|FR | ENGL|
|Ceramic tile pressing. Equipment and compaction programme|
|Dr. José Luis Amorós Albaro
INSTITUTO DE TECNOLOGÍA CERÁMICA DE LA UNIVERSIDAD JAUME I DE CASTELLÓN
Given the importance of ceramic tile manufacture by pressing, both worldwide (about 3500 million m2 per annum, approximately 45000 million tiles a year) and in the socio-economic region surrounding the Instituto de Tecnología Cerámica of Universitat Jaume I (385 million m2 per annum, accounting for 14% of world production), the presses and compaction programmes typically used in this industry have been chosen as examples of industrial facilities and their functioning.
At present, the presses used are of the hydraulic type; i.e., the pressure that reaches the punches is transmitted by means of a pressurised liquid, usually oil (Figure 6). The principal parts of a hydraulic press are schematically illustrated below:
(i.) Feeding system (Figure 7), whose objective is to fill the cavity with pressing powder (usually produced by spray drying), level it and remove the piece compacted in the previous cycle. The system consists of a rectangular metal frame, inside which some bars are fitted (normally arranged transversally to the feed direction), as separators, known as « reglets ». These are arrayed so as to obtain the most homogeneous possible powder distribution in the cavity. The system is charged by a filler that takes previously conditioned (sieved, mixed, etc.) spray-dried powder from a reception-hopper (Fig. 6).
(ii.) Hydraulic system, which is made up of:
(iii.) Pressing system (Figure 6), which comprises the mechanical elements that carry out pressing powder compaction and subsequent ejection of the piece. These elements are located in the rigid structure of the press. The main elements are:
(iv.) Automated control system, which regulates the pressing cycle electronically.
The combined synchronised actions performed by the principal press elements that act in the different pressing steps are known as the compaction programme or pressing cycle. The pressing cycle or compaction programme typically used in forming ceramic tiles includes the following steps:
The complete programme, which lasts about 3.5 seconds can be followed with the help of Figures 9, 10 and 11.
The cycle starts with the advance of the feeding system (A), appropriately charged with pressing powder, which moves towards the press cavity. At this moment the travelling frame and the bottom punch are at their highest position and there is no pressure in the main hydraulic circuit (Figure 10).
When the feed system is at a set point above the cavity (B), the first descent occurs of the bottom punches (C) and the cavity starts filling. The rate of punch descent (slope of the CD stretch) is very high, so that the punch rapidly reaches the end of its first drop (D), which determines the thickness of the pressing powder bed.
When the feed system moves back (E), a blade at the front of the system, known as a « scraper », removes excess powder from the cavity.
Once the cavity has been filled, the bottom punch drops to a second point (F), accommodating the spray-dried powder for compaction (Figure 11). This is when the travelling frame starts descending. On plotting the variation of the position of the travelling frame with time, three segments can be observed: a first rapid descent segment (EG), a second slowing segment (GH), hardly noticeable in this type of plot, and finally a horizontal segment (HI), in which the travelling frame rests directly on the pressing powder bed.
First pressing then starts, increasing the pressure on the powder bed (JL), while the frame descends (M).
On completing the first pressing stroke, pressure falls in the hydraulic circuit as well as in the powder bed, with a slight lift of the frame owing to the effect of the side shock absorbers (N). This is when de-airing time commences, in which the travelling frame rises and remains at a higher position to facilitate release of the air held in the powder bed.
After de-airing time, the second pressing (O) starts. The rising stretch exhibits two clearly separated areas: a first area, in which all the required pressure is provided by the central hydraulic unit (OP) and a second area, in which the multiplier group provides the necessary pressure (PQ) to complete the second pressing.
After reaching the maximum pressure programmed for the second pressing stroke, pressure falls in the main hydraulic circuit (QR) and the travelling frame lifts (ST), initiating tile ejection with the rise of the bottom punch (UV). In this case ejection occurs in unison, and the bottom punch lifts together with the travelling frame, until the compacted piece is outside the cavity.
The next cycle begins when the travelling frame has returned to its initial position, with feed system advance and withdrawal of the previously formed compact.
Table 1 shows the duration of each pressing cycle step at a pressing rate of 17 strokes/min.
Table 1: Duration of the different pressing operation steps
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