1 Background and problem description
Large-format, porosity-enhanced, vertically perforated clay bricks with high core-hole fractions and filigreed webs call for the use of materials with good plasticity. That, however, means that the freshly extruded brick green bodies will display diminished strength and be in danger of deforming and suffering structural damage on cutting. Moreover, their compositions are usually laced with fibrous porosity enhancers like sawdust or paper sludge that cause “smearing” of the filigreed core-hole structure due to cutting. Indeed, cutting imposes substantial loads on the highly perforated bricks – loads that can cause considerable deformation.
To a greater or lesser extent, both phenomena tend to close off the core holes. This impedes the kind of energy-conserving through-air drying that is becoming increasingly popular in modern masonry brickworks. Subsequent grinding of the fired bricks, in turn, presupposes tight dimensional tolerances in order to minimize the required amount of grinding.
For reasons of energy economy, producers try to use as little batching water as possible for extruding facing bricks. Thus, either less energy has to be expended for evaporating the water, or the freshly extruded brick green bodies can be loaded onto the kiln cars by the “direct setting method”. The force required for cutting stiff-extruded clay columns, however, imposes bottom limits on the reduction of batching water volumes.
The mechanisms of action involved in the use of knives as cutting tools were investigated with a view to reducing the applied cutting force. The overlapping effects of vertical and horizontal motion greatly reduce the effective cutting angle (with respect to the workpiece) as compared to the actual blade angle. The flatter the effective cutting angle, the more easily the blade penetrates the workpiece.
A similar situation applies to the cutting wire. A wire pressing vertically downward through the wet green body squeezes the latter with its full, circular cross section. Superimposing horizontal motion to the wire during the cutting process effectively gives the wire the nature of an elliptical cutter plunging its narrow side into the green brick. The faster the speed in the wire‘s direction of motion, the narrower the cutting angle and, hence, the easier the wire is able to penetrate into the green body to be cut.
If the blade (wire) is not always pulled in the same direction, but made to swing back and forth, it achieves exactly that effect. The limited length of knives and saws makes this practically self-evident.
If a cutter is made to change direction very quickly, the effect is that of a longitudinally vibrating wire. The following course of events can be anticipated for a high vibration frequency: Thanks to its horizontal motional component, the wire not only penetrates more easily, it even induces local fluidization via vibrating pressure at the cutting point and therefore leaves behind a particularly smooth, integumental film on the sliced face.
The implemented trials showed consistently positive results for the use of a vibrating cutting wire. Braided cutting wires, with their elevated inherent elasticity, were successfully employed as a means of preventing wire breakage (caused by operation at resonant frequency). With each increase in cutting speed, the frequency of vibration must be altered accordingly in order to maintain the positive “saw effect”. The positive results are exemplified by the cross-sectional photo of a vertically perforated clay brick in »1.
This project of the Research Alliance of the Clay Brick and Tile Industry Regd (FGZ) was funded under the project number AiF 16074 N by the BMWi through the German Federation of Industrial Research Associations “Otto von Guericke” Regd (AiF). and implemented by Brick Research Institute Essen Regd (IZF). The final report is 65 pages long and available for a handling fee from the Research Alliance of the Clay Brick and Tile Industry Regd in Berlin.