Assembly line feeding policies

Line feeding policies describe how parts are provided to assembly stations. An assembly line feeding policy thus concerns a sequence of logistical operations that need to be performed on the shop floor and the presentation at the location of assembly.

By reviewing literature on assembly line feeding, one could assume a quite high number of distinct line feeding policies. This can partly be explained by the use of different names for the same feeding policies. To avoid confusion in further research, we now present five different terms for line feeding policies and show them in Table 2.1 together with nomenclature previous literature.

Moreover, line feeding policies are sometimes mixed up with production control systems like by Satoglu and Ucan (2015), where Kanban and Polca production control systems are confounded with parts supply to assembly lines. This might also be due to the fact that boxed-supply is often described by JIT supply or Kanban-based-feeding (see Table 2.1).

In their literature review, Kilic and Durmusoglu (2015) distinguish four policies: line stocking, boxed-supply, kitting, and hybrid policies. In their framework, sequencing is not considered as a

line stocking boxed-supply sequencing kitting Table 2.1Line feeding policies in literature

separate policy. On the other hand, a hybrid line feeding policy is described, combining different policies within one assembly system. Within this review however, we do not consider the hybrid policy to be a distinct policy, since we assume line feeding policies to be selected at the individual part level, which usually results in a combination of multiple line feeding policies.

Thus, we define five policies in the context of assembly line feeding, namely line stocking, boxed-supply, sequencing and kitting, whereas kits can be stationary or traveling. These line feeding policies cover all described practices, found in literature and practice. In the following subsections we introduce and explain those line feeding policies. A first overview of the effect of line feeding policies on the provision of parts at the border of line (BoL), i.e. an area next to assembly stations which is dedicated to store assembly parts, can be seen in Figure 2.1.

Figure 2.1 shows that parts can be provided in homogeneously filled (line stocking and boxed-supply) or mixed containers (sequencing and kitting). Furthermore, they can be presented at the BoL or on the line itself. The latter happens in the case of traveling kits, where every product (grey boxes) on the assembly line is joined by a kit container (white box). For clarity, in Figure 2.1 only one line feeding policy is applied per station. However, in practice the simultaneous use of multiple line feeding policies at a single station is assumed to be beneficial (see Sali and Sahin (2016)).

Line stocking

Line stocking can be considered the simplest form of line feeding where a complete load car-rier like a pallet or box is directly delivered from storage to the border of line (BoL) (Battini et al., 2015; Lim`ere et al., 2012; Sternatz, 2015). To consider line feeding as line stocking, no further preparatory handling effort, except the transport of the load carrier to the line, should be necessary. The load carrier remains at the BoL until depletion.

Sequencing

Stationary kitting Travelling kitting Boxed-supply Line Stocking

Figure 2.1Line feeding policies

Boxed-supply

In comparison to line stocking, line feeding with a boxed-supply policy requires more logistical handling effort. The main characteristic of the boxed-supply policy is the reduction of container sizes. Feeding parts in boxed-supply starts with the retrieval of a pallet or container from storage, possibly transporting it to a preparation area, and repacking parts from this load carrier into smaller bins. An important characteristic of boxed-supply is that all bins are still filled homogeneously after repacking. Next, bins are transported to the BoL and stored there until depletion. Since in both policies, line stocking and boxed-supply, parts are stored in a larger quantity in homogeneously filled load carriers at the BoL, some authors do not consider these policies to be distinct. However, within the framework of the ALFP it is necessary to distinguish between both as the underlying logistical processes are distinct.

Sequencing

The increasing number of part variants is a big challenge for companies, since it rules out the possibility of line stocking for all required parts at the BoL, due to space restrictions. This is aimed to overcome by applying sequencing. Sequencing is initiated by the retrieval of pallets or boxes containing different variants of parts which are possibly transported to a preparation area. Next, variants of a part are sorted into a container according to the sequence of demand.

Subsequently, those sequenced parts are transported to the BoL in a container and stored there until depletion (Sali et al., 2015). In particular, in case of many part variants this policy is beneficial, as stocking an enormous amount of different part variants at the BoL can be avoided.

The location of the sequencing activity can vary. Especially in the automotive industry suppliers quite often already manufacture the parts needed by the original equipment manufacturer (OEM) in the appropriate sequence. According to Swaminathan and Nitsch (2007), the sequencing point can be located at the supplier, in an intermediate sequencing centre, in a sequencing area within the assembly plant or at the BoL. Within this framework, the last option is considered as line stocking. Boysen et al. (2015) also describe the location of a sequencing point at supplier(s), logistics provider(s) or in-house.

Stationary kitting

Kits are an extension of sequenced containers since not only one part and its variants but a combination of multiple parts and its respective variants are combined in a kit. A specific characteristic of kits is the depletion of one kit for one product. A kit container can, however, contain one (Sternatz (2015)) or multiple kits (cf. Lim`ere et al. (2015, 2012)).

The logistical process starts with retrieval of different parts from storage, if applicable, trans-porting and storing them in a preparation area, followed by repacking them into a common kit.

Stationary kit containers are afterwards transported to the BoL of a specific station, where all parts of that kit are used and the kit is depleted (Bozer and McGinnis, 1992). The composition of kits may vary and a distinction between uniform mix and variable mix kits can be made. In the latter case, kits are not necessarily always composed in the same way. For instance, some parts may be optional and therefore, not be used in every kit (Lim`ere et al., 2012). Another possibility for variable mix kits is that different product models require different kits and the demand for those is varying often (Bozer and McGinnis, 1992). Both issues may result in a different demand of kits and the corresponding parts. In contrast, uniform kits always contain the same amount of parts and they differ only due to different variants of the same part.

Note that, though kitting seems to entail a larger effort for preparation than other line feeding policies, in some scenarios it reduces effort for assembly workers (Hanson and Medbo, 2012).

Traveling kitting

Traveling kits have a similar characteristic to stationary kits since they also contain different variants of multiple parts. One important difference is, that traveling kits contain parts that are used at multiple stations whereas parts of a specific stationary kit are only used at a single station. The process of preparing traveling kits is the same as preparing stationary kits though it might cause even more outlay than for stationary kits due to a higher complexity. However, traveling kits are usually brought to the beginning of an assembly line, and reduce effort for assembly workers in comparison to stationary kits, since they do not need to walk to pick parts from the BoL (Bozer and McGinnis, 1992). In fact, a traveling kit can also enter the line at an arbitrary point but either way, it travels along the line with its dedicated product, until it is depleted. Once a traveling kit is depleted it might be removed from the line and eventually be replaced by another one or it simply travels until the end of the line. Thus, traveling kits contain parts for more than one station.

Sets:

I Set of parts S Set of stations

P Set of line feeding policies Variables:

χisp=

(1, part iis assigned to policypat stations 0, otherwise

ψ_sp=

(1, policypis used at stations 0, otherwise

Ωp=

(1, policypis used 0, otherwise

Parameters:

c^v_isp Costs to feed partiwith policy pto stations c^f_sp Costs to use policypat stations

c^f_p Costs to use policyp

λ_is Demand of partiat stations

Table 2.2Notation for a simple assembly line feeding problem

Similarly to stationary kits, traveling kits may also be distinguished into uniform and variable kit mix (see previous paragraph).