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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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Fig. 87.

in hot-air chambers, since the shoes have to be cured without an;^
protective covering. The stove is generally traversed by heating
coils on the floor and walls (fig. 86). The walls should be lagged
with some insulating material to prevent loss of heat, and so to
enable the temperature of the chamber to be kept absolutely constant.
Special care must be taken with the air-outlets, which must be dis-
tributed over the top of the oven in such a way that no variation
of temperature is brought about in the process of drawing ofl" the
air from the chamber, for the temperature must be kept absolutely
constant in every part of the oven. The rubber shoes are put on
mandrels on iron cross-rods, supported on trolleys (fig. 87), on
which they can be run into and withdrawn from the vulcanising
chamber. As is always the case with hot-air vulcanisation, where
the space is only slowly heated up, the length of time required for


a cure is comparatively long, from eight to ten hours being occupied
in gradually raising the temperature up to 130° to 140'' C., according
to whether the rubber is quick- or slow-curing. This temperature
must be maintained for at least an hour and a half, but must not
be allowed to exceed 140° C. ; in the first place, because the fabrics
would begin to lose strength and change in colour ; and, secondly,
because the polish of the varnish would be dulled thereby.

The so-called " sulphuring-up " does not often occur with shoes ;
but when it does, its eifect on their appearance is more objectionable
than in many other articles ; further, when insufficiently vulcanised
the varnish may remain tacky. The temperature of the heating
chamber is controlled by the use of a long thermometer ; in order
to make sure that vulcanisation is complete the chamber is provided
with flaps through which test-pieces can be withdrawn. Before
attempting to empty the chamber the foul air is exhausted from
it ; the trolleys are then run out into a room to cool down. When
cool, the shoes are examined for defects, sorted, branded (generally
with vermilion) and packed ready for dispatch.

16. Manufacture of Insulated Wires and Cables. — Except in the
case of submarine cables, vulcanised rubber is at present used for
the insulation of the metallic wires used for cables, etc. ; the principal
rubbers employed in the mixings used for this purpose are Para,
Mozambique, and the best Congos. In this manufacture it is
essential that the rubber should be very thoroughly washed ; but
the mistake should not be made of washing it for too long a time,
and in too thin a sheet, since the rubber suffers by overworking on
the washing-rolls. The rubber used should not be dried in the
vacuum-drier; this point has been specially referred to in the
chapter on the drying of raw rubber. The ingredients, French
chalk, china-clay, and zinc oxide, must be well dried and sifted.
In mixing, the formation of nibs must be avoided, and the rubber
should not be worked for too long a time. For calendering the
sheet the four-roll calenders, as specially designed by the firm oi
Haubold (p. 52) are employed, the chief aim being to avoid the
formation of . blisters ; to achieve this the mixing must be well
warmed up and calendered immediately. The calendered sheet is
allowed to remain wrapj)ed in its cloth for at least a day to cool
down, and is then lightly chalked on the machine already described.
The most usual and the most convenient method for covering the
copper wire is by pressing the rubber round it in the coating
machine (shown in fig. '88). This machine has been introduced
everywhere, and as many as twelve wires, or even more, can be



covered by it at a time. The ordinary machines in use are those
with three pairs of rollers, but there are others with as many as
six pairs, with diameters between 150 and 250 mm., and breadth
of face from 100 to 200 mm. Working under normal conditions,
the rollers are driven with a peripheral speed of from 7 to a
maximum of 19 metres per minute. The speed of the apparatus
for carrying away the cable as it passes from the macliine must be
capable of regulation, and the cable should not be drawn away
faster than it is paid out by the machine, or both wire and cover-
ing will be weakened, and the rubber insulation may leave the wire

Fig. 88.

or be torn off. The circular knives must be turned clean and free
from notches, otherwise the covering will not be very reliable.
Again, the rollers should not be set up too tight. The sheet to be
used for covering, which^ is from 02 to 03 mm. thick, is run, free
from blisters on the calenders, cooled down, and then rolled round
cylindrical mandrels and cut into rolls of strip of the desired width,
on automatic lathes. These rolls are inserted in the covering
machine ; the wire passes between two plates through the first pair
of cutting rollers, and above and below the wire are run strips of
rubber from the rolls inserted in the machine, and the wire is thus
covered by the strips. The wire and its covering pass between the
cutting rollers, running in grooves on their surfaces, where com-


pression takes place, and the excess of rubber is cut off at the edges
by the pressure of the sharp edges of the roller. On leaving the
first pair of rollers the wires pass on to the second pair, where the
second coating of rubber is put on, and so on for further coats of
rubber. The wires run through fine steel combs, which separate
the cut edges from one another, and from these to the drums, on
which they are wound evenly and without tension by means of
the winding apparatus.

For ordinary insulations, machines similar to tube machines
may be employed. Dr Baur's assertion that only inferior qualities
can be run on such machines is not accurate : the better qualities
can also be dealt with if the rubber mixing is properly treated and
suitable additions of ceresine and vaseline are made. Thin wires
are wound in layers on slightly tapered wrapping-drums and then
vulcanised, but the stouter sizes must first be lapped round w^th
tape, proofed on one side, on a special machine, so that the insulation
may not get pressed out of shape and become one-sided.

Vulcanisation is best carried out in the autoclave (fig. 32), for it
is of great importance that the steam throughout the heater should
be dry, and should circulate uniformly round about the wires wound
on the drum ; that this result cannot be attained in the ordinary
heaters must be known by everyone who has had to deal with them,
and is easily shown to be the case by determining coefficients of
vulcanisation. The drum being mounted at a sharp angle to the
horizontal, and kept in motion, prevents the formation of steam
banks in the heater, and in consequence the wires do not come into
contact with wet steam in the lower part of the heater. For this
reason the insulation absorbs a comparatively small amount of
water, and the insulation resistance of the conductor is higher than
in the other case. In addition to this, vulcanisation is completed
more quickly on account of the more thorough circulation of the
steam in the heater, and immediately the cure is completed the wire
can be dried in vacuo in the same heater. To obtain a uniformly
good insulation resistance it is most important that the rubber be
correctly vulcanised ; in no case should it be left too soft.

Mixings for cable insulation should contain rather a large
percentage of pitch which has been heated at 225^ C. For high-
tension cable this is strictly necessary, the rubber being rendered
more homogeneous thereby. The addition of pitch to the mixing
has the effect of enabling it to be run out into a firm, very dense
sheet, of low porosity, even if it contain a considerable percentage
of brown substitute (white substitute must not be used in cable


mixings). Brown substitute should not be despised as an in-
gredient of such mixings, more particularly if it be made from high-
grade blown oils. In cable mixings, differences in insulating power
result from uneven distribution of the compounds in a mixing, and
the aim in preparing such mixings should be to make the " breaking-
down " and the insulation resistance in different parts of the cable
equally good. The results obtained by these two methods of testing
should not greatly differ from one another. For this reason care
should be taken to produce a homogeneous mixing by the addition
of pitch, and to carry out the mixing process in such a way that an
almost air-free admixture of the ingredients is brought about ; the
attainment of this end is assisted by the addition of pitch which
displaces the air contained in the pores of the rubber.

17. Compositions of Asbestos and Rubber, (a) Vulcanised Asbestos.
— This insulating material consists of asbestos fibre felted together
with rubber and mixed with pigment and other fillers. By intimate
subdivision of the asbestos fibre a felted sheet is formed which
answers the highest requirements of electrotechnics.

This sheet can be made in the following way : — The washed
rubber indicated in the mixing given below is dissolved in a
Werner- Pfleiderer machine in 35 kilos, of light benzine. The
other ii*gredients named in the mixing are then gradually added
to the rubber solution, and the whole mass is worked until a
homogeneous solution is produced, a process which may occupy
about two hours. When that is complete, and not till then, the
finely frayed-out asbestos fibre is slowly added in the form of
fluff, and the whole mass is again worked for about an hour on
the machine. When the mass is thoroughly mixed it is spread
out in large frames, the benzine evaporated off in the vacuum
chamber, and the sheet dried. The sheets are then put between
zinc plates in a hydraulic press, where they are subjected to a
pressure of 40 kilograms per square centimetre. Finally the sheets
are passed through the glazing calenders, and are ready to be used
for making magnet bobbins, rollers, and sounding-boards. The
articles are afterwards vulcanised ; the sheets may also be vulcanised
as such in the press, under hydraulic pressure.



Massai . . . 2,500 gms.


. 4,000

Columbian . . . 2,500 ,,

.Fapan red .

. 4,500

Substitute (free from smell) 2,000 .,

Zinc oxide ,

. 4,500

Waste . . . 2,000 ,,

Linseed oil .

. 1,000

Sulphur . . . 2,500 .,

Magnesia usta


Golden sulphide . . 4,000 „

Asbestos fibre

. 16,000

China-clay . . . 4,500 ,,

Burgundy pitch .

. 1,000


A second process is based on the use of papermaking machinery,
the " solution " being dried off on the steam-heated drums, and
the mixture cut off in the form of a cylinder when it is thick

(b) " -Ite" Compositions. — The demand for ordinary rubber
packing-sheet for steam-packings has fallen off, as the practice of
using superheated steam and high pressures has grown amongst
engineers. At steam pressures of twelve atmospheres and at
temperatures of 180° to 186° C. ordinary rubber packing-sheet cannot
be used, since it is no longer able to meet the demands made
upon it.

In recognition of this change, in consequence of which a more
resistant packing material became a necessity, attention was turned
to new compositions, and the result was the " -ite " sheets, which
satisfied all requirements. The sheets as a class were given this
name in consequence of the number of names ending in " -ite " given
to different makes by the firms which made them, e.g., Klingerite,
Moorite, Metzelerite, Cooperite, etc. (The name "Itplatte" itself
has recently been protected by the firm of Gustav Adolph, Biebrich-
a.-Rh.). Indeed, blind competition has in this instance, as in others,
brought this composition, in principle a good one, to the very
limits of possibility within a very short time, and converted the
quondam speciality into a staple article of rubber manufacture,
a state of affairs which is of no more benefit to the consumer than
to the producer. The " -ite " compositions consist essentially of a
thoroughly felted mass, in which asbestos is used by preference as
the fibrous material. But it is not a matter of indifference what
kind of asbestos fibre is used. Blue Cape asbestos has proved to
be the most resistant towards high temperatures. Next on the list
comes Siberian asbestos, and this is to be preferred to the Canadian
product. The staple of the fibre is also an important point to
be considered, and only samples with long fibres should be used.
Waste fibre from spinning, which is in part richly mixed with
cotton, is not to be recommended for the production , of a good
sheet, the fibre being too short, on account of the frequent carding
and willowing to which it has been subjected, and so giving rise to
an imperfect felting. The composition is the better the more
intimately the fibre is mixed and bound together. For similar
reasons hand-picked asbestos sheet is to be preferred to machine-
made sheet. As regards other ingredients, " -ite " sheets generally
contain quantities of mineral fillers, such " as china-clay, barytes,
talite, iron oxide, etc., according to quality. On the other hand, the


rubber content is rather low, the rubber acting mainly as a cement.
Hydrocellulose is a particularly suitable ingredient.

It is clearly and easily conceivable that packing of this descrip-
tion must be able to withstand not only high temperatures but also
the high steam pressures which are now usual everj^where ;
because, indeed, the packing actually consists merely of an inti-
mately felted mineral mass, to which a somewhat elastic, leathery
grip is imparted by the rubber cement, and so it cannot be forced
out between the flano^es. A further advantaoje is the mechanical
stability of this kind of packing. It has already been rolled out
and firmly compressed in the course of manufacture, and therefore
cannot alter very much afterwards in tightening up the flange
bolts ; these bolts, therefore, do not require continually tightening
up. The composition vulcanises in a few minutes between the
flanges, and so forms a safe packing which can only be damaged
by blows.

Why the most inconceivable additions should be made to such a
simple composition passes comprehension, and reminds one of the
early days of rubber manufacture. Why, in fact, employ purpose-
less additions, only to add mystery to the subject ?

Two different methods are available for the manufacture of
these sheets, but a preliminary mixing in the dough mill is common
to both. The masticated rubber which is to be used is, first of all,
dissolved in benzine, and the thin solution obtained is mixed in
two lots with the mineral ingredients, a homogeneous paste being
formed. Into this the loose, carded asbestos fibre is introduced, and
the whole mass is worked until it is thoroughly felted together.
The more thoroughly the mixing is done the more homogeneous
will be the resulting composition. We consider it very advisable to
first card the fibre before adding it to the mixture, and recommend
the use in connection with the mixing-machine of a small special
opener with a belt conveyor to carry away the carded fibre. By this
means the fibre can be mechanically added to the mass in a uniform
way in the form of a light fluff* and not balled up together ; this
leads to more efficient felting.

The methods of working up the mixed compound into sheets
are as follows : — The sheet can with advantage be either rolled round
the rolls in single layers, the ends of the length being then brought
together and the sheet " doubled "up to the desired thickness, or
the special "-ite-sheet" rolls made by Haubold, and shown in
fig. 89, may be used. On these rolls sheet up to 3 metres in length
can be made. On the rolls shown in fig. 89a, on the other hand, it is




possible to work sheets as much as 5 metres in length, one after
another, without having to build up from a number of thin layers.


In principle the manufacture is similar to that of asbestos sheet.
The stiff pulp on leaving the mixing-machine is rolled out into

Fig. 89a.

wide sheet between the cylinders, taken up by the conveyor, and
carried forwai'd to the large cast-iron roll. Here the thin sheet is


firmly pressed by the press-roll on top. As soon as the correct
thickness is reached the sheet is cut off and removed. The rolls are
provided with arrangements for heating and cooling them, and also
with a spreading-knife and carrier rollers; the metallic .conveyor
passes through a containing vessel where, if necessary, it can be kept
moistened. By this means a solid "-ite" sheet is obtained, con-
sisting of thin separate layers intimately felted together. In both
cases the sheets are afterwards subjected to pressure in large
hydraulic cold presses, in order to produce a hard sheet.

18. Eraser Rubber. — The manufacture of this article is carried
out in the following way : — Mixings of suitable composition, con-
taining as fillers chiefly pumice powder, glass powder, whiting and
barytes in large quantities, are calendered into sheet and doubled
up to the requisite thickness as described on pp. 54*-56.

The sheet, which should be free from blisters, is compressed in
metal moulds measuring 60 x 60 cm., which are provided with the
•various impressions which it is desired to reproduce on the rubber.
A cold press is all that is necessary for this purpose, and in it as
many as twenty moulds or more can be dealt with at a time, the
pressure being maintained for about three hours. The compressed
sheets of rubber obtained in this way are cut up and vulcanised
in chalk at a low temperature. Another method is to vulcanise
the roughly-calendered sheet in the moulds in a multiple press.
Qualities containing much waste give better results when treated
by the latter process, for in the first method of production the
impressions on the rubber turn out indistinct and not so sharp as
is desirable, this defect being enhanced during vulcanisation by the
shrinkage which then takes place.

Union rubber — that is, rubber consisting of two different qualities
(one for rubbing out pencil and the other ink marks) — is prepared
in the following way: — The sheet, pressed smooth, is cut up on
the lathe into strips of suitable width, attention being mainly
directed towards getting a clean cut. The cut edges are allowed
to dry, and are then given two coats of a good vulcanising solution.
This is allowed to dry off, and the strips of the two kinds of rubber
are then stuck together alternately, compressed, and vulcanised in
suitable frames under the press.

These various kinds of eraser-rubber are cut up in automatic
cutting presses with smooth- or saw-edged knives, it being unneces-
sary in these machines to mark out the separate sizes.

Another kind of eraser-rubber comprises the polislied varieties,
such as " Velvet " and " Diamond " American rubber, pointed


rubbers, etc. "Velvet" rubber is cut from sheet, and the pieces
are rubbed down in a revolving drum, which is best made with
a hexagonal section, and contains powdered pumice. The shaped,
so - called, " Diamond " rubbers have first to be buffed on an
emery - wheel, each piece being separately pressed into a wooden
holder and held down on the wheel. In this way all the pieces
are obtained of exactly the same shape and size. Pointed rubbers
may be cured in moulds and then tubbed, or they may be cut out
of sheet and smoothed down and pointed on special machinery.

Grease rubber consists of a mixing: containino; con-lderable
quantities of dry white substitute and vaseline. Marbled rubber
is prepared by the non-homogeneous mixing and calendering of
mixings of different colours. Radifix is essentially only a chloro-
sulphide substitute containing magnesia, and has nothing in common
with rubber.

It was formerly the practice to use ordinary rubber stamps for
branding the pieces of rubber, but the whole sheet is now printed
in a press or by means of a roller.

19. The Manufacture of Para Sheet and Elastic Thread. — The
oldest method of making tliese goods, and the one still the most
widely used in the special factories which deal with them, consists
in spreading them on the spreading-machine (as opposed to calender-
ing the sheet). Both methods are equally difficult to carry out,
and good results are only possible after long experience and with
the help of a well-trained personnel. It is also essential that only
first-class raw material, consisting of selected "old Para" of the
highest quality, should be used. It is by no means a matter of
indifference what kind of Para is used, as is often incorrectly
assumed even by specialists. The Para used sliould contain less than
2-8 per cent, of resin, a figure which would indicate that the latex
was obtained from very old trees, and which would ensure a high
degree of elasticity. The protein content should not be higher
than 2-6 per cent. A good quality for sheet and thread should
stand the following test: — A piece of the rubber 0'2 mm. thick and
50 mm. long by 10 nun. wide, should be capable of extension to
570 mm., and should recover its original length, on releasing the
tension, to witliin from 3 to 8 mm. When the raw material has been
selected it is washed and dried, as described in an earlier chapter,
but obviously vacuum-drying is excluded in this instance, for well-
known reasons which liave been already discussed in Chapter II.,
p. 34. Wlien dr^^, the sheet of rubber is run tln^ough a cleansing
machine as a safeguard against the possible presence of any


particles of dust and other impurities, and only when the process
is completed is the rubber subjected to mastica,tion and mixing
on the rolls with sulphur. The rolls should not be worked too hot
at first, or the finely-powdered sulphur may melt on the rolls and
form granules or crystals. The addition of sulphur being com-
pleted, the rubber is rapidly worked between hot rolls, so that the
process may be completed as quickly as possible. The mixing and
working on the rolls necessitate great watchfulness on the part
of the workman.

When the mixing is completed the actual preparation of the
calendered or spread Para sheet commences.

(a) Calendered Sheet. — The calendering of this sheet requires,
in the first place, a carefully selected, capable personnel, and,
secondly, a set of four-roll calenders of the best construction, which
alone can find use here. This machine should in particular run
quietly and smoothly ; the rolls should be highly polished and as
smooth as glass. The central adjustment for the rolls is so arranged
that the third and fourth rolls (the two uppermost) can be adjusted,
not only individually, but also both at once from the middle roll
(the second), which is mounted in fixed bearings and serves as the
driving-roll. The fourth (bottom) roll is also capable of being
moved downwards. In connection with the third roll the necessary
double- wedge adjustment, which serves as an arrest, and prevents
the vibration of the rolls, should be looked for. In that way the
bank of rubber between the second and third rolls is kept equal ;
otherwise the third roll would be lifted by the rubber against the
fourth roll above it, a state of affairs which would be very detri-
mental to the uniform solidity of the sheet. The spindle of the
roll-adjusting apparatus should not be in front of the rolls, but
above the lower end of the frame top, since the rolls must be
easily accessible. In front of the calender-rolls, which should
always be regulated to a constant temperature with the aid of
thermometers, are small, easy-running rollers over which the sheet
can be drawn away. The four-roll calenders illustrated in the
chapter on calendering (p. 52, fig. 24) are, up to the present, the
most perfectly constructed set which it is possible to get for the
purposes of this branch of manufacture.

In making this sheet it is not at all a matter of no importance
at what temperature the rubber is introduced into the calender-
rolls, and variations in the different batches among themselves

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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 17 of 21)