"I designed this part, and it 3D Prints Great! What do you mean it can't be injection molded?"We hear this day in and day out. 3D Printing is fantastic, it gives everybody from 2nd graders to degreed engineers the ability to manufacture whatever they imagine. Unfortunately, it gives users the false sense of success for future manufacturing.
It is necessary to maintain a uniform wall thickness in the areas of the substrate that will be filled or covered by the second shot. The same design principles that apply to plastic parts apply to the voids in the substrate to be filled by the 2nd shot. These would include ribs being 60% of the thickness of intersecting walls, and generous radii and fillets for flow. Typically there is no shrinkage applied to the 2nd shot mold since the substrate will not shrink after the over-molding process.