large piece need to be ultrasonic welded

Sir
I need a Sonotrod for a 450 x 155 MM freezer door in the refrigrator. It is 2 piece Gpps plastic injection door. We have 20Khz branson machines. they are telling us to weld 6-8 times by making 160 x 40 x 140 mm tall sonotrod off coarse it need to be tuned and calculated.
Why this can not be weld by one sonotrod? I like to get a controle of this calculation in our own hand rather than waiting outside help. How we can do it? What we need to test the frequency ? what we need to calculate the maximum size of sonotrod we can use?
Regads
Surjit Virdi
Mold2Specs