Dipl.-Ing. (FH) WILLI BENDER, D-Mühlacker

Clay roofing tiles are one of the most successful structural ceramic products with a clearly marked upward trend. This is shown not least by the large number of clay roofing tile works which have recently been built all over the world, are still under construction or are at the planning stage.

Corresponding to the production process a distinction is made between pressed and extruded clay roofing tiles, in which the former assume greater importance. They make up about 65% of the production in Germany. In many countries in fact the share taken by pressed clay roofing tiles is even greater.

The production of pressed clay roofing tiles requires complex and exacting shaping technology which - more than with other types of product - is of key importance in the production process. For flat and accessory tiles different product-specific adapted shaping systems have thus been developed for the purpose. The following Market Survey is designed to give an overview of the various pressing systems currently on offer in the international market for the production of flat and accessory clay roofing tiles.

The pressing systems

The manufacturers who have taken part in this Market Survey are representative of the pressing systems given in Table 1, in which a distinction is made between flat tile presses and accessory tile presses. Certain systems - in modified form - are used as presses for both flat and accessory tiles. The individual systems are briefly presented in general form below.

This is the revolver press model which was standard up to 1958. Manual means that the placing of the bats and the removal of the green tiles are performed by hand. This type of press naturally no longer occupies a place in modern clay roofing tile works of industrialized countries but it still finds justification for its existence in developing countries and those at a transitional stage. For countries in which the production of interlocking tiles is still performed with simple hand lever presses and manually activated screw rod or spindle presses, as is still advertised in a brochure of the GATE (German Appropriate Technology Exchange), such a press in fact represents real progress. In the industrialized countries also people are increasingly aware that "high-tech" does not create the jobs which are needed to give work to the growing number of unemployed, but that above all simple technologies and those of medium complexity are required, such as is represented for example by this revolver press. It finds simple peripheral equipment adequate for its needs, is inexpensive to procure and maintains jobs.

The presses have a 5-face drum on which the moulds are set in a transverse arrangement. The angle setting of the drum surfaces to the horizontal level is 36°. This is particularly advantageous for the manual take-off of the green tiles, because the flatter the angle setting, the easier is the release of the tile from the mould and it falls without slipping or deformation onto the tile setter.

The 6-face drum has been developed for automatic operation, with large surfaces on which the moulds are no longer arranged transversely but on a linear layout. The angle setting of the drum faces here is therefore 60°. As well as the single-head press there are multiple head presses, in which, depending on the press model, 2, 3, 4 or 5 moulds are arranged next to each other on a single drum face and accordingly up to five tiles can be pressed per stroke.

The single-head revolver press, owing to its relatively low output, is suitable for special individual cases, e.g. for clay roofing tile models for which the model park should be kept to a minimum or otherwise the normal press operation would be disrupted, or for works with mixed production, at which clay roofing tiles represent only a byproduct. Among the multiple presses the 2- and 3-head presses are the most widely represented. The technical limit is reached with the 5-head press, and it is therefore no longer generally on offer. The 3- and 4-head presses, instead of having three or four top moulds for roofing tile models of normal format, can also be fitted with 2 or 3 top moulds for large-format models.

The pressure head with the top moulds carries out a lifting and lowering movement. It is of great importance here to adapt the speed of the pressure head during pressing and during the release of the green tile from the top mould to the proper-ties of the clay. This means minimum speed during pressing and an occasional pause of the pressure head in the bottom position. For the some output of the press this can only be achieved by an accelerated upward strike period which does not form part of the pressing. The revolver press is therefore designed as an eccentric press. First of all there was a joint lifting and pressing cam located in the middle. Later on lifting and pressing movements were carried out by separate eccentric cams. In the large

multiple presses these normally comprise two lifting stroke and two pressure cams. The cams are designed and manufactured for a specific pressing curve, in which considerable co-ordination is made between the working body, the tile model and the mould material. The cam can also, according to option, be designed with or without an intermediate stroke, the so-called ventilating stroke. In the specification of the cam the pressing characteristics are also rigidly specified and cannot be changed at short notice, i.e. a change in the pressing curve is only possible by means of a time-wasting conversion.

In these presses the required pressure at the pressure head is applied via hydraulic cylinders. They were developed in order to obtain a freely programmable pressing system with flexible pressing characteristics which permits rapid adaptation and co-ordination to the tile model, working body, mould material and number of cycles. The hydraulic system is infinitely variable by means of a computer-aided change in the valve setting. This means that for every tile model empirically determined pressing and production characteristics are stored and can be called up when needed. The setpoint and actual pressing curves can be indicated on a screen.

The use of the hydraulic revolver presses for flat tile production is only just beginning. Apart from the advantage of flexible pressing characteristics, compared with the mechanical revolver press there is considerably higher expenditure on the controls, the highly exacting maintenance and the higher installed capacity. Many producers are still therefore taking an attitude towards the system of "wait and see". The advantages however would lead one to expect that the hydraulic presses will become increasingly established.

The anvil press can likewise be designed as a single- or multiple-head press. It consists of a stationary pressing table, the anvil, on which the bottom moulds (1- to 5-fold next to each other) are permanently arranged, and two side standards with crossbeam in which the pressure head, which takes the top moulds is conducted. The pressure head, the punch, is connected with multiple toggle levers as the pressure transfer system, while the pressure generation can be mechanical or hydraulic.

The press unlike the revolver press, only executes the lifting and lowering movement of the plunger. The result is that the three process sequences: bat placing, pressing, green tile removal, which occur simultaneously next to each other in the revolver press, have to be carried out in succession on the anvil press. This concept requires a rapid, operationally reliable and fully automatic handling system for the placing of the bats and removal of the green tiles, normally with the use of suction plates.

The anvil press represents a pressing system which is particularly suitable for pressing with closed moulds, stiff pressing and high applied pressure.

Usually metal moulds are used with lubrication, in some cases with air ejection. The advantage of the anvil press is that it dispenses with the rotary drum, and thus achieves a considerable simplification of design. Because of the fixed, stationary pressing table, movements during the pressing process, which are possible with a drum, are excluded. The pressing curves can be adopted quickly to individually needs. Model-specific pressing curves can be stored and called up as required.

This type of press, particularly familiar in Japan and the For East, is normally designed as a multiple head press and is specially conceived for operation with closed moulds, i.e. an almost waste-free and burr-free pressing process. It consists of two side standards with crossbeam, in which the pressure head travels up and down with the top moulds and is supported on the swivelling beam with the bottom moulds below. After the pressing process the swivelling beam with the bottom moulds fastened to the surface of the swivelling beam swings from the horizontal position outwards, in which the swivel angle amounts to about 60°. After removal of the green tiles by suction heads, the swivelling beam swings back into the horizontal plane, and the bats from the other side are pushed onto the bottom forms, via feed conveyors. With this type of press the working cycles proceed in succession, i.e. bat feed, pressing and mould removal.

The original design concept of the turntable press was based on the exclusive use of the press for accessory tiles, it is therefore also examined in more detail in that section. With certain models however such a high output is obtained, from about >12 strokes/min., that they can also be used for flat tile production. There are a few clay roofing tile works which operate exclusively with turntable presses. The power transmission for turntable presses is normally via hydraulic pressing cylinders.

The principle of the sled press is the oldest existing since clay roofing tile presses were constructed. In the modern sled presses all process sequences are automated. The machine is conceived as a four-column press. Four columns support the crossbeam in which the pressure head with the top mould is installed. The pressing force is applied by means of a hydraulic cylinder. Underneath this is located the sled with pressing table, onto which the bottom mould is fastened. This is moved backwards and forwards, it is also hydraulically activated and thus brought to the pressing and loading/unloading station. A sled is to be found on the front of the machine, on which are mounted both the bat feed device with suction head and also the green tile removal device, with suction head and integral knifebox. Both devices are activated by this sled and operate in a tandem process.

This produces the following work flow:

The suction head of the bat feed device picks up a bot placed ready, parallel to this a pressed green tile is deburred by the lift-off device, subjected to suction and lifted out of the bottom mould. After this the two suction heads move upwards and are driven by the sled into the placing position. The bats are placed simultaneously on the bottom mould and the green tile on the tile setter. After this the two suction heads again travel up while the sled with the bottom mould travels under the top mould to carry out the pressing process.

The tandem sled returns to its initial position and after the bottom mould is moved out the operation is repeated.

The high-lift revolver press was developed in order to utilize the revolver press also for production of accessory tiles. Whereas the pressing stroke in normal revolver presses is at about 140 mm, in the high-lift presses it fluctuates approximately between 180 and 250 mm. The high-lift revolver press is always designed cis a single-head press. This type of press does not invariably prove to be ideal for accessory tile production. The reason for this is the high mould requirement, as for utilization to capacity of each tile model six bottom moulds are needed. As with the usually large accessory range of the clay roofing tile works the mould park is mostly already very extensive, the acquisition of so many moulds in most cases would be too expensive. The use of such a high-efficiency press is therefore to be recommended chiefly for the production of accessory tile models which are needed in very large runs.

Turntable presses are usually hydraulic presses. They operate with one top mould and, depending on the particular press model, with 2, 3 or 4 bottom moulds. The bottom moulds are supported on a turntable, which moves to the individual work stations, so that after completion of all work cycles they will have passed through a complete rotation of 360°.

For the press models with three bottom moulds there are three work stations: bat feed, pressing, green tile removal (consisting of deburring, suction application, lift-off). In this case all three work cycles run parallel. With the press models having two or four bottom moulds a 4th station is also available, which is optional for a further cycle, e.g. it can be used for perforation or punching. In the designs with four bottom moulds all work cycles likewise run parallel. With two bottom moulds there are two sequences, in which two work cycles respectively run simultaneously: Sequence 1: Pressing and green tile removal, Sequence 2: Perforation and bat feed. A special feature displayed by the WEGA-AS model is a decoupling of the two pressing tables which saves time and is reflected in higher output. Here the following work flow is obtained. While the pressing process is taking place at the pressing station on Table 1, Table 2 is turned as for as the take-off station and, after removal of the green tile, swivels back to the bat feed station, where a bat is placed on the bottom mould. Meanwhile the pressing process is now completed on Table 1. Both tables continue to turn together. Table 2 stops under the pressing station to carry out the pressing process, while the process flow previously described for Table 2 is repeated for Table 1.

The pressed green tile is deburred by a suction head with integral knife frame, exposed to suction and lifted off. Parallel with this the suction head takes up a bat placed ready from the bat feed device. In a joint work cycle the suction head travels into the green tile placing position and the bat placer to the bat placing position above the bottom mould. The suction head and bat placer then move back in another joint work cycle to their initial position. The pressing work cycle also runs parallel to the work cycles green tile/pick up/placing and bat pickup/placing.

The anvil presses used as accessory tile presses are designed as single-head presses. Otherwise the working principle and sequence correspond to the design already described.

Tables 2, 2.1, 3. and 3.1 give the most important characteristic data and technical features of the flat tile presses and accessory tile presses of the manufacturers who took part in this Market Survey. They are intended to give a general overview of the international range available. The basis taken was a specific questionnaire circulated to the respective manufacturers.

An examination of the design details, in which the manufacturers in some cases differ widely even for the some type of press, is not a function of this Market Survey and would exceed its terms of reference. just a note on the details given on the max. possible number of strokes and the theoretical output derived from this. They are based on different mould materials, such cis plaster, rubber, metal, which have a considerable influence on the capacity, just as the tile model and the nature of the production body, so that general comparability is not available.

Bender, Willi: "Entwicklungen und Tendenzen beim Bau von Dachziegelwerken - Developments and trends in the construction of clay roofing tile works", Ziegelindustrie International Nr.8/97, S. 485-496 und Nr.9/97, S. 562-571

Lepsien, R. u.a.: "Dachziegeldesign verlangt viel Know-how - Clay roofing tile design demands considerable know-how", Ziegelinustrie International Nr.1-2/97, S.5-10

Schreiter, Bernd u. Lorenz, Artur: "Die maschinelle Formgebung von Dachziegel-Zubehör - Mechanical shaping of accessory clay roofing tiles", Ziegelindustrie International, Nr. 10/97, S. 714-718

Ulrich, Lothar u. Lorenz, Artur: "Dachziegelfer-tigung mit der Revolverpresse - Clay roofing tile manufacture with the revolver press", Ziegelindustrie International, Nr. 9/97, S. 573-578