EP0496471A1

EP0496471A1 - Manufacturing method and mould made according to such method for moulding ceramic articles, in particular sanitary articles

Info

Publication number: EP0496471A1
Application number: EP92200147A
Authority: EP
Inventors: Roberto Nassetti
Original assignee: NASSETTI ETTORE SpA
Current assignee: NASSETTI ETTORE SpA
Priority date: 1991-01-23
Filing date: 1992-01-20
Publication date: 1992-07-29
Also published as: IT1247096B; ITMI910146A1; ITMI910146A0

Abstract

A mould for manufacturing ceramic articles comprises shells (17, 17') made of porous material, of a generically constant thickness, internally reproducing the matrix of the article to be moulded and externally provided with a plurality of interconnected grooves (19, 19'). Said shells are covered by casings (22, 22') made of non-porous material to form a generic parallelepipedon to enable them to be pressed together between press plates (27). The interconnected grooves (19, 19') form a network of ducts interconnected with the outside by means of ducts (23, 23') made in the non-porous casings (22, 22'). A method for manufacturing the mould comprises the steps of moulding the shells (17, 17') made of porous material with a network of grooves on their outer surface, of closing said grooves (19, 19') on the surface of the shells by means of strips (20, 20') to form a network of ducts, and of moulding a casing (22, 22') onto each shell (17, 17') with at least one duct (23, 23') leading into the network of grooves (19, 19').

Description

A method normally used in the known technique of moulding ceramic articles, such as in particular, sanitary articles, consists of using presses fitted with permeable moulds made by using microporous plastic materials. The press holds the two halves of the mould tightly closed and a concentrated suspension of clay, kaolin, feldspar and other additives are injected under pressure into the cavity. The water seeps out through the porous walls of the mould and a solid layer consisting of the elements in the suspension is deposited on the inner walls to form the article. By subsequently blowing compressed air through the pores in the mould, the article becomes hard enough to enable it to be removed from the mould and sent on to be fired.
The above-mentioned technique is satisfactory as far as the quality of the finished product is concerned, however the same cannot be said of the procedure required to produce the moulds and of the mould themselves obtained by means of this procedure.
In fact, the moulds of known technique are made by pouring a polymer suspension (generally based on polymethyl methacrylate) into two boxes containing the male and female matrices to be reproduced, so as to obtain two parallelepipedons of microporous plastic internally reproducing the geometric shape of the matrix. The plastic parallelepipedons then have to be subjected to an exacting process consisting in drilling a series of holes starting from the outside of the mould to within a few centimetres, orthogonally, from the surface reproducing the matrix. The drilled sides are then closed by metal plates whose surfaces are provided with grooves and ducts which match with each hole, thus creating a uniform network for removing water from the mould and for subsequently blowing compressed air into the mould. The holes are necessary in order to remedy the different thicknesses in the various areas of the mould, so as to ensure a sufficient water removal capacity whatever the thickness.
It is clearly evident that an embodiment of this kind is both expensive and time-consuming. Moreover, drilling the holes is a very delicate phase of the process because of the risk of accidentally drilling through the surface reproducing the matrix and ruining the mould. In addition, the holes must be made very close to one another and yet the removal of water and introduction of air through them is never very uniform since they are necessarily perpendicular to the surface.
Lastly, the join between the plastic part of the mould and the metal plates which, in addition to performing the function of connecting the holes, also perform the function of bracing and strengthening the mould, is a weak point of the mould. Consequently, after a certain number of moulding cycles, cracks and breaks begin to appear in the structure of the mould, due both to the mechanical stresses produced by the press and to the different coefficients of thermal expansion of plastic and metal.
The general scope of this invention is to obviate the aforementioned problems by providing a method for rapidly and inexpensively manufacturing moulds which are both sturdy and reliable. A further scope is to provide a mould made according to the aforesaid method.
These scopes are achieved, according to the invention, by providing a mould comprising at least two shells made of porous material which when coupled together define a moulding chamber corresponding to the article, characterized by the fact that the shells are housed in complementary recesses in casings made of non-porous material, networks of ducts being provided in correspondence with the faying surfaces of the shells and said of casings, said networks being provided with at least one outlet duct through said casing.
A method is also provided for manufacturing a mould for moulding ceramic articles comprising shells internally shaped, when coupled together, to form a cavity for moulding the article, comprising the steps of moulding the shells made of porous material with a network of grooves on their outer surface, of closing said grooves on the surface of the shells to form a network of ducts, and of moulding a casing onto each shell with at least one duct leading into the network of grooves.
The innovatory principles of this invention and its advantages with respect to the known technique will be more clearly evident from the following description of a possible exemplificative embodiment applying such principles, with reference to the accompanying drawings, in which:

figure 1 shows a schematic cross-sectional perspective view along a vertical plane of a first mould shaping element;
figure 2 shows a schematic cross-sectional front view of a first mould shaping step, using the element of figure 1;
figure 3 shows a schematic cross-sectional perspective view along a vertical plane of the element resulting from the shaping shown in figure 2;
figure 4 shows a partial cross-sectional front view of a complete mould made according to this invention and positioned in a press for use.

With reference to the figures, figure 1 shows a perspective cross-section of a first element 10 used in manufacturing a mould according to the invention. The element 10 has a surface 11 reproducing the surface of the matrix (shown in the accompanying drawings with a shape intended merely for illustrative purposes) from which the mould is to be obtained, but spaced at a certain distance from it, for example approximately three centimetres.
Figure 2 shows the element 10 assembled with the matrix 12, so as to define an interspace 13 of a substantially constant thickness, where substantially constant is understood as an even thickness except for local variations due, for example, to particularly elaborate conformations in localized areas of the article. In these areas, in fact, it is not always convenient to follow the exact shape of the article with the surface of the element 13, due both to manufacturing problems and to problems of unmoulding. The surface 11 is furrowed by ribs interconnected to one another.
At least one inlet duct 15 and one outlet duct 16 communicate with the interspace 13, so as to enable it to be filled with a a known type of mixture which, when hardened, gives rise to a porous plastic mass. The mixture can, for example, be based on polyester or acrylic resins.
This produces a porous plastic element or shell 17, shown in cross-section in figure 3, having a surface 18 which is an exact copy of the shape of the matrix and an opposing surface furrowed by a network of interconnected grooves 19.
As partially shown in figure 3, the interconnected grooves 19 are protected by means of glued strips 20 slightly wider than the grooves, so as to be able to cast, on the side opposite the surface 18, a generically parallelepipedal-shaped casing 22 of known non-porous plastic without fear of obstructing the grooves 19. The non-porous plastic can, for example, be based on thermosetting resin, such as a polyester resin or, preferably, an epoxy resin due to its low shrinkage on hardening and to its close adherence to the surfaces.
The first half of the innovatory mould of this invention is made in this way. The other half of the mould is then obtained by means of a similar procedure. Figure 4 shows the complete mould, generically indicated by reference 24, where the two halves are indicated by references 21 and 21', respectively. The component parts of the half 21' are indicated by the same numbering as the corresponding parts of the half 21, described previously, with the addition of the suffix "prime".
During casting, the blocks of non-porous plastic (generically indicated by references 22 and 22') are provided with ducts 23, 23' to connect the network of grooves 19 and 19', respectively, with the outside, and with ducts 26 communicating with the inside of the moulding cavity.
To use, the mould 24 is placed in a press 27 and the ducts 26 are fitted with pipes 28 for introduction of the aqueous clay solution to produce the article. The ducts 23, 23' are fitted to pipes 25, 25' for the usual operations of removing the water and subsequently blowing in compressed air.
The operation of moulding the article then proceeds according to the known technique by injecting the clay suspension, draining off the water through the porous layer 17, 17', blowing in air under pressure to harden the article and lastly raising the press to open the mould and remove the article ready for further processing.
At this point it is clear that the intended scopes are achieved. By means of this innovative method of producing the mould it is possible to avoid time-consuming operations while at the same time obtaining a very sturdy and inexpensive mould. Moreover, since the the network of ducts is parallel to the internal surface of the mould, it is much more efficient and ensures a more homogeneous fluid extraction compared to the known technique with holes perpendicular to the internal surface of the mould.
Since the plastics used for the porous parts 18 and non-porous parts 22 can be of mutually compatible types, the interface between them is extremely solid and practically inseparable.
The mould is consequently very sturdy and free from stress raisers, which are normally found in the embodiments of known technique with metal plates for retaining and connecting the ducts.
The foregoing description of an embodiment applying the innovatory principles of this invention is obviously given by way of example in order to illustrate such innovatory principles and should not therefore be understood as a limitation to the sphere of the invention claimed herein.
For example, the network of ducts made in the interface between the porous part and the non-porous part of the mould can differ from the one shown. It will be obvious to any expert technician that the network can be made with different densities from one part of the mould to another according to particular moulding requirements due, for example, to the particle shape of the article to be obtained. Likewise, the generically constant thickness of the porous shells can be subject to local variations due to particular moulding requirements.
The network of grooves in the interface can obviously also be achieved by machining the wall of the shell initially moulded smooth.
Lastly, although the network of ducts is described as a single communicating network, as will be obvious to any expert technician, it can also be made in the form of a plurality of separate networks each provided with its own ducts connecting it to the outside, so as to enable the blowing in and drainage operations to be differentiated both as regards pressure and as regards temporal sequence.

Claims

A mould comprising at least two shells made of porous material which when coupled together define a moulding chamber corresponding to the article, characterized by the fact that the shells are housed in complementary recesses in casings made of non-porous material, networks of ducts being provided in correspondence with the faying surfaces of the shells and said of casings, said networks being provided with at least one outlet duct through said casing.
Mould as claimed in claim 1, characterized by the fact that the thickness of the shells is substantially constant.
Mould as claimed in claim 1, characterized by the fact that the casings are made of thermosetting resins preferably based on polyester or epoxy resins.
Mould as claimed in claim 1, characterized by the fact that the shells are made of plastic material preferably based on polyester or acrylic resins.
Method for manufacturing a mould for moulding ceramic articles comprising shells internally shaped, when coupled, to form a moulding cavity for moulding the article, comprising the steps of:
moulding the shells made of porous material with a network of grooves on their outer surface, of closing said grooves on the surface of the shells to form a network of ducts, and of moulding a casing onto each shell with at least one duct leading into the network of grooves.
Method as claimed in claim 5, characterized by the fact of comprising, for each shell, the further steps of obtaining a moulding element with its surface traversed by a network of ribs and defining a cavity, when coupled with a matrix reproducing the article to be moulded, and of filling said cavity with the porous material to form the corresponding shell with its grooves formed by said ribs.
Method as claimed in claim 6, characterized by the fact that the moulding element is made with said surface at a generically constant distance from said matrix to obtain said cavity with a generically constant thickness.