Each time you travel out of the U.S. you need to have a valid travel signature. you can request a valid travel signature by completing a travel form provided by international center:

www.ic.uci.edu/General/travel.php

To request a travel signature on your I-20 (F-1) or DS-2019 (J-1) you will need to drop-off a Travel Form and your I-20/DS-2019 at the International Center. You can submit your documents any time during International Center office ours Monday through Friday 8 am to 5 pm (except holidays). You will then wait 5 business days, and pick-up your signed I-20/DS-2019 with a Photo ID at the IC.