起源故事 · 折信封游戏
Origin Story · The Envelope-Folding Game
这是丰田与无数精益培训里百试不爽的折信封游戏：四个人坐成一排，分别负责折纸、装信、封口、贴邮票。
第一轮"批量"做 —— 每人把一摞全做完再传，结果第一封信迟迟出不来，桌上堆满半成品。
第二轮改"单件流" —— 折一张立刻传下去，四道工序同时忙起来，第一封信几秒就完成，总时间也短得多。
学员往往瞠目结舌：动作一模一样，只是改了传递方式，速度竟天差地别。这就是"流"的威力。
This is the envelope-folding game Toyota and every lean trainer keeps in the back pocket. Four people in a row — fold the paper, insert the letter, seal the flap, stick the stamp.
Round one runs in batches: each person finishes a whole pile before handing it down. The first finished envelope takes forever, and the table is covered in half-finished work.
Round two switches to one-piece flow: fold one and pass it along immediately. All four steps are working at the same time, and the first envelope is done in seconds — total time drops too.
The trainees are stunned. The motions are identical; only the handoff changed; the speedup is enormous. That is the power of flow.
1 两条产线，同时开跑 Two Lines, Same Starting Gun
计时 0.0 sClock 0.0 s上方是批量产线：一整批工件挤在一道工序里，做完才整批往下搬。下方是单件流产线：工件一个接一个流过三道工序。按"开始"看谁先出货。 Top track is the batch line: the whole batch piles up at one station and only moves once everything is done. Bottom track is the one-piece flow line: parts move through the three stations one at a time. Press Start to see which line ships first.
2 前置时间对比：差距有多大 Lead-Time Comparison: How Big Is the Gap
两根条对比全部完工的总耗时。单件流不仅总时间短，第一件出货更是快得惊人 —— 这意味着资金回笼快、缺陷暴露早。 The two bars compare total time to finish the order. One-piece flow not only wins on total time — its first-piece delivery is dramatically faster, meaning cash comes back sooner and defects surface earlier.
3 现实里的批量 vs 流 Batch vs Flow on the Real Shop Floor
小批量生产：批量缩小一半，前置时间和在制品几乎也减半，换型频次成为下一个改善点。
Small-batch production: Halve the batch size and both lead time and WIP roughly halve too — changeover frequency becomes the next thing to attack.
单件流改善：把工序物理靠拢、去掉中间货架，让工件不落地直接传递。
Building one-piece flow: Move the stations physically closer, delete the buffer racks, and hand parts off without them ever touching the floor.
缩短前置时间：单件流让首件极快交付，客户体验和现金流同时改善。
Shorter lead time: One-piece flow makes the first-piece delivery dramatically faster — customer experience and cash flow improve together.
降在制品：在制品越少，资金占用越低，缺陷一出现就能被立刻发现、立刻止损。
Lower WIP: Less WIP means less tied-up cash, and any defect is caught and contained the moment it appears.
一句话In One Line
批量为什么慢？因为一件做完后，它只能干等着整批兄弟都做完，才能一起去下一道 —— 大把时间耗在排队上。
单件流把工序并联起来：当第 2 件在做工序一时，第 1 件已经在做工序二了，三道工序几乎同时运转。
关键洞察是：单件流并没有让每个动作变快，它只是消灭了批内排队的等待 —— 这正是上一页时间梯里那条最长的下台阶。
Why is batching slow? Because once a part is done it has to sit there waiting for all its siblings to finish before the whole batch can move on. Most of the time is burned in that queue.
One-piece flow runs the stations in parallel: while part 2 is on station 1, part 1 is already on station 2 — all three stations are working at once.
The key insight: one-piece flow does not speed up any single operation; it just eliminates the in-batch waiting — exactly the tallest drop on the lead-time ladder from the previous page.
常见误用Common Mistakes
以为大批量摊薄成本一定更划算。算上库存、前置时间和缺陷蔓延，单件流总成本常更低。
Assuming big batches always win on unit cost. Once you add up inventory, lead time, and defect propagation, one-piece flow usually has the lower total cost.
强推单件流却不先压缩换型时间。先做 SMED 快速换型，单件流才跑得动。
Forcing one-piece flow without attacking changeover time first. Run SMED to slash setup time — only then can one-piece flow actually move.
只盯单件加工速度，忽略批内排队。真正的浪费是等整批做完的那段空耗。
Optimizing only single-piece cycle time while ignoring the in-batch queue. The real waste is the dead time spent waiting for the rest of the batch.