Stack inserter

The stack inserter is an inserter upgrade that moves items in larger batches than a normal inserter. Its defining feature is that it can pick up and drop multiple items at once, with its stack size determined by researched inserter capacity bonus technologies and by any manually set stack override, up to the current maximum unlocked value.

Its default stack size starts at 6 items. Researching inserter capacity bonus increases that size further: bonus 1 raises it to 7, then 8, 9, 10, 12, 14, and finally 16 at bonus 7. The stack size can also be overridden to a smaller value for a specific inserter, but not above the highest researched limit. Because stack inserters are already late-game logistics tools, these upgrades are often available by the time they are built.

When inserting onto belts, stack inserters interact specially with belt stack size research. They always try to place items in stacks matching the current maximum belt stack size, and the hand size is effectively rounded down to that same multiple when dropping onto a belt. This means they usually place full stacks even when the item count in hand is larger than the stack they are about to create. On belts, the maximum stack size increases from 1 to 2 with Stack inserter research, and later to 4 with two further researches. If the hand size would round down to 0 because it is smaller than the belt stack size, the inserter instead uses the exact hand size and creates a partial stack.

There are a few important exceptions to the usual belt behavior:

• If the inserter is holding a spoilable item and it spoils, it will immediately drop that item into its destination, building whatever stacks it can on a belt.
• Changing the stack inserter’s filter can also force it to drop its current hand.
• If belt stack research changes while the inserter is moving toward the belt, it will use the new belt stack limit when it places the items.

When inserting into containers or machines, the stack size is respected exactly rather than being rounded to fit a belt stack.

A useful feature of stack inserters is that they normally do not turn away until their hand is full. This is often desirable for maximizing throughput, but it can also make them hold items longer than wanted. The usual way to force a stack inserter to discard its hand is to change its filter so that the item it is carrying is no longer valid. Then it immediately turns to drop the contents.

This is especially effective with circuit-network control. If a stack inserter is wired to a container or machine and set to filter based on circuit conditions, it can automatically stop taking an item once the source no longer provides it, which causes the inserter to drop the items already in hand. This works well for containers and for machines that broadcast their contents, including input slots, though recipes with more than 5 total relevant item signals can become awkward because inserter filtering can only consider 5 items at once. For spoilable-item recipes, possible spoil products also count toward that limit.

Stack inserters can also use a belt’s read contents signal when taking from a belt, but doing so removes the normal waiting behavior and can cause extra insertions beyond what is strictly needed.