Adolf Heil.

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

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Another process which may be employed for inflating hollow
articles consists in the use of ammonium nitrite, which, as is well
known, is decomposed, on heating, into nitrogen and water, accord-
ing to the equation —

NH,N02 = 2H20 + N2.

The ammonium nitrite may be prepared in situ, by the action of
alkali nitrite on ammonium chloride, in accordance with the
equatioi^ —

NHvCl -h NaNOg = NaCl -1- NH^'NOs.

The most convenient method of inflation is to utilise the production
of CO2 from carbonates.' Nitrogen can also be prepared from
chloride of lime and ammonia, and ammonium carbonate may be
used instead of the latter. In mixings which have not the proper
composition, and especially if the rubber has not been worked until
perfectly homogeneous, and in which therefore the micro-porosity
of the rubber is greater than it should be, ammonia may easily
pass through the walls of the article, or fill the pores and decom-
pose the rubber. In this place a crystal of ammonia is formed on
the outside of the ball, and the gases and air are allowed to escape.
It is therefore considered desirable to add to ball mixings a
quantity of pitch, or a mixture of ozokerite and pitch. The use of
finely-ground, dry, pulverised waste should not be permitted. If
such be used there will always be produced a number of spots
which are permeable to air or other gases, whereas none of these
can be noticed if reclaimed rubber and pitch be used. The pitch
certainly darkens the colour of the rubber a little, but in the case
of painted balls that is of no consequence. The following mixings
are some which have been used with good results : — •



(1) Mozambique
Massai . .
Sulphur .
Zinc white
Whiting .
Pitch

(2) Upper Congo
Nicaragua
Sulphur .
Reclaimed



Rubber


Balls.


5,000 gms.


Lithopone


5,000 .,


Talite


1,200 ,,


Pitch


12,000 ,,




15,000 ,,


(3) West Indian


300 ,,


■ Ozokerite.





Sulphur .


4,500 „


Reclaimed


5,500 ,,


Zinc white


12,000 ,,


Whiting .


6,000 ,,


Pitch



12,000 gms.

10,000 ,,

300 „

10,000 „

400 ,,

1,200 „

8,000 ,,

12,000 „

i5;ooo ,,

200 ,,



The manufacture of hollow rubber toys, such as dolls and
animals, is carried out in the same way, the separate sections being



138 RUBBER MANUFACTURE.

cut out of sheet, and joined together in the form of the portion to
be reproduced. They are made up in the same way as balls, but
it is preferable to use water as an inflating agent during vulcanisa-
tion, or else capsules of gelatine containing sodium bicarbonate and
tartaric acid, 'from which mixture carbon dioxide is evolved.

Vulcanisation proceeds exactly in the same way as in the case of
other moulded goods, but the rise of steam pressure should be as
rapid as possible, and care should be taken that the temperature and
pressure do not recede during the process, so that the hollow articles
may not drop in. Of late, ball-presses, (fig. 59) have been coming




Fig. 59.



into use in place of single moulds, for curing balls both hollow and
solid, of reasonably small dimensions, say up to 2 J inches in dia-
meter ; with such an arrangement as many as two hundred balls can
be cured in a single operation. The simplicity of handling and work-
ing this apparatus, together with the fact that means are provided
for cooling down rapidly by a flow of cold water through the press-
plates, constitute very great advantages. The moulds are opened
while they are being vigorously cooled down ; by this means the
gases present in the ball under considerable- pressure are prevented
from blowing it out still further, and, on the other hand, from
contracting owing to the fall of temperature. For this reason it
is also necessary to re-inflate the ball after vulcanisation ; this is



MANUFACTURE OF SOFT-RUBBER ARTICLES.



139



only done on the grounds already given, and it is incorrect to
assume that all gases diffuse through the ball. After the inflating
gases have been removed from the ball it is filled by means of
compressed air, which should be as cold as possible, the object of
the plug previously referred to being to close up. the hole made in
it by the hollow needle when the latter is withdrawn. For safety's
sake it is best to push a small piece of rubber, soaked in turpentine,
into the plug in addition.

In toys the after-inflation is unnecessary, because they are
provided with air- valves. The holes into which these are fitted
are drilled out on a drilling machine.

Balls and toys are painted by hand. In the case of balls the
ground- work and lining can be put in with the aid of a small
painting apparatus. The arrangement is extremely simple ; the
ball, resting on a disc, is rotated whilst a fixed brush marks the
lines according to the position in which it is put and the colour
with which it is filled. The pictures on the balls are put on with
transfers.

The mixings used for the manufacture of dolls should be very
soft and elastic, since they must be able to take the impression of
the sharpest intricacies of the various moulds. The following may
be given as examples : —





Rubber Dolls.




(1) Columbian


6,000 gms.


(2)


Lopori
Columbiiin


6,000 gms.


Sulphur .


1,400 ,,




4,000 ,,


Ceaia


. 4,000 ,,




Golden sulphide


3,000 ,,


Zinc white


. 10,000 ,,




Whiting .


8,000 „


Whiting .


8,000 „




China-clay


4,000 „


China-clay


. 4,000 ,,








Paraffin wax


250 ,,









3. Manufacture of Seamless Tubing and Shaped Cord. — In the
manufacture of these goods the tube machine depicted in fig. 60
occupies an all-important position, and a very great variety of
articles can be produced by means of it according to the die
employed. The machine consists of the cylindrical casing, which
can be heated, enclosing the screw, and the box, in which a spindle
is fixed ; this spindle, the form and size of which depend on the
article being made, can be centred by means of set-screws, and its
thickness corresponds to the internal diameter of the tubing to be
made. In addition, we have at the end the die with a circular hole
which determines the outside diameter of the tube. If the spindle
be removed the machine produces a cord, which may be of circular
section or angular, according to the die employed. The machine



140



RUBBER MANUFACTURE.






works as follows : — The mixed rubber is warmed up and run into
thin sheets which are filled into the hopper fitted over the cylinder
at the after-end of the screw (this hopper is, in some forms of
machine, substituted by feeding-rolls), and is then forced through
the screw towards the box in the front. It will be clear that
during the operation the rubber mass is powerfully kneaded, and
a breaking-down of the rubber molecule may readily occur — first,
if the cylinder be too strongly heated ; and secondly, if its length be
too great, since the mass is then worked for too long a time before
reaching the die. Tube machines with short cylinders and
powerful screws of great depth are to be' preferred for the pro-
duction of a suitable pres-
sure. The extent to which
the machine must be heated
up in order to obtain a
smooth surface — the so-
called "mirror" {Spiegel) —
is greater or less according
to the quality of the rubber
mixing, ^his warming-up
refers less to the cylinder
than to, the box of the
machine, and better results
are obtained by using a
gently-heated cylinder and
a hot box than if the re-
verse plan be adopted.
Very tough qualities, which
cannot be run smoothly, are mixed with from 1 to 5 per cent, of
vaseline or paraffin wax ; surprising results are obtained in this
way. The cord or tube leaving the machine passes through French
chalk, carbon black, or other powder, according to its colour, and is
either run into the vulcanising chamber in front of the machine, or
on to wooden trays. In the latter case it may be either worked up
for use in the manufacture of moulded goods, or vulcanised direct.
These articles are laid in French chalk just as they are made, the
chalk serving to keep them in shape and prevent them from getting
flattened, and also to prevent the direct penetration of moisture ;
the goods must therefore be made from qualities which can be
cured in the open without becoming rough on the surface. Many
qualities, especially those which contain soft rubbers and a large
proportion of softening ingredients, such as vaseline, paraffin wax,



Fig. 60.



MANUFACTURE OF SOFT-RUBBER ARTICLES.



141



fatty substitute, etc., need a suitable addition of magnesia usta to
enable them to be cured in chalk in the open ; only those makes of
magnesia which consist of oxide alone, and do not contain carbonate
or hydrate, are to be recommended for this purpose.

In the case of large cord care must be taken to have a perfectly
homogeneous mixing ; otherwise these goods become porous from
the centre outwards, a result produced in many cases by faults in
mixing or by the use of unsuitable ingredients.

Linings for canvas hose are, as already mentioned, also made on
the tube machine. For this kind of tubing either a machine with
a hollow screw or one with a transverse end is used, so that chalk
can be introduced by blowing it through by means of an air-blower
attached to the machine. Inner tubes
are run on the machine in a similar
manner. In order to be able to regu-
late the pressure of the rubber, a cock
is fitted into the box of the machine
through which the excess of rubber is
allowed to pass out; this is a point
which must be specially observed if
one desires to produce tubing with
uniform thickness of wall.

For the manufacture of flower-
tubing (covering for the stems of
artificial flowers) machines are used
with very small cylinders and with a
special arrangement of the box (fig. 61),
enabling tubes up to three in number
to be run- at the same time. The use of hydraulic pressure in
tube machines does not seem to have met with approval. Experi- ,
ments initiated some years ago have not yielded satisfactory results,
for it is hardly possible to keep the temperature of the rubber
constant, and defective places are easily produced. On the other
hand, there has recently been a leaning towards two-spindled
machines, which consist, in principle, of two tube machines fixed
opposite to one another, and united in the middle by the box,
so that the rubber is forced both from the right and from the
left into the box and die.

4. Preparation of Sheet for "Mechanicals." — For the manufacture
of these articles, which constitutes an important branch of the rubber
industry, calendered sheet and the proofed fabrics for insertion
form the central point, as the basis for making up. Packing-sheet




Fig. 61.



142



nUBBER MANUFACTURE.



with insertion is generally prepared by rolling the rubber coating
on to the proofed fabric. This process can be simpliiied by uniting
the proofed insertion and the coat of rubber direct while the
latter is being run on the calenders, thus preparing the finished




\



Fig. 62, a.

sheet, firmly doubled, in the same time as would be necessary
merely to run the coating of rubber. The whole process is exactly
similar to that employed in the preparation of sheet rubber which
has to be doubled, and has been fully dealt with in the chapter on
the calenders. Before curing, the finished sheet is tightly wrapped




Fig. 62, b.

between cloths on a wrapping-machine (of which fig. 62, A and B,
illustrate an improved form), in order to prevent its blistering,
and to cause the various layers of rubber and insertion to become
firmly united during vulcanisation. Packing-sheet may also be
cured under the press, but the cost of vulcanisation then comes out
considerably higher.



MANUFACTURE OF SOFT-RUBBER ARTICLES.



H3




144



RUBBER MANUFACTURE.



Manhole washers, and flat rings made up of strips, are cut
from uncured sheet by means of a band saw, and the strips joined
up together, covered with cloth where necessary, and cured on
sheet-iron moulds. The manufacture of round cord is carried out
cheapest and best by means of a machine made by Francis Shaw of
Manchester (fig. 63). The separate sheets, united with their inser-




FiG. 64.
tions, are rolled up, and any number of such strips up to twelve
are put under the rolling machine, formed of two bench-tops. The
upper bench-top, tilting increasingly as it advances, presses on the
strips in its forward motion and rolls them into the form of round
cord; insertion and covering can be rolled up in one operation.
Where smooth ' cord is desired the covering is left out, and, if
necessary, a layer of rubber is run on to the cord by means of



MANUFACTURE OF SOFT-RUBBER ARTICLES. 1 45

the tube machine. The cord is vulcanised in a straight length,
in a hose-heater, after having been previously wrapped in the same
machine. Square and flat Tuck's cord is first made up in the form
of round cord in the same way, and is afterwards passed through
the shaping-machine (fig. 64) and pressed into shape. This machine
possesses the great advantage over the ordinary grooved rolls, that
by means of it cord of any dimensions can be made by merely
changing the upper wheel. Tube-rings and similar articles are
made of the required size on the tube machine, and are then
drawn on to mandrels of corresponding size, wrapped in cloths,
and vulcanised.

5. Press-cured Goods. — Under this heading press-cured sheets and
pump valves first of all claim our attention. These articles are
made by doubling sheet up to the required thickness, care being
taken to get rid of all air- bubbles, and then cutting it down to the
required sizes. Lengths of 3 metres by 1 J metre wide are what are
generally asked for in the case of press-cured sheet, and such sheet
is built up on the press-table and vulcanised in one piece of that
size. In the case of circular valves, and valves of other shapes,
planed wrought-iron rings and frames of corresponding shapes
are used; by these the rubber is supported during vulcanisation,
which is carried out under suitable pressure. In calculating the
sizes of moulds it must be remembered that rubber contracts some-
what on vulcanisation, as a result of the shrinkage of the raw
rubber and the way in which it has been stretched by the processes
previous to vulcanisation. Pure qualities contract to the extent of
from 2-65 to 2:9 per cent., lower qualities proportionately less, and
the latter decrease in thickness, whereas the former thicken up
slightly; in preparing moulds these facts must be specially taken
into consideration. Another article which is cured under the press
is dealt with in the following section.

6. Rubber Belting. — The manufacture of this special article
necessitates special machinery, if it is to be carried out in a rational
manner and durable, serviceable goods are to be produced. The belt-
ing is prepared in the following way : — The cotton fabric, spread on
the calenders, and into which a highly adhesive layer of rubber is
frictioned, is folded, on a folding machine, into belts of the requisite
thickness and the desired number of ply. The excess of fabric is cut
oft' on the machine ; or, if it is too narrow, another piece is stuck on.
In this way a considerably more even product is obtained than by
hand-folding, a process in which, for example, it is impossible to
get rid of air-blisters. Unfortunately, German manufactui-ers are

10



146 RUBBER MANUFACTURE.

not yet very familiar with this machine, and therefore prefer hand-
work. When the insertion has been folded the belt is run through
the belting calenders (fig. 65) in order to press it and straighten it




out. If the method of making up necessitates it, a covering sheet
of rubber may now be put on, but on properly spread cotton fabric
this is not at all necessary. The belting is vuk*anised either with
squared edges, in moulds, or with rounded edges, free under the



MANUFACTURE OP SOFT-RUBBER ARTICLES.



147



hydraulic press. In order to prevent the belting from stretching
while in use, it is necessary to stretch it while it is under the press
being cured, and this also is done by means of hydraulic power, the
necessary apparatus being attached to the press. To produce a
good solid article a pressure of at least 20 kilograms per sq. cm.
should be put on the belting by means of the press while it is being
cured, and the temperature should not be raised above 140° C. so that
the fabric may not be weakened. This temperature is quite high
enough to bring about rapid vulcanisation with the particular
mixing employed, which contains litharge, and to render the article
firm and stiif ; at higher temperatures the belting would become
too hard. A particularly suitable mixing is one of the following
composition : —



West Indian


10,000 gms.


Sheet for cover.




Sulphur


. 1,500 „


Congo


. 10,000 gms


Litharge . .


1,800 ,,


Sulphur


. 1,500 „


Whiting .


15,000 „


Lithaigc


. 1,000 ,,


Pitch .


1,000 ,,


Reclaimed waste


. 10,000 „


Barytes


. 5,000 „


W biting


. 15,000 „


Carbon black


200 „


Talite .


. 7,500 „



It is advisable to employ considerable hydraulic pressure, because
the fabric, although previously dried, always contains a certain
amount of moisture, which is given off in the form of steam during
vulcanisation, and may be the cause of the belting splitting into
layers later on.

7. Rubber Eollers. — The preparation of rubber rollers is a
department of manufacture demanding great experience and know-
ledge, for the uses of rubber rollers are so diverse that a particular
method of manufacture and a special mixing are demanded for each
particular class. In the case of all rollers, however, there is one
point of great importance which is seldom observed, viz. that the
workshop should be free from dust and kept at a uniform. tempera-
ture. Convenient lifting apparatus (overhead travelling cranes)
should be provided, so that the rollers, which are often very heavy,
may be easily handled. The stands upon which the rollers are
supported, as well as the wrapping apparatus, are made of galvan-
ised ironwork. Wooden supports and trestles should be avoided
as far as possible. Paper rollers — so called " wet-press " rollers —
which need special treatment on account of their size, constitute a
special department. All rollers have iron cores, which may be
either solid castings, or made hollow of sheet iron, according to
what the roller is to be used for, or, as in wringer rollers, the core
may be simply a turned iron spindle. Large solid cast-iron cores



148 RUBBER MANUFACTURE.

are not well adapted for covering with rubber, on account of the
unevenness of the heating-up.

Before anything further is done with the core it must be warmed
up in order to remove any traces of moisture that may be present.
After this operation has been carried out — it may be done in a hot-
air oven, or over a charcoal fire, etc. — the iron roller is cleaned. Any
pores found in it must be either plugged or filled with lead, to pre-
vent the formation of air-bells between the inner coat of rubber and
the iron core. If the rollers to be covered are large hollow ones it
is advisable to connect them up with a steam pipe so that they can
be kept warm while the roller is being made up ; by so doing even
a very slight formation of air-bubbles is provided against. When
the core has been examined it is washed with benzine, and is then
repeatedly brushed over with hard-rubber solution in thin layers.
The coating of hard rubber to be put on the top of this should be
hard and tough in order to unite well with the metal ; a rubber
insufficiently so might result in the rubber coming away from the^
iron, a tendency which is assisted by the unequal expansions of
rubber and iron within the limits of temperature dealt with. The
layer of hard rubber which is put round this innermost coating,
and the thickness of which varies according to circumstances, some-
times reaching as much as 15 mm., is in its turn covered by a mass
of hard rubber of a firmer quality, to form the junction with the
soft outer layer of rubber, and serve as a firm foundation for it.
Single sheets of rubber are doubled up to about 1'5 mm. thick, and
are then worked on with a roller, and the seams pressed well
together. It is best to dry-wrap the hard-rubber layer firmly
over-night, and to prick any bubbles which are evident on the
following day. A preliminary heating of the hard-rubber layer is
not necessary in good work. The process of doing so often brings
about a loosening of the innermost coating from the iron during
the second vulcanisation. The best way of rolling on the outer
coatings is without doubt in the form of sheet which has been
carefully crossed and doubled up to 1"5 mm. in thickness. The
method according to which the layers of rubber are put on in the
form of a spiral band is uncertain in its results.

In doubling tlie sheets and rolling the different layers of rubber
on to the core, the roller should be kept continually at a uniform
temperature, so that any air-bubbles can be seen to be driven off.
When completely built up the roller is wrapped, on the power-
driven wrapping apparatus provided for the purpose, in a close-
woven cotton fabric, capable of standing a great strain, and is then



MANUFACTURE OF SOFT-RUBBER ARTICLES.



149



first heated up very gradually during three to six hours, according
to the size of the roller or the thickness of rubber, being afterwards
slowly vulcanised at 135° C. for from eight to twelve hours. The
temperature should not, however, be allowed to drop before vulcani-
sation sets in, or the outer coatings may become porous or spongy,
places of unequal hardness being formed. After vulcanisation the
roller should be allowed twelve hours to cool down, or more, accord-
ing to its size, after which it can be unwrapped and turned up and
buffed on the lathe.

It may here be mentioned that the process of manufacture has




Fig. 66.

latterly been rendered much simpler and quicker by suitable addi-
tions of magnesia to roller mixings. The magnesia is added to
the hard rubber as well as to the soft rubber of the roller covering,
and its use is quite permissible in rollers which are to be used in
the manufacture of food stuffs.

In the case of large rollers the lathe-tool is used until the roller
has been turned quite true, when the buffing is finished off with an
emery-wheel ; wringer-rollers, on the other hand, are finished off
direct upon specially constructed buffing lathes, on which three
articles can be buffed at a time. Fig. 66 illustrates one of the
newest forms of this machine.

The compositions of the mixings used are determined by the



I50



RUBBER MANUFACTURE.



requirements which they have
few examples : —



to satisfy. The following are a



Hard rubber inside-coating for hard-press \


Leather press rollers—cont.




rollers.






Barytes .


6,000 gms.


(1) Congo .


6,000 gms. 1


Sulphur . .


1,000 ,,.


Canvas waste .


2,000


,,


(2) Para


5,000 ,,


Reclaimed


3,000


J J


Mozambique ,


5,000 ,,


Sulphur .


1,500


J5


Litharge .


4,000 ,,


Magnesia usta .


1,250


>>


Barytes .


7,000 ,,


Litharge .


1,000


99


Sulphur .


1,500 „


Barytes .


5,000


,,


r

Ceresin


500 ,,


Pitch


500


V






(2) Congo


10,000


>5


Lacquering rollers, for lacquer and tur


Reclaimed


10,000




pentine.




Sulphur .


5,000


>5


(1) Para


10,000 gms


Magnesia usta .


1,000




Sulphur .


900 „


Litharge .


2,000


>5


Magnesia usta ;


150 „


Covering sheet.






(2) Para

Golden sulphide


10,000 „
1,100 „


(1) Para


10,000


>>


Magnesia usta .


100 „


Zinc white


8,000


)>


Vermilion


600 „


Barytes .
Sulphur .
Magnesia usta .


. 2,000

. 1,000

300




. (3) Para

Litharge .
Sulphur .


1,000 „

3,500 „

800 „


(2) Para


. 6,000


) i


Pitch


500 „


Lopori

Golden sulphide (17


4,000
%) 3,000


>>


Sleeves for printing-machines.


Barytes .


7,000


,,


Kassai (Congo)


10,000 „


Magnesia usta .


250


M


Substitute, white
Zinc white


. 6,000 ,,
. 10,000 „


Leather press-rollers.






Barytes .


. 3,000 „


(1) Para


. 10,000


))


Talite .


. 5,000 „


Pitch preparation


. 1,000




Sulphur .


. 1,500 „


Zinc white


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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 13 of 21)