A casing for a writing instrument comprises an outer shaft, an inner shaft, and a nib. The outer shaft is formed using dual injection molding. An outer surface of the outer shaft includes a raised hard part and a raised soft part. The inner shaft is formed of a thin-wall tube having an upper end and a lower end, the inner shaft being configured for insertion inside the outer shaft. The inner shaft further includes a fastener protruding on its upper end that engages an upper end of the outer shaft. The nib is configured to engage the lower end of the inner shaft to help secure the inner shaft within the outer shaft. The raised hard or soft parts on the outer surface of the outer shaft may include an oblique loop-shaped part, a strip-shaped part, and/or an arc-shaped part.