Paper Mate InkJoy Ballpoint Pen Ink Refill for 300/550 RT, Medium, 3-Pack (1807421) This is an ink refill for Ink joy 300/550 RT The technology of Ink joy: a unique blend of ultra-low viscosity inks and optimized writing tips gives you crisp, clean lines and unparalleled smoothness. The art of Ink joy: a range of sleek and modern pen designs houses a rich palette of vivid ink colors to dazzle your writing imagination. Experience Ink joy quality time after time. Using ink refills is more economical and produces less waste than buying pens over and over again.