Extrudate shrinkage

In this section the phenomenon is descrihed as extrudate swell. Althongh the practical problem is nsnally with componnds, the swell of both gum rubbers and compounds will be discussed. 

Between gum rubber and compound there are number of differences in the extrusion mechanisms . The gum rubber usually fractures in extrusion but with compounds there is no melt fracture or very slight, when it occurs. The extrudate swell of gum rubber is very large but it is considerably reduced in the presence of filler. 

With the plastic melt it was shown that the extrudate swell is a recovery of memory, which was introduced by the deformation of the melt at the entrance to the capillary rheometer. As the melt flows through a capillary, the memory fades by relaxation. Therefore, the shorter the residence time in the capillary, the less memory is lost. This results in the higher swell at the higher shear rate, where the residence time is shorter. Also, the swell decreases with a longer capillary, because the residence time is longer. 

Reprinted from N. NakaJIma and E. A. Collins, Polymer Engineering and Science, 1974, 14, 2, 137. Copyright 1974, Society of Plastics Engineering. 

Only for the lower MW rubber. A, at low shear rate, did the swell increase with increasing shear rate. This shear-rate region of sample A is in the flow state and therefore, the behaviour is similar to that of plastics melt. However, at the higher shear rate, and particularly after the melt fracture the swell decreased significantly. The extrudate going over maximum swell is not generally recognised. 

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NDB should be, as observed, more pronounced at small 0 2 extrudate shrinkage was also reported (24, 38). Blends with were extruded, then placed In an oil bath at 190°C for about one hour. The shrinkage was calculated as... 

Figure 2. Extrudate shrinkage (In Hencky strain units) vs. composition of polypropylene/low density polyethylene blends. (Adapted from ref. 38.)...

Smoothness Dimensional stability Extrudate shrinkage Die swell Extrusion rate Pinholes Scorch... 

Dried granules may be injection moulded, blow moulded and extruded without undue difficulty with melt temperatures of 220-260°C. The moulding shrinkage (0.003-0.004 cm/cm) is very low and a somewhat greater than normal draft may be required in deep draw moulds. 

Computer-aided flow-simulation programs are also available for dies. All the programs can successfully predict a certain amount of shrinkage under specific conditions that can be applied to experience. The actual shrinkage is finally determined after molding or extruding the products. When not in spec process control changes can meet the requirements unless some drastic error had been included in the analysis. 

ISO 14236 2000 Plastics pipes and fittings - Mechanical-joint compression fittings for use with polyethylene pressure pipes in water supply systems ISO 14616 1997 Plastics - Heatshrinkable films of polyethylene, ethylene copolymers and their mixtures - Determination of shrinkage stress and contraction stress ISO 14632 1998 Extruded sheets of polyethylene (PE-HD) - Requirements and test methods... 

If the pH is lowered to about 6.0 the SFV particle undergoes a dramatic decrease in diameter of about 70 A which is due to the contraction of the nucleocapsid (Sfiderlund et al, 1972 von Bonsdorff, 1973). The viral membrane apparently adheres to the nucleocapsid during the contraction, and excess membrane is extruded in the form of blebs. Interestingly, few spike proteins are seen on these blebs suggesting that they contain only lipid and that the spike proteins remain bound to the nucleocapsid during shrinkage. 

Recall that in the process of low-pressure moulding, thermoplastic polymers (polyolefins, polyamides, or their compositions) are loaded into a cylinder of the adiabatic extruder, plasticized and injected into a mould at low pressure. The formed article is cooled in the mould and removed due to shrinkage phenomena. 

Orlando, FL, 7th-llth May, 2000, paper 67 PREDICTING HOW THE COOLING AND RESULTING SHRINKAGE OF PLASTICS AFFECT THE SHAPE AND STRAIGHTNESS OF EXTRUDED PROFILES Brown R J... 

Extruded compounded pellets were converted into strips (1 in. X 0.05 in.) using a % in. Brabender laboratory extruder. ASTM-D 638 Type I 0.05 0.005 in. tensile specimens were tensile cut from the extruded strips. Extrusion conditions used were typical for this class of materials. Extrusion take-off rates were controlled to minimize orientation in the extruded strips. Annealing experiments showed shrinkage to be approximately 2% in the longitudinal direction. 

The expansion at the die is not the sole determinant of final product specific volume. Guy and Horne (1998) also report the extent of shrinkage on cooling of extruded maize grits (Figure 18.7). 

Figure 18.7. Expansion and shrinkage for products extruded as in Figure 18.5, diameters of extrudates when hot O and after cooling (a) 4 mm dies (b) 3 mm dies. (From Guy and Home 1988 with permission.)...

As described previously, the process, especially if long dies are used in the extrusion process will result in compaction of the system. If the spheronization of the extrudate follows that described earlier, there will be further consolidation of the ma.ss. In fact. Chapman (52) used evidence of an increase in density of the pellets as part of his proof of the mechanism of spheronization. The liquid content of formulations is quite high, often 35-40%, hence the rate of removal of this and the potential influence of shrinkage can all influence the final structure. In some formulations, not all the liquid is removed from the pellets, which again can influence the final density. The density of the materials incorporated into the formulation will also influence the density of the pellets. Hence, it is not always possible to predict the final density of the pellets. Because of the influence of the material density, it is preferable to express the property as porosity."... 

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