Difficult blunging of clays or Too high residue

Blunging the plastic materials (clays) is the first step of the casting slip preparation. This is a key step, to ensure a consistent formulation with correct rheological parameters.

To have an efficient clay phase preparation, the blunging system (power installed , propeller type, sieving capacity) and the clay type have to be compatible.

The potential issue is mainly a difficult sieving due to un-dispersed clay particles, leading to unconsistent formula, wastes of good material, longer blunging and sieving time. A high residue level in the clay could lead to similar issues.

To have a quicker clay dispersion and lower residue , we recommend the use pre-processed clays.

 

Difficulties to adjust the density or rheology

Basically, sanitary ware casting requires high solid percentage (density around 1800-1820 g/L for vitreous china, and 1960-1980 g/L for fine fireclay) together with good fluidity. The slip has to present low viscosity with adjusted thixotropy, in order to cast difficult and complex pieces with good yield.

In some cases, the use of poor quality or un-adequated plastic raw materials (clays with high illite or smectite / montmorillonite content) could avoid the achievement of the proper rheological parameters.

 

Too low density or too high viscosity

In order to increase the density, we recommend to use products with higher fluidity, that will enable to reach higher solid content of the casting slip. The same recommendation could be done if an increase of deflocculant do not help reaching the proper viscosity, or if the slip contains too much deflocculant, causing casting cords, mould corrosion.

High fluidity clays

High fluidity kaolins

 

Too high thixotropy

A too high thixotropy is generating many issues during the casting process of sanitary ware pieces: soft and flabby pieces causing bad drainage, deformation and cracks.

When the thixotropy could not be adjusted with the defloculants, you have to look to soluble species (salts), like sulfates (SO₄²-, Cl-, NO₃-…), coming either from the water (and especially if it’s well water), or from the raw materials themselves. The return slip is potentially contaminated with plaster from the moulds, bringing sulfates, and giving thixotropy.

The use of low sulfates clays from selected feeds, or even better fully refined clays (soluble salts removed by filter pressing the clay) could help in decreasing the total soluble salts of the casting slip.

 

Ageing

Ageing is an important step to stabilize the rheology of the slip before casting. Nevertheless, some important issues could appear during the maturation of the slip :

Slip sedimentation:

Slip sedimentation usually comes from too low density. Apart from accidents like excess of water, and if an increase of density is not possible (slip already at optimal defloculation), we recommend to replace a low fluidity clay or kaolin by a material with better properties:

High fluidity clays

High fluidity kaolins

Thixotropy increase:
Uncontrolled thixotropy increase during the maturation process usually comes from increase of soluble salts like Sulfates (SO₄²-), Chlorides (Cl-).

Thixotropy Decrease:
Too high speed of agitation and/or defloculant type used can decrease the thixotropy of the casting slip.

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