0 25 50 100 150 Increasing moisture content (%)

PRESSING

PLASTIC

CASTING

MIXING

Slip density

(kgm3 x 103)

Tiles, Electrical porcelain Flatwares, Tablewares

Electrical Porcelain, Tablewares, Earthenwares

Sanitarywares, Tablewares, Glove formers

RELATIONSHIP BETWEEN FORMING METHODS AND MOSITURE CONTENT

4 Important Range

2 ranges with higher moisture content:Casting and Mixing

- Refer to region in fluid form (involving deflocculant to maximise solid content in water)

2 ranges with lower water content:Dry pressing and Plastic forming

Relationship between porosity and moisture content

Dry Powder Plastic SlipBody (Press) Body (Casting)

Moisture

Pores

Solid

Volume

35

65

The role of water with reference to forming methods (rujuk carta bar): Powder pressing – water as binding of powders Plastic Body – water fills pores and provide slip

between particles (pelinciran) and improve plasticity

Slip casting – water improves flowability of slurry

Typically :Jasad selepas pengeringan mengandungi

30 – 40% liang

Controlling moisture level in bodies of whitewares ?Raw materials

BasahKering

Tambah Air Aduk dan campur

Campuran Jasad

Tekan Perah

Kek Plastik

Adun

JASAD PLASTIK

Kering leger

Kering

Kisar

PENEKANAN SERBUK

Kering sembur

Terus

Tambah air dan penyahkelompok

TUANGAN SLIP

Flowchart of body forming30 20 6 – 10 % air

Water in slip casting (30 –35 %)

Requires 2 important criterion :

1. Slip (slurry) with high solid content and controlled amount of water (high solid to water ratio) with suitable level of viscosity

- to enable flowability, transportation, casting etc

- however, the viscosity level (fluidity) is prepared without sacrificing high solid content (about 70% SOLIDS)

(2) Produced cast (tuangan) with good properties such as good density, solid and with good green strength and not easily crack

To produce quality product – Control rheology

Whats rheology?

Hence 3 parameters that need to be controlled :

 (i)    slip density : (mass/volume)

(ii)    viscosity : relative internal resistance in relation to

movement at other positions

(iii) thixotropy : the ability of slip to be more viscous when remain undisturbed

Generally : rheology is controlled by the charactersitics of the clay used

Theoretically, the influence of clay on rheology is difficult

The addition of electrolyte (known as deflocculant

or dispersant) will reduce the viscosity to a minimum point before increasing the viscosity again

Relative Viscosity

Amount of deflocculant

The graph of relative viscosity against amount of deflocculant

Common deflocculant usedNa SilicateNa Hexametaphosphate (or Calgon)Na carbonateDispex CMC

Deflocculant enables the slip to be prepared with high solid content in a small amount of water

without compromising the viscosity

% solid content

% deflocculant

Graph of % solid content against % deflocculant

Therefore, the density and viscosity of slip would produced different state of the cast

product

Flocculated slip and deflocculated slip produced different state of the casts.

  Deflocculated slip :

cast body will be denser and solid and upon firing, the physical and mechanical properties will be better (the arrangement of the particles in body will be effective)

However, the yield point and thixotropy are two other important parameters

A B C D E

Strain rate

Tegasan Ricih

Increasing slip density

Yield stress

A – Pure viscous fluid (in accordance to Newton’s Law) example : thick syrup

B - Bone China: special characteristics due to presence of phosphate

C – behaviour of casting slip typically with no yield point, and gradient of curve increases when strain rate increases (i.e. viscosity decreased). This is known as thixotropy ( assist in thickening of cast products)

B & C – deflocculated ceramic slips.

A B C D E

Strain rate

Tegasan Ricih

Increasing slip density

Yield stress

D & E – slip without deflocculant.

High viscosity. Shows yield stress, that is - it requires a small force before flowing (close to plastic making body)

Therefore, 3 slip properties need to be measured : 

Slip densityFluidity (kebendaliran) or viscosity thixotropy

How to measure???? Use of viscometer:-

Manual (old method):

Torsional viscometer (Viskometer kilasan)

Digital (new):

Brookfield Viscometer

There is no specific values to indicate that the slip is suitable (pending on products)

Generally (using torsional visc.):

Slip density 1.8 oz/pint

Viscosity 260 – 300 over - swing

Thixotropy 10 – 60 over-swing/minute

% deflocculant

Fluidity and thixotropy

General characteristics, the relationship between fluidity and thixotropy against % deflocculant

Fluidity (propportional 1/viscosity)

Tiksotropi

Increasing slip density

How??

(i) Maintained high solid content

(ii) Moderate thixotropy so as to have a strong body

The characteristics can be controlled by

  water raw materials deflocculant

More More important ..why?important ..why???