Total Shrinkage (Ceramic)

Total shrinkage (ceramic)

Total shrinkage TS = (L_wet − L_fired)/L_wet·100% is what really matters to the mould designer: the sum of all the shrinkage a piece undergoes, from the moment it leaves the press (wet, at mould size) until it becomes the finished product (dry and fired). It combines the two sequential shrinkages — drying (loss of the forming water) and firing (sintering in the kiln) — which together can reach 10–15% in a porcelain tile. That figure settles a fundamental practical problem: the customer wants a piece of exact size (a 60×60 cm floor tile, a piece of tableware to a standard measure), and the piece shrinks along the way, so the mould cavity (or the pressing die, or the extrusion profile) has to be made larger, in the exact proportion of the total shrinkage, so the fired product comes out on size. At 12% shrinkage the mould must be around 13.6% larger than the finished piece (the arithmetic is not symmetric: for a finished piece of 88 mm with 12% shrinkage on the wet dimension, the wet mould measures 100 mm). Mistakes here yield out-of-size pieces — and recutting a steel mould is expensive and slow. Total shrinkage must also be constant and predictable batch after batch; swings in moisture, in body composition or in the firing curve shift it, causing the dreaded calibre problems (pieces of the same model at slightly different sizes, which have to be sorted and sold in separate lots). Enter the wet length and the fired length.