The invention relates to the general field of powder metallurgy and compression molding with particular reference to forming complex structures.
The production of metal or ceramic components using powder injection molding (PIM) processes is well known. The powder is mixed with the binder to produce a mixture that can be molded into the desired part. The binder must have suitable flow properties to permit injection into a tooling cavity and forming of the part. The molded part is usually an oversized replica of the final part. It is subjected to debinding where the binder is removed without disturbing the powder orientation. After the binder is removed, the part is subjected to sintering process that results in part densification to a desired level.
The parts produced by PIM may be complex in geometry. They also tend to be made of a single material. For example, an orthodontic bracket can be made of 316L stainless steel using PIM technology.
There is, however, a need for objects, formed by PIM, that contain multiple parts, each of which is a different material whose properties differ from those of its immediate neighbors. The prior art practice has been to form each such part separately and to then combine them in the finished product using costly welding operations or mechanical fitting methods to bond these different parts of different materials together.
The basic approach that the present invention takes to solving this problem is schematically illustrated in FIGS. 1a and 1b. In FIG. 1a, 11 and 12 represent two green objects having different physical properties and formed by PIM. FIG. 1b shows the same two objects, after sintering, joined to form a single object. In the prior art, the interface 13 between 11 and 12 was usually a weld (i.e. a different material from either 11 or 12). Alternately, a simple press fit between the 11 and 12 might have sufficed so that the final object was not a continuous body.
An obvious improvement over welding or similar approaches would appear to have been to sinter 11 and 12 while they were in contact with one another. In practice, such an approach has usually not succeeded due to a failure of the two parts to properly bond during sintering. The present invention teaches how problems of this sort can be overcome so that different parts made of materials having different physical properties can be integrated to form a single continuous body.
A routine search of the prior art was performed with the following reference of interest being found: In xe2x80x9cComposite parts by powder injection moldingxe2x80x9d, Advances in powder metallurgy and particulate materials, vol. 5, pp 19-171 to 19-178, 1996, Andrea Pest et al. discuss the problems of sintering together parts that comprise more than one material. They show that control of shrinkage during sintering is important but other factors (to be discussed below) are not mentioned.
It has been an object of the present invention to provide a process for the formation of a continuous body having multiple parts, each with different physical properties.
This object have been achieved by using powder injection molding together with careful control of the relative shrinkage rates of the various parts. Additionally, care is taken to ensure that only certain selected physical properties are allowed to differ between the parts while others may be altered through relatively small changes in the composition of the feedstocks used.
Another object has been to provide a process for forming, in a single integrated operation, an object that is contained within an enclosure while not being attached to said enclosure.
This object has been achieved by means of powder injection molding wherein the shrinkage rate of the object is caused to be substantially greater than that of the enclosure. As a result, after sintering, the object is found to have detached itself from the enclosure, being free to move around therein.