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then withdrawn from this receptacle and the interior heated by means of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