Melt fracture

Melt fracture is a severe distortion of the extrudate, which can take many different forms spiraling, bambooing, regular ripple, random fracture, etc. see Fig. 7.118. 

It is not a surface defect like shark skin, but is associated with the entire body of the molten extrudate. However, many workers do not distinguish between shark skin and melt fracture, but lump all these flow instabilities together under the term melt fracture. There is a large amount of literature on the subject of melt fracture (e.g., [152-164]). Despite the large number of studies on melt fracture, there is no clear 

However, there is relatively uniform agreement that melt fracture is triggered when a critical wall shear stress Is exceeded in the die. This critical stress is in the order of 0.1 to 0.4 MPa (15 to 60 psi). A number of mechanisms have been proposed to explain melt fracture. Some of the more popular ones are  

More recent work by Utracki and Gendron on pressure oscillations in extrusion of poiyethyienes [231] led them to conclude that the pressure oscillation does not seem to be related to elasticity or slip. They conclude that the parameter responsible for pressure oscillations is the critical strain (Hencky) value of the melt. For LLDPE, Sc 3, for HDPE, s 2, while for LDPE, s, 3.5. The instability seems to be based on the inability of the polymer melt to sustain levels of strain larger than the critical strain. 

Streamlining the flow channel geometry has been found to reduce the tendency for melt fracture in branched polymers. Increased temperatures, particularly at the wall of the die land, enable higher extrusion rates before melt fracture appears. The critical wall shear stress appears to be relatively independent of the die length, radius, and temperature. The critical stress seems to vary inversely with molecular weight, but seems to be independent of MWD. Certain polymers exhibit a superextrusion region, above the melt fracture range, where the extrudate is not distorted 

The problems discussed in this section are those observed in the him either visually or by a property measurement. In the case of visual problems (defects), it is good practice to maintain samples for extrusion personnel to refer to when judging the existence or severity of a problem and for training new personnel. For example, a notebook can be kept in the plant quality assurance area or supervisor s office that contains samples of him with gels, die lines, melt fracture, etc. When a problem exists with a measured him property, personnel can perform a standardized test and compare the results to documented specihcations for that product. 

Melt fracture is an aesthetic defect that appears as roughness on the him surface. It has names such as orange peel and sharkskin. It may also appear as wavy lines in the him. Most researchers identify the source of these problems as excessive shear stress on the melt as it passes through the die so reducing this stress can eliminate this type of defect. 

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Melt flow rate Melt-formed ceramics Melt fracture Melting temperature... 

Non-Newtonian Fluids Die Swell and Melt Fracture. Eor many fluids the Newtonian constitutive relation involving only a single, constant viscosity is inappHcable. Either stress depends in a more complex way on strain, or variables other than the instantaneous rate of strain must be taken into account. Such fluids are known coUectively as non-Newtonian and are usually subdivided further on the basis of behavior in simple shear flow. 

For primary insulation or cable jackets, high production rates are achieved by extmding a tube of resin with a larger internal diameter than the base wke and a thicker wall than the final insulation. The tube is then drawn down to the desked size. An operating temperature of 315—400°C is preferred, depending on holdup time. The surface roughness caused by melt fracture determines the upper limit of production rates under specific extmsion conditions (76). Corrosion-resistant metals should be used for all parts of the extmsion equipment that come in contact with the molten polymer (77). 

Like other thermoplastics, they exhibit melt fracture (32) above certain critical shear rates. In extmsion, many variables control product quaUty and performance (33). 

Extrusion. Like other thermoplastics. Teflon PEA resin exhibits melt fracture above certain critical shear rates. Eor example, samples at 372°C and 5-kg load show the following behavior ... 

Injection Molding. Any standard design plunger or reciprocating screw injection machine can be used for PEA 340, although a reciprocating screw machine is preferred (32). Slow injection into mold cavities avoids surface or internal melt fracture, and control of ram speed is important at low... 

At a holdup time longer than 10—15 min at a high temperature, resin degradation is avoided by keeping the rear of the cylinder at a lower temperature than the front. At short holdup times (4—5 min), cylinder temperatures are the same in rear and front. If melt fracture occurs, the injection rate is reduced pressures are in the range of 20.6—55.1 MPa (3000—8000 psi). Low backpressure and screw rotation rates should be used. 

Whilst the origin of such turbulence (melt fracture) remains a subject of debate it does appear to be associated with the periodic relief of built-up elastic stresses by slippage effects at or near polymer-metal interfaces. 

The narrow molecular weight distribution means that the melts are more Newtonian (see Section 8.2.5) and therefore have a higher melt viscosity at high shear rates than a more pseudoplastic material of similar molecular dimensions. In turn this may require more powerful extruders. They are also more subject to melt irregularities such as sharkskin and melt fracture. This is one of the factors that has led to current interest in metallocene-polymerised polypropylenes with a bimodal molecular weight distribution. 

Injection moulding and extrusion may be carried out at temperatures in the range of 300-380°C. The polymer has a high melt viscosity and melt fracture occurs at a lower shear rate (about 10 s ) than with low-density polyethylene (about 10 s ) or nylon 66 (about 10 s ). Extruders should thus be designed to operate at low shear rates whilst large runners and gates are employed in injection moulds. 

Example 4.4 A blow moulding die has an outside diameter of 30 mm and an inside diameter of 27 mm. The parison is inflated with a pressure of 0.4 MN/m to produce a plastic bottle of diameter 50 mm. If the extrusion rate used causes a thickness swelling ratio of 2, estimate the wall thickness of the bottle. Comment on the suitability of the production conditions if melt fracture occurs at a stress of 6 MN/m. ... 

Since this is less than the melt fracture stress (6 MN/m ) these production conditions would be suitable. These are more worked examples on extrusion blow moulding towards the end of Chapter 5. 

To calculate the inflation pressure it is necessary to get the melt fracture stress. From Fig. 5.3 it may be seen that this is 4 x 10 N/m ... 

So the suggested pressure would cause melt fracture. 

Narrow molecular weight distribution, which is characteristic of metallocene-based polyethylene (Fig. 7), causes processing difficulty in certain applications due to increased melt pressure, reduced melt strength, and melt fracture [14,15]. This problem can be overcome by blending the metallocene polymer with other prod-... 

EPDM-ZnO-stearic acid systems could not be extruded even at 190°C. This is not unexpected since the material, in the absence of zinc stearate, shows no transition from the rubbery state to the viscous flow state (Fig. 1). In the presence of 10 phr of zinc stearate, the m-EPDM-ZnO-stearic acid system could be extruded but melt fracture occurred at a lower temperature (150°C) at all shear rates. At 160°C and 170°C, however, the extrudates showed melt fracture only at high shear conditions. At 20 phr loading of zinc stearate, melt fracture of the extrudate occurred at high shear conditions at 150°C, but at higher temperatures no melt fracture occurred and the extrusion was smooth under all shear conditions. At 30 and 40 phr loadings of zinc stearate, the extrudates were smooth under all shear conditions at all temperatures. 

Melt fracture of the extrudate was studied using M-45 wild photoautomat. Blend morphology was studied by SEM after differential solvent swelling. 

Disclosed is a crossUnked ethylenic polymer foam structure of an ethylenic polymer material of a crosslinked, substantially linear ethylenic polymer. The ethylenic polymer in an uncrossUnked state has (a) a melt flow ratio greater than or equal to 5.63 (b) a molecular weight distribution defined by a given equation and (c) a critical shear rate at onset of surface melt fracture of at least 50% greater than the critical shear rate at the onset of surface melt fracture of a linear ethylenic polymer having about the same melt flow ratio and molecular weight distribution. Further disclosed is a process for making the above foam structure. 

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