These are used in small factories for making hollow bricks and drain pipes. One of the best-known types (Fig. 564) consists of two iron boxes in which two pistons, moving in opposite directions, work, one compressing the clay while the other returns. Advantage is taken of this return to introduce the clay, which has been previously blended and prepared. If it contains no impurities such as roots, hard lumps, etc., the machine is double-actioned, that is to say, each end is furnished with a die and cutting-table; but if the clay contains impurities, it is better to sift it by passing it through a metallic sieve sufficiently fine to retain the foreign substances. This is fixed to one of the boxes of the machine; the clay is caused by the compression to pass through it, and is afterwards placed in the other box, whence it issues in the form of piping. The machine in this case becomes a single-action one.
I. Machines expressing horizontally 2. Machines expressing vertically. They may be worked by hand or power.
Fig. 564. Piston Expression Machine, worked by Hand (Whitehead).
In the Whitehead machine, the compression of the clay is always effected by means of a piston worked by cog-wheels (Fig. 565), which are so arranged that the return, that is the waste movement is made at great speed. As there is no lid to be raised before introducing the clay, the production is increased. The machine is filled during the return motion of the piston.
Fig. 565. Piston Expression Machine, worked by Steam (Whitehead).
The diameter of the pipe which can be made with this machine does not exceed 6 inches, but by means of what is called an expansion mouthpiece (Figs. 566, 567), which is fitted to the box, we may increase the diameter to a foot
Fig. 566. Pipe cut and raised (Whitehead).
Fig. 567. Pipe issuing from the Die (Whitehead).
The cutting-table is provided with a cradle of the same curvature as the pipe. The frame E carries the cutting-wires, and the swinging shelf B receives the pipe.
The preceding machines only produce smooth pipes. To form the socket, we use a table provided with a metal mould, which consists of two parts turning on a hinge and fixed together by a bolt. At one end of this mould, a movable mandrel of the same diameter as the socket is moved horizontally by a system of levers (Fig. 569).
Fig. 568. Pipe before.
Fig. 569. Apparatus (or forming the Socket of Pipes (Whitehead).
Fig. 570. Pipe after.
The smooth pipe (Fig. 568), having a mandrel inside it, is placed in the mould, which is closed over it; the lever in front of the table is then moved, the movable mandrel is forced into the pipe, and the socket is formed (Fig. 570).
The expression machines used for making bricks, if provided with suitable dies, may be used for pipes of small diameter. As in the case of hollow bricks, it is better to choose machines of small or medium production, in preference to large ones which produce more than is necessary for hollow articles.
Fig. 572. Vertical Expression Machine, worked by Hand (Boulet).