Apparatus to accumulate automatically articles traveling between downstream and upstream machines in a production line. The accumulator includes an input magazine, a bin, and an output magazine. When the downstream machine ceases operation, a vertical plunger feeds articles into the input magazine, forming a stack. A loading plunger pushes the completed stack into the bin, where multiple stacks may be held, the stacks being moved from one position to another within the bin by a powered belt. To feed articles from the accumulator back into the production line, a stack is moved into the output magazine. There, a discharge plunger feeds successive lowermost articles in the output magazine onto an output conveyor. The output conveyor has at least twice the throughput capacity as the upstream machine, and articles from the accumulator may be merged into the production line stream, thus making up production time lost during shutdown of the downstream machine.