GB2424610A

GB2424610A - Forming method

Info

Publication number: GB2424610A
Application number: GB0506349A
Authority: GB
Inventors: Ihsan Ali Hussein Al-Dawery; Giuliano Tari; Jonathan Graham Peel Binner; Yan Yin
Original assignee: Ceram Research Ltd
Current assignee: Ceram Research Ltd
Priority date: 2005-03-30
Filing date: 2005-03-30
Publication date: 2006-10-04
Also published as: GB0506349D0

Abstract

A method of forming a hollow ceramic body comprises supplying a pourable suspension of ceramic material into a hollow mould, and rotating the mould as the ceramic material solidifies. The rate of solidification of the ceramic material in the mould may be accelerated using coagulation casting or gel casting. The suspension of ceramic material may have a high solids content and heat may also be applied to increase the rate of solidification of the ceramic material in the mould. The article may be formed with a plurality of layers by introducing subsequent ceramic materials into the mould and allowing each layer to dry.

Description

Forming Method This invention relates to a method of forming hollow

ceramic articles.

A number of techniques are currently used for forming hollow ceramic articles, and these techniques include slip casting, pressure casting, extrusion, injection moulding and pressing. A number of disadvantages are encountered with each of these processes. For instance, with slip casting it is necessary to use a porous mould, and plaster of paris moulds are regularly used. Such moulds only have a limited life and can provide disposal problems following use. Also with such a method it is necessary to tip away excess slip once full casting has taken place. Whilst excess slip can be re- used it may need rheological adjustment before blending with virgin material.

According to the present invention there is provided a method of forming a hollow ceramic article, the method including supplying a pourable suspension of ceramic material into a hollow mould, rotating the mould such that the ceramic material locates on the inner surface of the mould to form a shaped article, and subsequently removing the shaped article from the mould once the ceramic material has become substantially solid.

The suspension of ceramic material preferably has a high solids content, and the percentage volume solids content is preferably greater than 20%, desirably greater than 40%, and may be in the range 40% - 85%.

The mould is preferably simultaneously spun about more than one axis.

A plurality of moulds may be simultaneously spun.

The method may be carried out such that a substantially constant thickness layer of ceramic material is formed on the inside of the mould.

In one embodiment the mould may be spun about two axes, and the relative speed of rotation about one or both axes may be varied to provide a non constant thickness layer of ceramic material on the inner surface of the mould.

The method may be carried out such that parts of the shaped article are non hollow.

The article may be formed with a plurality of layers, with the first outer layer of a first material provided on the inner surface of the mould by rotating the mould with the first material therein, and subsequently after the first material is substantially solid, introducing a second material into the mould and rotating the mould such that a layer of the second material is provided on the inside of the first outer layer. The first outer layer could be of glaze.

Further materials may be introduced into the mould to provide further layers on the interior of the second layer.

Steps may be taken to increase the rate of solidification of the ceramic material in the mould.

The mould may be heated before and/or whilst the suspension of ceramic material is located therein. The ceramic suspension may be heated prior to supply into the mould.

Heat may be applied to increase the rate of solidification of the ceramic material in the mould, and the temperature of the ceramic material could be raised to within the range 40 - 60 C.

Any of coagulation casting, gel casting, albumin casting, freeze casting, or the addition of polysaccharides, may be used to increase the rate of solidification of the ceramic in the mould.

The mould may be made of metal, or may be made of plastics material.

The mould may be made of a porous material, and fluid may be blown through the mould to aid release of the shaped article therefrom. Fluid may be blown through the mould to clean and/or condition the mould prior to forming an article.

The mould may comprise a plurality of parts, which parts can preferably be selectively separated to aid removal of a shaped article therefrom.

Where an opening is provided in the article, this may be cut subsequent to formation of the shaped article. Alternatively, a part of the mould inner surface could be provided with a non-wetting inner surface such that the ceramic material does not locate thereon.

A pair of articles may be formed in a single mould such that when the articles are separated from each other the separation provides an opening into each of the articles.

The ceramic material suspension is preferably deflocculated prior to supply into the mould.

Embodiments of the present invention will now be described by way of

example only.

Hollow ceramic bodies were formed by a method according to the invention. The rate of solidification of the ceramic material in the mould was accelerated either using coagulation casting or gel casting as follows. The invention was usable with clay based bodies or alumina ceramics. The clay based bodies were traditional ceramic industry bodies containing a mixture of clay quartz and fluxing materials such as feldspar. The alumina ceramics used an industry standard high purity (99.8%) alumina powder.

All of the articles were formed using the following basic techniques.

Ceramic powder was mixed with water and dispersants, and the suspension was dispersed by mixing, followed by addition of appropriate additives. The mixture was then poured into a mould and the mould closed, and rotated whilst maintaining a required temperature. During rotation of the mould the mixture is urged onto the inner surface of the mould. Solidification of the material on the inner surface of the mould provides a green body which can be removed from the mould and subsequently fired to provide a sintered body.

Coagulation Castiflg This could be achieved via two routes. The first direct coagulation casting (DCC), is based on the controlled destabilisation of highly concentrated aqueous suspensions by shifting the pH towards the isoelectric point and/or increasing the ionic strength of the suspension.

The second, known as In-Situ Coagulation Casting, involves destabilisation of a suspension by a time-dependent in-situ hydrolysis of lactones. The hydrolysis forms carboxylic acid derivatives with the acid again shifting any negatively charged particles in suspension towards the isoelectric point. The processing parameters affecting coagulation casting with the invention include: mould material, lactone mixing time, entrapped air, amount of slip, rotation speeds, rotation time, surrounding atmosphere, de-moulding, drying and reproducibility.

Successful moulding of high density clay slips into a variety of component shapes has been carried out. For example a cylinder with one closed end is formed from a suspension solids content clay slip with 1 wt% GDL. The mixture was biaxially rotated in a mould initially at 20 rpm for 2 minutes, and then for a further 8 minutes at 10 rpm, with a maximum temperature of the mixture of 40 C.

A small milk jug was found from the same clay slip with a similar quantity of GDL lactone. This was initially biaxially rotated for 2 minutes at 16 rpm, and then 8 minutes at 20 rpm, again maintaining the maximum temperature of the mixture of 40 C.

A shape of an alumina suspension was made using a suspension with a solids content of 70 wt% GDL lactone. This was biaxially rotated at 14 rpm for 2 minutes, and 10 rpm for 8 minutes, again with a maximum temperature of 40 C.

Both porous epoxy resin and porous metal moulds may be used. The green bodies obtained were subsequently successfully fired and glazed.

Gel Casting This process involves the addition of a monomer to a well dispersed suspension that polymerises to immobilise the suspension. With gel casting the following paremeters have to be considered: the order of mixing of the components; mould material; gelling agent mixing time; entrapped air; amount of slip; rotation speeds; rotation time; surrounding atmosphere; de-moulding and drying.

An alumina slip was moulded to a variety of components with the following constituents. An alumina suspension with a solids content of 70 wt%; and 0.05 wt% gelling additives comprising 20 wt% monomer, 0.1 - 0.3gms ammonium persulfate (APS) initiator, and tetramethylethylenediamine catalyst. This mixture was rotated for 2 minutes at 10 rpm, and then for 10 minutes at 5 rpm with a maximum temperature of 30 C.

A clay slip was used to provide a variety of hollow ware articles. The clay slip suspension had a solids content of 70 wt%, and 0.05 wt%, gelling additive as outlined above was used. This mixture was initially rotated at 10 rpm for 2 minutes, and then for a further 10 minutes at 5 rpm, maintaining a maximum temperature of 30 C.

There is thus described a method of forming hollow ceramic articles which provides for a number of advantages and in most instances is more sustainable relative to existing techniques. In general less moulds will be required, and moulds with a long working life would generally be usable. A lower energy requirement would normally apply. The method is relatively quick, and would generally be quicker than existing techniques, and particularly relative to slip casting. In most instances there is no requirement for the moulds to be porous. Also, in contrast to slip casting it is not required for excess liquid to be tipped from the mould and be subsequently dealt with.

In contrast, in the present invention all of the suspension poured into a mould is used in forming the article.

Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims

1. A method of forming a hollow ceramic article, the method including supplying a pourable suspension of ceramic material into a hollow mould, rotating the mould such that the ceramic material locates on the inner surface of the mould to form a shaped article, and subsequently removing the shaped article from the mould once the ceramic material has become substantially solid.

2. A method according to claim 1, in which the suspension of ceramic material has a high solids content.

3. A method according to claim 2, in which the percentage volume solids content of the ceramic material is preferably greater than 20%.

4. A method according to claim 3, in which the percentage volume solids content of the ceramic material is greater than 40%.

5. A method according to claim 4, in which the percentage volume solids content of the ceramic material is in the range 40% - 85%.

6. A method according to any of the preceding claims, in which the mould is simultaneously spun about more than one axis.

7. A method according to any of the preceding claims, in which a plurality of moulds are simultaneously spun.

8. A method according to any of the preceding claims, in which the method is carried out such that a substantially constant thickness layer of ceramic material is formed on the inside of the mould.

9. A method according to any of claims 1 to 7, in which the mould is spun about two axes, and the relative speed of rotation about one or both axes is varied to provide a non constant thickness layer of ceramic material on the inner surface of the mould.

10. A method according to any of the preceding claims, in which the method is carried out such that parts of the shaped article are non hollow.

11. A method according to any of the preceding claims, in which the article is formed with a plurality of layers, with the first outer layer of a first material provided on the inner surface of the mould by rotating the mould with the first material therein, and subsequently after the first material is substantially solid, introducing a second material into the mould and rotating the mould such that a layer of the second material is provided on the inside of the first outer layer.

12. A method according to claim 11, in which the first outer layer is of glaze.

13. A method according to claims 11 or 12, in which further materials are introduced into the mould to provide further layers on the interior of the second layer.

14. A method according to any of the preceding claims, in which steps are taken to increase the rate of solidification of the ceramic material in the mould.

15. A method according to claim 14, in which the mould is heated before and/or whilst the suspension of ceramic material is located therein.

16. A method according to claims 14 or 15, in which the ceramic suspension is heated prior to supply into the mould.

17. A method according to any of claims 14 to 16, in which heat is applied to increase the rate of solidification of the ceramic material in the mould.

18. A method according to claim 17, in which the temperature of the ceramic material is raised to within the range 40 - 60 C.

19. A method according to any of claims 14 to 18, in which any of coagulation casting, gel casting, albumin casting, freeze casting, or the addition of polysaccharides, are used to increase the rate of solidification of the ceramic in the mould.

20. A method according to any of the preceding claims, in which the mould is made of metal.

21. A method according to any of claims 1 to 19, in which the mould is made of plastics material.

22. A method according to any of the preceding claims, in which the mould is made of a porous material.

23. A method according to claim 22, in which fluid is blown through the mould to aid release of the shaped article therefrom.

24. A method according to claims 22 or 23, in which fluid is blown through the mould to clean and/or condition the mould prior to forming an article.

25. A method according to any of the preceding clams, in which the mould comprises a plurality of parts, which parts can be selectively separated to aid removal of a shaped article therefrom.

26. A method according to any of the preceding clams, in which where an opening is provided in the article, the opening is cut subsequent to formation of the shaped article.

27. A method according to any of claims 1 to 25, in which where an opening is provided in the article a part of the mould inner surlace is provided with a non-wetting inner surlace such that the ceramic material does not locate thereon.

28. A method according to any of the preceding claims, in which a pair of articles are formed in a single mould such that when the articles are separated from each other the separation provides an opening into each of the articles.

29. A method according to any of the preceding claims, in which the ceramic material suspension is deflocculated prior to supply into the mould.

30. A method of following a hollow ceramic article, the method being substantially as hereinbefore described.

31. Any novel subject matter or combination including novel subject matter disclosed herein, whether or not within the scope of or relating to the same invention as any of the preceding claims.