Under Pressure


I still remember the first time I walked into a press room. In my first tile factory, all of the pressing was done in one area that was called (you guessed it) the press room. Row after row of imposing-looking machines were whumping away every few seconds, producing tile. That sense of wonder hasn’t left me, especially when I walk by a modern hydraulic tile press that is so massively heavy that it had to be brought in on a special truck with an engine on both ends. When that press makes an impression, the earth at your feet literally moves.

Granted, there are other ways to form a tile. You can slip cast them or isostatically press them, but the most common method is the dry press. Last month, I told you about the joys of playing with fire, but somehow the tile had to be turned into a square or rectangle first. That step happens at the press.

Tile is formed in much the same way that a diamond is formed—through incredible pressure. What takes nature millions of years to achieve underground takes a few seconds in the factory. The press itself is just a really fancy hydraulic piston that moves up and down. The real trick is in the die. The die (also sometimes referred to as a form or mold) is a hardened metal case with cutouts in the shape of the tile. A top punch is mounted to the hydraulic piston and the case is attached to the press bed. The clay is charged into the holes in the die case, and then the top punch pushes down at incredible pressure to form the tile. The bottom punches, then pushes up and out of the die case to eject the tile, which is pushed onto a conveyer belt to be brushed and decorated.

In general, the harder you can press a tile, the better. Pressing the tile helps form the general shape of the piece and removes much of the air (porosity) from the clay. The harder you press it, the closer it is to its finished size and porosity. Also, the harder the tile is pressed, the more rugged it is when chugging around hundreds of feet of conveyors and decorating equipment.

What the press giveth, the press can also taketh away. If there are areas of the tile that are less dense (usually because of the distribution of the clay), these areas will shrink more in the kiln. Modern porcelain tile can shrink more than 10% during firing. This produces a tile that is not square, not flat, or both. Another defect is called lamination (or windblown), where the air coming out of the tile during pressing causes distinct layers to form, leaving a horizontal crack in the body of the tile. The tile can also develop cracks from the surface down into the body, or it can get chipped as it gets ejected from the die.

With careful control of the clay fill, pressing cycle and handling after the press, today’s rugged tiles are born. The press can give the tile character; it is here that surface texture and chiseled or rustic edge effects are programmed into the pieces. The press also imparts strength to the tile—that break strength or MOR rating in the technical specifications is formed here.

The next time you see a harried-looking tile engineer who tells you they’re under a lot of pressure to produce your tile exactly right, they mean it…literally.