Production planning involves the design of a production process that allocates resources effectively, to ensure that the production demands of the system are met [42]. The scheduling of the on-demand FxMC is an important contribution of the research. The inputs to the scheduling method are based on the parameters fed to it from the other activities of the production system. The production planning architectures that exist in current manufacturing systems was investigated to formulate the FxMC and its scheduling method within a framework that would be relevant for a MC production system. These paradigms also include the systems and concepts currently investigated for the fourth industrial revolution [1].
17 Production planning systems can be classified as either push systems or pull systems. Hopp and Spearman [43] define a pull production system as one that explicitly limits the amount of Work in Process (WIP) that can be in the system, whereas a push production system is one that has no explicit limit on the amount of WIP that can be in the system.
2.6.1 Push Production Systems
Material Requirements Planning (MRP) is a classical production planning and inventory control system.
MRP aims to maintain sufficient levels of inventory to ensure that the required materials are available when necessary. As a push production system, the focus of MRP is to push the production of items such that inventory levels are kept to a minimum [43], [44].
Manufacturing Resource Planning (MRP II) is an expansion of MRP; it factors in influences from other functional areas of an organisation, to create a feedback loop that aims to comprehensively control the system [45]. MRP II is defined as a computer-based system for planning, scheduling, and controlling the materials, resources, and supporting activities required to satisfy the production demands [42].
Alternative production planning systems that evolved from MRP include: Enterprise Resource Planning (ERP); Customer-Oriented Manufacturing Management System (COMMS); Customer-Oriented Management System (COMS); and Manufacturing Execution System (MES) [42]. These systems were not considered for the research, due to their increased focus on business-oriented functions that were beyond the scope of the objectives.
Production planning systems are driven by the Master Production Schedule (MPS). The MPS is a plan that specifies which items are to be produced and when they are due. The MPS is formulated from a combination of sales forecasts, customer orders, safety stock and internal orders. The Bill of Materials (BOM) is also a crucial element of MRP and MRP II. The BOM contains information on the materials and components required to create the products that must be produced. The BOM disassembles the products into their basic components to summarise the inventory requirements [44].
MRP and MRP II focus on push production. In a push production system, there is an emphasis on maintaining minimum inventory. The production is pushed out at a rate that is based on the processing ability of the first station of a production process. This first station processes the raw materials from inventory and pushes its completed work to the next station, regardless of whether that station is available or not. If any station other than the first is the bottleneck of the system (has the longest lead time) the WIP increases. This causes an increase in total lead time of the system [46]. A schematic representation of the push system is shown in Figure 2.7, demonstrating the flow from one workstation to the next, without intermediate control.
Figure 2.7: Push system schematic [46]
The Production Planning and Control (PPC) of a mass customisation production system with a fixture manufacturing cell is elaborated in Chapter 4, with reference made to MRP, MRP II and their
18 subsystems. The PPC system provides an overview of the functions at higher levels of the production system, from which the inputs to the scheduling method are retrieved. The higher levels of the PPC system were derived from push production systems, but the specific operation of the manufacturing system was derived from pull production systems (elaboration in Section 5.2).
2.6.2 Push Production Systems
Just-in-Time (JIT) manufacturing is an alternative system to MRP and MRP II. JIT is an example of a pull production system. JIT manufacturing systems have the primary goal of continuously decreasing (ideally eliminating) all forms of waste [47]. The method to achieve this goal depends on the ability of the system to deliver the right parts in the right quantity to the right place at the right time.
Kanban is a facilitator of JIT production systems. Kanban is the name given to the plastic cards used in the system, which contain the necessary information required for the fabrication and/or assembly of a product at each stage of its production process. The cards are attached to containers of a specific size, which can only hold the specified number of parts. Kanban is used in pull production systems [46].
There are two types of kanban systems:
1) Single card system, or Production Order Kanban (POK); and
2) Two-card system, where both a POK and a Withdrawal Kanban (WK) are used.
In the single card system, it is assumed that the distances between workstations are such that a single buffer can be used as both the outbound buffer of the previous workstation and the inbound buffer of the next workstation [46], [48]. A schematic diagram of the single card kanban system is shown in Figure 2.8 for a Workstation A and Workstation B.
Figure 2.8: Single card kanban system [48]
The two-card system uses a separate outbound buffer and inbound buffer due to greater distances between workstations. The POK card is used for the same purpose as the single card system, while a WK card is added to retrieve containers from the outbound buffer. A schematic diagram of the two- card kanban system is shown in Figure 2.9 for a Workstation A and Workstation B.
19 Figure 2.9: Two-card Kanban system [48]
The buffer storages for the workstations have limited capacity. As per the kanban system, a workstation is incapable of processing parts when either of its buffers are full; this is known as blocking. This is the mechanism by which kanban systems limit the WIP, and are thus classified as pull production systems.
The limit on WIP (and hence the buffer capacity) is determined by the Toyota formula [49]:
𝐾 ≥𝐷𝐿(1 + 𝛼)
𝐶 (2.1)
Where:
K = number of kanbans, D = demand per unit time, L = lead time,
α = safety factor, and C = container capacity.
The focus of the Toyota formula is to find the optimum number of kanbans for the system, as overstock occurs for a high value, and understock for a low value. Kekre and Karmarker [50] found that a decreased container size with increased kanbans lead to superior results. However, practical factors limit the ideal implementation in industry.
Lean production is defined as the production of goods or services that is accomplished with minimal buffering costs [43]. This means that there is minimal waste in the system as a whole, producing the required products by utilising the minimum of total resources. Buffering may refer to traditional waste, such as slow and unreliable machines; it may also include variability of the system, as this is a significant cause of waste in systems that rely on precise forecasts that vary in practice [43].
Lean production and JIT principles require a high standard in manufacturing. Decreased set-up times for reduced batch sizes, and stable and reliable production operations are prerequisites. Implementation and maintenance of these objectives can be difficult to achieve in practice. Human errors can be reduced through higher degrees of automation [51].
JIT, Kanban and lean manufacturing reveal the improvements in manufacturing efficiency obtainable by reducing buffering and batch sizes. The operation of the FxMC layout implemented in the research (Section 3.5) and the production system workflow (Section 3.6) are based on JIT and lean manufacturing. A kanban policy is implied through the unit workflow that was employed; fixtures and parts are retrieved one at a time. This is imposed through the instructed dispatch of parts and fixtures.
20 JIT characteristics are observed via the workflow, which is dependent on the completion of the prior operation; fixtures are not buffered, but idle time is minimised for improved utilisation of both fixture and workstations instead.