"I am assisting students in a photo silk screen We scan the photo and
then
copy it onto inkjet transparency film. The transparency does not appear
to have a moire pattern but when we expose the transparency on the photo
sensitive silk screen, a moire appears and show up on the resulting
print."
It sounds like you are screening it twice, once when you scan it which
adds pixels and again when you expose it onto silkscreen. If you expose
a black and white photo or a black and white zerox copy, there are no
dots in the original you are exposing onto silkscreen.