8.1 Material
8.2 Strength
8.3 Weight
8.4 Drainage
8.5 Nesting
8.6 Stacking
8.7 Useful Life
8.8 Dimensions and Capacities
8.9 Fish Box Washing
8.9.1 Hose wash
8.9.2 High pressure cleaning
8.9.3 Box washing machines
Fish boxes are commonly made of wood, aluminium or plastic. Wood was the traditional material for fish boxes but is being increasingly replaced now by plastic for reasons of cost and problems of usage. Wood is extremely difficult to clean properly, has a short life compared to plastic and requires frequent repair. Aluminium is easier to keep clean but is prone to damage by rough handling making them difficult to stack. Aluminium boxes are also very noisy to handle on a concrete market surface. Plastic (thermoplastic high density polyethylene) is easy to clean, will withstand rough handling and allows detail design requirements to be incorporated in the mould.
Boxes used in the fishing industry are often subjected to abuse by rough handling and should be designed to withstand such treatment. At sea they are often subjected to fierce gravitational forces caused by vessel motion. Particular attention should be given to the design of the handles or recesses for lifting, which should be strong enough to carry the load of three full boxes as vessels often discharge this many boxes using hooks to the handles of the bottom box of three. A stack formed from loaded boxes should be rigid and not tend to flex.
The tare weight of plastic and aluminium boxes is constant and not subject to age or environmental conditions. Wooden boxes will absorb water and can almost double in weight when saturated making accurate weighing of fish in the wooden boxes difficult and increasing transport costs. Typical box weights are given in Table 20.
Fish boxes should be designed to have good drainage in order that the melt water, which contains bacteria from the fish, is allowed to drain away. Some designs enable the melt water to drain through the box so that melt water from one box will then pass through the box beneath it thereby providing the maximum cooling effect from the cold water. The disadvantage of this design is said to be cross contamination by bacteria carried by the water from box to box. Other designs prevent this by drainage to the outside of the box but with the loss of the cooling effect of the water.
The ability of a box to nest when not in use is very useful when storing empty boxes, particularly for applications within fish markets and box pools. Nesting boxes will give a reduction in storage volume of up to 60 percent depending on design compared to stack-only types. The nesting type of box does however suffer the following drawbacks, particularly for use at sea:
An advantage of some nesting designs is the ability to lift boxes by the lips by a modified fork lift or pallet truck which eliminates the need of pallets. Figure 23 shows the stack/ nest box.
Figure 23. Stack/nest box
Boxes that stacking only should incorporate interlocking grips between boxes to prevent sideways movement of boxes in the fish room at sea or during transportation on shore as shown in Figure 24. Plastic and aluminium boxes can be designed with this facility but not wooden boxes. The grips should allow staggered patterns of stacking for better utilization of fish room space. Stack only boxes are volumetrically more efficient than nesting boxes but require far greater storage volume when empty.
Figure 24. Stack only box
The life of any box is plainly dependent on factors of design, material, construction, degree of use and methods of handling but as a comparison aluminium and plastic boxes might last 6-10 years, respectively, whereas a wooden box might last only 2 years and require repair during that period. In the calculation of requirements of boxes over a period, pilfering and loss should be accounted for, and in this respect, plastic boxes offer an advantage over wooden or aluminium inasmuch as the material is of little use for alternative purposes.
The dimensions and capacity of a fish box should reflect factors of fish size, packing density of fish and ice, use of mechanical handling equipment and whether a one-or two-man lift is desirable. It may also need to be compatible with existing boxes, possibly wooden, and of a capacity that is a traditional unit of sale. The length should be such that fish will not overhang the boxes or have to be forced into it and the depth should be such as to allow for fish and ice without crushing. If the box is to be used on a roller conveyor care must be taken to ensure a continuous bearing surface along the base of the box in the direction of intended motion. The volumetric capacities and dimensions of different types of boxes are given in Table 20.
Table 20 Dimensions, capacities and costs of fish boxes
Type |
Material |
Volume |
Cost a/ |
External |
Tare weight |
Capacity of |
Stack |
Plastic |
30 |
9.80 |
800 x 450 x 150 |
2.5 |
17 |
Stack |
Plastic |
42 |
6.90 |
600 x 368 x 214 |
2.6 |
25 |
Stack c/ |
Plastic |
25 |
6.90 |
560 x 415 x 150 |
1.6 |
20 fillets |
a/ Individual box cost based on an order of 1 000 boxes ex-works (1979)
b/ Dry weight which can almost double when water-logged
c/ Fillet boxes for use in the processing factory
d/ Approximate capacity of fish assuming a 2:1 fish: ice ratio (crushed block ice)
It is essential that after use fish boxes should be thoroughly cleaned of all dirt, fish slime and scales. The method of cleaning to be employed will depend on the number of boxes involved and whether or not the debris has been allowed to dry onto the box.
With small numbers of boxes, a simple cold water wash may be all that Is required but if the debris has been allowed to dry on the boxes then an overnight soak In a detergent solution with scrubbing and hosing the following day will be necessary. Such treatment is not suitable for wooden boxes which cannot be satisfactorily cleaned.
Portable high pressure cleaners can be used effectively to clean plastic and aluminium boxes but should not be used on wooden boxes. The units are available with cold or hot systems operating at pressures. typically in the order of 70 kg/cm�. Water consumption is about 12 litres/min and fuel consumption (hot water models) between 7-11 litres/h depending on temperature. Installed power is typically 2-3 kW. Approximate cost of cold water unit is US$ 1 500 and for a hot water unit US$ 3 500 (1979).
Box washing by machine is by far the most efficient and quickest method of handling large numbers of boxes with capacities in the range of 800-1 200 boxes/h. Boxes are normally fed through the machine hung on a monorail conveyor or upside down on a roller or mesh belt conveyor. The machines are typically made up of three sections having a cold or warm water pre-rinse followed by a hot water detergent wash and finally a cold water rinse. A pre-wash is necessary to remove salt from the boxes as it reduces the effectiveness of the detergent. The washes are performed under pressures of 4.6-5.6 kg/cm� if the debris is dried on, or slightly less if not. Fresh water consumption for a unit of 1 000 boxes/h capacity would be about 5 t/h. This can be reduced by using pumped sea water for the pre and post-wash rinses but is not recommended because of possible contamination. Power consumption will depend on throughput and water pressures involved, but is typically in the range 15-30 kW for a 1 000 box/h unit. Boiler oil consumption for heating water will be about 40 litres/h for a throughput of 1 000 boxes/h with a wash temperature of 70 �C in a temperate climate. A mildly alkaline detergent should be used for wood, aluminium or plastic boxes at a concentration of 1-2 percent solution. For the 1 000 box/h machine a minimum labour force of four men will be required; two feeding the machine and two removing boxes plus additional labour depending on distances involved fetching and stacking boxes. Price for complete unit, 1 000 boxes/h, including boiler and installation would be in the order of US$ 50 000-60 000 ex-works. The space required for the machine is not large (10 m long x 3 m wide x 2 m high), but relatively large areas must be allowed for handling of the boxes which will be dependent upon the type of box and maximum height, Stack/nest boxes in this situation have a big advantage.
