Stack inserter

A Stack inserter is a long-reach inserter variant that picks up and places multiple items at once. It is used to move larger quantities between belts, chests and machines more efficiently than basic inserters. Its pickup amount (hand size) is determined by the Inserter capacity bonus research levels and can be manually overridden up to the current researched maximum.

When inserting onto a belt, the stack inserter builds stacks according to the currently researched maximum belt stack size rather than strictly by its hand size. The game research increases the maximum belt stack to 2 at the first relevant technology, with later researches raising it to 4; the stack inserter will attempt to form stacks of that maximum whenever placing onto a belt. For non-belt targets (machines and containers) the inserter always respects its exact hand size.

The stack inserter’s hand size increases with the Inserter capacity bonus research. Base total hand sizes by research level are: 6 with no bonus, then 7, 8, 9, 10, 12, 14, 16 at successive upgrade levels. The hand size can be set to any value up to the current researched maximum to tune throughput.

Because belt stacking uses the researched belt stack limit, the effective items moved when inserting onto belts is subject to rounding: the inserter’s hand size is treated as rounded down to the nearest multiple of the current belt stack size for belt insertions. This causes the inserter to almost always build full stacks even when that results in moving fewer items than its nominal hand size indicates. Exceptions where full stacks are not built onto belts include:

• If the hand size is smaller than the current belt stack size (rounding down would yield zero), the inserter respects the exact hand size and places partial stacks.
• If the inserter is holding a spoilable item that spoils while in its hand, it will immediately insert the spoil item into its destination and place whatever partial stack it can onto a belt.
• If the researched maximum belt stack size changes while the inserter is mid-move, it will use the new limit for that insertion, possibly resulting in a different stack size.

Practical notes and common tactics:

• The stack inserter typically will not rotate to place items until its hand reaches the configured pickup amount. This behavior increases throughput consistency but can cause delays when you need the inserter to drop a partial hand.
• You can force an inserter to drop its current contents immediately by changing its filter set so that the held item is no longer valid; the inserter will then turn and deposit the items regardless of how full the hand is.
• Wiring a stack inserter to its source container or belt and using circuit-controlled filters lets the inserter auto-drop when the source no longer contains that item. This is useful when inserting from chests or machines: when the container stops broadcasting a positive signal for an item, the inserter’s filter will clear and the inserter will drop its hand.
• Machines that broadcast contents can be used similarly, but beware inserter filter limits: inserter filters accept up to 5 item signals. If a machine’s inputs and outputs produce more than 5 different signals, the automatic filtering may not include the desired output and the system requires combinators to reduce signals to the intended set.
• Wiring a belt segment to the inserter and reading that belt tile’s contents can force immediate drops for belt-fed inserters, but this prevents the inserter from waiting for more items and can increase the number of insertions beyond what is strictly necessary.
• For maximal throughput, match hand size and belt-stack research: higher Inserter capacity and belt stacking research together increase the effective items moved per insertion.