation weakens the pipe transversely and the burst strength will suffer. In addition, the tubular form necessitates use of a ‘spider’ to support a central die, or mandrel, which causes the melt to split and coa- lesce before entering the die. The resultant weld lines introduce weakening interfaces. A die system is now available in which one or both dies are rotated. Their shearing action has the beneficial effect of inclining fibres at an angle to the extrusion direction. Weld lines are also reoriented so that they are placed in shear rather than tension when in service.