MATERIAL-HANDLING SYSTEMS:CONTAINERIZATION AND UNIT LOADS

CONTAINERIZATION AND UNIT LOADS

Perhaps one of the least understood, yet critical, aspects of material handling is the notion of con- tainerization and unit loads. Many design drivers in materials handling, such as frequency of flow, load dimensions and weight, type of equipment that can or cannot be used, are often dictated by the type of container(s) and unit load(s) handled by the system. However, there is no clear-cut definition of a container or a unit load; a container in one application may very well be a unit load in another. Generally speaking, a container serves as a receptacle to keep individual / loose parts or packages together, confined in the same physical space, such as a cardboard box, a wire basket, or a tote box, among others. A unit load, on the other hand, is generally defined as the unit to be moved or stored at one time; it may consist of a single part or a bundle of parts / products, or it may consist of one or more containers. Also, the unit load includes the carrier or support needed to store or move the load. For example, a group of cardboard boxes (i.e., containers) stacked on top of a pallet would constitute a unit load, which includes the pallet itself and, say, stretch wrapping to stabilize the individual boxes.

Another example would be a 6-pack of soft drink cans. The container is the can and the bundle of six cans may be the unit load. Many people would find it difficult or awkward to carry six (loose) cans at one time. When the cans are held together with the white plastic holder, however, it becomes much easier to handle them. This underlines the significance of the unit load concept and the inclusion of support structures in its definition. Of course, a 12-pack carton is another example of a unit load. Multiple 12-pack cartons stacked on a pallet (as one might see in a grocery store) would be yet another example of a unit load. Clearly, multiple types of unit loads may flow through one or more systems as cans are bundled into 6-pack or 12-pack loads, which are then stacked on pallets.

The size and configuration of the unit load often determines how it can be moved and stored. For example, a forklift truck will typically be used if the unit load is palletized. If the unit load is chained, strapped, or stringed, however, a hook or similar lifting device, attached to a crane / hoist, may be used to lift and move the load. The configuration as well as the dimensions and weight of the unit load will dictate or rule out certain types of material handling equipment. Since it is not possible to treat the subject of unit loads and handling equipment in a comprehensive manner due to limited space, here we will show only some of the basic type of equipment typically used with palletized loads or with containers that have smooth surfaces, such as tote boxes, cardboard boxes, and the like. For more information, the interested reader may refer to Chapter 6 in Tompkins et al. (1996), among others.

The size and configuration of the unit load also determines how often it must be moved and stored. For example, if 100 boxes must be moved per hour from point A to point B, and a trip-based material-handling system is used (i.e., devices such as lift trucks must perform trips to move the

loads), then 100 trips / hr would be required to move the boxes one at a time. However, if the boxes are palletized, for example, and moved in batches of 20 (i.e., a unit load consists of 20 boxes), then 5 trips / hr would be performed from point A to point B. In many cases, the speed of the handling device would not show a significant difference, whether it is moving 1 box or 20 boxes, as long as the unit load is within the weight / volume capacity of the device and standard safety precautions are followed.

Hence, from a material-handling perspective, it may appear that moving the boxes in batches of 20 is the preferred approach since the device(s) would have to perform only 5 trips / hr instead of 100 trips / hr. However, moving the boxes in batches of 20 also leads to a phenomenon known as transfer lot delay. That is, if the transfer lot size is 20 boxes, the first box at point A must wait for the remaining 19 boxes, the second box at point A must wait for the remaining 18 boxes, and so on, before they can be moved to point B. In many systems (including manufacturing systems), such a delay increases the time required to move the boxes through the system even if the throughput (i.e., the rate at which the boxes are moved through the system—in our case 100 per hour) remains the same. Such an increase in time is also accompanied by a proportional increase in the number of boxes in the system at any one time, which would be work-in-process, WIP, or work-in-progress, depending on the application.

The average time to move a box through the system (say, the cycle time per box) and the average level of WIP would be minimized if the boxes are moved one at a time, provided there is enough material-handling capacity to move the boxes one at a time. Whether there is sufficient capacity to achieve one-piece flow or not depends on the volume of flow, as well as the distance, from point A to point B. If the flow volume is high and the distance from A to B nonnegligible, then we may have no choice but to use a conveyor to achieve one-piece flow.

However, conveyors require a large initial investment, and once they are installed, they are difficult to modify or relocate. Hence, as is the case in lean manufacturing, one-piece flow is often achieved by reducing the distance from A to B to such a short distance that the material-handling function is essentially eliminated. That is, no handling device or conveyor is required; instead, either the human operator moves the part with him or her over a very short distance or simple slide / roll mechanisms are used to slide or roll the parts from A to B. This approach is generally known as setting up manufacturing cells to put machines A and B within very close proximity of one another, which works reasonably well for manufacturing applications. In other applications, such as warehouses and airports, however, there are cases where the cell analogy does not apply and the ideal solution may indeed be a conveyor or a trip-based handling system. Furthermore, in facilities such as warehouses and airports, the handling system is also often used to sort the loads, and for high-volume, high- speed sortation, conveyors may be the best solution.

Thus, material-handling decisions are often complex decisions that involve the configuration and size of the unit load(s), the determination of transfer lot size(s), the type of handling systems available (trip-based, conveyors, or robots), the volume of flow, the frequency of flow, and the distances involved. In the next section, some basic handling equipment is presented. We note that the material presented here applies largely to discrete-parts material flow systems, where there is a discrete unit of flow. In some applications, such as oil refineries and sugar-processing plants, the flow is a contin- uous flow and until the product is unitized (by putting it in, say, a barrel or package), there is no discrete unit of flow. Such systems generally fall under bulk material-handling systems, where flow is often measured as gallons / hr, tons / shift, and so on.

The reader may refer to Fruchtbaum (1988), Shamlou (1988), and Woodcock and Mason (1987), among others, for bulk material-handling. It is interesting to note that some systems may be a combination of bulk and discrete-parts flow. For example, in a soft drink bottling plant, the preparation or mixing of the drink may be in bulk form, but once the drink is filled into cans or bottles via the filling machines, the flow will be a discrete flow. Likewise, in a pasta factory, the preparation of the durum wheat would typically involve bulk handling, but once the dough is mixed and pressed into various forms and shapes through the dies, the remainder of the flow will be in discrete units. (Of course, individual pieces of pasta may still be treated as bulk flow until they are packaged.) Both of the above examples happen to involve food handling and preparation, which is often subject to more strict and specialized handling standards.