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Variations and hybrid strategies is either a modification to an existing lot release control strategy or a combination of more than one strategy to achieve sustainable improvements in performance over a single strategy. Examples of variations and hybrid strategies reviewed in this work are: m-CONWIP, Multi-CONWIP, Hybrid Kanban-CONWIP, Paired-Cell Overlapping Loops of Cards with Authorization (POLCA), and Generic Paired-Cell Overlapping Loops of Cards with Authorization (GPOLCA), which are briefly discussed in the following sections.

m-CONWIP

m-CONWIP, where m stands for multiple CONWIP, is a lot release control strategy that regulates releasing lots to manufacturing systems having more than one route, where, a CONWIP loop for every routing is introduced to control the release of lots as displayed in Figure 2-10.

Hence, it is considered as route specific control lot release strategy [49, 50]. It should be noted that m-CONWIP balances the workload among the routings by constraining the number of lots that are released separately to each route [50].

Figure 2-10: m-CONWIP lot release control strategy.

Germs et al [50] analysed the cycle time performance at a make-to-order manufacturing system under the control of CONWIP, m-CONWIP, and POLCA (Paired-Cell Overlapping Loops of Cards with Authorization) taking into consideration workload balancing capabilities, they concluded that cycle time was successfully reduced and that workload balancing capability exists for m-CONWIP and POLCA but not for CONWIP.

Multi-CONWIP

Multi-CONWIP (also known as segmented CONWIP) mixes between the Kanban and CONWIP, in which the WIP cap is controlled by a number of loops, each loop has a constant WIP level independent from the other as presented in Figure 2-11. The number of loops must be more than one loop (CONWIP) and less than the number of stations (Kanban). This lot release control strategy can be found in real manufacturing, such as semiconductor manufacturing [51, 52].

Figure 2-11: Multi-CONWIP lot release control strategy.

It should be noted that the first loop must include the first station, the last loop must include the last station, and all other stations must be part of loops.

Hybrid Kanban-CONWIP

Hybrid Kanban-CONWIP was introduced by Bonvik et al. [53] in order to overcome the disadvantages of loose coordination between production stages in a CONWIP line. They also stated that the hybrid strategy proposed is a better way than the minimal blocking strategy in facilitating machine recovery from failures and keeping bottlenecks working even if there are failed machines downstream.

In Hybrid Kanban-CONWIP, as in CONWIP, an overall cap is placed on the amount of inventory allowed in the manufacturing system. In addition, inventory is controlled using kanban cards in all stations except the last station as shown in Figure 2-12. CONWIP can be considered as special case of Hybrid Kanban-CONWIP, in which there is an infinite number of kanban cards distributed to each station [54].

Figure 2-12: Hybrid Kanban-CONWIP lot release control strategy.

As a further variation to CONWIP, Hybrid Kanban-CONWIP was combined with DBR and applied to an assembly production line. Simulation results showed that the method was indeed able to solve the bottleneck problem effectively, enhanced the productivity, and reduced the delay time of the line [40].

Paired-Cell Overlapping Loops of Cards with Authorization (POLCA)

POLCA is a hybrid push-pull strategy that combines the best features of card based pull systems and push systems for quick response manufacturing (QRM) [28-30, 42]. The QRM strategy is best applied at companies that make custom designed products in small batches (or even one of a kind), and companies that don’t custom design each product, but still have such a wide variety of options and combinations of specifications that they cannot afford to store inventory for all these options at various stages of their manufacturing system [27-30, 35, 42].

POLCA uses signal cards, called POLCA cards to communicate and control the lot movement between cells (stations) in order to authorize the progress of a lot. The release of lots is authorized by specifying the release dates using high-level Materials Requirements Planning system (HL/MRP), which might be accomplished by calculations from the due date and planned lead times. Similar to an MRP system, there are times when each cell may begin work on a particular lot. However, unlike in a standard push system where a cell should start work at that time, POLCA simply authorizes the beginning of the work, but the cell cannot start unless the corresponding POLCA card is available [27-30, 35, 42].

Although this may seem similar to Kanban, however there are some important differences. First, the POLCA cards are only used to control movement between cells, not within cells (For material control between workstations within a cell, cells have the freedom to use various other procedures) [28-30, 35]. Second, the POLCA provides a route specific control of the lot flow, while Kanban provides a product specific control. In other words, in POLCA the cards are assigned to pairs of cells instead of being specific to the product type, as in Kanban. The third difference from Kanban is that the POLCA cards for each pair of cells stay with a lot during its journey through both cells in the pair before they loop back to the first cell in the pair [27-30], and an additional card needs to be attached to the lot before entering the second cell of the first pair to signal the availability of capacity at the first cell of the second pair within the routing [27, 49].

Generic Paired-Cell Overlapping Loops of Cards with Authorization

(GPOLCA)

GPOLCA is an adaptation of the POLCA mechanism developed as part of the QRM strategy [20]. GPOLCA requires that a released lot must possess cards for all POLCA loops in its complete routing before processing may begin at its first cell [25].

The characteristic of GPOLCA, non-existent in POLCA is that, GPOLCA releases lots only after the GPOLCA cards necessary by a lot at each pair of cells in the job’s routing become available and are allocated to the lot. POLCA, on the other hand, undertakes the lot release as long as cards for the first pair of cells in the job’s routing are available [20].