GB2030509A - Process and apparatus for the manufacture of ceramic mouldings by injection-stamping - Google Patents

Abstract

In a process for the manufacture of ceramic mouldings e.g. a dish by injection-stamping, an excess amount of ceramic material is injected into the cavity 8 of a mould 1 consisting of at least two parts 2,3 before the mould parts are completely closed. Upon or during closing of the sprue, the mould parts are moved together. In order to avoid variations in density and the formation of parts of deficient composition in the ceramic moulding, there are exerted different pressures on the material in different zones. These can be produced by extruding excess material from the cavity 8 through throttling outlets 10a into overflow spaces 12 so that pressure is produced in the edge zone through the throttling action and an increase in density obtained. The height of the outlet 10a may vary along its length. In addition to mould closure, moulding material may be expelled by urging a pin 7 mounted on a screw 6 towards the mould cavity. <IMAGE>

Description

SPECIFICATION Process and apparatus for the manufacture of ceramic mouldings by injection-stamping The invention relates to a process of and apparatus for the manufacture of ceramic mouldings by injection-stamping. More particularly the invention relates to a process and apparatus in which a predetermined volume of a plastic ceramic material is injected or pressed through a sprue into the cavity of a two or more part mould before the final closing thereof, and subsequent to, or simultaneously with, the closing of the sprue the mould parts are moved together into their final position and simultaneously exert pressure on the ceramic material present in the cavity.

it is known that in processing plastic ceramic materials by injection moulding or injectionstamping it is difficult to transmit the pressure applied from the sprue so as to produce sufficiently uniform compression of the ceramic material in the mould cavity. Particular difficulty is encountered with edge zones which are relatively remote from the sprue, and thinly tapering zones.These difficulties are more pronounced when relatively dry ceramic materials, which for example contain less than 16% of moisture, are processed, and are due to the fact that plastic ceramic materials, and in particular relatively dry materials differ from other flowable moulding materials in that they transmit the moulding pressure only to a very limited degree, so that at, for instance, the edge zones remote from the sprue there is a substantial loss of pressure, and hence a correspondingly lower compression.

A process of the above type has been proposed in which, after introducing the plastic ceramic material into the cavity of the mould prior to closure thereof, the cavity is closed from the sprue side and thereafter, or simultaneously, the mould parts are moved together completely into their final position. In this process, an amount of ceramic material which is accurately matched to the material requirement of the moulding to be produced is introduced into the cavity, so that at the time of its introduction the cavity is only partly filled. As a result of the subsequent moving together of the mould parts into their final position, a moulding pressure is exerted, in particular in the zones of the cavity remote from the sprue so as to completely form the moulding.

Due to this pressure being exerted, the ceramic material is subjected to a compression, even in the zones remote from the sprue, which is substantially greater than the compression which is possible by exerting pressure only from the direction of the sprue.

Although this process leads to improvements in the density distribution in the ceramic moulding, it has nevertheless been found that it continues to be difficult to obtain distortion-free mouldings. The distortion results from the variations in shrinkage on baking, which, in turn, is more manifest in zones of lower density than in more highly compressed zones.

According to the present invention there is provided a process for the manufacture of ceramic mouldings by injection-stamping, in which a predetermined volume of a plastic ceramic material is injected or pressed through a sprue into the cavity of a two or more part mould before final closing thereof, and subsequent upon or simultaneously with, the closing of the sprue the mould parts are moved together into their final position whilst simultaneously exerting pressure on the ceramic material present in the cavity, wherein, in zones of the moulding which are particularly prone to density variations, prearranged pressures which influence the density of the material are exerted on the ceramic material in the cavity.

The present invention is based on the recognition that in the known process the pressure distribution on the moulding subjected to pressure, when the mould halves are moved together, seems to depend on the ability of the ceramic material in the mould cavity to move out of the way, and hence indirectly depends on the shape of the moulding. The result of this is that, for example, in places where there is an abrupt change in cross-section, corresponding changes in density also occur, since the zones of the moulding of greater cross-section are compressed to a different extent from those of lesser cross-section.

It follows that edge zones and changes in crosssection in the moulding are to be regarded as zones prone to distortion, that is density abnormality or variations. Based on this recognition, the invention proposes that pressures, which may differ from zone to zone and which permit influencing the denseness of packing ofthe moulding in the cavity, should be deliberately exerted locally in such zones. The invention thus uses the low ability of the material to transmit pressures deliberately for the purpose of generating pressures in the ceramic material, in a locally confined manner, in those zones which, compared to the adjoining zones, require additional application of pressure.

A particularly advantageous embodiment of the invention provides that the plastic ceramic material is introduced into the cavity in an amount which exceeds the amount required to form a moulding, and that the excess material is extruded from the mould, as it is closed, in a controlled manner, in zones of the moulding which are particularly prone to variation, whereby to set up said prearranged pressures in the material present in those zones.

This embodiment of the process is notably advantageous because it is very simple to carry out and furthermore allows a very simple construction of the mould. Advantageously, the said prearranged, locally confined additional pressures are generated in that the opposing pressure against which the excess amount of ceramic material is extruded is produced by creating throttling resistances in the flow path of the material to be extruded. The throttling resistance can for example be controlled by appropriately choosing the size of the channels or gaps through which the ceramic material flows on being extruded.

However, it is also possible to allow an opposing pressure to act against the extruded material, this opposing pressure being adjustable, or even controllable during the course of the moulding process.

The invention also provides a process for the manufacture of ceramic mouldings by injectionstamping, in which a predetermined amount of a plastic ceramic material is injected or pressed through a sprue into the cavity af a two or more part mould before final closing thereof, and subsequent upon, or simultaneously with, the closing of the sprue the mould parts are moved together into their final position whilst simultaneously exerting pressure on the ceramic material present in the cavity, wherein an amount of the plastic ceramic material is introduced into the mould cavity which exceeds that required to form the moulding and the ceramic material which forms the flow front is discharged from the cavity in an amount corresponding to the excess amount.

This process can be performed, for example, with outflow orifices provided in edge zones of the cavity or in dead spaces which are present in respect of the flow path of the injected ceramic material in the mould cavity, through which orifices the material can, after complete filling of the mould cavity, flow out and, if appropriate, be led into an overflow space. It has in fact been found that as a result of demixing phenomena occurring during injection a structure forms in the flow front of the ceramic material, which cannot be homogenised however high the pressure applied and is in most cases the cause of edge distortions during baking, or even of cracks.

As a result of the procedure described, the structured material in the zone of the flow front is simply removed from the cavity, so that the corresponding zone of the moulding to be produced is formed by the homogeneous material which follows up. In this way, the quality ofthe mouldings to be produced can be improved relatively simply.

In another aspect the invention provides apparatus for the manufacture of ceramic mouldings by injection stamping including a mould which consists of at least two parts which can be brought together in a controlled manner, and an injection or pressing device for injecting or pressing plastic ceramic material into the cavity ofthe mould, the mould having a device or devices for exerting a locally confined pressure on the ceramic material present in the cavity, the device or devices being associated with one or more wall zones of the mould cavity which correspond to the zones of the moulding to be manufactured which are prone to density variations.

In order to carry out the particularly preferred embodiment of the process, described above, the devices for the locally defined exertion of pressure can be formed by outflow orifices leading from the mould cavity in the wall zone or zones, which orifices are either in communication with a space which is under a particular opposing pressure, orwhich are of such dimensions that they exert a throttling action on the ceramic material flowing through them and built up a pressure, corresponding to the throttling action, in the material.These outflow orifices which, for example, in rotationally symmetrical mouldings can also be arranged rotationally symmetrically, for example as a gap in the zones of the cavity wall which forms the zones of the moulding prone to distortion, can be in communication with an overflow space in which the extruded ceramic material collects. These overflow orifices can also be formed, for example, by gaps which result from the fact that the mould parts or mould halves have not moved together completely into their final position but are still at a certain distance from one another.

In order that the invention may be more clearly understood, the following description is given by way of example only with reference to the accompanying drawings in which: Figure 1 is an axial part section through a mould for the manufacture of an approximately square dish, and through the adjoining part of an injection cylinder, with the injection screw present therein; Figure 2 shows an end-face view of a mould half, viewed in the direction of the arrows ll-ll in Figure 1; and Figures 3 and 4 show sections along lines Ill-Ill and IV-IV respectively in Figure 2.

Figure 1 shows a so-called three-plate mould, marked 1 in its entirety, and having mould halves 2 and 3. The mould half 3 consists of two parts 3a and 3b, which can be moved relative to one another. The mould half 2 is also movable, as indicated by the arrow 4. The mould halves 2,3 are actuated by a mould closing device which is not shown, of an injection moulding machine for ceramic materials, of which machine only the end, on the nozzle side, of an injection cylinder 5 is shown. A screw 6 is mounted so as to be rotatable and axially displaceable in the injection cylinder 5, and in the screw 6 there is an axially adjustable pin 7 which acts as a seal for the end of the injection cylinder 5 on the nozzle side and hence also as a seal for the sprue.

The end of the injection cylinder 5 on the nozzle side is firmly connected to the part 3b of the mould half 3 in a manner which is not shown.

The drawing shows the mould halves 2 and 3 in their final position when moved together. It follows from this that between the mould halves 2,3 there is a narrow gap 10a which terminates, in the outermost edge zone 11 of the mould cavity, in an outflow orifice 10. The gap 10a is a plane annular gap, which is of a shape approximating to the circumferential contour of the dish to be moulded in the cavity 8. It is obvious that in a moulding formed in a rotationally symmetrical manner relative to the central sprue, for example a plate, the annular gap has a correspondingly rotationally symmetrical shape. As explained at the outset, the edge of the dish is to be regarded as a zone prone to distortion, this zone being moulded by the edge zone 11 of the cavity 8.

The gap 10a terminates in an overflow space 12 which is formed in the part 3a of the mould half 3.

The plan view of the opened mould 1 according to Figure 2 shows the shape and dimensions of the gap 10a and of the overflow space 12 which surrounds it.

It can be seen that the radial length x of the gap 1 Oa on the longitudinal sides of the cavity 8 (see Figure 4) is significantly smaller than the gap lengthy (Figure 3) in the corner zones of the cavity, as a result of which - assuming a constant height for the gap 10a an increased throttling action is to be expected in the corner zones.

For reasons of clarity, the dish to be produced in the cavity is shown in Figure 2 to 4.

As may further be seen from Figure 1, another slot-like outflow orifice 10, with an adjoining gap 1Oa leading to an overflow space 12, is also formed in the zone 11 a ofthe cavity 8, which zone moulds the foot of the dish. The overflow space 12 is formed between the parts 3a, 3b ofthe mould half 3.

The process according to the invention takes place as follows, using the apparatus shown in the drawings: Starting from the open position of the two mould halves 2,3, which is notshown,the parts 3a, 3b of the mould half3 are first moved into the mutual position shown in Figure 1. Thereafter, the mould half 2 is brought into a position which is shown in dot-dash lines in Figure 1, so that the final position shown has not yet been reached. A certain weight of plastic ceramic material (having a moisture content of, preferably, 16%) is then injected or pressed into the cavity 8 by advancing the screws, and this shot weight exceeds the amount required to form the dish and hence to fill the cavity 8 as defined by the end position of the two mould halves 2, 3.The cavity 8 is thus already completely or almost filled -whilst the mould half 2 is still in the position shown by dot-dash lines. The sprue side of the cavity 8 is now sealed by advancing the pin 7 and, of appropriate, a pressure is exerted on the ceramic material contained in the cavity 8 by axially applying a force to the pin 7 in the direction of the cavity 8. Atthe same time, the mould half 2 is moved into the final position shown in Figure 1. Due to this movement towards the mould half 3, pressure is exerted on the ceramic material which is present in the cavity 8 and largely fills the latter, and the excess of ceramic material is extruded through the gaps 1 0a into the corresponding overflow spaces 12.During mould movement, the gap 10a correspondingtothe outer edge zone 11 of the cavity 8 narrows more and more as the mould halves are brought togethe-r and -only reaches its minimum size in the final position ofthe mould halves 2,3 which is shown in Figure 1.

As a result ofthe impact of the ceramic material, injected from the sprue into the cavity 8, on the cavity wall opposite the sprue, an initial demixing occurs in the ceramic material, which leads to the ceramic material present in the fl ow front having a great deal of structure. A part of this ceramic material possessing structure deposits on the flow path in the zone 1 la of the cavity 8, which zone forms, to some extent, a dead space relative to the flow path, whilst the predom inant part of the ceramic material possessing structure is pushed with the flow front as far as the outermost edge zone 11 ofthe cavity 8.

As a result of the extrusion, described above, of the excess material through the gap 1 Oa, the part of the ceramic-material which possesses structure tends to be removed from the cavity 8 and pass into the overflow spaces 12 corresponding to the gaps Idea.Since the flow front spreads n concentrically to the sprue but the cavity 8 is not rotationally symmet -rical, the material in the flow front, which possesses structures, will reach the edge zone 11 at the longitudinal sides of-the cavity 8, and be pushed out through the gap 10 provided there,earlier than at the corner zones. When therefore, homogeneous mate rial is already present in the edge zone 11 which corresponds to the longitudinal sides of the cavity 8, material possessing structure is still present in the corner zones. It is therefore necessary to expel still more material over the-entire periphery of the cavity, and in particular to do so until homogeneous material -is also present in the corner zones.Since, by virtue ofthe longer-lasting throttling phase atthe longitudinal sides, greater compression of the material results, this increased-compression must be compensated by a correspondingly greaterthrottling action during the throttling phase of shorter duration in the corner zones. This is achieved by the factthatthe gap lengthy (Figure 3) in the corner zones of the cavity.8 is -greater than the gap length x .(Figure 4) in the longitudinal side zones.As a result -of the pressure build-up which this causes in the cornerzones, an -equaily high density to that in the longitudinal side zones is created in the ceramic material in the corner zones, in spite of the shorter duration ofthethrottling phase, so that overall a -uniform --density is obtained in the edge zone ofthe final moulded dish.

The-gap 10a thus exerts a throttling action on the outflowing ceramic material, which leads - restricted to the edge zones 11 and 11a-ofthe cavity 8 - to a .corresponding pressure rise in the ceramic material and to itscorripression which, however, because of the poorpressuretransmission ability of the ceramic m-aterial, remains generally locally confined. This additional compression can be controlled by adjusting the-dimensions of the gap 1-Da, that is to say by adjusting its throttling action.Conse-quently it is possible, for each moulding to be manufactured, to find ind the pressure distribution ofthe ceramic material which gives an undistorted moulding. When the corresponding setting ofthe mould has been found, undistorted mouldings-ean -be mass-produced with this settin:g.

The -ceramic.material which flows into the overflow spaces 12 isremov d during each mould 'release process-and can be-directly returned to the screw 6.

Because ofthe outflow ofthe ceramic material, a flash is produced-on the finished-moulding at the edges which have been formed by the zones 11 and 11 a of the cavity 8. -However, this flash can be removed in the known manner by grinding and/or wiping with a sponge.

It is obvious that the outflow gaps need not have a constant gap height-over their entire length. Rather, it is possible to generate and control the throttling action byelements provided in the gaps.

Claims (13)

1. A process for the manufacture of ceramic mouldings by injection-stamping, in which a predetermined volume of a plastic ceramic material is injected or pressed through a sprue into the cavity of atwo or-more-part-mould before final closing thereof, and subsequent upon, our simultaneously with, the closing of the sprue the mould parts are moved together into their final position whilst simultaneously exerting pressure on the ceramic material present in the cavity, wherein, in zones of the moulding which are particularly prone to density variations, prearranged pressures which influence the density of the material are exerted on the ceramic material in the cavity.

2. A process according to claim 1, wherein an amount of plastic ceramic material is introduced into the cavity which exceeds that required to form the moulding, and the excess material is extruded from the mould as it is closed in a controlled manner, in zones of the moulding which are particularly prone to density variations, whereby to set up said prearranged pressures in the material present in those zones.

3. A process according to claim 2, wherein the excess material is extruded against a specific opposing pressure, and the opposing pressure can be varied during the process of extrusion.

4. A process according to claim 2 or 3, wherein the opposing pressure against which the excess material is extruded is produced in a controlled manner by throttling resistances in the flow path of the material to be extruded.

5. A process for the manufacture of ceramic mouldings by injection-stamping, in which a predetermined amount of a plastic ceramic material is injected or pressed through a sprue into the cavity of a two or more part mould before final closing thereof, and subsequent upon, or simultaneously with, the closing of the sprue the mould parts are moved together into their final position whilst simultaneously exerting pressure on the ceramic material present in the cavity, wherein an amount of the plastic ceramic material is introduced into the mould cavity which exceeds that required to form the moulding and the ceramic material which forms the flow front is discharged from the cavity in an amount corresponding to the excess amount.

6. Apparatusforthe manufacture of ceramic mouldings by injection-stamping including a mould which consists of at least two parts which can be brought together in a controlled manner, and an injection or pressing device for injecting or pressing plastic ceramic material into the cavity of the mould, the mould having a device or devices for exerting a locally confined preessure on the ceramic material present in the cavity, the device or devices being associated with one or more wall zones of the mould cavity which correspond to the zones of the moulding to be manufactured which are prone to density variations.

7. Apparatus according to claim 6, wherein outflow orifices leading from the cavity in the wall zones are formed permanently in the mould or on moving the parts thereof, said orifices communicating with a space which is under a predetermined pressure.

8. Apparatus according to claim 6, wherein outflow orifices leading from the cavity in the wall zones are formed permanently in the mould or on moving the parts thereof together, such orifices being of such size that they exert a throttling action on ceramic material flowing throughtthem and build up a pressure corresponding to the throttling action in the material in the mould.

9. Apparatus according to claim 8, wherein the outflow orifices allotted to a given wall zone are adjoined by outflow gaps and the pressure built up locally in the material is determined by the gap length.

10. Apparatus according to any one of claims 6 to 9, wherein an outflow orifice and an adjoining outflow gap are defined by a gap between the mould parts which remains even in the finally closed position of the mould parts.

11. Apparatus for the manufacture of ceramic mouldings by injection-stamping including a mould which consists of at least two parts which can be brought together in a controlled manner, and an injection or pressing device for injecting or pressing plastic ceramic material through a sprue into the cavity of the mould, wherein outflow orifices are provided in the edge zones of the mould cavity remote from the sprue and/or in dead spaces which the mould cavity possesses in respect of the flow path of the injected ceramic material, which outflow orifices are in communication with an overflow space located in the mould outside the cavity.

12. A process for the manufacture of ceramic mouldings by injection stamping, such process being substantially as hereinbefore described with reference to the accompanying drawings.

13. Apparatus for the manufacture of ceramic mouldings by injection stamping, such apparatus being constructed and arranged substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.