By Default Excel does not print out gridlines with your Excel data.  Here is how to make Excel print out the gridlines for easier document readability.

When you are ready to print out the Excel document click Page Layout tab on the Ribbon, then click on Page Setup

In the Page Setup dialog box click on the Sheet tab.  Place a checkmark next to Gridlines and click OK.  When you print your document you will now have the gridlines appear with your data!