Lost wax casting method

1. A method of manufacturing a multilayer ceramic shell mould with at least one contact layer, one intermediate layer and several reinforcing layers, out of a master pattern, said method comprising the following steps:

dipping said master pattern in a first slip containing ceramic particles and a binder to form a first layer, depositing sand particles onto the first layer and drying said first layer, in order to form said contact layer,

dipping said master pattern in a second slip containing ceramic particles and a binder to form a second layer, depositing sand particles onto said second layer and drying said second layer, in order to form said intermediate layer,

dipping said master pattern in at least a third slip containing ceramic particles and a binder to form a third layer, depositing sand particles onto said third layer, drying said third layer, in order to form a reinforcing layer, forming reinforcing layers being repeated until obtaining a shell mould of a set thickness, and

baking the shell mould by heating up the shell mould to a temperature ranging between 1000 and 1150° C.,

wherein the ceramic particles of the slips comprise mullite or alumina, or a mixture of mullite and alumina, whereas no layer contains any zircon, and

wherein said second slip comprises in weight percentage:

a 50-75% mixture of alumina flour,

a 5-20% mullite flour,

a 20-30% colloïdal silica binder,

0-5% water, and

a wetting agent, a liquefier and a texturing agent,

wherein said sand particles are composed of mullite grains,

wherein the sand particles are applied so that the shell mould exhibits an after-baking porosity ranging between 20 and 35%, and

wherein the second and third slips comprise a mixture of alumina and mullite flours and mullite grains.

2. The method according to claim 1 , wherein the particles of the slips comprise one of mullite or alumina.

3. The method according to claim 1 , wherein the binders for the different slips are based on mineral colloïdal solutions.

4. The method according to claim 1 , wherein the grains have a size distribution ranging between 80 and 1000 microns.

5. The method according to claim 1 , wherein for the first three layers, the sand particles are applied by sprinkling.

6. The method according to claim 1 , wherein the sand particles are applied by fluidised bed.

7. The method according to claim 1 , wherein the first slip for oriented solidification contains mullite flour, in an amount ranging from 40 and 80% in weight, alumina flour, a colloïdal silica-based binder, and organic admixtures.

8. The method according to claim 1 , wherein the first slip for equiaxed solidification, comprises a mixture of alumina and mullite flours in amounts ranging respectively between 40 and 80% in weight of alumina and between 2 and 30% in weight of mullite flow, a colloïdal silica-based binder, a germinative and organic admixtures.

9. The method according to claim 1 , wherein the second and third slips comprise a mixture of alumina and mullite flours in amounts ranging between 45 and 95% in weight, and mullite grains in amounts ranging between 0 and 25% in weight.

10. The method according to claim 1 , further comprising a baking cycle of the finished shell mould, wherein the baking cycle comprises heating up to a temperature ranging between 1030 and 1170° C.

11. The method according to claim 1 , wherein said third slip comprises in weight percentage:

a 30-45% mixture of alumina flour,

a 15-30% mullite flour,

14-24% mullite grains,

a 10-20% colloidal silica binder,

5-15% water, and

a wetting agent, a liquefier, a texturing agent and a sintering agent.

12. The method according to claim 11 , wherein said first slip comprises in weight percentage:

a 40-80% mixture of alumina flour,

a 2-30% mullite flour,

a 0-10% germinative, cobalt aluminate,

a 18-30% colloidal silica binder,

0-5% water, and

a wetting agent, a liquefier and a texturing agent.

13. The method according to claim 11 , wherein said first slip comprises in weight percentage:

a 2-30% mixture of alumina flour,

a 40-80% mullite flour,

a 18-30% colloïdal silica binder,

0-5% water, and

a wetting agent, a liquefier and a texturing agent.

14. The method according to claim 1 , wherein said depositing of said sand particles is performed so as to control said after-baking porosity thereby controlling the shell mould's sensitivity to thermal shock to comply with casting conditions meeting stresses of a solidification method selected from the group consisting of an equiaxed solidification (EX), a columnar structure oriented solidification (DS) and a mono-crystalline structure oriented solidification (SX).

15. The method according to claim 1 , wherein the step of baking the shell mould consists of heating up the shell mould to a temperature ranging between 1000 and 1150° C. such that said method is free of any heating of said shell mould to a temperature greater than 1150° C.

16. The method according to claim 1 , wherein said method is free of a step of including a ceramic based mat of reinforcing material in said shell mould.