Do iron drain fittings clog more readily than plastic

I recently replaced drain piping beneath my Mom's kitchen sink, following a stoppage downstream from her garbage disposer. The old pipes had been in place perhaps 15 to 20 years. I noticed severe clogging at iron elbows. The plastic nipples were not nearly so bad.
http://img18.photobucket.com/albums/v55/jbecich/plumbing/45_degree_elbow_from_left_sink.jpg Hopefully, the picture will speak for me.
Old houses have much iron sewer piping. Is plastic less inclined to stop up?
TIA John
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The lining on plastic is smooth, and if installed correctly will offer years of trouble free use. Its a common problem when you have nipples, or reducers from one size to another, clogging. Its always best to use the same type of material, as far as you can to the fixtures. There are many types of fittings that would work in this situation, but for the do-it-yourselfer A great connector would be a "no-hub" or a Ferco type rubber connector. These can be found at most local hardware stores. Good luck
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