When cutting mats for picture framing you will enjoy better results if you use a slip sheet. Sometimes referred to as a backing sheet or an underlayment, a slip sheet will insure sharp, clean edges on your mats' windows and effectively increase the longevity of your mat cutting blades.
A slip sheet is nothing more than a piece of scrap mat board, a remnant left over from a larger sheet when you cut it down to size. You will acquire plenty of potential slip sheets from these remnants. Ideally it should be about 5-1/2" wide and the length of your mat cutter's cutting bed, but it doesn't have to be. It really only needs to be wide enough to cover the area where the blade will penetrate the mat.
The purpose of the slip sheet is to provide support for the face paper of the mat that you are cutting. By providing this support it has the desirable affect of producing cleaner cuts with fewer rough or ragged edges.
To understand the benefits of a slip sheet it's best to consider what happens when it is not used. With most mat cutting systems it is possible to cut a mat without using a slipsheet. The slot that runs down the middle of the cutting bed provides airspace for the blade to enter after penetrating the mat, so damaging or chipping the blade is not the issue.
However, since most mats are cut face down, the blade penetrates the back of the mat first. If the blade is starting to become dull, as it passes through the core of the mat and emerges from the face, it will tend to erupt the face paper into the airspace provided by the slot as it breaks through. This erupting and breaking of the face paper is manifested as a rough or ragged edge on the window of the mat.
Those who fail to use a slip sheet will find themselves changing blades often, thinking the blade has gotten dull. Those who use a slip sheet, on the other hand, will get more use out of each blade. That's because the slip sheet provides support for the face paper of the mat.
With a slip sheet in place, even if the blade is dull it cannot erupt the face paper of the mat as it breaks through. The slip sheet provides stability for the face paper so the blade is made to slice through rather than break through and the result is a clean, sharp edge on the mat's window.
It's important to move the backing sheet every so often so you don't cut repeatedly into the same place, but if you do this, and replace your slip sheet when it has too many cuts in it, you will find that you are getting better results generally. You will find that rough and ragged edges are fewer and further between.