IN A FRUIT CANNING FACTORY the cans are handled by a system of conveyers and the fruit is placed in the containers without being touched by hand. At the inspection belts behind the conveyers fruit is being examined before it passes to the fillers. The fillers are operated by girls, one of whom is seated at the foot of each conveyer.
THE tin can enables vast numbers of people to obtain inexpensive, pure and varied food produced thousands of miles away. It enables a wide variety of solid and liquid articles to be stored, transported and marketed. It is a prime example of large-scale engineering and coordinated large-scale production. To produce a container which costs, on the average, about a penny, there arc great rolling mills and batteries of automatic machines which have cost millions of pounds. Every step in the production and application of the tin can, from the metallurgy of the steel to the shape of the container and the methods of packing its contents by machinery, is the subject of continuous research and experiment.
Food canning began at the beginning of the last century when Nicholas Appert won a prize offered by the French Government for a method of preserving food for victualling the French army. In 1810, Peter Durand, a Briton, was granted a British patent for an invention communicated to him by a “certain foreigner residing abroad” for preserving food and perishable articles. The foreigner is believed to have been Appert.
This process of sterilizing food by heat and sealing it in receptacles referred to as tin canisters became known as canning. The tinplate canisters were called “tin cans”, or “tins” in Great Britain and “cans” in the United States. Seven years after Durand had taken out his patent another Briton, William Underwood, went to Boston and began the canning industry in the United States.
Canning stimulated the tinplate industry and began remarkable developments. The manufacture of tinplate began in Bohemia in the Middle Ages, and flourished later in Saxony. Several attempts were made to start the industry m Great Britain, and by 1730, when tinplates were being made from rolled iron sheets at Pontypool (Mon.), it was definitely established. The workers became so efficient that Monmouthshire and South Wales developed into the centre of the world industry.
At the end of the last century, however, the canning industry of the United States was importing such enormous quantities of tinplate that the United States put a tax on imported tinplate. The Americans therefore built up their own industry, so that to-day the United States is the largest producer as well as the largest consumer of tinplate. About 98·5 per cent of the metal in tinplate is steel and about 3,000,000 tons of the metal are used every year. This steel passes through the stages of being mined, smelted, refined, rolled, annealed and tinned. Various methods of making tinplate are used in America and in South Wales. In the days when wrought iron was the basic material this was smelted and refined by charcoal. When coke was introduced impurities due to the coke caused a poorer quality of blackplate, as the untinned basic plate is called. Nowadays the term “charcoal plate” refers to tinplate with a comparatively thick coating of tin, and “coke plate” applies to that which has a thinner coating; neither term has anything to do with the smelting. By far the greater proportion of tinplate is rolled from open hearth steel, which is preferred, because of its softness, to Bessemer steel, although some Bessemer steel is used.
Ductility is essential for making cans, and therefore the steel used in tinplate is mild, with little carbon. If the sheet is to be made by the process of pack rolling, or doubling the sheets repeatedly, a small amount of phosphorus is added to the molten steel before the ingots are cast, so that the sheets will not adhere too firmly when they are being rolled.
If the sheet is to be continuously cold-rolled without doubling, the phosphorus is not necessary and is not added. Even at this early stage in the making of the can various problems have to be considered. The presence of metalloids such as phosphorus may influence the mechanical process or cause corrosion.
There is no universal method of rolling the steel ingots to finished sheet, and details differ in almost every plant. In general the huge steel ingot, weighing many tons, the parent of a multitude of cans, is placed for several hours in the soaking pit where furnaces heat it to a white-hot temperature for preliminary or rough rolling. The ingot is lifted out by tongs, operated from a crane, and is carried on a conveyer to the blooming, or cogging rollers. Scale is removed, generally by a water spray, and the rollers reduce the ingot to the size and shape required. The dimensions vary according to the rolling process that is to follow. The ingot is rolled to a long slab and is sheared into, convenient lengths, which, in turn, pass through a series of rollers which reduce them further. They are cut by revolving disks into lengths, called tinplate bars. The rolling improves the ductility of the steel and is the prelude to another series of rolling and heating processes.
In the pack rolling process the bars, which may be about 15 feet long, 10 in. wide and ½-in. thick, are sheared to a length about equal to the required width of the finished blackplate and are hot-rolled to approximately the finished gauge. The operations of reheating, rolling, doubling and shearing produce packs of eight sheets, each sheet being uniform in size and substance. The gauge is generally 0·01 in. for ordinary tins, or if for what is called “taggers’ plate”, which is used for the tops of cigarette tins and other purposes, only 0·006 in. To prepare them for tinning, the packs are opened by hand; should the sheets stick they are passed through a roller to separate them. Then they go into a hot acid bath to be “black pickled”, the name of this operation reflecting the old days when the operators put the sheets in vinegar. The plates are dipped by various types of machinery which ensure that each sheet is treated by the acid, and are then washed.
CAN ENDS ARE MADE SEPARATELY, the bodies being made by a different process. The tinplate, cut into strips, is fed into this press, which automatically stamps and forms the disk which will make the top or bottom of the finished can. In a second operation the same machine curls the edge of the disks ready for seaming on to the body.
After they have been taken from the vats the sheets are piled flat on stands in an annealing box. In some plants the annealing boxes move slowly through heating, soaking and cooling zones. In other plants portable furnaces, composed of electrically heated or gas heated covers, are placed over the boxes containing the stands, so that the furnace is moved and not the box. The packs, having been annealed, are opened and cold-rolled. The sheets are then annealed at a lower temperature than before, the second operation being called “white” annealing, to distinguish it from the first, or “black” annealing. Then the plates are “white pickled”. and are stored in tanks of slightly acidified water to await tinning.
The tinning apparatus comprises a subdivided cast-iron pot, fitted with the tinning machine. The pot is generally heated by gas. The molten tin fills the lower part of the pot; a thin layer of molten zinc chloride flux floats on top of the tin in one compartment and a deeper bath of palm oil floats on top of the second, or exit compartment. In some installations gas heaters of the immersion type are inserted in the first compartment so that the temperature can be closely regulated.
The tinning machine is a system of rollers for conducting the sheets singly through the bath. The sheets are placed in water in a trough near the plant and are fed into the rollers by hand or by an automatic machine. The sheets pass first through the flux, then through the molten tin and finally through the palm oil, which serves to keep the tin molten while the sheets pass through the final rollers, which press off the excess and regulate the thickness of the coating. If a heavier coating of tin is required the tinned sheet is hand-dipped in another tin bath. Articles such as milk cans, pails and kitchen utensils which are made from blackplate before tinning are hand-dipped repeatedly, the excess tin being removed by blowing, draining or wiping. The tinned sheets pass to the cleaning units, where they are cleaned by being dipped in various solutions, or by sheepskin rollers fed with bran. Finally the tinplates are sorted and packed. Generally the film of tin is only 0·0008 in. thick, about one fortieth of the thickness of a human hair.
Continuous Strip Method
Another method of rolling steel for tinplate is the continuous strip method. The ingots are hot-rolled to a wide strip one sixteenth of an inch thick, which emerges in a long coil from the last stand of rollers at a speed of over twenty miles an hour. These coils are cold-rolled to tinplate thickness at about half this speed. One coil may be up to 3 feet wide and over 2,000 feet long. The thickness is generally about 0·01 in. as in pack rolling, but 0·006-in. taggers’ plate can be produced accurately. The strip is uncoiled and cut to size for tinning. This method is newer than pack rolling and requires heavy rollers exerting tremendous pressure and capable of hair-breadth adjustment. Such a mill, which costs about £3,000,000, produces strip and sheet steel for motor cars, for corrugated steel and for other purposes, in addition to producing tinplate, so that a humble tin can and the wings and other parts of a costly motor car often come from the same mother ingot of steel.
COLD-ROLLING STEEL FOR TINPLATE. In this tandem line the hot-rolled strip, from 1/16 in. to ⅛-in. thick, is quickly and accurately reduced to a thickness of about one hundredth of an inch. The operation is automatic and the operators’ chief duties are to watch the instruments and to throw the switches when necessary.
There are two main types of mills for the continuous rolling method. One uses a number of individual stands of rollers operating in tandem on one strip of steel. The other uses a set of rollers that can be reversed. The reversing mills are less expensive but are slower, because there is a slight pause in the work when the direction of the rollers is altered and they are tightened. The continuous tandem mill is sometimes a quarter of a mile long to accommodate all the processes of transforming the ingot into blackplates. The ingot is rolled and cut to length as in a pack roll mill, and the slabs are taken to the head of the continuous line where they are hot-rolled by a series of huge rollers and wound into coils while still hot. The coils are cooled and pickled and then pass to the cold roller stands, after they have been oiled to prevent rust. Having left the cold rollers the coils are freed of oil by being passed through a bath; then they are cut to pack size, annealed, and given a final cold rolling to polish them. They are then ready for tinning.
The reversing mill uses strip, but when the strip has passed through the rollers in one direction the machine is stopped. The adjustment is altered to tighten the rollers and the strip is passed through again. After about five passes the operation is complete.
Fascinating as is the making of tinplate, it leaves millions of flat sheets of bright tinplate which have to be made into containers for a thousand and one different foods or into articles of a multitude of shapes and sizes. The can may have to be filled and sealed hermetically, or made so that it can be used for years as a tobacco, cigarette or sweet tin. Above all it must be attractive in shape and must be labelled either by printing direct on to the metal or by using a paper label. There are also the problems of filling the cans, fitting the lids and so designing them that they can be opened easily. The first canners had to work by hand. Tinned iron sheets were marked out and cut by tinsmiths, and were bent into shape on a roller so that the edges of the body overlapped. These were soldered, so that the body was a cylinder open at either end. Then came the problem of the top and the bottom.
Two disks were cut, slightly larger than the opening of the cylinder, and a hole was cut in the disk for the top. Each disk was fixed by turning the overlapping edge to form a flange, and by hammering it to the body and then soldering it. The can was filled through the hole in the top, and was closed by soldering a small disk over the hole. It took a man a day to make a hundred cans by hand.
Then presses were evolved for cutting disks and flanging them, and mechanical aids for assembling tins were introduced. Gradually the process of can manufacture became automatic. Some machines turn out 300 or 400 cans a minute. Then the open-top, or “ sanitary ” can was introduced and came into general use except for condensed milk and some products suited to the filler-hole can. The early canners could not depend on the thickness or the quality of tinplate, so that some cans were so thick and tough that considerable physical strength and an exceptionally stout tin opener were required to enable the buyer to get at the contents. Inventors concentrated on making cans easier to open. One successful type is opened by a slotted key. The lid of the can projects in a tongue across one corner, and the key is passed over of the tongue and wound, so that a strip of the lid weakened by scored lines is torn off. The “tagger top” can is another type. The top of the can is a sheet of thin tinplate, and above it is a strong loose cover fitted with a cutter.
The open-top can consists of a body with the side-seam hooked, flattened and sealed, to which the bottom has been attached. It can be filled quickly without damage to the contents and the top is sealed by automatic machines at the rate of about 150 a minute.
SIDE-SEAM SOLDERING MACHINE. The bodies of the open-top cans are made in five operations, the normal rate of a line of body-making machines being 300 cans a minute. Blanks, already notched and with edges turned to form the side-seam, are bent into a cylindrical shape round a mandrel so that the turned edges engage and the seam is flattened by a blow from a mechanical hammer. The seam is then automatically soldered.
Can making is so competitive that the average profit on a can is only about a hundredth part of a penny. A huge output is therefore essential, the normal rate of a line of machines being 300 cans a minute. There are generally five operations for making bodies and three for making the ends. The first stage in body making is to cut and trim the tinplate by automatic machines which have rotary cutters of hardened tool steel which cut the tinplate without leaving much of a burr. The plate is cut for the body blanks and passes to the head of the body-making line, where the lock-seam body maker operates. The blanks are fed into the magazine and stacked. Vacuum cups feed them to bars which convey them to notching and edging stations where the edges are turned, one up and one down to form the side-seam.
The blanks are bent into cylindrical shape round a mandrel so that the turned edges engage and the seam is flattened by a blow from a mechanical hammer. Next the seam is soldered, the excess solder is wiped off, and air from a blower cools the seam. The
body moves on to the next stage, flanging, by rolling down a chute. The flanging machine bends the two ends of the body by pressing them against steel dies, and at the same time rollers revolve against the body and roll out any kinks as it turns on a rubber cylinder. The next stage is the double seamer, where the bottom is attached by double-seaming rollers.
The same type of machine is used later to put the top on a filled can. The body enters the machine upright and an end, previously prepared, is dropped over it as it enters the rotary seamer. Here it is held firmly while grooved rollers press the edges of the body and the bottom disk tightly together. A gum or paper gasket is inserted to make the seal airtight. The final stage is to test the can for airtightness. Rubber vacuum pads place the cans in receptacles set horizontally on a revolving wheel, and the open end is sealed by a rubber pad. Compressed air is forced into the can and if the can leaks as the wheel revolves the air passes into the chamber round the can and operates a diaphragm. This actuates an electrical device which releases the can.
The tops and bottoms of the cans are made in an auxiliary line. The tinplate is fed to the scroll shears or slitter, where it is cut into strips. Then it passes to the press and curler, which stamps and forms the disk in one operation, and in a second operation curls or bends the edge ready for seaming to a body. There is a third operation for inserting the gasket. Sometimes the gasket is of paper, sometimes of a plastic gum. The machine applies the compound and the ends are stacked and passed into a heated chamber, where they are dried. Then they are either fed as bottoms to the double seamer or are packed to send to the canners as tops for filled cans.
Each individual machine does not operate at the same pace, and to maintain an output of 300 completed cans a minute some of the machines are in duplicate to keep up with those that operate faster.
Square, rectangular or tapered cans used for asparagus, corned beef and other commodities are more difficult to make. For many years the ends were soldered, then double-seamed ends came into use, but a vent hole was left in corned beef cans to be closed by solder in a vacuum chamber. The cans are now often closed by double seaming under vacuum and solder is not used. The process has been accelerated almost to speeds at which cylindrical cans are made.
When foods are sealed in cans under vacuum, the filled cans pass through a vacuum chamber in which the double seaming is performed by automatic machinery. Some foods were found liable to corrode cans and the difficulty was overcome by varnishing the insides of the cans with lacquer. One class of lacquer resists acids and another resists sulphur compounds. It was not until a suitable lining for the cans was found that it was possible to can beer successfully. Enamels made from synthetic resins are used to coat the interior after the beer can has been made. An alternative is a wax which is applied molten and allowed to set.
CANS FED BY GRAVITY to the flanging machine. This device bends out, or flanges, the top and bottom edges of the open body of the can ready for the ends, which are made by separate processes. The edges in this machine are flanged by pressure against steel dies.
The tinplate used is stronger than usual because of the pressure which may be developed by the beer during pasteurization, and the side-seam has to be particularly well soldered. The cans are made either in the usual shape or with a conical top closed by a crown cap, the former type being better for packing and the latter more adaptable to bottle-filling apparatus.
The enamelling, lacquering and lithographing of tinplate are an important branch of the industry. In almost all instances the tinplates are treated before they are made into cans. The first step is to examine the tinplates and to remove inequalities at the edges for the purpose of ensuring accuracy as the sheets pass through the enamelling rollers.
Automatic feeders are used to feed the plates to a variety of machines. The tinplates are stacked in a pile to a height regulated by a governor and are lifted in rotation from the top of the pile. Two rubber vacuum lifters raise the sheet from the back while a blast of compressed air helps to separate it from the sheet under it. As the sheet tilts, rubber forwarders grasp the front edge by vacuum and push the edge under rubber drop rollers, which force it forward on to belts that convey it to the machine for the next operation.
Before the tinplates are enamelled or printed they are cleaned by vacuum, heat, chemicals or cloth rollers. Then they are carried by conveyers to the rotary enamelling machine, where a set of rollers transfers enamel from a bath to one side of each tinplate, but not coating it entirely if it is to be used for can bodies. If the tinplate were entirely covered, trouble would follow when the side-seams of the can were soldered. Strips of uncoated tinplate are therefore left at the exact intervals where solder will afterwards be applied. Tinplates for can ends are coated entirely, as no solder is used. Some machines coat both sides of a tinplate at once, and if a coat is required only on one side two sheets are fed into the machine together so that the two inner faces are not enamelled.
Colour Printing on Cans
The wet sheets are automatically tilted to a vertical position and carried by conveyers through a continuous baking oven. On emerging, they are cooled by air and then stacked ready for use.
Sometimes several coats of enamel are applied, or coats of lacquer are put on over the enamel. As the cans for food are often heated it is essential that the lining shall withstand heat without impairment of its resistance to acids or sulphur: hence the care and attention paid to enamelling. The lining of beer cans, and of a few other cans, is applied after the can has been fabricated. This is more expensive than treating the tin plate, and is done to ensure that there is no risk of damage to the lining.
Any picture or design printed on paper can be reproduced on tinplate. This fact has been turned to good advantage, for some large tinplate containers are decorated with reproductions of famous pictures. Colour printing on cans, however, is much slower than on paper, because only one colour is normally applied at a time, and has to be baked on before the next is applied. Fifteen or sixteen separate operations of sizing, lithographing, varnishing and baking may be required for a picture with delicate shading. The tinplate is trimmed, cleaned and given a base coating of lacquer as for can bodies, the base coating being varied according to the conditions.
ROLLER CONVEYERS help to reduce the labour of handling sheets and sheet-packs of tinplate. This roller conveyer loop brings partly roiled sheets to the back of the furnace, through which they are conveyed ready for the finish-rolling process.
The printing is done by rotary offset machines, the lighter colours being printed first and then the darker ones. As the tinplate does not absorb ink, each colour has to be dried or baked. The baking is done in continuous ovens, but at a lower temperature and for a shorter time than for enamelling, as the ink dries quickly. The finishing varnish is applied by roller machines and is baked on, forming a protection to the picture when the plate is being made into containers, and also when the container is in use. Thousands of tons of tinplate are used every year for making caps, crowns, screw-tops and lids for bottles, jars, cardboard and other non-metallic containers. The crown cork type of closure used for many kinds of bottles is produced in thousands of millions and accounts for 80,000 tons of tinplate annually in the United States. The tinplate is trimmed, coated or printed, and punched into shells for the caps by automatic stamps which produce up to 50,000 caps an hour. Each punch cuts a disk and presses it against a die which shapes it. The shells pass to the assembly machine, which places a spot of adhesive inside each shell and then presses in a .cork or paper insert. Sometimes the cork inserted is coated with paraffin wax to protect it, and at others a disk of tin or aluminium foil is thrust in so that the cork lining does not come into contact with the contents of the bottle.
Screw caps which can easily be replaced on cans or bottles are first punched out of the tinplate as disks and pressed into hat-shaped shells. The threading is added by a second operation. In some types the exterior is smooth and inside it is a threaded liner. Both shells are made separately and then joined together.
Although the British canning industry is small compared with that of the United States, it has grown surprisingly during recent years, particularly for canning fruit and vegetables. Can manufacture has increased correspondingly. The output of one firm in 1926 was a million cans a year — in 1931 the automatic plant in the same factory was capable of making a million cans in a day.
Tremendous strides have been made in canning peas, and mechanization extends even to the shelling. The complete plants, including the stems, leaves and pods, are put into revolving drums. Inside these the pods are opened and the peas fall through holes in the drums and pass to sorting machines. One machine is devised to pick out maggoty peas. The peas pass over drums equipped with fish-hook barbs, and if a pea contains a hole it is caught by a hook.
Before fruit and vegetables reach the cans for cooking they are blanched, a preliminary process in which hot or boiling water helps to remove light organic matter and gas. After blanching the food is cooled and again washed before it reaches the filling station.
Developments in Great Britain
Here the cans are sterilized by steam jets and drained ready for the food. The food is placed in the cans by automatic machines set to supply either solid or liquid material in exact quantities through nozzles, which are opened by each can opening a valve as it moves into place under the nozzle.
It is essential to exhaust the air and gas from the space between the top of the food in the can and the rim. With dry food this is done by vacuum, and with liquids by conveying the cans through tanks containing hot water and team. The can lid is then seamed on and the can is ready for the cooking retorts.
In contrast to the automatic machinery of the canning factories is the development of home canning, which is of growing importance in the United States. In many rural communities the American housewife no longer bottles fruit in glass jars: she cans it.
Recent developments in British canning are due to the co-operation of various trade and industrial organizations with Government departments. Much valuable work is done by the International Tin Research and Development Council, a body established for the purpose of acquiring and disseminating scientific and technical knowledge relating to tin and its applications. Research chemists in Government and other laboratories study every aspect of food canning, and every stage of the industry is subjected to rigorous inspection.
TANDEM COLD-ROLLING MILL for rolling steel for tinplate by the continuous strip method. Ingots are first hot-rolled into a wide strip which emerges in a coil. The coils are about 3 feet wide and 2,000 feet long. They are cold-rolled to tinplate thickness at a speed of about 10 miles an hour. The continuous tandem mill is sometimes a quarter of a mile long.