Filter manufacturing machine, filter manufacturing method using the machine, and hollow filter

A filter manufacturing machine comprising:

• a feed path along which filter tow is continuously fed;
• a forming device connected to a tail end of said feed path, said forming device being configured to form the filter tow into a hollow rod-shaped continuous filter body and send out the formed continuous filter body,
• wherein said forming device includes:
• a forming tube having a tubular forming path through which the filter tow is passed while being brought together to be formed into the continuous filter body,
• a mandrel arranged in said forming tube and forming the tubular forming path in cooperation with said forming tube, and
• a heat treating section provided in said forming tube and configured to heat the filter tow while the filter tow passes through the tubular forming path,
• wherein said mandrel has a small-diameter portion extending from an inlet of said forming tube toward an outlet of said forming tube, a large-diameter portion connected to the small-diameter portion and located downstream of the small-diameter portion, and a diameter expansion portion formed between the small- and large-diameter portions, and
• the diameter expansion portion of said mandrel is within said forming tube and located downstream of said heat treating section as viewed in a moving direction of the filter tow.