GB2292106A - Method of releasing a moulded clay product from a mould - Google Patents

Abstract

In the production of a moulded clay product 20, for example a cup or bowl, in particular when producing such products by mass-production methods, the clay article 20 is released from the mould 12 or 21 by irradiating the article with radiation from a catalytic infra-red heater 15 or 23. By this method, mould-release times may be significantly reduced. <IMAGE>

Description

Method of releasing a moulded clay product from a mould The present invention relates to the manufacture of moulded clay or ceramics articles, such as cups or bowls for example, and in particular to a method used for removing those articles from the moulds in which they are formed, prior to firing.

In the manufacture of ceramic, clay or china objects, it is common for an article to be cast in a mould in order to produce the desired shape of article.

It is then necessary to remove the wet clay article from the mould before it is fired and then finished e.g. by applying a glaze. The method most commonly used for releasing clay-type products from their moulds consists simply of removing water from the clay material so that it shrinks away from the surface of the mould and thus frees itself. This is usually achieved by drying the clay in the mould in an oven heated by air-convection heating. When the clay articles are mass-produced, the moulded articles in their moulds are conveyed through a large oven for sufficient time to effect release of the articles from their moulds. In order to maintain a continuous production of articles by this method, a very large number of moulds is produced, effectively to fill the conveyor which carries the moulded articles through oven.This is because for a given rate of production and therefore rate of conveyance through the oven to be maintained, the length of the oven is chosen to give the required drying time. A longer oven used for longer drying, and conveyor therefore requires more moulds to fill it to maintain the required rate of production. Apart from requiring a considerable investment in moulds, the above-described method of mould release also requires a considerable amount of down-time when the moulds upon a conveyor are to be changed to produce a different line of goods. The hot air used to effect the removal of water from the clay sufficiently to release the moulded product unfortunately causes a fairly high rate of deterioration of the plaster moulds which are conventionally used.Although this problem has been avoided to some extent by using synthetic materials instead of plaster, reducing the time for which the moulds are exposed to air-convection heating is also beneficial in prolonging the life of plaster moulds.

For these reasons at least, a method of releasing clay articles from moulds more quickly is highly desirable for clays and ceramics-ware manufacturers.

According to the invention a method of releasing a moulded clay product from a mould comprises applying heat in the form of the infra-red radiation supplied by a catalytic infra-red heater to a clay article whilst still within or upon a mould. Radiation of the particular wavelengths produced by such a heater is well matched to the absorption bands of water molecules and so the radiation is absorbed by and heats up the water in the clay more than other materials which may be in the vicinity, such as a mould, for example.

Thus the removal of the water in the clay by this form of heating is much faster than with air-convection heating, resulting in quicker mould release times, which allows smaller ovens and therefore fewer moulds to be used.

Catalytic infra-red heaters operate by reacting an organic gas, such as butane or natural gas for example, with oxygen over a platinum-based catalyst. The products of this reaction are carbon dioxide, water vapour and long-wave infra-red radiation. A preferred type of catalytic infra-red heater for this application is naturally oxygenated, i.e. it is able to operate by using the oxygen present in the air surrounding it.

The reaction proceeds at relatively low temperatures in this type of heater and it is flameless. One example of this preferred type of heater is sold by BRUEST INDUSTRIES INC.

The infra-red radiation may be used alone or in combination with another form of heat. For example, it may be advantageous to use a combination of catalytic infra-red heating and air-convection because the movement of air may serve to remove water vapour from around the articles to be released and thus improve conditions for the removal of further water from the articles.

It is preferred to provide a flow of dry air around the article(s) to be released, whether or not that air is heated, in order to remove water vapour given off by the article(s). The air may be recirculated by passing it through a drying process.

More than one catalytic infra-red heater may be used, for example to provide a source of sufficient length or width to cover an article or line of articles. In certain applications it may also be advantageous to apply the infra-red radiation from more than one direction. This may be the case when the thickness of the clay article to be released varies; a relatively thick base of an article may thereby be subjected to a relatively large amount of radiation, e.g. from overlapping sources, whilst its relatively thin rim receives radiation from a single source only.

By selecting an appropriate number of catalytic heaters and placing them in appropriate positions in this manner a fairly uniform rate of water removal from all parts of the article may be achieved and thus the failure rate of articles by cracking caused by uneven rates of shrinkage may be reduced.

It may be beneficial when synthetic moulds are used to irradiate the mould also to heat it up and thus assist in driving off water from the clay product formed on or in the mould. This benefit is seen more particularly when using deep-set moulds because it is more difficult to remove the water from those parts of the clay article within a deep set mould which are further from the radiation source. Conversely, a plaster mould is susceptible to damage when it is heated and so the radiation is advantageously directed away from the mould itself. A reflective coating, cover or shield may be employed to prevent excessive heating of a plaster mould.

Specific forms of apparatus which may be used to apply the method according to the invention of releasing a moulded clay product will now be described, by way of example only and with reference to the accompanying drawings, to illustrate how the method may be applied.

Fig.l is a transverse section through an oven for the release of flatware and Fig.2 is a transverse section through an oven for the release of deep set clay products.

Fig.l shows in schematic form a sectional view through an oven 10 for releasing flatware, from moulds i.e. plates, shallow bowls etc. The length of the oven is sufficient to allow a number of moulded clay articles to be conveyed through it at a sufficient rate for a desired rate of production of the flatware articles to be achieved.

A moulded clay article 11 on its mould 12 is conveyed through the oven upon a holder 13 which, in this case, is able to rotate about its vertical axis.

The holder is placed upon a conveyor 14 which may either be indexed or set to move through the oven continuously at a fixed or variable speed. A catalytic infra-red heater 15 is located above the conveyed article to emit long wavelength infra-red radiation downwards on to the article 11. The walls 16 of the oven are insulated to retain heat. Air is circulated within the oven by a fan 17. The air is passed through drying units 18 before re-entering the main oven through ducts or perforations in the wall 19.

The moist air may alternatively be extracted from the oven.

As the clay article passes through the oven underneath the catalytic heater 15, the water in the clay absorbs the infra-red radiation from that heater, heats up and evaporates, which in turn causes the clay article to shrink away from the mould. The infrared radiation impinges mainly upon the central part of the article, which is thicker than the surrounding edges, which receive less radiation. However all edge regions are covered evenly due to the rotation of the article under the heater. The circulating air ensures that the catalytic heater 15 receives sufficient oxygen to operate properly.

Fig.2 shows a similar view of an oven for releasing deep-set clay articles such as cups for example, from their moulds. The article 20 is set in a synthetic mould 21 which is carried through the oven on a moving steel frame or holder 22. Catalytic infra-red heaters 23 are mounted above the article offset at an angle from horizontal. The infra-red radiation emitted from these heaters overlaps within the article so that additional heat may be absorbed in these regions compared with the rim of the article.

Hot dry air is blown down into the article through duct 24. The air flows into and then out of the article and out of the oven via duct 25. The air may then be dried and recirculated or extracted. The hot air is able to reach the base of the article to assist in removing water from this region of the article which may otherwise be slow to release. the heaters 23 are placed so that they irradiate the exterior of the mould in addition to the clay article 20. In this way the mould itself is heated, to approximately 600and this assists further the removal of water from the parts of the article 20 deep within the mould.

As will be appreciated from the above examples, the number and position of the catalytic heater(s) used may be varied to suit the particular characteristics of an article to be released from a mould.

Claims (10)

1. A method of releasing a moulded clay product from a mould, comprising applying heat in the form of the infra-red radiation supplied by a catalytic infra-red heater to a clay article while the article is still within or upon the mould.

2. A method as claimed in Claim 1, wherein the catalytic infra-red heater is naturally oxygenated.

3. A method as claimed in Claim 1 or 2, wherein a flow of dry air is provided around the article to be released.

4. A method as claimed in Claim 3, wherein the dry air is heated.

5. A method as claimed in any of the preceding claims, comprising applying heat to the article from more than one catalytic infra-red heater.

6. A method as claimed in Claim 5, wherein the heat is applied to the article from more than one direction, whereby to supply different amounts of radiation to different parts of the article.

7. A method as claimed in any of the preceding claims, wherein the mould is of a synthetic material and infra-red radiation is also directed to irradiate the mould.

8. A method as claimed in any of Claims 1 to 6, wherein the mould is of plaster and a reflective coating, cover or shield is provided to divert radiation from the mould.

9. A method as claimed in any of the preceding claims, applied continuously to a succession of clay articles passing through an oven.

10. A method of releasing a moulded clay product from a mould, which method is substantially as hereinbefore described with reference to the accompanying drawings.