METHOD AND APPARATUS FOR FORMING ARTICLES

Technical Field

This invention relates to a moulding method and apparatus for forming articles and refers particularly, though not exclusively, to such a method and apparatus for producing thin hollow tubes and plates.

Background

For the production of articles such as ceramics tubes and tubes with multiple holes, there is a known method using a sliding sleeve with a core. During forming, the core blocks a portion of the flow path of the material entering the mould (forming the holes), while the sliding sleeve helps to keep the core in position to minimize thermal and bending stresses within the formed tubes, reducing problems of distortion and deformation. The core can have one or many pins, depending on the number of holes to be formed, and the resultant article has good material uniformity throughout, even for articles with high aspect ratios.

To produce ceramic articles with reduced thickness at the tens of microns level, tape casting may be used. In tape casting, thin layers of ceramic-loaded polymers are rolled out and can be used as single layers or can be stacked and laminated into multilayered structures. They may be in batch or continuous casts. The prime dried thickness range for tape casting is reported to range from 5 microns to slightly over lmm.

Despite the availability of the above methods to produce hollow tubes and thin sheets, there is still a need for a method and apparatus that can produce ceramic tubes with walls that are only tens of microns thick up to several millimetres, but with a more uniform stress distribution during part forming. Tape casting can produce only flat sheets. The sliding sleeve and core method is limited to forming articles with less stringent requirements on internal stress distribution of the formed parts. For thin parts, however, uneven distribution of stress or pressure during forming may cause distortion especially for parts that have high aspect ratio, e.g., length over thickness. In such cases, although high pressure forces material to fill up a cavity from one end to another end to

form an article, forming pressure varies along the cavity and affects stress distribution along the length of the article.

Summary According to a first exemplary aspect, there is provided a method of moulding an article. The method comprises injecting a material into a cavity of a die set comprising a movable cavity tool and a stationary cavity tool; advancing the movable cavity tool relative to the stationary cavity tool while injecting the material so that the cavity elongates together with formation of at least a first portion of the article, the movable cavity tool receiving and retaining injected material as the cavity elongates.

The method may further comprise injecting a second material into the cavity of a die set; advancing a second movable cavity tool while injecting the second material so that the cavity expands together with formation of at least a second portion of the article, the second movable cavity tool receiving and retaining the second injected material as the cavity expands.

According to another exemplary aspect there is provided apparatus for moulding an article. The apparatus comprises a die set having a cavity, the die set comprising a movable cavity tool and a stationary cavity tool configured to form the cavity therebetween, the movable cavity tool being movable relative to the stationary cavity tool. The movable cavity tool being configured for elongating the cavity during injection of a material to form at least a first portion of the article, and for receiving and retaining injected material as the cavity elongates.

The apparatus may further comprising a second movable cavity tool for expanding the cavity during injection of a second material to form at least a second portion of the article; the second movable cavity tool receiving and retaining the second injected material as the cavity expands.

For all aspects, the cavity may include a core for forming at least one elongate opening in the article. The core may be attached to the movable cavity tool. The core may comprise a plurality of pins for forming a plurality of elongate openings in the article.

The core may be slideably supported by a core support to minimize bending of the core during injection of the material. The core support may be integral with the stationary cavity tool. The core support may include at least one extension for additional support of the core.

The movable cavity tool may be advanced by pressure of the material being injected into the cavity.

The material may be injected at an angle less than 90° to provide a force component in the direction of advancing the movable cavity tool. The angle may range from 25° to 60°.

The movable cavity tool may be advanced by mechanical actuation of the movable cavity tool.

According to yet another aspect there is provided an article produced by the above method. The article may be a thin plate. The thin plate may comprise at least one through channel and/or at least one surface channel.

Brief Description of the Drawings In order that the present invention may be fully understood and readily put into practical effect, there shall now be described by way of non-limitative example only exemplary embodiments, the description being with reference to the accompanying illustrative drawings.

In the drawings:

Figure 1 is a schematic cross-section of an exemplary embodiment at the beginning of the moulding process;

Figure 2 is a schematic cross-section of the embodiment of Figure 1 during the moulding process;

Figure 3 is a schematic cross-section of the embodiment of Figures 1 and 2 at the end of the moulding process; Figure 3 A is a schematic cross-section of an alternative exemplary embodiment with a core during the moulding process;

Figure 4 is a schematic close-up cross-section of the embodiment of Figure 2;

Figure 5 is a perspective view of a core support;

Figure 6 is a schematic perspective view of an exemplary embodiment of the cavity tools;

Figure 7 is a schematic perspective view of another exemplary embodiment of the cavity tools;

Figure 8 is a schematic perspective view of a further exemplary embodiment of the cavity tools; Figure 9 is a schematic side view of yet another exemplary embodiment of the cavity tools;

Figure 10 is a schematic side view of a still further exemplary embodiment of the cavity tools;

Figure 11 is a collage of perspective views of products able to be produced by the process; and

Figure 12 is a schematic illustration of the process steps with an exemplary embodiment.

Detailed Description of the Exemplary Embodiments Figures 1 to 3 show a die set 10 that forms a cavity 12 in which a green article is to be formed (prior to debinding and sintering). The die set 10 includes a movable cavity tool 14 and a stationary cavity tool 15. The movable cavity tool 14 is supported by a base plate 18. Bearings 20 between the base plate 18 and the movable cavity tool 14 facilitate smooth movement of the movable cavity tool 14 between the base plate 18 the stationary tool 15. To allow free movement of the movable cavity tool 14, clamping force on the die set 10 is not directly applied to the movable cavity tool 14.

Preferably, the movable cavity tool 14 has a head 17 to substantially seal with the stationary cavity tool 15, while a gap between a tail 19 of the movable cavity tool 14 and the stationary cavity tool 15 forms the cavity 12.

During moulding/forming, a material 16 from feedstock in a material barrel 21 is injected via a runner 26 through a gate 27 into the cavity 12 (100, Figure 12). The material 16 may be injected by means such as a powder injection moulding machine, or a hydraulic press. Depending on the shape of the green article to be formed, the runner 26 may take the shape of a nozzle or a slot covering a part or a whole width of the article to be formed.

While the material 16 is being injected, the movable cavity tool 14 is advanced relative to the stationary cavity tool 15 (as indicated by the arrow) so that the cavity 12 elongates (102, Figure 12). Newly injected material is preferably continuously received and retained by a receiving section 24 on the tail 19 of the movable cavity tool 14. The receiving section 24 is always directly at the site of the injection gate 27.

Because the movable cavity tool 14 is continuously advancing during injection, the receiving section 24 is continuously moving back along the tail 19 during injection. There is thus practically no pressure gradient in the material 16 within the cavity 12 since the receiving section 24 is always a "fresh" location on the tail 19 of the movable cavity tool 14. In this way, the material 16 can readily fill the cavity 12 under relatively low injection pressure for a theoretically infinite length of the cavity 12.

This method and apparatus are thus particularly suitable for producing high-aspect-ratio (i.e. long and thin) articles such as thin plates and other articles with highly uniform walls that are just tens of microns thick or up to millimetres thick with uniform stress distribution along the length of the article. Green articles such as sheets produced by this method and apparatus can be uniform with isotropical shrinkage.

The movable cavity tool 14 may be advanced simply by pressure of the injected material 16 acting against the head 17 of the movable cavity tool 14. This is facilitated

by having the runner 26 at an injection angle α less than 90°, more preferably between 25° and 60°, to provide a force component in the direction of the arrows on Figures 1 and 2. Alternatively, the movable cavity tool 14 may be advanced by simple mechanical actuation. A combination of material pressure and mechanical actuation may also be used.

While it has been described that the movable cavity tool 14 advances, movement of the movable cavity tool 14 is relative to the stationary cavity tool 15. The same relative movement can be achieved by moving the stationary cavity tool 15 together with the material barrel 21 and the runner 26 in the opposite direction to that shown by the arrow in Figures 1 to 3, while immobilising the movable cavity tool 14.

To form green articles having elongate openings therein, the cavity may include a core 22. Preferably, the core 22 is attached to the movable cavity tool 14 so that it advances together with the movable cavity tool 14 during moulding. Figure 3 A shows an alternative exemplary embodiment of the movable cavity tool 14 with attached core 22, movable together relative to the stationary cavity tool 15. This embodiment may be used for the formation of cylindrical and other shaped articles, including tubes.

The core 22 may be supported by a core support 28, as shown close up in Figure 4. The core support 28 may be separate or integral with the stationary cavity tool 15. The core support 28 is for minimising bending of the core 22 due to downward pressure of the material entering from the runner 26 in the direction as shown by the arrow in Figure 4.

The core 22 may comprise a plurality of pins so that a plurality of elongate openings is formed in the green article. Accordingly, the core support 28 may comprise an appropriate number of elongate openings 29 therein for slideably supporting the pins of the core 22. Where preferable, the core support 28 may include at least one extension 30 to provide additional support to the core during material filling, as shown in Figure 5. The length of each extension 30 is preferably about or greater than the width of the runner 26.

Depending on the cross-sectional size and shape of the cavity 12 and the core 22, articles of various shapes and sizes having a varying number of openings therein may be formed. By controlling the length and positioning of the core 22, the openings may be through or blind holes as desired.

Figures 6 to 10 show various exemplary embodiments of the cavity tools 14, 15. In each of these figures, the stationary cavity tool 15 as shown is simplified for clarity. In Figure 6, the movable cavity tool 14 is a relatively simple plate for producing thin sheets. In Figure 7, the movable cavity tool 14 has additional side walls 14s. In both cases, when used together with an appropriate core 22 (Figures 1 to 3), the cavity tools 14, 15 allow for the formation of thin walled tubes. The side walls 14s are an example of a design variation to help control external wall thickness of the green article formed.

Li the embodiment of Figure 8, the movable cavity tool 14 comprises a first movable cavity tool 14a and a second movable cavity tool 14b. The movable cavity tools 14a, 14b can be independently advanced for the injection of the same or different materials associated with each movable cavity tool 14a or 14b. Using the embodiment of Figure 8, a first material is injected into the cavity 12 (100, Figure 12) while only the first movable cavity tool 14a is advanced (102, Figure 12). When the first material is sufficiently solid, a second material is injected into the cavity 12 (103, Figure 12).

Sufficiently solid is to be taken that the first material is of sufficient solidity to minimize transference at the interface of the first and second materials. While the second material is being injected, only the second movable cavity tool 14b is advanced so that the cavity 12 expands (104, Figure 12). The second material fills the expanded cavity alongside the first material 16, thus coping with any irregularities in the interface surface of the two materials. In this way, a green article having a first portion made of the first material and a second portion made of the second material can be formed.

Various other configurations of the movable cavity tools 14a, 14b may be devised according to the desired green article to be formed. For example, Figure 9 shows the two movable cavity tools 14a, 14b in an alternative configuration, with the first movable cavity tool 14a atop the second movable cavity tool 14b, and the stationary cavity tool

15 in between. This configuration is suitable for the production of articles comprising double layer sheets, with each sheet ranging from hundreds of microns to millimetres thick. Each sheet may be made of the same or of different materials. In this embodiment, the injections gates are preferably coplanar with each sheet to be formed, such that sheet thickness is dependent on sheet width.

Embodiments having two movable cavity tools may also comprise a plurality of pins for forming elongate openings in the green article. A number of the plurality of pins may be attached to the first movable cavity tool 14a, while another number of the plurality of pins may be attached to the second movable cavity tool 14b. In this way, both the first portion and the second portion of the green article formed can have elongate openings therein depending on the number and location of the pins.

Articles having additional features such as surface or through channels may also be formed using appropriately styled cavity tools 14, 15. Figure 10 shows an exemplary embodiment comprising protrusions 36 on the movable cavity tool 14. Corresponding recesses 38 on the stationary cavity tool 15 may be provided. Depending on the height of the protrusions 36, surface or through channels may be formed in the moulded article. Alternatively, protrusions may be provided on the stationary cavity tool 15 while corresponding recesses are provided on the movable cavity tool 14.

Depending on the intended use of the article after subsequent processing such as debinding and sintering, the first material and the second material may be different, or they may be the same material but having different material properties such as different particle size. In the case of ceramic tubes, the first material and the second material may have different percentage volume or particle size of the same or different ceramic particles in a polymer binder.

The polymer binder, for example wax in the feedstock, may provide a self lubricating effect to facilitate material flow in the cavity during moulding. A limited mixing of the two materials at the interface between the first portion and the second portion of the

green article during moulding may also be advantageous. Mixing at the interface may promote better fusion of the two portions upon sintering.

Examples of green articles that can be formed by various embodiments are depicted in Figure 11.

Although one and two movable cavity tools have been described, multiple movable cavity tools may be provided to form green articles having more than two portions, each portion having a different material property from the others. Cores having various cross- sectional shapes and sizes and numbers of pins may be provided in various combinations with various numbers of movable cavity tools having various cross- sectional shapes and sizes, so that different complex structures having reduced thickness may be formed.

Whilst there has been described in the foregoing description exemplary embodiments, it will be understood by those skilled in the technology concerned that many variations or modifications in details of design or construction may be made without departing from the present invention.