Movement of Materials
5.11 Accumulation and Intermediate Storage of Materials .1 Accumulation
If supply were matched to demand at every stage of manufacture, distribution, and sale, there would be no need for intermediate storage. The “Just-in-Time” (JIT) philosophy of manufacture seeks to approximate this situation by appropriate design of the production and supply systems. It utilizes a clear flow of information in the opposite direction to the product flow to enable downstream circumstances to control upstream operations. Where, as frequently happens, there is a temporary mismatch between the rate of supply from one operation and the rate of demand from the next, an accumulation system is needed to maintain a buffer stock. At its simplest, products may be allowed to back up along the conveyor linking the two operations. Alternatively, items on the conveyor may be diverted by sweep bars on to a second belt, moving parallel to the first but in the opposite direction. A second set of sweep bars subsequently re-diverts them back on to the original conveyor, thereby completing a circulatory accumulation system. Alternatively, an accumulating table may be provided.
5.11.2 Intermediate Storage
Intermediate storage is another example of accumulation. Perishable products are often stored prior to order picking and/or dispatch, and it is important to maintain a “first in–first out” system of stock rotation. In intermediate storage, this is achieved automatically through the use of a racking system, which allows product to be delivered at one side of the racking, move through it in sequence automatically, and be retrieved as required from the other side.
The same concept may be applied to the handling of complete palletized unit loads. However, the advent of computer-based retrieval systems offers an alternative. Pallets can be placed in any location in a racking system, their location being automatically recorded. They may subsequently be retrieved in the order of production.
5.11.2.1 Bins and Tote Bags
Bins should stack conveniently when filled to allow unit loads to be built up. They should also nest neatly when empty. Plastic moldings can be shaped so that one bin will stand firmly on another when in one orientation but will nest when the orientation is reversed.
Collapsible cubic bins may also be mounted on pallets. A disposable plastic liner is placed inside the erected bin, which is subsequently filled with a liquid product and sealed. Once emptied the bin can be collapsed for return and storage before reuse.
Tote bags are often used to store in-progress materials as well as bulk ingredients. These bags, often containing loads up to 1 t, can be lifted and moved using forklift trucks, cranes and hoists, or other suitable handling devices.
5.11.2.2 Racking Systems
While pallets can be stacked one on top of another, the number of units so stacked is limited by the compressive strength of the unit loads. If each pallet is supported in one cell of a racking system, this limitation is removed and a better utilization made of the building height. The simplest racking systems consist of three-dimensional support scaffolds, and systems 30 m high are not unknown.
To make fullest use of the building capacity, the highest possible fraction of the floor area must be devoted to pallet storage. However, the aisles must be sufficiently wide for handling equipment such as forklift trucks to maneuver and be positioned at right angles to the rack face when inserting or removing loads. To limit the aisle width required, reach trucks, which have forks that can move forward to insert pallets two deep, may be used, thereby halving the number of aisles required.
Alternatively, mobile racks with limited movement perpendicular to their face may be employed.
Space need then only be provided for one aisle, which can be opened up as required between any two racks.
Another strategy is to use automated racking and handling systems where the pallets are moved in and out of the racking by dedicated, unmanned equipment, mounted on guide rails operating in an aisle little wider than the unit load itself.
5.12 Weighing
Manual weighing using counterbalancing weights has now been replaced, in most cases, by systems utilizing transducers linked to microprocessors. Several different types of transducer are available, but the load cell incorporating strain gauges is the most widely used.
5.12.1 Load Cells
A load cell comprises a stainless steel or aluminum alloy block to which strain gauges are rigidly bonded. The strain gauges are usually arranged to form a Wheatstone bridge. Flexing of the cell generates an output voltage, which can be related to the applied load. The cell may be constructing in the form of a short beam, which is rigidly clamped at one end; the load is applied at the free end.
Alternatively, the load may be suspended from the load cell. Shear-type load cells in which the strain gauges are attached to the inner web of a short hollow beam measure shear force rather than bending moment.
Modern load cells are fast in response, robust, and tolerate considerable overloads. They are simple to attach, easy to protect against adverse environmental conditions, cheap and easy to maintain, and suitable for rapid and frequent auto-zeroing. They are, however, sensitive to vibration, although microprocessor-based damping and filtering devices can effectively counteract this.
A recent development is that of load cells which can be fitted to the forks of forklift trucks without interfering with their normal operation. This allows pallets to be weighed during handling.
5.12.2 Automatic Weighers
These fall into two main groups, namely, static and dynamic devices. Static weighing is used to checkweigh finished units or to weigh batches of ingredients, which are subsequently charged to the manufacturing process.
5.12.2.1 Checkweighing
Checkweighing involves the rapid weighing of individual units (200–300 per min). The items to be weighed must be correctly spaced (using photocells) and presented to the weighing head so as to avoid shock and vibration. The load cell readouts are fed to a microprocessor, which has been preprogrammed to execute a number of different functions. Thus, modern checkweighers automati-cally record totals of acceptable, overweight, and underweight items; display mean weights, standard deviations of weights, and weight change trends; reject underweight and overweight units; and provide feedback or activate filling or dispensing equipment.
5.12.2.2 Static Weighing
This usually involves fast feeding the bulk material into a tared vessel (the load cell having automatically weighed the empty vessel) until the required weight of material is approached, when the fast feed is automatically replaced by a fine feed device, which adjusts the batch to its target
weight. Weighing vessels for static automatic batch weighing are usually supported on three points.
For liquids or self-leveling solids, only one of these points needs to be a load cell. For other materials, three load cells may be employed; the microprocessor computes a mean weight.
5.12.2.3 Dynamic Weighing
This involves the assessment and control of a continuous flow of material. One such system measures the rate of loss of weight from a vessel containing material being charged continuously to a process.
The vessel is suspended from a continuously transmitting load cell and is fed through a flexible gaiter. Feeding and discharge of the dispenser vessel is via a screw feeder.
Another form of dynamic weight control uses the belt weigher. In this, the material to be continuously weighed is fed at a controlled uniform depth on to a carefully mounted short section of belt passing over a servo-controlled zero-displacement weigh platform. The weigh signal is used to vary the belt speed and thus to control the weight delivered per unit time. Systems of this type are increasingly used in the dry blending of multicomponent ingredients. While considerable process flexibility is possible, continuous automatic weighing cannot yet match the accuracy of continuous batch weighers.