CERAMIC PROCESSING MLZ

Prof. Dr. Ferhat KARA

CERAMIC PROCESSING MLZ 218

Slip Casting

What is a slip?

A slip is a suspension of colloidal particles in a liquid and

usually water is used as the liquid.

What is Slip Casting?

Slip casting process involves

a-) Pouring the slip into the porous gypsum mould.

b-) Capillary suction of the mold

causing the liquid to be filtered from the suspending medium and densely packed layer of the desired particles to be deposited aganist the mold wall.

c-) Drainage of the excess slip

d-) Removal of the part

What is Slip Casting?

Slip casting is a cheap shaping technique,

especially used to produce complex shape parts from a wide range of ceramic materials.

Slip catsing is the shaping technique used to shape sanitrayware articles.

Porous gypsum mould

Gypsum Mould

CaSO

. 1/2H

O + 3/2 H

O CaSO

.2H

O 100 : 18

but we use 100:80 plaster:water ratio to make porosity and to increase suction of water!

 Low cost

 Ease of manufacturing complex shapes

 Good dimensional accuracy

 Repeatable water absorbtion properties

 Softness because of plaster

 Water solubility

Advantages Disadvantages

Gypsum Mold

Pore diameter (m) Porevolume(cm3 /g)

Water/plaster weight ratio

Drycompressivestrength(MPa) Adsorption(%)

Control Parameters of Slip Casting

 Slip rheology

 Casting rate / speed

 Slip density

 Slip rheology during drainage

 Shrinkage and removal of the part from the mold

 Finishing operations

Surface Chemistry and Rheology

 Particle surface chemistry is a dominant factor in the slip casting because it regulates interparticle attraction and repulsion forces, which, in turn have significant effect on

 slip rheology

 casting rate and

 microstructure evolution.

Surface Chemistry and Rheology

The control of surface chemistry while preparing low-viscosity suspensions that are highly concentrated with powders is a major requirement of slip casting. How to control surface chemisty is addressed previously.

Low viscosity (<1 Pa.s) are needed for pourability and high solids

concentrations (up to 60 vol% solids) are needed to maximize the

casting rate and «green» density

Effect of interparticle forces on slip structure

Stable slurry

(Repulsive forces dominant)

Unstable slurry

(Attractive forces dominant)

A B C

B A

C

Viscosity

Deflocculant content

Effect of interparticle forces on slip structure

C B

A

Time

Viscosity

Thixotropy

A B C

B A

C

Viscosity

Deflocculant content

Effect of slip structure on casting

For technical ceramic slurries such as alumina, slip should be prepared as in Point A. The particles are well stabilised and they accumulate on the mould surface individually and form dense cake.

For traditional ceramic slurries

such as sanitaryware, slip should be prepared as in Point B.

Sanitaryware body formulation

Clay-1 Clay-2 Clay-3 Kaolin-1 Kaolin-2 Quartz

Na-Feldspar ^{25-30 %}

90% of the surface area of the recipe is due to clay and kaolin and thus rheological properties is controlled by clay and kaolin.

The most important property of the sanitaryware cast is its plasticity. This requires retaining of some water in the cast and this is achieved by forming flocs in the slip.

20-25 % 20-25 % 25-30 %

A B

C

Low casting rate Low retained water Hard cast

High casting rate High retained water Plastic cast

Good drained surface

Very high casting rate Very high retained water

Flabby (soft) cast, poorly drained surface

Effect of slip structure on casting of sanitaryware

Casting rate is controlled by permeability of the cake. As the thixotropy of the slip increases casting rate increases and the cake retains increasing amount of water.

Influence Of Deflocculation On The Shape Of The Slip Cast Whiteware Part

J.S. Reed, Principles of Ceramic Processing, John Wiley and Sons

Thickness of cast (L)

Time Time

Thickness of cast² (L²)

Slip Casting

Slip Casting vs Pressure Casting

Slip Casting

 Gypsum mold

 3-4µm pore size

 Don’t need pressure

 2 hrs casting time

 Need to dry in the mould

Pressure Casting

 Polymer mold (Polymethyl methacrylate)

 20-25µm pore size

 Casting under pressure ≈15-16bar

 20min casting time

 No need to dry

 Every product must have same geometrical properties!

For that reason;

 Rheology of slurry must be under control

 Because of clay and kaolin have high surface area rather than quartz and feldspar, one must control clay and kaolin properties

Reproducability

Reproducability

Good Casting

 Reasonably fast casting rate

 Casting time must be as low as possible

 Quality of Cast

 Good plasticity

 Good trimming behaviour

 Enough strength

Reproducability

S = Soft

G = Good Cast

H = Hard

Process flow diagram for shaping by

injection molding

Injection molding

Injection molding products

slurry cylindric greenbody

Industrial pug mill with deairing chamber and extrusion auger

Extrusion

Tape casting

slurry

doctor blade liquid absorbing, porous film

green body in form of a film

Compositions of a alumina and titanate tape cast (vol%)

Powder Al₂O₃ 27.0 BaTiO₃ 28.0

Solvent Trichlorethylene 42.0 Methylethylketone 33.0 Ethylalcohol 16.0 Ethylalcolhol 16.0 Deflocculant Menhaden oil 1.8 Menhaden oil 1.7 Binder Polyvinylbutiral 4.4 Acryllic emulsion 6.7 Plasticizer Polyethylene glycol 4.8 Polyethylene glycol 6.7

Octyl phthalate 4.0 Butylbenzlphtalate 6.7 Wetting agent Cyclohexanone 1.2

Such slurries exhibit also shear thinning. The quality and thickness of the tape is controlled by the size of the blade oppening, the speed of the tape, the rheology of the slurry and the shrinkage during drying. Industrial tape casting machines are up to 25m long, several meters wide and run with speeds. Up to 1.5m/min to produce tapes with thicknesses between 25 and 1250mm.

Tape casting

A doctor blade assembly. The ceramic slurry is held in the reservoir behind the blade [middle of the micrograph]. The twin micrometers [right] control the blade height above the carrier film. More sophisticated versions feature double blades and pumped metered slurry flow to keep the height of the slurry reservoir constant.

Example of a tape drying on the Mistler laboratory- scale batch tape caster. Industrially the process is often continuous with the tape being force dried prior to removal from the carrier, dicing and further processing.

Tape casting

3D PRINTING (ADDITIVE MANUFACTURING)

https://www.3dprintingmedia.network/the -additive-manufacturing-market-2019/

3 BOYUTLU YAZIM TARİHİ

- 1980 yılında Dr. Kodama Japonyada ilk patent başvurusunda bulundu.

- 1986 yılında Charles Hull’un stereolitografi parçaları için yaptığı patent başvurusu kabul edildi.

- 1987 yılında stereolitografi yöntemi tanıtıldı.

- 1989 yılında Carl Deckard seçimli lazer sinterleme yöntemi için patenti kabul edildi.

- 2004 yılında Dr. Bowyer açık kaynaklı her seviyeden kullanıcının kullanabileceği 3B yazıcıları tanıttı.

- 2007 yılında fiyatı 10 000 doların altındaki 3B yazıcı sistemleri tanıtıldı.

- Günümüzde 3B yazıcıların pazar değeri 1 milyar doları geçmiştir.

3 BOYUTLU YAZICILARIN ÖZELLİKLERİ

- Bilgisayar destekli yazılımlarla çalışılabilir olması (CAD) - Birden fazla malzemenin kombine edilebilmesi

- Çok hızlı bir şekilde üretim yapabilmesi

- Ekstra bir kalıp yada fikstüre ihtiyaç duymaması - Uygun fiyatlarla alınabilir olması

- Kompleks şekillerin elde edilebilmesi

3 BOYUTLU YAZICILARIN KULLANIM ALANLARI

Sağlık Sektörü Eğitim Uzay Araştırmaları

Elektronik Devreler Seramik Sektörü Oyuncak Sektörü

3 BOYUTLU BASIM METODLARI

Stereolitografi (SLA)

Eriyik Yığma Modelleme (Fused Deposition Modelling- FDM)

Seçmeli Lazer Sinterleme (Selective Laser Sintering )

Direk Mürekkeble Yazma (Direct Ink Writing - Robocasting)

Tabaka Halinde Çamur Biriktirme (Layer-Wise Slurry Deposition – LSD)

Seçmeli Lazer Sinterleme (Selective Laser Sintering -SLS)

- Lazer tabanlı, toz malzemeler ile çalışan katmanlı imalat tekniğidir.

- Lazer ile toz sinterlenir ve kaynaşır. Kaynaşan partiküller katı bir şekil alır. Arta kalan toz temizlenir - Düşük yoğunluklu ürün üretimi temel dezavantajları arasındadır.

Stereolitografi (SLA)

- 3 boyutlu objelerin lazer yardımıyla fotopolimerlerden (UV ışık altında katılaşan malzemeler) şekillendirilmesi metodudur.

- Katman kalınlığının küçük olmasında dolayı yavaştır ve bu nedenle küçük boyutlu parçalar ile sınırlıdır.

- Yoğun seramik üretilebilmektedir.

- Bu teknikte kullanılan reçineler pahalı olup, litresi 30-210 $ aralığında değişmektedir.

Tabaka Halinde Çamur Biriktirme (Layer-Wise Slurry Deposition – LSD)

- Son yıllarda geliştirilmiş olup, henüz ticari değildir.

- Yüksek hacim yüklemeli çamur katman olarak bir düzleme yayılır ve yazılmak istenen ürün kesiti lazer ile kurutulur ve proses tekrar edilerek biriktirme yapılır.

- Yüksek ham yoğunlukta yaş bünye ve sonrasında tam yoğunlukta ürün üretilir.

- SLA’ya göre daha hızlı ve büyük parçaların imalatına uygun bir tekniktir.

Eriyik Yığarak Modelleme

(Fused Deposition Modelling – FDM)

- Sıcaklık kontrollü bir nozzle vasıtası ile termoplastik veya metalik malzeme katman katman üretim tablasına serilir.

- Malzeme olarak makara halindeki termoplastik malzeme (filament) yada metal çubuk kullanılır.

Direk Mürekkeple Yazma (Direct Ink Writing – DIW)

- Nozülden belirli bir kayma oranıyla akan mürekkebin (akma noktasına sahip seramik çamuru) üst üste yığılmasıyla şekil verilmesi prensibine dayanır.

- Büyük parçaların yüksek ham yoğunlukta ve hızlı şekilde yazılmasına olanak tanır.

- Yüksek yoğunlukta seramik eldesine imkan verir.

- Üretim amacına bağlı hazırlanan mürekkepler nispeten düşük maliyetlidir.