I have several non-accumulating slug conveyors that feed in to a merge. (I will either use a merge controller or the fixed-gap process flow posted in 2016 by @anthony.johnson (Article name "Process Flows for Two Types of Merge Controller"). The only difference is I (or rather our controls engineer) want the slug to advance to the head of the conveyor once it it built, so that it is closer to the merge when it is released. In other words, once the conveyor gets to (e.g.) 85% full, it stops accepting new cartons and moves all the cartons on it forward for the remaining 15%, so that the first carton is at the discharge end of the conveyor, before the slug is considered ready for release.