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This sentence, however, does contain redundant parts.
Primary as used here is an adjective.
Primary is defined as follows:
http://dictionary.reference.com/search?q=primary
adj.
1. First or highest in rank, quality, or importance; principal.
2. Being or standing first in a list, series, or sequence.
3. Occurring first in time or sequence; earliest.
4. Being or existing as the first or earliest of a kind; primitive.
Thus, "only primary" doesn't make sense or is redundant.
You could rewrite this as the following:
The primary function of the printing process is putting the ink on paper.
Or perhaps even better,
The primary function of the printing process is to put ink on paper.