I have a car model, movies [movieseriesworld.com] 123 movies [www.movieseriesworld.com] that I want to turn into an RBD Object, but when I do, it removes all Materials from that object.
The model that I'm using is a solid model, where I have grouped parts of the primitives to assign different material.

It seems that when making the object RBD it gets an “assemble” node, which is the cause of the materials disappearing. I'm not sure what to change. Any help?

Your object is being packed before going to DOPs… the materials are likely still there, but you'd have to unpack them to see them in the spreadsheet.

If your renders aren't working, you could either unpack after the sim (which can be slow) or you can instruct Mantra to make sure all shader definitions are being included in the IFD. When you generate IFDs for rendering, Houdini is going to try to figure out what shader definitions it needs to bake into the IFD. Packed objects don't always tell Houdini what materials are packed into them, so the materials are considered unneeded and left out. To force Houdini to include them, on your Mantra ROP > Rendering > Render set “Declare Materials” to “Save All Materials”.

You could also use stylesheets to assign materials to the already-packed primitives, and that avoids having to enable that flag on the Mantra ROP. If you have a ton of materials, this will speed up IFD generation greatly, since including ALL materials in the IFD can bloat it quite a bit.