Dies are the orifices by which the clay escapes when compressed by the motion of the screws or cylinders. These dies are movable, and are fitted to the machines either by collar-bolts, or nuts and bolts, or any other means. As the shape of the orifice is variable, it may be seen at once that brick-machines working by expression can make bricks of any dimensions or shape. Let us consider how clay issuing from a die behaves. According to Schlickeysen (experiments made in 1856) -
1. Well prepared and ductile clay driven vertically through a rectangular orifice in sheet-iron with narrow walls, comes out without cohesion; the prism opens out at the four angles as far as the centre.
2. If the thickness of the walls is increased so as to make a cylindrical pipe, the prism acquires cohesion, but breaks perpendicularly at short intervals.
3. If the thickness of the walls, or the breadth of the pipe, is still more increased, the prism of clay then splits into two in a plane passing through the angle of pressure. This separation is due to a slackening of the speed of the exterior particles, due to their friction against the sides of the pipe. The two pieces curve outwards. If this effect is produced in consequence of the thickness which must be given to the face-plate, the influence of friction can be reduced by piercing conical holes from the outside to the inside, which diminishes the surface of friction.
4. If the piping is conical from inside to outside, so that the largest section is towards the machine, the cohesion of the prism is increased, but the surface is covered with flutings and the edges are jagged.
The effects above described are very variable according to the richness of the clay. With a paste of a proper degree of dampness and plasticity there is no tearing at the angles when it comes through the die. If the clay is too damp, or if it changes from less to more moist, the prism is deformed and increases in volume as it issues. Finally, thin clays require special dies called hydraulic.
To summarise, what must be avoided is the friction caused by the walls of the piping, especially at the angles, which tends to produce a slackening of speed of the outer molecules as compared with that of the inner ones, If this slackening occurs the prism of clay undergoes one of the above-mentioned deformations. This inconvenience is less to be feared with rich clays, which glide more easily; it is otherwise with thin clays, and for them hydraulic dies are required (Figs. 136, 137, 138). Inside a hydraulic die, on the four faces, is placed a pervious skin fitting closely to the prism of earth. This skin is surrounded by an empty space bounded by the sides of the die. This space is furnished with a pipe to which is attached a tap communicating with a water-conduit or reservoir frequently placed above the machine over the die (Figs. 120, 121, 124, 129). The water passes into the die, percolates through the skin, and lubricates the clay, thus facilitating its expression. The flow of water is regulated by the speed of the prism.
Figs. 130-138. Various Dies.
The shape of dies varies with the maker, as can be seen by an examination of the different machines we have described (Figs. 100 to 128); but in all of them arrangements are made to ensure that the prism shall issue with smooth faces and clear-cut edges. We give sketches of some dies for hollow bricks (Figs. 130, 132, 134), for solid bricks with water-face (Figs. 136, 137, 138) or without water-face (Figs. 131, 133, 135). When several thicknesses of brick are to be expressed, well stretched threads are placed in front of the die and divide the prism of clay (Figs. 130, 135). The tube of the die has the dimensions and shape (Figs. 133, 138) of the brick to be manufactured. In calculating these dimensions, allowance must be made for the contraction which the clay will undergo in drying and firing; 10 per cent, is usually allowed for semi-firm pastes.
Several cases present themselves according to the position in which the brick comes out.
1. The prism forms one brick.
A. Flat (Figs. 131, 132, 136), 0.105 x 0.06 - (x-contraction) (y-contraction).
B. Up on edge (Figs. 137, 138), 0.220 x 0.105 = (z- contraction) (x- contraction).
The dimensions of the die will then be found by x - (x/10) =.105; therefore, x=.116 y - (y/10) = .06; therefore, y = .066
S - (2/10) = 0.22. therefore, C = .244
2. The prism forms several bricks -
A. Flat (2 bricks): (o. 105 x 2 x 0.06) = (x1 - contraction) (y1 - contraction).
B. Up on edge (3 bricks) (Figs. 130, 135): (0.105 x0.06 x 3)-(x1 -contraction) (y1 -contraction).
The dimensions are therefore found by x1- (x/10) =.21; therefore, v1 = .233 y1 - (y1/10) =.06; therefore, y1 = .20
It must be noted that bricks being placed on edge in firing contract more in height than in the other two dimensions; thus, we must increase x but diminish y and especially z. For example, we shall take x = .120 instead of .116 y =.065 ,, ,, .066 z = .240 „ „ .244
Whenever it is possible to judge by experiments made with moulding clay, we should not fail to do so; they give the most accurate information.
These are divided into two classes according as they are worked by hand or automatic. In the first class we shall make a distinction between cutting machines with trolleys and those without, also between those cutting obliquely and those cutting perpendicularly.
1. Hand-cutters -
A. With movable threads
(2) With trolley
( a) Cutting ob-liquely.
(b) Cutting per-pendicularly.
B. With fixed threads
2. Automatic cutters.