Stack inserter

The stack inserter is an advanced inserter that can move multiple items at once instead of a single item, making it especially effective for high-throughput transport on belts and in factories. Like other inserters, its stack size depends on the Inserter capacity bonus technology, and the maximum stack size can be overridden by the inserter’s settings up to the highest researched level.

Its base stack size is 6 items, and research increases it as follows: capacity bonus 1–4 raises it by +1 each time, while 5–7 raise it by +2 each time, for a total stack size of 7, 8, 9, 10, 12, 14, and 16. When a stack inserter inserts onto a belt, it tries to form stacks up to the current researched belt stack limit. The Stack inserter research raises the maximum belt stack to 2, and two further researches raise it to 4.

When putting items onto a belt, the stack inserter has special behavior with its hand size: the amount in hand is rounded down to the nearest multiple of the current belt stack limit. This usually causes it to place full stacks even when its hand contains more items than that exact multiple. If the hand size is smaller than the belt stack limit, it will instead place a partial stack. The same happens if the stack inserter is carrying a spoilable item that spoils before it is inserted, or if its filter changes while it is holding items. A very rare edge case is that if the researched belt stack limit changes while the inserter is already moving to insert onto a belt, it will use the new limit for that insertion.

When inserting into containers or machines, the inserter respects its hand size exactly rather than rounding it to belt stack sizes.

A few practical points are worth keeping in mind:

• The stack inserter usually does not turn until it has a full hand. This is useful for throughput, but it can be inconvenient when you want it to dump its current load early.
• The most direct way to force it to drop what it is holding is to change its filtered item list so that the item in hand is no longer a valid filter.
• This can be automated with the circuit network. If the stack inserter is set to filter from a wired container or machine, and the available item disappears from the network signal, the inserter will immediately turn and drop its hand.
• This technique also works with crafting machines that broadcast their contents, including input slots, but it can run into the inserter filter limit of 5 items. If a recipe has more than 5 total relevant items, the filter selection may exclude the desired output, and spoilable items also count their spoil products toward that limit.
• When reading belt contents through circuits, a stack inserter can be made to react to what is on the belt tile it is drawing from, but this removes its ability to wait for a larger stack and can therefore cause extra insertions.