Channel die compression

Fig. 9.5 A sketch of a channel-die compression device used in producing highly textured semi-crystalline polymers in plane-strain compression.

Highly textured samples of HDPE and Nylon-6 obtained by channel-die compression discussed in Sections 9.3.3 and 9.3.4 having QSC perfection in bars of relatively macroscopic size, as has been established by Young and Bowden... 

The book concentrates heavily on research conducted at the Massachusetts Institute of Technology from the mid 1980s to the mid 2000s by the author and a group of collaborators. It reports on extensive experimental studies and related computational simulations. In the latter there is much emphasis on development of mechanistic models ranging from unit plastic relaxation events to the evolution of deformation textures in channel die compression flow to large plastic strains. At every level the experimental results are compared in detail with predictions from the models. 

It was shown that for most crystalline polymers, including polypropylene and other polyolefins, the tensile drawing proceeds at a much lower stress than kinematically similar channel die compression [10,17]. Lower stress in tension was always associated with cavitation of the material. Usually a cavitating polymer is characterized by larger and more perfect lamellar crystals and cavities are formed in the amorphous phase before plastic yielding of crystals. If the lamellar crystals are thin and defected then the critical shear stress for crystal plastic deformation is resolved at a stress lower than the stress needed for cavitation. Then voiding is not activated. An example of such behavior is low density polyethylene [10]. 

Fig, 19. (a) Sketch of the channel-die apparatus used for the deformation experiment. Dimensions are in millimetres. The compression stamp is moved along the deformation direction D. The flow of the sample is constrained by the rigid walls of the die in the direction C, and free flow is possible in the direction F. (b) Stress (cr)-strain(e) diagram resulting from channel-die extrusion of bisphenol-A polycarbonate at 300 K and a strain rate of e = 0.01 s l. (c, d) Dipolar DECODER spectra of 13C-labelled bisphenol-A polycarbonate before and after deformation. The spectra exhibit a characteristic star-like ridge pattern. Each of three types of corners (C, D, F) in the pattern corresponds to vectors oriented along a particular direction in the channel-die used for the experiment, (e, f) The anisotropy caused by the deformation becomes readily visible in the difference spectrum (deformed minus undeformed). For clarity, the negative (f) and positive contours (e) have been drawn separately. (Reproduced from Utz et al. with permission.)... 

Fig. 9.7 Small-angle X-ray scattering (SAXS) patterns of HDPE after several levels of plane-strain compression in a channel die (a) unstrained, (b) after = 0.9, (c) after Ce = 1.14, and (d) after e = 1-86. A new long period of restructuring occurs after Ce = 1.14 (from Gal ski et al. (1992) courtesy of the ACS).

The deformation, starting from an initial compression-molded rectangular plate, equilibrated by annealing at 170 °C in vacuum, was performed in an environment of relative humidity 60% at 20 °C to large plastic strains in a channel die similar to the one described for the work on HDPE in Section 9.3.3. The end result of the plane-strain compression history of the Nylon at a CR of 4.0 (se = 1.39), with a similar complement of intermediate-structure probes of TEM, WAXS, and SAXS, was a texture of similar perfection to that of HDPE, with orthotropic symmetry, but incorporating a dual symmetrical set of intermixed monoclinic components of indeterminable scales and form of special aggregation, as depicted in Fig. 9.14. As with the HDPE, the principal direction of molecular alignment... 

Lamellar orientation in thin films of a model diblock copolymer with symmetric poly(styrene)- -PLLA (PS-PLLA) was investigated by Chen et al. [62] in the molten state on silicon wafer supported surfaces. Stretching and compression were apt to induce orientation of PLA. Pluta and Galeski [63] studied the plastic deformation of amorphous and thermally noncrystallizable 70/30 PLA/PDLLA induced by plane strain compression in a channel die. The results revealed that plastic deformation transformed an amorphous PLA or PDLLA (thermally noncrystallizable) into a crystalline fibrillar texture oriented in the flow direction. 

Materials and Orientation. The HDPE samples used in this study were obtained by (a) compression in a channel die 15, 16) and (b) by orienting a slider 17) in friction transfer 18). PTFE surfaces were unidirectionally oriented by sliding a block of... 

Plane strain compression hi a channel die is kinematically very similar to drawing the sample is extended and its cross-section decreases accorduigly. However, the possibility of void formation is limited due to the compressive component of stress. It means that the differences in true stress-true strain dependencies of drawn and plane strained polymers should be attributed to the formation and development of cavities. Slopes of the elastic region of true stress-true strain curves are similar in tension and in plane strain compression. The difference in mechanical properties of polymers sets in at yielding in tension. The scale of difference depends on the particular polymer the yield in drawing for POM, PA 6, PP and HDPE takes place at a much lower stress than in plane strain compression. For polymers with low crystal plastic resistance, such as LDPEs and ethylene-octene copolymer (EOC), the stresses at selected deformation... 

The mechanisms associated with plastic deformation of crystalline polymers can be explained better on the basis of two examples of isotactic polypropylene subjected to drawing eind to plane strain compression in a channel die. The envisioned differences between defiarmation with and without cavitation are summarized in Figure 1.21. 

In the simplest and most often used form, the screw has a free channel cross-section that diminishes at a steady rate from the feed to the delivery end. The ratio of the channel depths from feed to die region along the screw is usually referred to as the compression ratio, since it gives a crude indication of the relative conveying capacities at feed and discharge. 

The mechanical stability of PSM and AMM-5 samples was investigated by pressing the sample in a die (having a diameter of 16 mm) under different pressures for 15 min. The effects of compression on the surface areas and pore properties of the materials are shown in Table 1. It can be seen that the surface areas of both PSM and AMM-5 samples decrease under high pressure compression. The decrease of surface area, which is proportional to the pressure exerted on the samples, is accompanied with the decrease of pore volume, with no obvious decrease of the pore diameter for both samples. The results indicate that, under high pressure compression, some of the mesoporous channels of MCM-41 have collapsed completely and not constricted to pores of smaller diameter. 

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