As you certainly understand already, the interaction of the gate with the core is creating the problem with the weld line.  Without understanding more about the geometry, injection parameters, etc. I cannot give you anything exact, but things to consider include the following:

1. Reducing the severity of the core geometry.  Is the core round?  Square/recatangular?  The more it departs from round, the worse the problem will be, due to residual stresses, etc.

2. Introducing multiple gates, in order to modify the flow regime around the core.  Depending on the size of the part, this may already exist.  Another option to consider is modifying the geometry to form a flow leader, which is basically a build-up in section size near the feature of concern that allows additional molten material to flow into that area.

3. In general, gating for CIM & MIM require much larger gates than for PIM.  Also, gating should be designed such that injection takes place along cavity walls, and does not directly impinge upon a mold surface or a core pin.  Although typically used in PIM, this causes particle/binder separation, and leads to poor weld lines.

A good article on the subject of tooling for MIM (very similar to CIM) appeared in a supplement to Injection Molding Magazine in December 1999.  I have included a link to the on-line article, which unfortunately does not include the figures.

http://www.immnet.com/articlelibrary/archive/getOneArticle.php3?getArtID=1057

Feel free to post additional details, and maybe we can help you further.  What would be most helpful is if you can post some photos on a website.  Then we can more specifically what to try.

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