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The manufacture of rubber goods : a practical handbook for the use of manufacturers, chemists, and others online

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provided with some means of closing them, in order to protect
the rubber from dust. Well-galvanised tubs or bins are generally
used to hold the weighed materials for the mixing; the bottoms
are protected by strong iron rims on which the receptacles can be
slid about. Wooden bins lined with sheet zinc are also used some-
times, and in certain cases these are divided into several compart-
ments, so as to keep the constituents of the mixing separate from
one another. More will be said as to the composition of the various
mixings in a special chapter.

The Mixing Mills. — The machinery for mixing is similar in con-
struction to the washing plant, but on a larger scale. An ordinary
mixing mill, such as would satisfy present-day requirements, has
rolls 110 cm. long and 450 mm. in diameter, and the approximate
weight of such a machine is 3500 kilograms. The cast-iron rolls
are hollow, and each is fitted with an independent steam and water
supply for heating and cooling respectively. The front, or so-called
working roll, runs at 18 revolutions per minute, whilst the back roll
makes 26. Fig. 20 shows the form of mill in most general use
at the present time ; this, on account of its firm build and large size,
gets through the work very rapidly, and only slightly afiects the
properties of the rubber. The setting-up screws should be of fairly
large size, and may be provided with pointers moving on discs
fixed to the ends of the bearings, and graduated in 5 mm. divisions.
This apparatus makes it easier for the mixer to set his rolls regu-
larly at the same distance apart, and when properly used ensures
that the two sets of bearings are an equal distance apart.

The sheet-iron boxes beneath the rolls should not be fixtures,
but should be provided with wheels so that they can be easily
moved backwards or forwards along two rails. The guides at the
sides should be of iron ; wooden or copper guides are not to be
recommended. It is advisable to provide for the removal from the
rolls of mixings which readily stick to them, by means of a " doctor,"
and for this purpose a steel bevelled rail screwed on to a strong
wooden base about 6 cm. thick may be recommended. This is
suspended, with the bolts fixed to the wooden side, in the bearing-
cases. The knife is depressed by means of a lever against the



middle of the roll, and cuts or scrapes off the mass of rubber
from it.

The mixing mill should, like the washing rolls, be provided with
instantaneous safety- clutches, as already described (figs. 12 and 12a),
the most suitable being those based on the friction-clutch principle.

A little further consideration may now be given to the mixing
mills fitted with revolving sieves, which have been already men-


Fig. 20.

tioned. (The rights of manufacturing these machines have been
acquired by C. G. Haubold, jun., a firm of engineers in Chemnitz.)
The rolls are constructed as in an ordinary mixing mill, but above
the front roll at a height of about 20 to 25 cm. (8 to 10 inches) there
is fixed a cylindrical sieve containing a revolving brush, driven
by toothed- wheel gearing from the axle of the roll (fig. 21). The
cylinder can be raised in its bearings in a backward direction by
means of an eccentric movement, in order to throw it out of gear.
The sifted material falls into a funnel-shaped sieve fixed below
the cylinder, and kept in continuous vibration by means of an



eccentric so that the materials passing through the cylindrical
sieve are passed through the second sieve on to the rubber. A
celluloid cover is fitted over the whole sifting apparatus in such
a way that it can be adjusted to the height of the front mixing
roll ; the edge of the cover presses lightly on the front roll so
that it readily adjusts itself to inequalities in the thickness of
the material surrounding the roll. The back part of the cover
is carried about half-way across the top of the back roll, and
is also adjustable. So that the cover may be transparent, only
pale-coloured celluloid should be used. This cover prevents the

Fig. 21.

inhalation of the fine magnesia dust produced when the sieve is
working ; any dust which tends to settle on the smooth surface
of the celluloid is at once shaken off" on to the rolls by the
vibratory motion.

The Calenders. — We have seen in the last few paragraphs
how the mixings are prepared on the mixing rolls, and we have
now to follow the mixed rubber through its further manipulation
until it arrives at the stage of hard- or soft-rubber goods.

Before anything more can be done with it, the mass of mixed
rubber has now to be run into the form of sheets, the thickness of
which, together with the greater or less degree of smoothness of their
surface, depends upon the details of the manufacture of the article


for which the rubber is to be used. The strange notion which was
very widely prevalent outside the industry for a long time, and
which is still in existence here and there, that indiarubber articles
such as tubes, balls, syringes, etc., are made by simply casting
liquid rubber in suitable moulds, in the manner in which metallic
castings are made, is, of course, quite an erroneous one. With
only a few exceptions, all rubber goods are prepared from 'plastic
material, partly by hand, partly by special machinery. But even
so it is the calendered sheet which has to be used for the purpose.
The calenders must therefore be looked upon as the centre of the
manufacture and of the machinery, and must accordingly be dealt
with fully.

As already stated, the calenders are used to run the sheets of
rubber, which are then worked up into the most diverse articles,
according to the kind of rubber. In whatever manner the sheets
are afterwards to be used, uniform thickness throughout the sheet
is the first essential to the achievement of good results. Con-
siderable experience is necessary in order to be able to produce
such uniformity in thickness, and there would appear to be very
few men who really understand the proper working of the
calenders. It most cases it is found that a three-roll calender has
only two of its rolls in use while the third is running empty ; so in
" f rictioning " insertions it is no uncommon thing to find only two
of the rolls in use. Taken as a class, the calender-hand knows
very little, or at any rate insufficient, about the principles which
underlie that method of working which involves the use of all the
rolls. The winding apparatus, which is a most important feature,
is in most cases left unused, or even thrown aside as worthless,
because the value attaching to the apparatus is not known, and
in most instances the fitter charged with the erection of the
machine is not clear himself as to the use of it. In order to be
able to understand things more clearly, the reader may first of all
be made acquainted with the constructive details of the calenders.

In all calenders, whether with two, three, or four rolls, the
following points must be specially observed, if the rolls are to work
perfectly, and accuracy in running the sheet of rubber is to be
secured: the rolls should be of sufficient strength not to spring,
and must in every instance be ground true. It is most convenient
to have cast-iron rolls 1250 mm. long and 500 mm. in diameter,
which have been ground on a Poole's grinding-machine. Three-
and four-roll calenders may be driven by a special motor to ensure
a uniform speed to the calenders, and to avoid the irregularities



brought about by driving the mixing rolls from the same main
shaftinof. Fig. 21a shows a modern set of three-roll calenders,
electrically driven, and provided with a friction-clutch.

In laying down new plant all the gear-wheels should have
double-helical, or at any rate helical teeth, the preference being
always given to the former, however, for when they are used the
motion of the calender rolls is absolutely uniform, as is most
markedly noticeable in calendering very thin sheet. Ordinary

Fig 21a.

teeth always give rise to a certain amount of back-lash, which
makes itself evident in the form of striped markings right across
the calendered sheet, where the rubber runs thin.

The arrangement for adjusting the top and bottom rolls should
be fixed in a convenient position about half-way up the machine,
and it should be possible to adjust either top or bottom roll by
manipulating a single wheel. The bearings of the middle roll are
fixed. The gearing, as also the worm-wheels for the spindles, and
the spindles themselves, cannot be chosen large enough to allow of
a certain and easy raising and lowering of the rolls. Chain-drive
should be altogether avoided, and the vertical shaft with bevel-



wheel gearing should be selected as the best arrangement. The
normal speed of the rolls is 4 revs, per minute, but for friction-
ing the lower rolls should be geared up so as to work with a

maximum speed-ratio of 1 : 3. In large factories it is, however,
always better to use, for proofing purposes, special calenders con-
structed for the purpose, as illustrated in fig. 22, and to use the
ordinary calenders only for running sheet. It is of fundamental



importance that every set of calenders should be provid.ed with
reliable apparatus for heating and cooling the rolls, and that the
controlling valves should be placed in a convenient position.
Finally, attention must be paid to apparatus for reeling and un-
reeling, which should always accompany a set of calenders. The
method still frequently employed for effecting these two purposes,
consisting in the use of a simple wooden roller, on to which the
rubber sheet is still more simply wound by several workmen, is
quite irrational. On a closer consideration of the process of run-
ning a sheet of rubber, it will be found that in the case of the three-
roll calenders the upper and middle rolls do the work, and that
the distance from the point where the sheet leaves the rolls to the
trestle, where it is taken up by the running cloth, is about IJ to If
metres. The faults of such a sheet, and the damage that may be
caused to the finished goods by them, are instructive. In particular,
the sheet draws up more or less, according to its quality, very
considerably and very unevenly ; whilst hanging free between the
calenders and the winding trestle the sheet gets stretched, and
the uneven tension of the winding which follows sufiices to pro-
duce great variations of thickness in the sheet, quite apart from
the creases and folds that are formed. If, however, every workman
understood the art of winding calendered sheet on the calenders,
and if the apparatus were always in position, these faults would
be remedied. Moreover, the use of the apparatus necessitates the
attention of only one man in addition to the foreman, whereas in
the older method at least two others besides these are necessary
to look after the winding, or to hold the cloth and the sheet of
rubber apart. The entire arrangement of the apparatus on the
calenders is extremely simple, but the bearings must be accurately
centred, and the friction-brake must work properly.

On the opposite side from where the rubber is fed in, the
unwinding apparatus is fixed to the calender frames, on a level
with the axis of the lower roll. The square iron axle, 35 mm.
thick, which carries the wooden roller 120 mm. in diameter, rests
in two bearings ; that on the right-hand side is closed on the
upper side by a liinged piece, with a fly-nut, so that it can easily
be opened, while that on the left-hand side is a closed bearing in
which runs the box which carries the axle. The box is provided
with a fixed friction-brake on the other side of the bearing, and by
adjusting this the roller is allowed to run free or is kept in check to
a greater or less extent. On the other side of the machine — that is
to say, on the feeding side, and at the same height as the apparatus


just described — is that for winding up the emergent sheet. One
bearing of this can also be opened, whilst the other takes the box
with the friction-drive. The roller is best driven from the middle
roll by means of toothed- wheel gearing to the friction coupling. If
a chain-drive is employed great care should be exercised in getting
a chain which is of an even make. Belt driving is quite unsatis-
factory, for as the roll of rubber sheet increases in diameter the belt
slips, destroying the even tension on the sheet.

When the wooden roller and axle have been put in position in
the bearings, the running cloth from the feeding roller is drawn
through between the middle and lower calender rolls, stretched
tightly over the receiving roller and wound round it a few times.
Care must be taken that the cloth lies smoothly on the roller, and
rolls up evenly. The calendering of the sheet may now be started.

The mixed rubber is warmed up to a uniform temperature on
the warming-up rolls situated close to the calenders ; these rolls
are quite similar in their construction to mixing rolls, but their use
is restricted to warming-up. Care must be taken that the rubber
does not become too doughy, and that it is not put into the
calenders in large masses, but that strips about 20 mm. (| inch)
thick and 200 mm. (8 inches) wide, cut from the warming-up rolls,
are fed in. In this way one ensures keeping the rubber at a uniform
temperature, and the pressure against the bearings of the upper
roll remains more constant than if the calenders were fed with
large masses of rubber ; for while the large mass is still warm, it
exerts little upward pressure, but as it cools its pressure increases,
causing the roll to lift ; on the other hand, as the rubber gets used
up the upward pressure again becomes less and the roll drops, the
net result being considerable unevenness in the thickness of the
rubber sheet. Assuming now that the rolls have been set to the
right distance apart, the rubber passes between the upper and
middle rolls, round the middle roll, to which it adheres closely until
it has gone half-way round, when it is taken up b;^ the running-
cloth, which, as has been mentioned, passes between the second and
third rolls. Immediately the rubber comes in contact with the
cloth the winding apparatus is started : that is to say, the friction
rim is drawn into the shell, and the roller begins to turn. The
speed of the roller is accurately regulated, so that when the brake
is on fairly hard it takes up exactly as much sheet as the calenders
pay out. As the diameter of the roll, with its alternate layers of
cloth and rubber, increases, the brake is slackened and the friction
pulley allowed to slip round, so that the speed of rotation of the



roll decreases as its circumference increases. After a little practice
the calender-man can adjust the brake to a nicety, so that the
sheet is rolled up without being stretched or creased. In this
way the thinnest Para sheet (0*2 mm. thick) can be calendered,
and a uniform thickness will be maintained, since the sheets are
not stretched, and cannot draw up unevenly. To entirely do away
with drawing up, the sheet should be left in the running cloth
until it is quite cold* and only then unrolled and made up.

It should be mentioned that the bottom roll should be so adjusted

Fig. 23.

as to gently press the rubber against the face of the running cloth.
Fig. 23 illustrates the principle of a modern set of three-roll
calenders. Such three-roll calenders are chiefly used for running
rubber sheet of the better qualities, which must be as free from air-
bubbles as possible. But they also come in for doubling the sheets
on the calenders, since it is only with great difficulty that, for
example, a sheet in a pure rubber quality, f mm. thick, can be
obtained in long lengths quite free from air-blisters. In that case
the calendering takes the following course : in order, for example,
to produce a sheet 1 mm. thick, a sheet J mm. thick is first run
as described, and wound on to the wooden roller. This is then
mounted on the axle of the feeding roller (unwinding apparatus),



and the running-cloth, with the «heet of rubber upon it, is passed
between the two lower rolls and taken up by the winding-up
apparatus; the bottom roll is now closed up towards the second,
until the second J mm. sheet, now to be run, is pressed tightly
on the first, so as not, however, to form blisters or streaks. The
whole is then wound up again, as already described, and yields

Fig. 25«

a sheet, built up of two layers which cannot be separated from
one another again. In this way the various thicknesses of sheet
can be built up of different numbers of layers according to the use
to which the sheet is to be put. For still thinner sheet, down to
0*15 mm., the four-roll calenders shown in fior. 24 have to be used.
The method of using them is similar to that described for the three-
roll calenders, only that one actually works with three rolls and
uses the fourth to cool the sheet as it is run out.

The two-roll calenders depicted in fig. 25 are chiefly used for



runnin^^ sheet which is to be cut up for solution, or where uniform
tliickness is not of importance. If, however, it be required to
produce on the two-roll calenders, sheet as even in thickness as
possible, for mechanical work, an automatic winding apparatus
must be provided, as shown in fig. 22, for the three-roll calenders.
The two-roll calenders (fig. 26) are also often used to run
thicker sheet free from blisters, to be used for surgical work, and
which must therefore liave a smooth surface. The method of doing

Fig. 26.

this is not to run the sheet out in lono- len2:ths, but to allow it to
run right round the roll ; a sheet equal in length to the circumference
of the roll is thus obtained, that is, about 120 cm. long. It is
advisable to rub a little vaseline over the roll round which the
sheet runs ; this helps to give the rubber a smoother surface and
allows the air which collects between the rubber and the roll to
get away at the sides.

It is also necessary to have a second set of two-roll calenders,
of similar construction to that shown in figs. 27 and 27a. This set
is chiefly used for running sheet, for ebonite manufacture and also
for moulded goods, which is required in thicknesses of from 3 to



10 mm., and which must be doubled absolutely free from air-
bubbles. These calenders are used in particular in the manufacture

Fig. 27.

of accumulator cases. The separate lengths are only 4 J metres
long, but this is quite enough for the purpose. As indicated in
tlie illustration, the machine consists of a two-roll calender with a

Fig. 27a.

doubler in front of it. The calender rolls are 450 mm. in diameter
and 1200 mm. long. The doubling drum is 1300 mm. in diameter,
and the conveying-cloth 5 metres long. The pressure-roll is rubber-
covered, and its distance above the drum can be adjusted by means



of movable bearings. Pressure is applied by means of weighted
levers. The drum is driven from the calenders, and the travelling
table derives its motive power from the same source. The rubber
mixing, which is run out between the rolls into the form of sheet,
passes over the cloth conveyor on to the drum, round which it is
wound until the correct thickness is reached, when it is cut off and
passed on to the table, by which it is carried away.

It may be mentioned that the rubber sheets can be cut into

Fig. 28.

strips on the calenders themselves, before being rolled up in the
running-cloth. To effect this a cutting apparatus is lixed between
the two vertical frames of the machine, about 18 cm. below the
level of the middle of the second roll. The knives, fitted in boxes
provided with grooves, are fixed upon the spindle between the
wheels, can be set to any desired width of strip, are kept in
position by means of set-screws and are pressed against tlie sheet
by means of a spring, the strength of which can be regulated ; this
arrangement results in the sheet being cut through clean.

The spreading calenders shown in figs. 28, and 29 are now
generally used for impregnating insertions and fabrics with rubber,



since by the use of these a considerable saving of benzine as well
as of time is effected as compared with the spreading machine.
As already mentioned, where the plant is a large one it is better to
use a machine exclusively for " frictioning." The use of threq rolls
is nowadays substituted for the American method, in which two
rolls only were employed, and which may easily lead to the cloth
getting torn. The uppermost and lowest rolls are driven slowly at
equal speeds, while the middle roll makes the ordinary four revolu-


Fig. 29.

tions per minute. The material on which the rubber is to be
spread passes, like the running-cloth mentioned above, between the
second and third rolls, when it is wound up. The process consists
essentially in feeding the rubber into the calenders between the top
and second rolls, allowing it to run round the much hotter middle
roll in a very thin sheet, the thickness of which is determined by
the substance of the cloth to be coated, and by means of this
pressing it into the cloth. The third roll, revolving slowly, rubs
the thin sheet of rubber deep into the meshes of the fabric, and
completely covers it over, filling up and penetrating all the inter-
stices with an even mass of rubber. In carrying out this process,


care must be taken to have the rubber mixing worked up and
warmed uniformly, so that it may he evenly round the roll, and
not in too soft or sticky a condition, so that the friction roll can
remove it without difficulty. Sticking to the middle roll must in
all cases be avoided ; it indicates that the particular mixing is not
suitable for calendering. With few exceptions the fault lies in
having the middle roll too hot. One of the chief aims should
always be to keep feeding in fresh quantities of rubber regularly,
and not too much should be fed in at a time. It is also essential
to success to have the cloth rolled up evenly and tightly on the
wooden roller before passing it through the calenders, so that no
creases are formed as the cloth is unrolled, and to see that the
fabric does not run crooked, which would lead to its getting torn.
In rolling up the cloth, as also in dealing with the ordinary running-
cloths, it is best to make use of a machine which should always be
provided with a calendering plant. This machine has a large
heating drum over which the cloth or fabric is carried in order to
dry it thoroughly, moisture being capable of great mischief under
these circumstances. Behind the drum is a mechanical brush
enclosed in a box, by means of which the fabric is freed from loose
fibres and dust. Into this the fabric passes as it comes from the
heating drum, emerging on the other side, and after passing over
the stretcher is tightly wound on the interchangeable rollers. In
this way one is always sure of getting clean, dry fabrics.

Having now shown how the mixed rubber is worked up into
the form of sheet, we propose to deal in the two following chapters
with the important subjects of Vulcanisation and Mixings, and shall
then proceed to consider in detail the various stages in the manu-
facture of a number of different articlea



The discovery of the process of vulcanisation by Charles Goodyear,
an American, in 183^, was not, as one often reads, an accidental
occurrence, but came about as the result of investigations which
had been carried on by him for years. Goodyear, like many other
men of an inventive turn of mind, kept on dabbling with rubber in
spite of his numerous failures, and studied the effect of mixing all
sorts of substances with it. Through this he was, undoubtedly of
set purpose, led to his discovery, and as a result of the experience
he had gained in the course of his investigations he recognised, on
observing the eifect of heat upon a mixture of rubber and sulphur,
that the heating was essential to the process. It is a remarkable
thing how this hard-pressed man, overwhelmed with debts, succeeded
in working out his invention. That he did work it out is clearly
demonstrated by the fact that in the description first published by

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Online LibraryAdolf HeilThe manufacture of rubber goods : a practical handbook for the use of manufacturers, chemists, and others → online text (page 5 of 21)