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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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The thickness of the sheet which can be made in this way varies
from ^Q mm. to 15 mm. Thicker sheet should receive a preliminary
vulcanisation under the press, as referred to below. In water-



bath vulcanisation the iron case in which the sheets are packed
should be large enough for the water to circulate freely round the
sheets, A thick cast-iron plate, planed smooth, serves as a base on
which the separate sheets of foil-covered rubber of equal size are
laid, care being always taken to avoid creases; it is not advisable
to vulcanise more than fifteen to eighteen sheets each 3 mm. thick,
in a single case. To prevent the sheets from being displaced, a
cast-iron plate, planed smooth, and of the same size as the sheets, is
laid on top of them. After vulcanisation — which, as stated at

Fig. 96.

the outset, is regulated according to the quality and thickness of
the sheet — the sheets are allowed to cool, and the tinfoil is then
taken off and melted down again. In the process of doing this
indications are obtained as to the correctness or otherwise of the
vulcanisation. If the sheets are under-cured the tinfoil adheres
tightly to them, and the sheets themselves are of a dull, dirty
greenish-black colour, and leathery in texture ; this mistake may be
partly rectified by re- vulcanising the sheet. If, on the other hand,
the sheets have been over-cured they are as hard as glass, and more
often still are porous and carbonised.

Sheets more than 15 mm. thick are covered on the hot-bench
with tinfoil ; this is not, however, pressed down at the edges, but


cut with the sheet itself to the right size for the frame mould which
is going to be used, and the sheet is then partly vulcanised in the
press, under hydraulic pressure. The size of the frames used is
determined by the thickness of the sheet, and the temperature at
which the partial cure is effected is at first a low one, but is
gradually raised until the sheet is about three-cjuarters cured. The
cure of the sheets is then completed in the water-bath for from
three to four hours, at a temperature not exceeding 130° C. ; in this
way the productive capacity of a plant is increased.

After vulcanisation the sheets are either worked up further,
that is, cut up into other articles, for which purpose special saws
are used, or they are sold as they are. , In the latter case the sheet
has still to be finely polished.

2. Manufacture of Tinfoil. — The consumption of tinfoil in a
hard-rubber factory being exceedingly great, and since the foil can
sometimes only be used once, its preparation is usually provided
for in the factory itself. The foil is rolled from pure block tin.

The manufacturing process may be divided into three parts : —

1. Melting down and casting.

2. Preliminary drawing out.

3. Kolling and glazing.

The tin is melted down in a special oven, and the scale must
be very carefully removed so that only a perfectly clean melt is
poured; the metal should not be overheated. It is cast in iron
Register moulds, 30 cm. square. Each mould is formed of a cast-
iron plate in which a cavity has been planed out, surrounded by a
rim 15 mm. thick. The back of one plate forms the cover of the
next. On one side of each plate the rim is missing, the edge being
slightly bevelled off. The plates are aU built up together to form
a mould. This is inclined at an angle of about 75°, the mould is
closed by a clamp, and after it has been warmed up is ready for
use. The casting is done in the ordinary way, the metal being
slowly poured into the moulds from a ladle, while the moulds are
cooled with water. The plates of metal obtained, which measure
300 X 300 X 15 mm., are now drawn out on the mill illustrated in
fig. 97, and are then rolled out on the tin-calenders, — which are
similar in construction to the ordinary two-roll calenders (fig. 25),
except that they must have rolls ground and case-hardened across
their whole breadth, — to thin doubled foil of 120 cm. ; soapy water
is used as a lubricant. For comb manufacture the foil is calen-
dered into narrow sheets about 15 metres long. This calender-
ing process is carried out in several stages, the sheet of foil being



run thinner and thinner by gradually closing up the rolls, using
soapy water as a lubricant. Finally, in order to improve the gloss
and smoothness, the sheet is glazed and is then ready for use.

3. Ebonite Rods and Tubes.— The manufacture of these articles
is in many respects distinct from that of sheet. Whereas sheet
was covered with tinfoil and vulcanised in the water-bath, it was
formerly the practice to vulcanise rods and tubing wrapped in
cotton binders, the tubing being of course first placed on mandrels.

Fig. 97.

It is customary now to cure both these classes of goods in chalk in
the open, or, as in cases where one is concerned with the production
of an absolutely true and smooth-surfaced article, in glass or metal
moulds, the inside walls of which are smooth. In carrying out this
process care must be taken that both the moulding tubes and the
machined ebonite cord or tube are evenly heated, so that the rubber
may distribute itself evenly in the mould and fit closely against
the walls. Some experience is, however, necessary here, in order
to prevent air-markings on the outer surface of the rubber rod or
tube. With the increasing use of magnesia usta this method of
vulcanisation has come into more general use, the results obtained


being quite equal to those produced according to the older method.
As a rule, the tube or rod is prepared on the tube machine ^already
described, the particular article required being run to the desired
dimensions. By working with the tube machiuQ one obtains a
much more uniform product, and the troublesome wrapping process
is done away with. Moreover, no defects can arise in such tubing
from faulty seams, whereas such defects are very likely to occur
in the case. of common, dry qualities with thin walls, made up by
the wrapping method; on the machined rod, again, there is no
seam-like marking showing where the edge of the rubber sheet
came when the rod was rolled up. The mixings should be of such
a composition that they do not tend to run out of shape during
vulcanisation; they should therefore set rapidly, and this is
generally ensured by the use of magnesia usta and ebonite dust
as ingredients. The finest possible dust should be used so as to
ensure getting very smooth surfaces straight away, and make
subsequent burnishing unnecessary. Furthermore, the magnesia
should be as free as possible from hydrate and carbonate, or porous
goods may be obtained as a result of the evolution of gas on heat-
ing. In very highly polished tubes and rod the magnesia is
omitted, the time of vulcanisation being correspondingly increased.

Tubing up to 15 mm. in diameter may also be vulcanised in the
open without mandrels, according to the thickness of wall, whilst
larger sizes for very accurate goods of smaller diameters, which
must measure correctly to a hair's-breadth, are either run on the
tube machine and then slipped on to mandrels, or run direct on to
the latter in the proper machines, and vulcanised in moulding tubes.
Rods more than 25 mm. in diameter after being run on the tube
machine must be cured in tubes, and larger sizes still in moulds,
since these larger sizes are very apt to swell and burst open. After
being suitably vulcanised the rods and tubing are either further
worked up, that is to say, turned, or they are regarded as finished
products, in which case they are in part polished or burnished,
processes which are dealt with later. Vulcanisation is carried out
either in the water-bath or in hot-air chambers.

The rods and tubes made in this way are quite smooth and exact
in dimensions.

4. Insulation Tubes. — Insulation tubes for electric conductors
form one of the chief objects of attention in the tube machine
department of an ebonite factory. The point to which greatest
importance has to be attached is the production of a good insulator
with as thin a wall as possible, and in this connection it is impor-



tant to put together the right mixing, so as to be able to make a
tube which is tough without being excessively hard, and which
can still be bent. The lengths of tubing should not be run from
the tube njachine on to trays, but in lengths of from 1 to 3 metres
on to belt conveyors, passing through water, if necessary, as they
leave the die of the machine, so as to cool them down rapidly and
keep them in shape.

The following is a suitable mixing : —


1,500 gms.


5,000 gms.


500 .,

Brown rape oil substi-

Dust .

6,000 ,,


8,000 „

Almeidina .

3,000 ,,

China-clay (free from

Reclaimed .

12,000 ,,


15,000 „

Pontianac mixture

6,000 ,,

Whiting .

6,000 „

Talite, Atmido or


Palm- or castor-oil

2,000 ,,


15,000 „

Magnesia usta

1,000 „


2,000 „

Tar ....

2,500 ,,

Time of vulcanisation, about one hour at 145° C.

5. Ebonite Accumulator Cases.— The method of manufacture of
these articles is not quite equal to the great demands put upon them,
and very unpleasant consequences have frequently resulted from
cases being badly made, or made from unsuitable mixings. Ebonite
cases have to fulfil the following conditions, and should be made
with this in view : —

1. They must withstand the action of sulphuric acid of 66**


2. They should not soften at a temperature of 65° C, and

must not be brittle at 10° C.

3. They must be reasonably tough to be able to withstand

shocks without breaking. •

4. They must be absolutely watertight.

5. They must not break down under a high-tension test at

30,000 volts.
The composition of the mixing will determine whether the first
of these conditions is fulfilled or not. The following mixings are
extensively used : —



10,000 gms.

B. Congo

10,000 gms

Sulphur .

4,000 „

Sulphur .

4,000 „

Ebonite dust .

2,500 „

Magnesia .

250 ,,

Powdered pumice

5,000 „

Ebonite dust .

4,000 „

Linseed oi

600 „

Powdered pumice
Linseed oil

7.000 ,.
800 ,,

Pure silica, produced in the manufacture of sodium silicofluoride,
may also be recommended as possessing advantages over pumice
in this connection.

In order to produce a good serviceable case the course of


manufacture described below must be followed ; attempts to econo-
mise by the use of light sheet-iron moulds instead of the heavier
castings are doubtful policy. Cast-iron moulds should, without
question, be chosen as the cores on which to make up, and as
moulds on which to vulcanise, the cases, which vary in size from the
smallest up to 70 cm. deep ; the moulds should have a slight taper
of about 1 mm. so that the finished cases can be easily taken off.
The surface of the mould is brushed over with a thin solution of
Para, in order that the rubber sheets may make intimate contact
with it. The sheet of the requisite thickness is placed on the
mould and rolled down with a rubber roller, starting from the
top and working downwards. That portion of the sheet of rubber
which projects beyond the corners of the mould is cut off square
with the face of the mould by means of a thin sharp knife,
and the adjacent surfaces are then applied. Finally, the bottom
is placed on the mould, rolled down firmly and cut round the edges,
and the case is now ready for vulcanisation. It is not necessary
to enclose the case in an outer mould, since the surfaces are securely
and firmly united, and the case cannot get out of shape on the
solid mould. The case is vulcanised in chalk, the moulds being
firmly embedded in it and covered with a good thick layer.
Vulcanisation is complete in eight hours at 145"* C. with the
proportion of sulphur given above, and this composition and time
of cure have been shown by a series of experiments to form an
advantageous combination.

While the cases are still warm they are loosened from the mould
in a small press, and are then taken right off and smoothed down
by means of a sand-blast. By this means the faces are made quite
even, and at the same time a considerable saving in wages is
effected as compared with the process of buffing by means of emery-
wheels ; these are only employed to grind down the cases to the
right height.

The cases should now be tested as follows : —

1. In a special apparatus under a water pressure of 1 atmos-

phere, to make sure that the seams are perfectly tight ; and

2, as regards their behaviour towards high electric tension, the

test being applied by means of a spark inductor.

The tested cases are varnished with an asphalt varnish, and are
then ready for use.

6. Moulded Ebonite Goods. — A large proportion of the articles
made in ebonite are vulcanised in moulds. The process is carried
out in cases where metallic fabrics are pressed into the rubber, in



the same way as described for moulded soft-rubber goods. But,
on the other hand, in order to economise in moulds and at the
same time to produce smooth surfaces, certain articles may be
given a preliminary pressing in steel moulds and the vulcanisation
completed in hot-air ovens, or in compo-moulds which, amongst
other things, may be dressed with collodion solution to prevent
sticking to the mould. This process has found employment
especially in the case of surgical goods ; such articles as enema
fittings, taps, screw-unions, cannulse and cigar holders are made in
this way, and it is not necessary to turn Ihem up on the lathe

Fig. 98.

afterwards, since they come from the moulds with ^ perfectly
smooth surface. All articles which are mounted on mandrels can
be cured with comparative ease b}^ means of hot air. Naturally
they are not very smooth after vulcanisation, and have to be turned
up before a polish can be put on. On the other hand, in the case
of certain articles which are covered in tinfoil and vulcanised,
or which are cured in moulds dressed with a special solution, a
brilliant gloss can be produced, wliich gives the article a peculiar
enamelled appearance, and does away with the necessity of
subsequent polishing. Combs are made in the same way. The
sheet rubber is pressed into the comb moulds, between sheets of
tinfoil, by means of heavy bent-lever presses (fig. 98), so as to
form a flat piece of the shape of the comb, but without the teeth.



The teeth are cut out after vulcanisation by means of automatic
saws, the ingeniouslj^-constructed machine working in such a way
that the slot in which the comb is fixed, according to the number
of teeth, moves towards the revolving saw, and is automatically
thrown out as soon as all the teeth of the comb are cut ; the comb
has then only to receive the final polish. By a special arrangement
of the mould the teeth can also be first of all pressed out.

7. Buffing and Polishing Ebonite. — The buffing of ebonite sheel

Fig. 99.

and surfaces of other ebonite goods must be done in a continuous
stream of cooling water, and the articles being buffed should be
pressed as lightly as possible on to the buffing wheel, in order
that the ebonite may not become too strongly heated. For sheet
ebonite an automatic buffing machine is used, in which emery-
wheels are moved evenly over the whole surface, rubbing down
the sheet, which is immersed in water, with fine Tripoli or pumice
powder. No cracks or streaks should show themselves ; if they do,
the sheet should be at once cooled down. In fig. 99 is illustrated a
special machine for buffing ebonite. Smaller articles are either
tubbed, or buffed with powdered pumice on emery buffs or maize-


leaf wheels working wefc. After buffing, the articles are scrubbed
with brushes in order to remove all the fine Tripoli dust or pumice
powder still adhering to them, before being oiled, dried, and polished.
If the article should remain dull in appearance it is rubbed down
with a little linseed oil ; on the other hand, in order to produce a
polish the ebonite must be worked on rapidly revolving flannel
buffs with fine Tripoli powder and a little linseed oil until a deep
gloss is produced. In carrying out this operation the ebonite
should not be rubbed too hard, or it will become overheated and
lose its capability of taking a polish.

Should spots and greenish-coloured patches show themselves on
the surface — defects which are due to the action of moisture during
the process of vulcanisation — they may be easily removed by
repeatedly rubbing them over with carbon disulphide.

8. Ebonite " Mechanicals." — Amongst the most important of these
goods are the coverings for the machinery and boilers used in
chemical works ; they constitute a branch of ebonite manufacture
which demands much experience and knowledge; the difficulties
are increased by the circumstance that the articles have to be made
without moulds. In covering heaters (Montejus) the following are
the points most worthy of note : — The surface to be covered must
be roughened as thoroughly as possible, after which it is carefully
warmed up and cleaned, and then dressed with a suitable solution.
When this is dry a layer of unvulcanised ebonite is rolled down
on to the iron; this layer of ebonite must fulfil the following
conditions : —

1. It must make good contact with the iron.

2. It must be tough and as hard as possible without being too

3. It must not contract to a greater degree than the, iron, or it
may break away from it.

On the top of this layer a suitable hard quality is rolled down ;
this also should show very little contraction ; finally, the actual
insulating quality is put on, and this must be absolutely stable
towards acids. If a partition is to be made in the heater only
such a quality can be used for it as shrinks very little, and this
may, with advantage, be partly vulcanised first of all by itself.

When covered, the heater is allowed to stand for some time, in
order that any blisters which make their appearance may be pricked
or pressed out. The inside of the heater is filled with chalk, tightly
packed, and vulcanisation is then carried out at 130° C. with a
gi^adual rise. Coverings for centrifugal machines are put on in


exactly the same way. The small holes which have to be covered
round, are filled with tubing of the right outside diameter, through
the inside of which passes a short nickel rod of the of the
finished hole. The whole centrifugal basket is packed in chalk and
cured. Vulcanisation may take anything up to fifteen hours at
135° C. according to the size of the basket. The chalk should only
be removed when the article has slowly cooled down. Taps, curves,
bends, and shaped pieces, of different inside measurements, c^n be
most conveniently dealt with on thin, stiffened, hollow, sheet zinc
cores; it is important to see that the individual parts will all
cure in the same length of time, for one cannot cure pieces of
over 30 cm. in diameter with a wall 8-10 cm. thick, of one quality.
They would be porous in the centre, and would blow, and cause
the article to burst. In order to remove the zinc core it may be
dissolved away in hydrochloric acid. By this means a much better
mould can be produced inside than if a corresponding plaster mould
were used. Plaster cores are, on the one hand, difficult to remove,
and, on the other hand, produce a very rough inside surface.

Quite recently it has become tlie practice to insulate ship's
screws and their bearings with ebonite, in order to stop the corrosion
due to local currents, which have been known to eat out even 25
per cent, nickel steel from the propeller shaft. Traun's protected
process for covering propellers,, used on ships of the German Navy,
may here be referred to. These qualities should be perfectly
homogeneous and durable, and, on the other hand, should adhere
firmly to the steel. Vulcanisation is carried out, in heaters of their
own construction, by means of hot air, in the docks, in order to save
the very considerable costs of transport.

9. Imitation Ebonite. — The various resinous compositions put on
the market under different names can be quite advantageously used
for the preparation of solid articles, used for insulating purposes in
the electrical industry, for surrounding and insulating all kinds of
metal parts, which are made in large quantities.

Th^ insulating material occurring in commerce under the names
" Ambroin," " Ebolite," etc., consists of an intimate mixture of resins,
fibre, and fillers, and is prepared by the method explained below.

The basis of the composition is resin. Lac resin (resina lacca)
or shellac is used by preference. Gum-lac comes from the branches
of several trees and shrubs, such as Ficus religiosa, L. ; Ficus
bengalensis, Ficus indica, Butea frondosa, Rb. ; Groton aromaticus,
Croton lacciferum, L. ; Ziziphus Jtijuha, Jj^m. The first-named of
these is a strong East Indian tree of considerable size, with pointed,


oval, somewhat heart-shaped leaves. The " fruits " are rounded and
elongated, and are actually none other than the enlarged receptacles,
to the inner walls of which the extremely small flowers and fruits
proper are attached. Gum-lac results from the perforations made by
a species of coccus, Coccus lacca, in the J^oung branches of the tree.
These insects, with their brood, become surrounded by the glutinous
liquid which exudes,^ later on boring their way out through the outer
shell, leaving behind the mass of resin still adhering to the branches ;
this is collected with the fragments of wood attached to it, and
is called stick-lac. The pieces are taken ofi" the branches, and
when freed from pigmenting material by treatment with sodium
carbonate, are known as button-lac. This button-lac is heated,
forced through linen bags, and pressed between Pisang leaves into
sheets of varying degrees of thinness ; this forms true shellac. It
is a yellowish-brown to brownish-red, hard, glossy, transparent sub-
stance, which clings together in large pieces. Shellac is an article of
commerce in Bengal, Assam, Siam, Annam, Sumatra, and China ; that
which comes from Bengal is, however, accounted the least valuable.

The more transparent a shellac, the greater is its value.
Shellac is only partially soluble in cold alcohol, leaving undissolved
a sticky mass, the so-called "'wax."

The other resin used in the preparation of imitation ebonite is
copal {resina copal).

There are various kinds of copal on the market, the chief sorts
being the following : — West Indian copal, obtained from Aifinyris
copallifera, Sprengel, a member of the Terebinthacese. In October
and November the resin exudes from the bark of the trees in drops,
and is collected and melted together. The fracture is highly
vitreous, the colour varying from light to dark.

Brazilian copal is derived chiefly from Hymencea Cottrharil,
Linn., and is of two kinds. The first kind oozes out of
the bark and forms tear-shaped, somewhat angular pieces of a
yellowish to yellowish-red colour, transparent, with a vitreous
fracture; while the other kind is found in the earth beneath
the roots, and forms egg-sized lumps, with a wrinkled surface.

Copal being cheaper than shellac, it is of advantage to use the
former in the preparation of imitation ebonite. Proceeding now to the
actual manufacture of the mass, the following details should be noted.

The resin is dissolved in solution mills (Pfleiderer's system), in

1 The now generally accepted statement of tlie case is that the lac is mainly,
if not entirely, a secretionary product of the coccus, elaborated by it from the sap
of the tree. — See Tschirch, Die Harze und Harzbehdlter, 1906, pp. 812, 813.



125 per cent, of its own weight of absolute alcohol ; the process of
solution is completed more rapidly if the resin be first of all
powdered in an Excelsior mill. The liquid is filtered, and the
residue on the filter is warmed up and treated with a further
quantity of solvent, this solution being afterwards added to the main
portion. In addition to the resin, the mass contains asbestos fibre,
jute fibre, and the various fillers, such as china-clay, magnesium

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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 19 of 21)