The problem? The Silhouette SD has been discontinued, and so has their custom sized mats. The SD mats are 12x9" while the Cameo's are 12x12" and the Portrait's are 12x8".
I read 3 options online and I'll tell you how each one worked for me.
Option 1: Wash the mats: I used this option for about a year until it no longer worked. You want to make sure you're not washing off the glue and definitely not drying the mats with something that will stick (I let mine air dry where nothing would disturb them rather than patting/rubbing them dry).
Option 2: Use repositional adhesive spray. The idea here is to put your mat into a paper bag, spray it with adhesive and voila, new sticky mat. Just remember that you need a small area on both sides of the mat without adhesive for the machine's runners. I tried this on one of the 2 mats that came with the machine. Turned out to be a BAD idea for me. Yes, it worked, but the adhesive came off the mat and stayed on my paper when I removed it. Now, if you're just gluing your project down and using a properly sized piece of cardstock, then this is great (though I suppose you'd have to keep reapplying the adhesive between uses). I, however, cut a design out of a full piece of cardstock. The leftover cardstock can't be put back in my stash or it will stick to other papers. So I decided this method isn't for me.
Option 3: Use a cutting mat from a different company. I researched this online before buying a double pack of Cricut 12x24" cutting mats. For about US $10 I'd get 4 mats (much better than the $25 the SD mats cost, assuming I could find even a place that still stocked them). There are several differences between the mats.
1) SD mats have adhesive going all the way to the edges, so there's a bit of plastic kept on the 2 sides, so the sheets don't get stuck on the rollers. With the Cricuit mats, the adhesive stops where the grid markings stop. All the way around. While that's not a problem for the sides, it means the top and bottom have less adhesive to keep your paper in place.
|You can see at the 3" mark the mat grids are already diverging.|
2) This is a bit more problematic, but the grid itself doesn't quite line up, despite each mat company using inches. They start out the same but the Cricut's grid slowly gets larger. If you're using scrap paper with the machine you need to know what grid to put it it so it matches the machine's spot for the cut. It's not a huge distance though, and I was able to add the proper markings with a permanent marker (just be sure to let the marker dry before you use the mat or it will smear all over the place - lesson learned the hard way).
3) The Cricut mat's a bit thicker than the SD, so you have to watch the loading to make sure it catches. Maybe even help it out a bit. Once it's in the machine it works just fine.
4) This is probably due to the fact that I haven't had a new mat in 2+ years and so don't remember how sticky the SD mats came, but the Cricut mats were so sticky I ripped one of my designs taking it off the backing. It was a somewhat more intricate cut than usual (definitely more intricate than anything I tried when my SD was new). Still, remember new mats are sticky and be careful taking your paper off.
If you're replacing your cutting mats I hope this post helps.