Solution 1:
Disable the "Bitmap Printing" option in the print dialogue of Adobe® Illustrator®.

1. From the Adobe® Illustrator® application, select [File]=>[Print].
2. Check off the "Bitmap Printing"option.

Solution 2:
Disable the "Improve Gray Printing" option in the printer driver.

1. From the Adobe® Illustrator® application, select [File]=>[Print].
2. Click the Properties button, and select the Advanced tab.
3. Select Manual at the Print Setting section and click the Setting button.

4. Check off the "Improve Gray Printing" option, and click OK.

5. Click OK.
6. Click OK.