FIG. 1 is a front perspective view of a portion of a mask frame according to a first embodiment of our design;
FIG. 2 is a rear perspective of the mask frame shown in FIG. 1;
FIG. 3 is a front view of the mask frame shown in FIG. 1;
FIG. 4 is a rear view of the mask frame shown in FIG. 1;
FIG. 5 is a right hand side view of the mask frame shown in FIG. 1, with the left hand side view being a mirror image thereof;
FIG. 6 is a top view of the mask frame shown in FIG. 1;
FIG. 7 is a bottom view of the mask frame shown in FIG. 1;
FIG. 8 is a front perspective view of a portion of a mask frame according to a second embodiment of our design;
FIG. 9 is a rear perspective of the mask frame shown in FIG. 8;
FIG. 10 is a front view of the mask frame shown in FIG. 8;
FIG. 11 is a rear view of the mask frame shown in FIG. 8;
FIG. 12 is a right hand side view of the mask frame shown in FIG. 8, with the left hand side view being a mirror image thereof;
FIG. 13 is a top view of the mask frame shown in FIG. 8;
FIG. 14 is a bottom view of the mask frame shown in FIG. 8;
FIG. 15 is a front perspective view of a portion of a mask frame according to a third embodiment of our design;
FIG. 16 is a rear perspective of the mask frame shown in FIG. 15;
FIG. 17 is a front view of the mask frame shown in FIG. 15;
FIG. 18 is a rear view of the mask frame shown in FIG. 15;
FIG. 19 is a right hand side view of the mask frame shown in FIG. 15, with the left hand side view being a mirror image thereof;
FIG. 20 is a top view of the mask frame shown in FIG. 15;
FIG. 21 is a bottom view of the mask frame shown in FIG. 15;
FIG. 22 is a front perspective view of a portion of a mask frame according to a fourth embodiment of our design;
FIG. 23 is a rear perspective of the mask frame shown in FIG. 22;
FIG. 24 is a front view of the mask frame shown in FIG. 22;
FIG. 25 is a rear view of the mask frame shown in FIG. 22;
FIG. 26 is a right hand side view of the mask frame shown in FIG. 22, with the left hand side view being a mirror image thereof;
FIG. 27 is a top view of the mask frame shown in FIG. 22; and,
FIG. 28 is a bottom view of the mask frame shown in FIG. 22.
Where included, broken lines showing environment or structure are for illustrative purposes only and form no part of the claimed design. Where included, dash-dot lines represent the border between claimed and non-claimed subject matter and form no part of the claimed design.