Flow laminations

Lambert s cosine law Lambic beer Lamb modes Lambs Lamb wave Lamellae Lamepon Laminar flow Laminated fabrics Laminated glass... 

The purpose of this study is to develop a simple model which retains some of the features of the above complex process to predict the lay-up thickness as a function of time during the squeeze-flow lamination of circular prepreg lay-ups. The prepreg of interest is of the type commonly adopted in the board manufacturing industry. It is composed of two outer resin layers and a fabric core constructed of interlaced yarns oriented in two directions perpendicular to each other (Figure 1). The fabric core is treated as a porous slab characterized by a constant Darcy permeability coefficient (see k in Darcy s law% i.e.. Equation 2 below) which can be estimated from fabric parameters such as the yarn diameter and the pitch distances. The lay-up thickness predictions provided by this model have been found to be in reasonable agreement with experiment for the lamination of up to five epo.xy prepreg layers. 

The characteristic feature of flow laminations is the directional orientation of the anisometric particles in the shearing speed gradient of the auger channel, the pressure head and the die. Figure 3 pictures the fracture surfaces of extruded clay-body columns after drying showing various flow laminations caused by different clay mineral content. 

Fig. 3 Flow laminations in cross-section of clay-body columns...

As certain properties of the crystalline components depend on the crystallographic axes, directional differences in the property values caused by the flow lamination can be determined at the extruded items. This holds true for example for the drying and firing shrinkage, the linear thermal coefficient of expansion, the thermal conductivity and the mechanical strength. 

Row laminations do, of course, also occur when using piston extrusion presses as the ceramic body is subjected to shearing stress in the pressure head and die. It follows that flow laminations are primarily linked to the nature of the material particles. There would be no flow laminations with... 

Flow laminations, laminations caused by inner gliding surfaces, and cutting laminations, all can be distinguished to varying degrees in the extrusion of plastic bodies, independent of the extrusion method employed, e.g. piston extrusion presses, single-shaft extruders, twin-shaft extrusion machines or rotory-type extruders. 

The work commissioned by the British Ceramic Research Association, carried out and published in 1962 and 1963 by Hodgkinson (6) was both scientifically and didactically pioneering, as can be seen by the accompanying instructional film, contributing then as now to a better understanding of flow laminations. 

Hodgkinson s work made and still makes many debates obsolete certain assumptions such as focusing on the extruder as the notorious creator of laminations had to be reconsidered and many patent cures documented in the literature revealed themselves on the basis of this work to be of no general use, only offering a suitable solution in individual cases. Hodgkinson was able to prove with the use of a highly sophisticated test device, which was ahead of its time, that the formation of flow laminations within... 

The so-called S-crack or Y-crack is typical for a screw extruder with a double-blade or triple-blade point auger. This type of lamination is a combined consequence of overlapping of flow laminations, the rotational movements of the body, as well as the hollow space created by the auger hub in the centre of the emerging body clog , which will not entirely rejoin as a result of body characteristics or the geometry of the pressure head and die, all as described above. 

The same holds true in relation to the high percentage of fibres as used in advanced ceramics products which, partly as a result of their structure, intensify the alignment by forming flow laminations and creating gliding laminations. 

Stratifiedflow a clear interface separates the two flows (lamination) and mixing occurs entirely by diffusion Vortex flow, the vortex creates secondary surface area Engulfmentflow the axial symmetry of the flow breaks up (see Section 4.2). 

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