What is a planning time fence
A time fence is the point in the planning horizon beyond which the plan stops changing freely. It splits the horizon into zones: frozen, slushy (negotiable) and liquid (free). The closer to execution, the tighter and costlier a change becomes.
There are two fences: the demand time fence (DTF), which sets how far out only firm orders count, and the planning time fence (PTF), which sets how far out the system does not reschedule on its own. They protect the master production schedule and prevent the constant replanning that disorganizes the plant.
Time fence visual map
The map shows the three zones, the rules for each and the 8-week example with a 2-week DTF and a 5-week PTF.
Planning horizon
The horizon is the total span the plan covers. It must be at least as long as the product's cumulative lead time, the longest path of purchasing, manufacturing and assembly. If the horizon is shorter, the plan cannot see when to buy the material.
Cumulative lead time = sum of lead times along the longest path
| Step | Lead time (weeks) |
|---|---|
| Purchased material | 2 |
| Machining | 1 |
| Final assembly | 2 |
| Cumulative lead time | 2 + 1 + 2 = 5 |
In the example, the minimum horizon is 5 weeks. In practice it is longer (8 weeks in the example, 13 in many companies) to cover capacity planning and S&OP. To measure each step, see lead time.
The three zones and their rules
| Zone | Where it sits | Demand considered | Who can change it |
|---|---|---|---|
| Frozen | Week 1 up to the DTF | Firm orders only | Exception approved by management, with material and capacity confirmed |
| Slushy | From the DTF to the PTF | The greater of forecast and orders | The planner, after checking material and capacity; the system does not reschedule by itself |
| Liquid | Beyond the PTF | The greater of forecast and orders | The system (MPS/MRP), automatically |
"The greater of forecast and orders" is a common convention; your company may use another, such as forecast consumption. What matters is to document the rule and apply it the same way for everyone.
Worked example: 8 weeks with DTF = 2 and PTF = 5
Illustrative example: opening inventory of 150 units, fixed lot size of 200 and a forecast of 100 per week. The DTF equals the final assembly lead time (2 weeks) and the PTF equals the cumulative lead time (5 weeks). Inside the DTF only orders count; after it, the greater of forecast and orders.
| Week | Forecast | Orders | Demand used | MPS | Projected on hand | Zone |
|---|---|---|---|---|---|---|
| 1 | 100 | 120 | 120 | 0 | 30 | Frozen |
| 2 | 100 | 110 | 110 | 200 | 120 | Frozen |
| 3 | 100 | 70 | 100 | 0 | 20 | Slushy |
| 4 | 100 | 40 | 100 | 200 | 120 | Slushy |
| 5 | 100 | 20 | 100 | 0 | 20 | Slushy |
| 6 | 100 | 10 | 100 | 200 | 120 | Liquid |
| 7 | 100 | 0 | 100 | 0 | 20 | Liquid |
| 8 | 100 | 0 | 100 | 200 | 120 | Liquid |
Projected on-hand calculation: week 1 = 150 − 120 = 30. Week 2: 30 − 110 = −80, so the MPS schedules the 200 lot and on hand becomes 30 + 200 − 110 = 120. Week 3: 120 − 100 = 20. Week 4: 20 − 100 = −80, a new 200 lot and 120 on hand. The pattern repeats in weeks 6 and 8.
Reading by zone: the week 2 lot is frozen (the material is already bought and machining is under way). The week 4 lot is in the slushy zone: its date or quantity can change, but only after checks. The week 6 lot is liquid and the system may reschedule it if the forecast changes. Lots follow the lot-sizing policy and become requirements in MRP.
Rush order: how to decide within the zones
Suppose the customer asks for 80 more units in week 3, raising the order from 70 to 150. Week 3 demand goes from 100 to 150, and projected on hand falls from 120 − 100 = 20 to 120 − 150 = −30. That is a 30-unit shortage.
| Where the order falls | What to do |
|---|---|
| Weeks 1 to 2 (frozen) | Accept only if stock, material and spare capacity exist; requires formal approval and a record of the expediting cost |
| Weeks 3 to 5 (slushy) | The planner pulls the week 4 lot into week 3 after confirming material (2-week purchase) and capacity, and tells sales the date |
| Week 6 onward (liquid) | Enters the MPS automatically; the system recalculates requirements |
In week 3 a new lot of 200 cannot be created from scratch: the 5-week cumulative lead time does not fit in 3. The only ways out are pulling in a lot already under way or using safety stock. This check is called available to promise (ATP) and must be done before giving the customer a date.
How to set the fences
- Calculate the cumulative lead time of each product family along the longest path.
- Set the DTF at least equal to the final assembly lead time (and test, if any). Inside it, material is already committed.
- Set the PTF at least equal to the cumulative lead time. Before it, a change means repurchasing or rescheduling a supplier.
- Set the total horizon longer than the PTF, to give visibility of capacity and long-term purchases.
- Write the exception rules: who approves, within what time, with what record.
- Review the fences whenever lead time, supplier or process changes.
- Measure how many changes occur inside each zone per month; the number inside the frozen zone should be minimal.
Use different fences per product family: a simple assembly item and one with an imported component do not share the same lead time. In levelled production environments, heijunka reduces the need to change the frozen zone.
Common mistakes
- Fence too long. It freezes the plan beyond what is needed, builds inventory and loses sales that could have been served.
- Fence too short. The plan changes every week, orders are reissued and the shop floor loses trust in the system (MRP nervousness).
- Unrecorded exceptions. If anyone can change the frozen zone, the fence exists only on paper.
- One fence for every item. It ignores lead time differences between families.
- Confusing horizon with fence. The horizon is the plan's reach; the fence is the limit of freedom to change.
- Promising a date without checking ATP. Sales promises and planning finds out later that there is no material.
- Not reviewing. The supplier lead time changed and the PTF stayed the same.
Frequently asked questions
What is a time fence?
It is the limit in the planning horizon beyond which changes to the plan become restricted. It divides the horizon into frozen, slushy and liquid zones.
What is the difference between DTF and PTF?
The DTF (demand time fence) sets how far out only firm orders enter demand. The PTF (planning time fence) sets how far out the system does not reschedule on its own and changes need planner approval.
How do you size the frozen zone?
Generally at least the final assembly lead time, because inside it material and capacity are already committed. In the example, 2 weeks.
What should the planning horizon be?
At least the product's cumulative lead time. In the example, 5 weeks; most companies use more to cover capacity and S&OP.
Can you change the plan inside the frozen zone?
Only as an approved exception, with material and capacity confirmed and the cost recorded. Without those conditions the change causes delays or expediting.
Is a time fence the same as the MPS?
No. The MPS is the production plan for end items; the time fence is the rule that defines how much of that plan can be changed at each point of the horizon.
Sources
- VOLLMANN, T. E. et al. Manufacturing Planning and Control for Supply Chain Management. New York: McGraw-Hill.
- ORLICKY, J. Material Requirements Planning. New York: McGraw-Hill, 1975.
- CORRÊA, H. L.; GIANESI, I. G. N.; CAON, M. Planejamento, Programação e Controle da Produção. São Paulo: Atlas.
- TUBINO, D. F. Planejamento e Controle da Produção. São Paulo: Atlas.
- APICS Dictionary. Chicago: APICS.
Keep studying: trusted sources and links
- ASCM: Association for Supply Chain Management
- ABEPRO: Associação Brasileira de Engenharia de Produção (in Portuguese)
- Wikipedia: Production planning
Save this address: Manufatura Enxuta is your school for lean manufacturing. Come back whenever you need to study, with a complete article, a visual map and trusted sources. Follow one of the learning paths or check the A–Z glossary.
Want real pull production?
Download the free pull production e-book, with kanban, supermarkets and leveling.
Download the e-book
About the author
Lean Manufacturing & Behavioral Management Specialist
Over 20 years in the automotive and metalworking industries (GM and Dana), Lean Manufacturing practitioner since 2006. SENAI instructor and mentor in Brazil’s Brasil Mais Produtivo program, delivering consulting, training and audits for 50+ companies, combining quality, productivity and people development.
