Technology Sheet: POLCA
POLCA stands for Paired-cell Overlap Loops of Cards with Authorization. POLCA is a production steering and control system for high mix/low volume and custom made products environments aimed at keeping the lead time at the shop floor short.
Setting up a planning and execution system is not easy in an environment where customer-specific products with complex routings are processed. POLCA, if properly set up, controls the shop floor in these difficult environments particularly well. A POLCA system shares similarities with a kanban system. However, a kanban system is more suited for environments with repetitive products.
In the case of production control, a 'push system' releases a work order based on the customer's demand.
In contrast, a 'pull system' releases work orders based on the status of the production system. For example, if the next workstation breaks down or is already flooded with work, then it’s often better to wait before starting a new order to avoid unnecessarily long queues at the next workstation. This also allows the steering system to prioritize orders for which there is capacity available at another subsequent station. As a result, the resources can be better used and flow through the system can be improved.
The Polca control system releases a production order on a workstation based on a combination of a push and a pull signal:
- Push signal: A schedule authorises the release of a production order at a workstation once a certain defined time (the authorization date) has come. This prevents that orders are started too early.
- Pull signal: POLCA achieves this by means of an ingenious card system with overlapping loops. Two consecutive workstations in a routing form a loop (ellipses in Figure 1). A limited number of 'POLCA' cards circulate in each loop. A POLCA-card is basically a capacity signal that indicates that processing capacity is available. A work order may only enter a loop if a free card of this loop is available. Once an order enters a loop, then a card is attached to the order. Once an order leaves a loop, then the card is returned to the first station of the loop and stored an a visual board. The card then becomes available for a next order. At the intermediate stations, (like D in figure 1), two cards will be attached to the order: an A/D card and a D/H-card. By overlapping the Polca loops, the actual order release is based on the actual available capacity further downstream.
POLCA started as a card based system made out of paper cards. Nowadays, several digital POLCA-system exist that remove the need of physical cards. As a results, cards don’t need to be returned and can’t be lost. Examples of commercial digital POLCA-systems are: PROPOS, Timeaxx (axxelia) and 3rdWave.
The classic kanban system works with product-specific cards, which means that the number of cards (and thus the work in process) throughout the production process increases dramatically when you have a large product variety. This stock explosion is prevented by the POLCA method. With POLCA, the number of card types is determined by the number of loops. This makes it possible to introduce a pull system in environments with a large product variety.
By limiting the work in process (WIP) on the shop floor, it becomes possible to keep the lead times at the shop floor short. This makes companies more flexible for late changes. Companies who have implemented POLCA have further reported a better delivery performance (people are working on the right things), better use of production capacity and lower indirect costs as the planning and steering becomes easier.
Several success factors will determine the success of a POLCA-implementation:
- Clarity of the rules and follow-up
This success factor speaks for itself. POLCA requires discipline and rigour:
- The rules must be correctly communicated and made visible.
- Operators must be properly trained.
- The operators must be involved in the implementation of the method
- The supervision must be quite demanding in terms of compliance with the rules
If this is not the case, POLCA cards are lost, are not returned to the upstream resource in time or are returned too early. As a result, the POLCA-system will break down.
- Link with planning
POLCA will not work if the planning is not done well. The POLCA method, regardless of the rigour with which it is applied, cannot work if the capacity is not adjusted to the demand. This must be anticipated at the planning level (S&OP/MPS).
As a reminder, the POLCA system facilitates the flow of materials and makes it possible to make better use of the available capacity. However, if the available capacity is below the required level, the possibilities for smoother flows will remain limited and the POLCA system will not solve this problem. It is therefore necessary to assess the feasibility of the production plan by evaluating the degree of utilization of the different cell resources and the processing times for the different products in the different cells.
- Coherence of objectives and indicators
This is undoubtedly the most difficult aspect: the underlying pull principle radically changes the way in which production performance is viewed. The goal is no longer to keep the machines busy but to create a smooth flow that minimizes the waiting time of the orders. If teams are solely judged on the basis of the utilization rates of their resources, then it will be difficult to respect the POLCA rules.