The invention relates to a segmented core for moulding of an impeller and a method for moulding an impeller.

In prior art, impellers are used for a variety of applications. For example, impellers may be used as a rotating component in pumps as, e.g., centrifugal pumps, for transferring energy from a motor that drives the pump to a fluid being pumped. Usually, impellers are made from metal and are configured as short cylinders with an inlet for incoming fluid, vanes for pushing the fluid radially, and a bore to accept a drive shaft.

Small impellers nowadays are made from plastic material by injection moulding. Impellers made of metal can be produced by casting or by welding metal sheets. Both methods are expensive. An effective and economic process known from prior art to shape parts and components in a single operation and in a high volume is the so called metal injection moulding (MIM) process according to which powdered metal is mixed with a binder material so that a feedstock is obtained which may be handled by plastic processing equipment by means of injection moulding.

However, a problem with respect to the above described metal injection moulding process is that complex three dimensional geometries may not be implemented. Therefore, fhe presenf invenfion is based on fhe object†o provide a segmenfed core†o be used for moulding an impeller, and a method for moulding an impeller which allow an impeller to be made by a MIM process and still allowing for a high design freedom to realize complex shapes.

This object is solved by a segmented core having fhe features according to claim 1 and a method for moulding an impeller having fhe features according to claim 1 1 .

According†o fhe present invenfion, a segmented core for moulding of an impeller, especially a pump impeller, is provided comprising af least three segments, all segments having the same form and each segment having connecting means for connecting the segment with fhe adjacent segments. Specifically, by segmenting the core info connectable elements, it is possible†o have free access†o each side of each segment during the moulding process. This is necessary for forming fhe impeller since it is technically impossible to mould fhe different three dimensional segments in one-piece. By providing a segmented core for moulding an impeller, a complex geomefry may be implemented and a great design freedom is thus provided.

According to a preferred embodimenf, two segments border the room for a blade of fhe impeller in the mould.

According†o a further preferred embodimenf, the core has an inner ring and an outer ring.

Preferably, fhe inner and/or outer rings of fhe core are destined for positioning the core in fhe moulding tool which allows an easy handling of fhe core during the moulding process. Further, it is advantageous, if the connection means of the segments are formed as self-locking parts of the inner and outer rings of the core. This also contributes to an easy handling and assembly of the core during the moulding process.

The connection means of the segments may preferably be formed as snap-locking parts which allow for an easy and reliable connection of the segments. Moreover, the segments may be nested and/or tensed up jointly.

According to a still further preferred embodiment, the segments are injection moulded which is a very cost effective method and which, moreover, is suited for mass production.

The segments may consist of a thermoplastic polymer, in particular, of polyoxymethylene (POM).

According to the present invention, there is also provided a method for moulding an impeller with the following steps: Providing a moulding tool, providing a core consisting of at least 3 identical segments, which comprise connection means for connecting the segment with the adjacent segments, assembling of the core and placing the core in the moulding tool, moulding the impeller, and ejecting core and impeller out of the moulding tool, removing the core from the impeller. The inventive method allows for an impeller to be formed in a moulding process with a complex three dimensional shape.

After the moulding process the core may be chemically and/or thermally removed. Preferably, the core segments are made by injection moulding and especially by moulding a thermoplastic polymer or a tin/bismuth alloy.

The impeller may be made by injection moulding and especially metal injection moulding (MIM) which process is well suited for mass production and which is very economical.

It is preferred that the impeller is baked-out, preferably sintered, after removing the core. This provides the desired properties for the thus finished impeller.

Also, it is possible to make the impeller by plastic injection moulding and it would be finished after removing the core. The above features and advantages of the present invention will become yet more apparent upon reading the following detailed description along with the accompanying drawings.

Fig. 1 A and 1 B are respective views of a segmented core according to an embodiment of the invention;

Fig. 2 is a perspective view of a single segment of the segmented core shown in Fig. 1 A and 1 b; Fig. 3 is a perspective view of preassembled segments of the segmented core shown in Fig. 1 A and 1 B;

Fig. 4 is a sectional view of a part of the segmented core with MIM feedstock moulded around the core in the moulding tool; is a perspective view of an MIM feedstock moulded around the core without the moulding tool; and Fig. 6A and 6B are respective views of a finished impeller.

Fig. 1 A and I B are respective views of a segmented core 1 according to an embodiment of the invention. Specifically, Fig. 1 A is a perspective view of the segmented core 1 with one segment 2 removed and Fig. 1 B is a partial sectional view of the segmented core 1 . As can be seen in Fig. 1 A, the segmented core of the embodiment consists of six single segments 2 which are connected to each other by connecting means 3. Specifically, there are respectively provided two first connecting means 3 arranged at an outer portion 4 of each segment 2 whereby the outer portions 4 of all segments 2 together form an outer ring 5, and there is respectively provided one second connecting means 3' provided at an inner portion 6 of each segment 2 whereby the inner portions 6 of all segments 2 together form an inner ring 7. The connection means 3, 3' of the segments 2 are formed as self-locking parts of the inner and outer rings 5, 7 of the core 1 . In the embodiment, the connection means 3, 3' of the segments 2 are formed as snap- locking parts. The inner ring 5 and the outer ring 7 of the core 1 are positioning aids for positioning the core 2 in a moulding tool. Further, each two adjacent segments 2 define and delimit the space for a blade of the impeller to be formed in a further process step in the mould by metal injection moulding. The segments 2 are also injection moulded and consist of a thermoplastic polymer, specifically, of polyoxymethylene (POM).

Fig. 2 is a perspective view of a isolated single segment 2 of the segmented core 1 shown in Fig. 1 A and I B. As can be seen in this enlarged view of the segment 2, each segment 2 comprises the two first connecting means 2 at the outer portion 4 and one connecting means 3' at the inner portion 6 which all are formed as self-locking means and are configured †o engage with correspondingly formed connecting means 3, 3' or neighbouring segments 2 when the core 1 is assembled. Fig. 3 is a perspective view of preassembled segments 2 of the segmented core 1 shown in Fig. 1 A and I B. As can be seen, in the assembling process of the core 1 which consists of six segments 2, in a first step, three segments 2 are preassembled by respectively connecting them via their respective connecting means 3, 3'. Then, in a second step, the two halves of the core 1 each consisting of three segments 2 will be assembled to form a complete core 1 . For assembly of the two halves each consisting of three segments 2, these are connected to each other by "sliding" them together. For assembly of the segments 2 and for handling the assembled segments 2 in insert moulding, it is necessary that all of the segments 2 are self-locking.

Fig. 4 is a sectional view of a part of the segmented core 1 with MIM feedstock 8 moulded around the core 1 in the moulding tool 14. When placed in the moulding tool 14, the segments 2 have been assembled as described above in connection with Fig. 3. After having been placed in the moulding tool 14, the segmented core 2 acts as an inner moulding part and the MIM feedstock 8 is moulded on the outside of the core 1 . The number of segments 2 of the core 1 represents the internal geometry of the impeller to be produced. As the number of segments 2 in the embodiment shown is six, the number of vanes or blades of the impeller to be produced will also be six. Further, as can be seen in Fig. 4, since it is essential that the segments 2 are sufficiently supported when loaded, there are provided supports 10. Also, larger segments 2 may have the tendency to deflect during insert moulding as a function of the injection pressure and thermal influence from the feedstock 8. To prevent this, ejectors 1 1 are provided which help to support the segments 2 during injection and retract these during the holding time. For larger MIM parts, there could be a critical parameter in the difference in thermal expansion between fhe MIM feedstock 8 and the segments 2. This would eventually generate cracks during de- binding due to fhe difference in thermal expansion. This can be avoided partially by heating the segments before fhe insert moulding in order to compensate for this expansion.

Fig. 5 is a perspective view of a metal injection moulded (MIM) feedstock 8 moulded around the segmented core 1 without the moulding fool. As can be seen, fhe feedstock 8 covers the segmented core 1 from fhe fop and is engaged af its oufer circumference by fhe oufer ring 5 of fhe segmented core 1 . Also, the inner ring 7 is fitted in an inner ring portion 9 of fhe feedstock 8. As mentioned above, fhe segments 2 of fhe core 1 are moulded in POM which is fhe main component of fhe binder system of fhe MIM feedstock 8 and which is the component which will react with the HNO3 in fhe de-binding process described below.

Fig. 6A and 6B are respective views of a finished impeller 12 wherein Fig. 6A is a perspective view and Fig. 6B is another perspective view with a front part cut away so that the inferior of fhe impeller 12 can be seen. Specifically, after the injection moulding process has been completed, †o obtain the finished impeller 12, fhe segments 2 of fhe segmented core 1 have been removed by a de-binding process which is a catalytic process in which the POM of fhe segments 2 reacts with HNO3 whereby formaldehyde is created which can easily escape from fhe metal particles. The remaining feedstock then is subjected to a sintering process where after the finished impeller 12 is obtained. As can be seen in Fig. 5B, the inferior of fhe impeller 12 in which fhe vanes or blades 13 are accommodated has a complex geometry. Nevertheless, the entire impeller 12 is formed as one metal injection moulded piece. I† is also possible†o produce the impeller 12 by a traditional injection moulding process using plastics. In this case, the inserted core 1 would normally be an alloy of Tin/Bismuth having a very low melting point. For releasing the core from the moulded impeller 12, the latter would be placed in an oil bath having a predetermined temperature so that the core would melt and leave the impeller 12 with its internal geometry created by the thus formed hollow spaces.

Reference Numerals

segmented core

segments

, 3' - connecting means

outer portion

outer ring

inner portion

inner ring

feedstock

inner ring portion

0 support

1 ejectors

2 impeller

3 blades

4 moulding tool