Flow Defect

Flow defects, especially as they affect the appearance of a product, play an important role in many processes. Flow defects are not always undesirable. 

A-B Viscoelasticity with slow deformation B Load removed B-C Viscoelastic recovery 

A-B Viscoelastic deformation (or creep) gradually occurs with sustained load. 

B-C Instantaneous elastic recovery occurs when load is renrwved. 

C-D Viscoelastic recovery gradually occurs where no permanent deformation (O ) o vith a permanent deformation (D -D ). Any permanent deformation Is related to type plastic, amount rate of loading and fabricating procedure. 

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When a molten plastic is forced through a die it is found that under certain conditions there will be defects in the extrudate. In the worst case this will take the form of gross distortion of the extrudate but it can be as slight as a dullness of the surface. In most cases flow defects are to be avoided since they affect the quality of the output and the efficiency of the processing operation. However, in some cases if the flow anomaly can be controlled and reproduced, it can be used to advantage - for example, in the production of sheets with matt surface finish. Flow defects result from a combination of melt flow properties, die design and processing conditions but the exact causes and mechanisms are not completely understood. The two most common defects are... 

Deformation contributes significantly to process-flow defects. Melts with only small deformation have proportional stress-strain behavior. As the stress on a melt is increased, the recoverable strain tends to reach a limiting value. It is in the high stress range, near the elastic limit, that processes operate. 

If the transmittance is > 90%, the material is called transparent, for lower values it is called translucent. Translucent bodies show loss of contrast and loss of detail. Haze or milkiness is defined as that fraction of the transmitted light that deviates from the transmitted beam by more than It may be caused by flow defects during processing... 

The major part of the scattered light originates at surface irregularities and other imperfections, such as scratches. Also in this case the dominant rimpling mechanism may be caused by flow defects (and their after-effects) and by crystallisation. Bennett and Porteus (1961) investigated the effect of surface roughness on the reflectance. This effect may be quantified by the equation... 

On the contrary, in the second binodal region (b) in Fig. 20.1-7, where the local mixing point finds the polymer-rich solution as the continuous phase, dispersed spharaids of nesrly polymer-free fluid are nucleated. These do meins then coalesce to p red nee a foam structure whose walls are composed of the solidified dispersed polymer phase. To obtain an open-cell foam with low resists ace to flow, defects clearly must occur in the waits of (be cells.39 Soch a structure is shown in Fig. 20.1-66. Tbe dense film on the surface can be promoted by a brief exposure of the cast or apan nascent membrane to air to obtain a more concentrated region at the surfhee prior to immersion in the nonsolvent precipitation hath, which then sets the dease layer in place and proceeds to nucleate the subetracture as described above, This evaporation step, however, is not required in all cases to produce acceptuble skins.56,65,66... 

Also associated with excessively hot processing parameters is the root-flow defect. This results from too much material flowing into the zone IV region and excessive penetration of the... 

Sharkskin occurs at a lower shear rate than melt fracture, but the term melt fracture is often applied to all regular flow defects including sharkskin. Sharkskin occurs at a critical linear extrusion speed that can be raised by increasing melt temperature. Melt fracture starts at a critical shear stress and can be reduced significantly by reducing the die inlet angle, but, like sharkskin, it is also reduced by raising melt temperature. Linear low-density polyethylene is particularly prone to these defects, but they are minimized by the addition of special additives or blends with other polymers. 

Surface defects in elastomer structural components include flow defects, air inclusions, specks in the material and many others (see Sect. Duroplastics ). 

As a melt is subjected to a fixed stress (or strain), the deformation vs. time curve will show an initial rapid deformation followed by a continuous flow (Fig. 1-6). The relative importance of elasticity (deformation) and viscosity (flow) depends on the time scale of the deformation. For a short time, elasticity dominates over a long time, the flow becomes purely viscous. This behavior influences processes when a part is annealed, it will change its shape or, with post-extrusion (Chapter 5), swelling occurs. Deformation contributes significantly to process flow defects. Melts with small deformation have proportional stress-strain behavior. As the stress on a melt is increased, the recoverable strain tends to reach a limiting value. It is in the high-stress range, near the elastic limit, that processes operate. 

Two developments have been reported (by Wacker Chemie) supplementing its range of foam-control systems Silicone Antifoam Emulsions SE 84 and SE 85. Based on organically modified silicone fluids, they reduce or prevent flow defects in polymer films. 

Harrison PJ, Newton JM, Rowe RC. Flow defects in wet powder masses. J Pharm Pharmacol 1984 37 81-83. 

Flow defects, especially as they affect the appearance of a product, play an important role in many processes. Flow defects are not always undesirable, as, for example, in producing a matt finish. Six important types of defects can be identified, which are applied here to extrusion because of its relative simplicity. These flow analyses can be related to other processes and even to the complex flow of injection molding. 

With the growing importance of plastics, more and more industrial processes involve extrusion of polymer melts through dies of complex cross section to form the final product. The shape of these dies and the pressure losses through them are important factors in preventing flow defects and controlling the quality of the end product. Attempts at predictions of extrusion pressure losses in dies of complex cross-sectional geometries are those of Lenk [45-47], White and Huang [48], Tiu [49], and Shenoy and Saini [50]. 

A number of the defects described previously involve flow driven by surface tension, but there also are flow defects where surface tension has little or no involvement. When a paint is applied, it is expected to flow out and level to produce a smooth film. Unfortunately, this does not always happen. Sometimes the viscosity is so high when the paint arrives on the part or increases so quickly after application that there is little flow and the result is a rough, bumpy surface... 

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