The ornamental design for a plastictubecutter, as shown and described.
FIG. 1 is front view of a plastic tube cutter illustrating my new design;
FIG. 2 is a rear view thereof;
FIG. 3 is a left side view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a bottom plan view thereof;
FIG. 7 is a top perspective view thereof; and,
FIG. 8 is a front view of the plastic tube cutter in an open position.
The broken line showing of the plastic tube in FIG. 8 is included for the purpose of illustrating unclaimed environment and forms no part of the claimed design.