The Changeover Sequence:
In which order the factory stands still the least
Every switch costs: cleaning from milk to dark chocolate, full clean-down after nut-containing items, start-up scrap. Today the shift lead sets the order from experience. An optimisation model finds the sequence with the fewest changeover minutes while honouring every allergen rule.
This module is part of a nine-part model calculation on a fictitious mid-sized chocolate manufacturer (€95 million net revenue, 140 items, 4 lines). All euro amounts are derived from the documented assumptions of the sample company. They are not results of real customers and not a performance promise. Which values are achievable on your data is shown only by a pilot.
01 The problem - many items, few lines, expensive switches
The number of possible sequences is astronomical, the number of good ones small
The sample company makes 140 items on 4 lines. Switching from light to dark needs a clean, switching from nut-containing to nut-free needs a full clean-down with release. Whoever puts nut items in the middle of the shift cleans twice.
With 900 changeovers per year of 40 minutes each, the lines stand 600 hours. Transition scrap comes on top. Choosing the order is a combinatorial problem no person solves in their head, but a solver solves in seconds - honouring delivery dates, shift limits and allergen order.
Plus 90 tonnes of transition scrap, partly reusable, partly lost material.
02 The model - sequence optimisation with hard rules
Changeover matrix from actual times per product pair, allergen order as a hard constraint, delivery dates and shift limits as the frame
Changeover time week: plan today 820 min -> optimised 650 min Allergen switches with full clean-down: 6 -> 4
Nut-containing items belong at the end of the shift, followed by the full clean-down. The model treats this rule as a hard constraint, not as a cost term to be weighed against changeover minutes. Result: fewer switches and no clean-down missing from the daily plan. The changeover matrix comes from measured MES times, not from standard values.
03 Business impact - line hours, scrap, overtime
Three levers from one sequence
Every assumption comes from the sample company and is stored centrally. With your figures the input changes, not the method.
| Position | Value |
|---|---|
| Changeovers / year × mean changeover time | 900 × 40 min = 600 h |
| Lever: Less changeover time (20 %) × value per line hour | 120 h × €500 = €60,000 |
| Transition scrap / year × material value | 90 t × €4,000 |
| Lever: Less transition scrap (20 %) | €72,000 |
| Seasonal overtime / year | €150,000 |
| Lever: Less overtime through a steadier plan (20 %) | €30,000 |
| Result: savings / year | €162,000 |
Assumptions of a sample company - in a real project your data replaces these values.
A gained line hour is only worth what can be produced and sold in it. That is why the sample company uses a value per line hour of about one third of the theoretical contribution margin per line hour - an assumption, not a calculation from your factory. In the season the hour is worth more, in summer less.
04 Next steps in your factory
From the changeover matrix to the Monday plan
Six months of MES data deliver the actual changeover times per product pair. Standard values are not enough; the deviation is often the real finding.
For one line the last weeks are recalculated with the optimised sequence. You see the changeover minutes it would have taken against those it took.
The sequence per line from the ERP orders, ready to execute; a short-notice order is rescheduled in seconds.