Part 42

_Washing._--When the crude rubber arrives at the factory of the rubber manufacturer, it is generally stored in bins in dark and fairly cool store-rooms, where it is kept until ready to be used. The rubber passes directly from the storage bins to the wash-room, where it is cut up into small pieces, put into large vats of warmed water and allowed to soak, in order to soften it sufficiently to be broken down in the machines. It is then fed into a cracker, a machine consisting of two rolls with projections on their surfaces shaped like little pyramids, the two rolls revolving with a differential, one going considerably faster than the other, and being adjustable, so that they can work close together or with some distance between them. The rubber is fed between these rolls and broken down into a coarse, spongy mass. Water flows on to the rubber during the process, bringing down sand, dirt, bark, and the many other foreign materials which come mixed with the rubber. The rubber is put through this machine a number of times, until it is worked into a uniform condition. Some of the rubbers, like the Ceylons and Paras, will sheet out into a coarse sheet by being put through this machine; others, like the majority of the African rubbers, will fall apart and come down in chunks and have to be fed into the machine with a shovel.

[Illustration: PREPARING CRUDE RUBBER FOR MAKING TIRES

CALENDER ROOM.]

After the rubber is broken down sufficiently in the cracker, it is next put through a washing machine, which is built very similar to the cracking machine, except that the rolls are grooved or rifled, so that their action is not so severe on the rubber. A large quantity of water is kept constantly running over this machine while the rubber is being put through, and the rolls work very close together, so that the rubber is finely ground and run out into a thin and comparatively smooth sheet, allowing the water flowing between the rolls to take out practically all of the foreign matter that remains. The rubber is run through this machine a number of times until the experienced inspectors in charge are satisfied that it is thoroughly washed. Some types of rubber, such as Manicoba, which have large quantities of sand in them, are washed in a special form of washing machine known as the beater washer. This is an endless, oval-shaped trough with a fast-revolving paddle-wheel. In this machine the rubber is submerged in water, after being broken down in the cracker, and the sand is literally knocked out of it by the paddle-wheel. The sand drops to the bottom of the machine, where if is drained off, while the rubber floats to the top and is there gathered and then put through a regular washing machine for the final sheeting out.

_Drying._--From the wash-room the rubber goes to the dry-room. Before the rubber can be used in any articles of commercial value, it must be thoroughly dried, as any moisture in the stock would turn to steam during the vulcanizing process and cause blisters or blow-holes to form in the goods. There are two ways in which rubber is usually dried. The method mostly used, and which is generally practiced with all the better grades of gums, is to hang the washed strips on horizontal poles and space them in aisles, so that air can freely circulate all around the surface of the rubber, the dry-room being kept at a constant temperature. To properly dry the rubbers by this method takes from four to six weeks. The other method of drying is by means of a vacuum-drier. Low-grade rubbers which have a comparatively large percentage of resin in their composition cannot bear their own weight when hung on horizontal poles, but drop off and stick in piles on the floor. Hence, these rubbers have to be dried in a peculiar manner. They are laid in trays which are placed into a large air-tight receptacle. The air is then withdrawn from this receptacle and the interior heated by means of steam coils. This allows the water to be evaporated off from the rubber at a considerably lower temperature than that at which water boils under atmospheric pressure, and at such a low temperature, and in such a short time, that the rubber is not affected. By this process these rubbers can be dried in a few hours.

_Mixing._--After the rubber has been thoroughly dried, it is ready to be mixed in proper proportions with the various ingredients which are used in rubber compounding, to give the desired quality of rubbers for the various products for which they are intended. In order that rubber shall vulcanize, it is necessary to mix with it a certain proportion of sulphur, vulcanizing, or curing, as it is sometimes called, being merely the changing of a physical mixture of rubber and sulphur into a chemical compound of these ingredients, by the application of heat. Besides sulphur, some of the more important ingredients used in compounding rubber are:

_Zinc oxide._--This toughens the rubber and increases its wearing properties and tensile strength.

_Barium sulphate._--This stiffens the rubber and adds weight, so reducing the cost.

_Lithopones._--This whitens the stock and makes it soft, and is used extensively in druggists’ sundries.

_Antimony sulphide._--This makes the stock red and is a preservative against oxidation.

_Litharge._--This has the same action as antimony sulphide, but makes the stock black.

_White lead._--This hastens the cure and is extensively used in gray and black stocks, and is a good filler or weight adder.

_Magnesia oxide and carbonate._--These are used as fillers for white stocks.

_Oxide of iron._--Used for coloring red and yellow stocks.

_Lime_ (unslacked).--This hastens vulcanization and chemically removes any water left in the rubber.

_Whiting._--This is used only as a cheap filler to increase quantity and lower cost.

_Aluminum silicate._--This is used chiefly as a filler.

There are also used in compounding what are known as the various substitutes. These are chiefly linseed oil products and mineral hydrocarbons which are more or less elastic, and act somewhat as a flux.

Why Don’t We Use Pure Rubber?

There seems to be a general impression that the various ingredients which are mixed with rubber are put into the compounds merely to cheapen the product and to lower the grade of the material. This is true in many cases, such as the general line of molded goods, rubber heels, bicycle grips, automobile bumpers, etc., but in many cases, such as tires, packing, belting, etc., these ingredients are added to toughen the gum, increase its wearing qualities, to make it indestructible when subjected to heat, or to make it soft and yielding so that it can be forced into fabric, etc.

~PROCESS NECESSARY TO MAKING RUBBER GOODS~

In the general process of manufacture the sheeted rubber is sent directly from the dry-room to the compound-room, where the various ingredients are weighed out into proper proportions along with the rubber to make up a batch, and placed in receptacles ready to be mixed. The batch is then sent into the mill-room to be mixed into a uniform pasty mass, which is the characteristic uncured, or so-called green, rubber compound. The mixing is done in the mill. This is a very heavy machine, constructed similarly to a cracker and a washer except that it is much larger and heavier, and the rolls are perfectly smooth and run closer together. No water at all is used on the batch during the mixing. There are steam and cold water connections to the mills which are connected with hollow spaces inside the rolls, so that the latter can be kept at any temperature desired. The general process of mixing is as follows:

First the rubber portion of the batch is thrown into the mill and is worked and warmed up until it takes on a very sticky and plastic consistency. When it has arrived at a certain stage of plasticity, the various compounds in the batch, which are always in the form of very fine powders, are thrown in the mill, being worked by the rolls into the rubber. The compounds are generally thrown on, a small amount at a time, until they are all taken up by the rubber. The batch is then allowed to go through and through the mill, over and over again, until the mixture is absolutely uniform throughout the whole mass. The consistency of the rubber, during this operation, is such that the batch can be made endless around one of the rolls of the mill, so that it is constantly feeding itself between the rolls.

After the batch is properly mixed, it is cut off the rolls in sheets and rolled up and sent to the green-stock store-room. In this store-room the compounded, uncured gums are kept in different bins, according to the nature of the compound, and are there allowed to season a certain length of time, after which they are delivered to the various departments of the factory in which they are going to be used.

Another form in which rubber is used is the so-called Rubber-Cement. Rubber or any of its compounds are readily soluble in naphtha. In this process, the compounds, after being milled, are chewed up and washed in specially constructed cement-mills and there mixed with a certain proportion of naphtha which gives a thick solution.

_Spreading and calendering._--Rubber which is used for the general line of molded goods, solid tires, some kinds of tubing, etc., goes directly to the various departments from the green-stock store-room, while rubber used for boots and shoes, waterproof fabrics, many of the druggists’ sundries, belting, pneumatic tires, inner tubes, etc., has to be sheeted out, and some of it forced into fabric before it goes to the various departments. This sheeting-out of the gum, as well as applying the rubber to fabrics, is done generally by two methods; either by spreading a solution of the rubber and naphtha onto the fabric, or by calendering the rubber between heavy rolls in a rubber calender.

In the spreading process, a machine called a spreader is used. The fabric to which the rubber is to be applied is mounted in a roll at one end of the spreader and from the roll passes through a trough of rubber-cement, and then up over a so-called doctor roll, and under a knife edge, which allows only enough cement to pass through to fill the pores of the fabric. From this knife the cemented fabric passes over a steam drying chest and is then rolled up with a roll of liner cloth to prevent its sticking together. Fabric treated in this manner must be put through the spreader a number of times before it has sufficient rubber on it to be used in the products for which it is intended.

For calendering rubber, a machine called a rubber calender is used. This machine is made with three and sometimes four heavy rolls, which are capable of very fine adjustment. The rubber from the green-stock store-room is first warmed up on a small mixing mill and is then fed between the rolls of the calender, coming through in a thin sheet of required thickness, and is wound up in a liner cloth and sent directly to the departments, where it is used for inner tubes, druggists’ sundries, etc., where only rubber and no fabric is used. Where the rubber is to be applied to fabric, the fabric is put through the calender rolls with the rubber, and the rubber is literally ground into the fabric. Fabric treated in this manner is known to the trade as friction, and is generally used in the manufacture of pneumatic tires, belting, hose, etc. For boots, shoes, and other special work, calenders are used which are equipped with rolls engraved with the shapes of the soles and other parts of the articles in question, so that the sheet of rubber coming from the machine has imprinted on it the shapes and thickness of the articles for which it is intended.

After passing through such of the above processes as are required the rubber is ready to be made up into the various articles known to the rubber trade, such as boots and shoes, mackintoshes, waterproof fabrics, for balloons, aeroplanes, tentings, etc., mechanical goods, such as rubber heels, horseshoe pads, packing, tiling, automobile and other bumpers, artificial fish bait, etc., druggists’ sundries, such as nursing-bottles, nipples, syringes, bulbs, hot-water bottles, tubing, etc. tobacco pouches, rubber belting, golf and other balls, insulated wire, fire and garden hose, inner tubes, tires, and the many other commodities into the manufacture of which rubber enters.

[Illustration: TRADING ROOM]

How Are Automobile Tires Made?

From the calender room of the rubber factory the stock is received in the automobile tire department, in the form of large rolls of rubber-coated fabric, and in rolls of sheeted rubber of various thicknesses and widths. The rubber-coated fabric is first cut into strips of proper widths so that the edges will extend from bead to bead over the crown of the tire. These strips are always cut on the bias, generally at a 45-degree angle, with the edge of the roll, and were formerly all cut on a cutting-table, a table about 50 feet long and 6 feet wide, covered with sheet metal. The cutting was done by two men, each having a knife and each cutting half-way across the cloth along the edge of a straight-edge so arranged as to be always set at 45 degrees with the edge of the table. This method of cutting is gradually being put aside by the use of the bias cutter, an extremely up-to-date machine having jaws which ride up to the end of the fabric and pull it for a certain distance under a knife set at a 45-degree angle, the knife being set to cut just when the jaws have arrived at the limit of their motion. The action is repeated so that the machine cuts about eighty strips a minute. These strips are fed onto a series of belts which carry them to where they are placed, by boys, into a book having a leaf of common cloth between each strip of gum fabric, to prevent the strips from sticking together.

[Illustration: CURING ROOM--SOLID TIRES.]

[Illustration: MAKING A PNEUMATIC TIRE

CURING ROOM, FIRST CURE--PNEUMATICS.]

[Illustration: SPREADER ROOM.]

The majority of automobile tires to-day are machine built, but there are still a great many built by hand and this is the process we shall describe first. In this process the books of fabric are laid up and spliced into proper lengths to go around the tire and allow a proper lapping for the splices. The proper number of these laid-up pieces, or plies, as they are called, are placed together with cotton cloth between and taken to the tire builder. The tire builder mounts the core, upon which the tire is to be built, on the building stand, generally cementing it so that the first ply of fabric will stick in place. The first ply is then stretched onto the core and spliced, rolled down with a hand roller onto the sides of the core, and trimmed with a knife at the base. The following plies are put on and rolled down in the same manner, the beads being put in at the proper time, according to the size and the number of plies to be used. After all the plies have been put onto the core the so-called cover rubber is put on. This cover rubber is generally a sheet of rubber about one-sixteenth of an inch thick or more, and of the same compound as the rubber on the fabric.

[Illustration: HOW THE TREAD OF A TIRE IS MADE

TREAD LAYING ROOM.]

In the case of the machine-built tire, the result is the same, but the stock is handled as follows: After the rubber-coated fabric has been cut on the bias cutter, the strips are spliced and rolled up in rolls on a spindle which is placed in the so-called tire-building machine. The tire core is mounted on a stand attached to the machine, so that it can be revolved by power, and the fabric is drawn onto the core from the spindle under a certain definite tension. The tire-machines roll the fabric down by power, and the beads are put into place before the tire and core are removed from the machine. Thereafter the process is the same as in the case of the hand-built tires.

After the cover rubber is in place the tire is ready to have the tread applied. The tread is made up independently of the tire by laying up narrow strips of rubber, in different widths, in such a way that the center of the tread is thicker than the edges. In the case of the so-called single-cure tires, which are wholly vulcanized at one time, this tread is applied to the tire directly after the cover, a strip of fabric called the breaker-strip generally being placed underneath, and the building of the tire so completed.

In the general method of curing, the tire is allowed to remain on the core, and is either bolted up in a mold and put into an ordinary heater, or it is laid in a mold and put into a heater press, where the hydraulic pressure keeps the two halves of the mold forced together during the vulcanizing process. After the vulcanizing is completed, the tire is removed from the mold, the inside is painted with a French talc mixture, the tire inspected and cleaned, and so made ready for the market. In some methods of curing, instead of the tire being put in a mold, it is put into a so-called toe-mold, which is virtually a pair of side flanges only reaching up as high as the edges of the tread on the side of the tire. After the flanges are fastened into place, the whole is cross-wrapped, the cross-wrapping coming in direct contact with the tread. The tire in this condition is then put into the heater and vulcanized, giving the so-called wrapped tread tire. Still another form of curing is to inflate a kind of canvas inner tube inside the tire and place the whole in a mold. This is known as the air-bag mold process.

[Illustration: PNEUMATIC-TIRE ROOM--SHOWING TIRE-BUILDING MACHINES.]

How Are Inner Tubes Made?

Inner tubes for pneumatic tires may be classed under three headings, according to the methods used in their manufacture, viz., seamed tubes, rolled tubes, and tube-machine tubes. By far the greater number of tubes come under the first two headings. For seamed tubes, the rubber is taken from the calender in the form of sheets from one-sixteenth to three-sixteenths of an inch in thickness. These sheets are cut into strips of proper length and just wide enough to make a tube of proper cross-section diameter when the two long edges are folded over and fastened together with rubber cement. These two long edges are cut on a bevel so that they make a good lap seam. The tube is then pulled over a mandrel of proper size and a thin piece of wet cloth rolled around it, and then it is spirally cross-wrapped with a long, narrow piece of wet duck for its entire length. The whole is then put into a regular heater and the tube vulcanized. After vulcanizing the wrapping is removed and the tube stripped from the mandrel, turning the tube inside out, so that the smooth side which is vulcanized next to the mandrel appears outside, and the rough side showing the marks of the cross-wrapping is inside. The valve hole is then punched in the tube, the valve inserted and the open ends of the tube buffed down to a feather edge. The tube in this state passes to the splicers, who cement the buffed ends and splice them together, placing one open end within the other, making a lapped seam around the tube about 2¹⁄₂ inches long. The cement used in splicing is generally cured by an acid which chemically vulcanizes the rubber without the application of heat. The tube is thus finished and ready for the market. Rolled tubes are made from very thin sheet rubber by rolling same over a mandrel of proper size, until the required number of layers of thin rubber have been rolled on to give the tube the desired thickness. The tube is then wrapped, cured and spliced, in exactly the same manner as a seamed tube.

What Is Rubber?

Crude rubber is a vegetable product gathered from certain species of trees, shrubs, vines and roots. Its characteristic peculiarities were early recognized by the natives of the tropical countries in which it is found. Records of the earliest travelers in these countries show that the natives had used various articles, such as receptacles, ties, clubs, etc., made from rubber, but it was not until about 1735 that rubber was first introduced into Europe. In civilization rubber was first used for pencil erasers and in waterproof cloth, and finally in cements. Vulcanizing, or the curing of rubber, was not discovered until 1844, and thereafter the development of the rubber industry was very rapid, especially in Great Britain.

[Illustration: WRAPPING ROOM--PNEUMATICS.]

There are many kinds and grades of rubber, and to-day these can be divided into two chief classes, wild and cultivated.

[Illustration: PNEUMATIC-TIRE ROOM, SHOWING TIRE FINISHING.]

[Illustration: HOW THE CRUDE RUBBER IS SECURED

Gathering Rubber in South America.]

[Illustration: 1. Tapping Axe. 2. Tin Cup to Catch the Rubber Milk. 3. The Beginning of a Rubber “Biscuit.” 4. A Palm Nut.]

[Illustration: Making Balls of Crude Rubber.]

[Illustration: Tapping the Trees in Japan.]

[Illustration: How the Rubber Looks when it comes to Market.]

[Illustration: Carrying Balls of Crude Rubber to Native Market.]

Pictures herewith by courtesy of The B. F. Goodrich Company, Ltd.

What Is Wild Rubber?

~WHERE RUBBER COMES FROM~

The first class, or wild rubbers, are collected from trees which have grown wild and where no cultivation processes whatsoever have been used. These rubber-producing trees, shrubs, etc., are found mostly in Northern South America, Central America, Mexico, Central Africa and Borneo.

The finest rubber in the world is Fine Para, and is gathered in the Amazon regions of South America. This rubber has been gathered in practically the same way for over a century. The natives go out into the forests and, selecting a rubber tree, cut “V”-shaped grooves in the bark with a special knife made for the purpose, these grooves being cut in herring-bone fashion diagonally around the tree, with one main groove cut vertically down the center like the main vein in a leaf. The latex, or milk-like liquid, of the tree, from which the rubber is taken, flows from these veins and down the center vein into a little cup which the natives place to receive it. After the little cups are filled they are gathered and brought into the rubber camp, and there the latex is coagulated by means of smoke. This is done by the use of a paddle which is alternately dipped into a bowl of the latex and then revolved in the smoke from a wood or palm-nut fire. This smoke seems to have a preservative effect on the rubber as well as drying it out and causing it to harden on the paddle, each successive layer of the latex causing the size of the rubber ball or biscuit to increase. When a biscuit of sufficient size has been thus coagulated it is removed from the paddle and is ready for shipment to countries where rubber products are manufactured.