Plastic containers are used for some very different tasks within the fish industry. Although some of them are all-rounders that are suitable for many different uses, containers are usually tailored to specific functions with regard to their size, design and the exact material they are made of. In general, a container’s appearance and the design of many details will depend on its intended function as on where it is to be used. Tubs for dry products must meet different requirements than containers for liquids, for example, tubs for frozen fish products usually have to be more resilient than for fresh fish, and in the food industry hygiene standards are higher than in the technical or industrial sectors. There are many factors that have to be considered when choosing a container but the range of plastic tubs and bins on the market today makes it possible to find a product that will meet the specific requirements of almost every possible application.
The materials used for making tubs and bins today mainly consist of two plastics: polyethylene or polythene (PE) and polyurethane (PU, PUR). The thermoplastic material polyethylene was developed already in 1898 but only gained importance for containers as from 1933 with the industrial production of the first pressure-resistant high density polyethylene (HDPE). And HDPE has actually only been used in larger quantities since the late 1950s, for example for pipelines, tubs and bins as well as for flexible packaging materials. Depending on the molecular fine structure, a distinction is made between several PE types, for example low-density PE (LDPE), linear low-density PE (LLDPE), or ultra-high molecular weight PE (UHMW), of which, however, for the manufacture of tubs and bins only HDPE is of importance. The properties of PE can be modified by using appropriate additives to meet the intended use. Polyethylene has a high mechanical and chemical stability, and it is resistant to acids, alkalis and other chemicals. Depending on its composition it can withstand temperatures in the range of about -85°C to +90°C. It becomes softer at higher temperatures and can easily be moulded. Without pre-treatment PE does not stick well but it can be easily welded. Although the plastics used are often equipped with some UV protection they tend to become brittle if exposed to strong sunlight over a longer period of time.
First synthesized in 1937, polyurethanes can be hard and brittle or soft and elastic depending on the manufacturing method used, and their possible applications are thus correspondingly diverse. They are used for mattresses, insulation materials and cast floors, textiles and potting compounds, for making rubber boots, adhesives and varnishes. Polyurethanes are particularly suitable for applications which require high wear resistance, such as for pipes or transport containers. It is possible to get an idea of the hardness of foamed PUR (whose density varies between about 1000 and 1250 kg/m³) by taking a closer look at bowling balls, the outer layer of which is usually made of polyurethane. Foamed polyurethane is not only lightweight, but also has a first-class insulating effect against heat and cold. That is why the material is often used for the construction of double-walled temperature insulated tubs and bins. The inner and outer walls of the containers are made of hard, durable PUR, and in between there is an insulating layer of foamed polyurethane.
Elaborate production using rotomoulding
Both PE and PUR containers are today mostly manufactured using the technique of reaction injection moulding (RIM), also called "rotomoulding", which is technically very complex. For the RIM method, a gimballed, heated mould is rotated continuously around different axes, during which the plastic material is distributed evenly in the mould right up to the final hardening. In this way it is possible to give complicated products, such as large, fully insulated, double-walled, seamless containers, a very uniform wall thickness, even at the edges and in hardly accessible nooks and crannies. This is one of the main reasons for the high stability and durability of the containers produced using rotomoulding. However, the constant movement, the heating and the gimballed positioning of the moulds make very large plant sizes necessary and when manufacturing large tubs and bins these can quickly exceed the dimensions of a detached house. During the construction of large containers, however, rotomoulding has asserted itself over the injection moulding technique. It requires reaction masses with relatively low viscosity and good flow properties to enable longer flow paths, and thus uniform wall thicknesses.
The processing of thermoplastic polyethylene using rotomoulding is comparatively easy because the raw granules really only have to be melted to allow the material to flow into the heated mould. Polyurethane plastics demand much more effort because the raw material mass hardens very quickly. That is why the reaction components for polyurethane synthesis cannot be mixed in until immediately before introduction into the mould. This is carried out under high pressure of between 100 and 200 bar in a mixing chamber from which the mixture is immediately injected into the shaping tool. There the mass hardens within just a few minutes and can then be removed from the mould.
Double walled fish containers can be insulated with polyurethane or polyethylene foam. The inner wall is often thinner than the outer wall which has to bear more mechanical stress. Containers with PUR insulation are 25 to 30% lighter and are characterized by a strong insulating effect which is three times better than that of PE foamed containers. PUR insulated containers are therefore ideal for conditions with very high or very low ambient temperatures, since their excellent insulation keeps the conditions inside the container constant for a long time. In cases where insulation is not so important the walls can be lined with a thinner layer of polyurethane insulation which makes the container lighter. The recommended amount of ice to keep fish inside the container cool and fresh depends primarily on the ambient temperature and the intended duration of storage. A simple rule of thumb is that a tub should contain at least 30% ice. Compared to the highly insulating PUR containers which keep the cold inside the container constant over a long period the insulating effect is significantly lower in PE containers. That is why they are sooner used in areas that have their own cooling system and so keep the fish cool "from the outside" so to speak. An advantage of PE tubs is, however, that they are particularly durable, long-lasting and can even reach the age of five, ten or more years even under the daily rigours on board fishing vessels, at auctions, in the processing area and during transport. How durable a container will actually be depends upon the concrete conditions of its use, and upon whether it enjoys careful treatment and regular maintenance.
Foamed plastics have optimum insulation effect
The inner and outer walls of tubs and bins made of plastic are smooth and hygienic and comply with all requirements in the food industry. PE containers can be cleaned with hot water (70 to 80°C) and at high pressures of up to 200 bar. Polyurethane is slightly more sensitive and often endures water temperatures of only 40 to 50°C. The use of these large plastic containers in the frozen products sector is basically possible, but they should be specifically made for this purpose. In the case of "normal" double tubs, the solid inner and outer walls might separate from the inner insulating layer, which will reduce their service life. Of course, “normal" containers are suitable for the storage of fish on ice.
The range of tubs and bins made of PE or PUR is immense and includes containers of nearly all sizes, shapes and designs for various purposes. A common standard is transport containers with a capacity of about 300 litres to over 1,000 litres, which usually have the same sized base and are thus easily stackable. The external dimensions of the tubs are often based on existing EU standards to enable effective use of the space in containers and trucks. Usually, the containers have two or more closable 2-inch openings in the base area to enable the drainage of melt water and other liquids. With their matching insulated lids that fit precisely onto the containers the tubs are very well suited to storage and transport of temperature sensitive goods. Under the climatic conditions in Central Europe iced fish can be stored about eight to ten days in insulated tubs. That is why these containers are often used on small fishing vessels in order to store the fish after the catch at sea. Plastic tubs make it possible to sort the fish by species and sizes, they reduce the pressure that under other storage conditions would weigh on the fishes in lower layers, and they allow for rapid unloading of the sensitive cargo after arrival in the port.
In the fish industry it is mainly single and double-walled plastic containers that are used for storing the fishes themselves and also slaughter products. Tubs are used for salting, maturing and marinating, containers for storage or thawing of frozen products, containers that can be closed tightly and have their own oxygen supply for transporting live fish, almost hermetically sealable containers for dead fish and offal, plus many others.
Manufacturers of bins and tubs try to modify their products in some design details so that they can be used for as many purposes as possible or for very special purposes. A relatively easy way to achieve this is to produce the container in different sizes that, depending on the amount of fish, always makes optimal use of the available space. Anyone who stores 30 kg fish in a 300 litre tub or transports it by truck is wasting space and energy. That is why the Icelandic company Borgarplast produces double-walled PE or PUR insulated fish containers in over a dozen different sizes. One of them is a fully insulated 1250 litre container that is designed for extremely large species such as tuna and swordfish. Nearly two metres long and 1.20 metres wide, it is probably currently the world's largest container made using the rotomoulding technique. Containers larger than these are hard to imagine, because that would require the heating chamber in which the mould rotates to have almost gigantic dimensions.
All tubs and bins in the range are designed to fit the standard dimensions used in the regions in which they are to be primarily used. In the case of fish containers for the European market the insertion openings at the base are for example the same size as in euro pallets, which facilitates transport of containers by forklift. Apart from that, the openings in the pallet-like substructure of the containers are particularly large and slope at the edges to facilitate the insertion of the forklift truck and prevent damage. Depending on the size, design and how full the container is, up to eight containers can be stacked on top of one another. Tub and bin manufacturers often stamp the company logo, name, etc. on the plastic during production since such stamps usually rub off quickly and so don’t last long.
Special products extend the range of applications
A lot of manufacturing firms do not only have insulated tubs in their production programme, but also non-insulated tubs, multi-purpose containers with or without drainage holes in the bottom, and multi-compartment containers for sorting shrimp on board ships, for example. A sought-after product is defroster tanks that have been developed for thawing frozen fish products. In some of these, special nozzles have been embedded into the base, through which water and compressed air of a few bar flow into the interior. The corresponding connections are located outside of the defroster tank where they are easily accessible. When air and water are pumped in simultaneously this creates a whirlpool-like effect which shortens the thawing time of the frozen food by about half. Special cooling bins with particularly tight fitting lids that can be optionally equipped with their own refrigeration unit are also available for aquaculture farms where dead fish occur regularly or where slaughter and processing waste occurs. These products must, of course, be isolated until their final disposal to prevent infection. Such bins mostly consist of high-quality HDPE which meets all HACCP standards and can be cleaned and sterilized with hot steam.
One special application for insulated containers is to use them as mobile stalls for selling seafood and other temperature-sensitive products. The user just has to place a counter top made of glass or Plexiglas onto the container. Such stalls have the advantage that they do not require a separate cooling unit. The products are pre-cooled and packed in the insulated containers and will maintain their temperature then for many hours, especially if more ice or dry ice is added. These simple and relatively inexpensive stalls enable traders to carry out additional promotion campaigns quickly and easily.
Although the tubs and bins are made of plastic they are relatively heavy, especially the larger containers. The main reason for this is that they must be built to last and need thick walls because they are often exposed to countless mechanical stresses on land and at sea. During transfer from one place to another by a forklift the tubs are often knocked or put down sharply, and when stacked they have to bear significant loads. In order to be able to occasionally move smaller containers between 200 and 500 litres by hand, the industry also offers ergonomic carts and buggies on wheels for in-company transport. For small tubs the carts are often made of plastic, for larger ones usually of stainless steel.
Many manufacturers optionally equip their containers today with RFID chips (Radio Frequency Identification). The chips can be read by sensitive sensors at a certain distance without direct contact, which enables continuous control and better monitoring of the internal flow of goods by computer. Such sensors are available as flexible hand-held readers, but they can also be attached directly to forklift trucks, conveyor belts or hall doors. Once an RFID-tagged container passes one of these sensors, its data is collected and the retrieved information transmitted to the computer.
Meanwhile, some service companies have even specialized in the care and maintenance of tubs and bins. They provide a professional repair service or take over the cleaning of soiled containers, for example. This is particularly worthwhile if the containers do not belong to a specific company but circulate among different users as rented property from a central pool. Centralized cleaning guarantees that each member of the rental system can always rely on clean, hygienic containers.