Mixed-Model Production

Mixed-model production is one of the harder lean transitions in small shops, and one of the highest-leverage. The instinct in most shops is to run all of variant A before switching to B, because the long batch amortizes changeover and the operators stay in the same mode. The cost is that customers wanting variant B during the A run have to wait, or the shop carries enough B in inventory to cover the wait. Mixed-model trades that long-batch comfort for short lead times and lower inventory across the variant mix.

"Stop running all of A before any of B. The customer who wants B today does not care that A's batch is not finished."

How mixed-model production works

The mechanics start with line design. Stations, fixtures, and tooling have to support multiple variants without long setup. The ideal is common fixtures across variants, with small quick-change adjustments. Where common fixtures are impossible, quick changeover is the prerequisite. A line that takes 30 minutes to switch between variants cannot run mixed-model in any meaningful way. Twelve changeovers a shift would consume six hours and leave nothing for production.

The schedule is the second piece. Mixed-model lines almost always run on heijunka, with the variants interleaved in a leveled pattern. The pattern is set by demand: if variant A is twice as much volume as B, the pattern might be AABAB instead of just ABABAB. The schedule is posted at the line, usually as cards in a heijunka box, and the operator runs the next card.

The third piece is standard work for each variant. Operators are now switching modes every few units, and without clear visible instructions for each variant, defects spike. Visual work instructions, color-coded fixtures, and labeled material kits all help operators move between variants without errors. Standard work that is clear for one variant is not enough; each variant needs its own visible standard.

The benefit is customer responsiveness with low inventory. A mixed-model line that runs every variant every shift can fulfill any variant order within a day or two, without carrying weeks of finished goods. A long-batch line that runs each variant once a month either carries a month of finished goods per variant or has month-long lead times on whichever variants are not in production this week. Mixed-model replaces both costs with a more demanding production discipline.

Where mixed-model production fits on the shop floor

Picture a small contract assembly shop building three variants of an industrial control box. Variants share a common chassis but have different internal wiring and labeling. Without mixed-model, the shop ran one variant per day: variant A on Monday, B on Tuesday, C on Wednesday. Customer orders for any specific variant could wait up to three days for the next batch. Total finished-goods inventory was about a week of demand across the three variants.

The shop redesigns the assembly cell to support all three variants. Wiring kits get color-coded; labels live in three labeled bins at the labeling station. A heijunka box gets installed at the cell entrance with cards for each pitch interval, interleaving the three variants in a pattern that matches steady demand: AABCABCAABC. Changeover between variants is now under two minutes, mostly swapping the wiring kit and label bin.

After two months, finished-goods inventory drops by about 60 percent. Customer lead time on any specific variant drops from up to three days to a half-day. The cell produces a steady output mix matching the steady demand mix. The shop has not added equipment or workers. The changes were in line design, standard work, and the cards in the heijunka box.

Common mistakes with mixed-model production

• Skipping the changeover work first. Mixed-model is impossible if each changeover takes an hour. Quick changeover has to come first.
• Mixing variants that are too dissimilar. If two variants need different tooling, different operator skills, and different fixtures, they probably do not belong on the same line.
• No visible standard work per variant. Operators switching variants without clear visible instructions make more mistakes. Visual work instructions are essential.
• Treating it as a scheduling change only. Mixed-model is a line-design change. The schedule depends on the line being able to switch quickly without quality drift.
• Ignoring demand mix. The pattern in the schedule should match the demand mix. A 2:1:1 demand ratio needs a pattern that delivers that ratio, not equal counts of each variant.

Mixed-model production and related Lean tools

Mixed-model production is enabled by heijunka scheduling and made possible by quick changeover. The cadence at which the line cycles through every variant is its every part every interval. Mixed-model lines typically use runner-repeater-stranger classification to decide which variants belong on the line and which should be scheduled separately, and the scheduled point in the value stream that paces the mixed-model line is the pacemaker process.