Improved Manufacture of Rubber Moulds, Patterns, or Models.
Year: 1898

page_text: COMPLETE SPECIFICATION.
Improved Manufacture of Rubber Moulds, Patterns, or Models.
A communication from THE FRANCO AMERICAN CASTING Co., Philadelphia, United
States of America.
I, ALFRED JULIUS BOULT, of 111, Hatton Garden, in the County of Middlesex,
Chartered Patent Agent, do hereby declare the nature of this invention and in what
manner the same is to be performed, to be particularly described and ascertained
in and by the following statement:-
The object of the invention is first an easy and reliable method of forming
elastic moulds say of rubber of objects which are to be reproduced in numbers by
means of casting either in plaster of Paris, Portland cement or other material.
Secondly to produce elastic say rubber patterns or models of objects or articles
which are to be reproduced by casting them in metal. Such elastic patterns or
models (to be used instead of rigid metal or wooden patterns) so as to permit the
formation of moulds presenting undercut parts or projections, without resorting to
the slow and expensive method of the socalled false core work.
In order to clearly explain the novelty and great utility of this method of pro-
ducing elastic moulds and patterns, an example or illustration will be given. For
instance an artisan has to make a number of plaster of Paris castings from an alto
relief which is finely finished and presents numerous undercuts, cavities and bold
projections so as to bring out the artistic effects of the design.
To reproduce copies of such a relief the artisan will resort to the old known
method of making from the relief a gelatine or glue mould into which he will cast
the plaster. When the plaster is hardened the elastic gelatine mould can be
easily removed from the plaster cast, but this casting will by no means be as
sharp and perfect as the original model. The plaster acts on the gelatine and
injures the fine surface of the gelatine mould, and besides, only a few casts can
be taken from a gelatine mould. The mould has to be renewed after a few
imperfect castings have been obtained from it. To make a casting of such a relief
and of Portland cement in a glue or gelatine mould is impossible because cement
takes a considerable time to become hard and destroys a gelatine mould before it
becomes hardened.
A vulcanised rubber mould cannot be injured in the least by the action of plaster
of Paris or by any kind of Portland cement. Hundreds of perfect and sharp
castings can be produced from a good rubber mould.
I wish to state however that my correspondents do not intend to claim as their
invention the use of a rubber mould or pattern for such uses, but the hereafter
described method of making such moulds and patterns. This method enables such
rubber moulds and patterns to be produced of such a shape that they are practically
useful in the art of casting. A rubber mould or a rubber pattern which is to be
used for making castings of an object presenting many deep undercuts and projec-
tions is offly practically useful if it is made thin like a shell, so that it is equally
pliable in all directions, that is to say, it must be nearly the same contour on the
back or wrong side as it is on the face side.

page_text: The pattern or a mould if made solid or too thick is too stiff and cannot be
removed from the casting without breaking the projections or undercuts of the
casting. If a rubber mould or pattern is made thin and of nearly uniform thick-
ness it can be drawn or rather peeled off the casting, whether it is a relief or
intaglio.
Only those who have the knowledge and skill of making elastic rubber products
can understand the difficulties and in most cases the impossibility of producing
such moulds and patterns from irregularly shaped objects.
Having made the above necessary explanation I will proceed to describe the
method of making rubber moulds and patterns. In order to give a comprehensible
description a certain object is selected from which a rubber mould is to be made.
The same description of the manipulations is likewise applicable in the production
of a rubber pattern. The difference is only that a mould is an intaglio impression
of the object, while a pattern is a relief or an exact copy of the object. For the
purpose of example a high alto relief is selected presenting in the design many
undercut parts and bold projections, and it is made of plaster of Paris as an
original model. This relief being of plaster of Paris is consequently very easily
broken or damaged. It would not bear any great pressure unless such pressure
was applied uniformly and equally at every point, high or low.
Indiarubber or caoutchouc is a very tenacious material and it is difficult to force
it into cavities especially so when applied to a relief in a thin sheet. If the
impression and shape are to be retained permanently, the rubber or caoutchouc
must be vulcanised under a continued pressure during the time required for
vulcanisation.
This essential point and several others are obtained by means of this invention
and the use of the special vulcanizing apparatus, which will now be described,
and then conclude with the description of the operation.
In the annexed drawings the apparatus is represented by two figures.
Fig. 1 is a plan or top view of the apparatus when closed,
Fig. 2 is a cross section of the same.
The same letters of reference denote the same parts.
The apparatus consists of two principal parts, a base and a cover. Each has
certain accessories which serve certain functions. In general appearance the
apparatus is like other similar vulcanizing vessels, but its details and use are
different. It is constructed of sufficient strength to resist a pressure of six
atmospheres with safety.
The construction is as follows:-
To a base A a cover B is hinged or connected by hinge lugs C, held in position
and swinging the cover on a connecting bar D. passing through the hinge lugs. The
base A is provided on its lower side with a duct or hollow space E, in communica-
tion with a pipe E2. This pipe is provided with a stop valve E³, and continues
beyond and leads say to an air tight vessel of about cubic metre capacity. The
pipe E2 is so arranged that it can be quickly detached beyond the stop valve E³.
In the pipe E a check valve is interposed between the stop valve E³ and the air
tight vessel. An air exhaust pump is connected with the air tight vessel, for the
purpose hereafter described.
From the duct or hollow space E several small apertures F lead to the upper
surface of the base A. At one corner of the base A a pipe G enters from the under
side. By this pipe G steam can be admitted into the interior of the vulcanising
chamber when it is closed. At another corner of the base A a similar pipe H
provided with a cock or stop valve I provides for the exhaust for the superfluous
steam and the water produced by a partial condensation of the steam.
At the top of the cover B is a steam pressure gauge J for the purpose of
indicating the steam pressure in the vessel when it is in operation. When the
vulcanising vessel is closed the base A and cover B are held together by means
of several bolts K, in a firm and steam tight manner.
The operation of making a mould or pattern is as follows:--To the surface of

page_text: the pattern from which a mould is to be taken a very thin coating of diluted
silicate of soda is applied, for the purpose of forming a film which prevents the
adhesion of the rubber to the pattern after vulcanization. The pattern (or the model
as the case may be) from which a rubber mould is to be obtained must be prac-
tically free from all moisture. The pattern, or mould, must be perfectly straight
and flat at the bottom so that it will rest equally on all points if laid on a straight
surface. When the pattern is so far prepared as stated the deepest and most
abrupt cavities are partially filled with small pieces of unvulcanized rubber, only
to such an extent as to remove and fill the most abrupt stops, the purpose of
which will be seen hereafter.
When this is done the pattern is placed with its flat base or surface on the flat
upper surface of the base plate A, in such a manner that it will cover several of
the small apertures or holes F leading from the duct E to the upper surface of the
base B. All the other apertures not covered by the pattern are then hermetically
plugged or closed in such a manner as to be readily reopened at will.
When this is done a sheet of unvulcanized rubber is taken of such a size and
shape that it will fully cover the pattern but with enough margin all round, not
only to cover the surface of the pattern, but also to extend over the edges and
leave a surplus to lay over on the surface of the base A, to which this overlaying
part is securely fastened by means of good rubber cement, care being taken that
all joints are hermetically closed all round the pattern. This arrangement is
clearly shown in Fig. 2 in which P denotes the pattern and R the rubber sheet.
When the rubber sheet is first laid on the pattern it lays loosely over it like a
cover. The drawing shows it when closely pressed on the pattern. The rubber
sheet is pressed in close contact with the pattern in the following manner:-The
stop valve E2 is closed. Then the air exhaust pump is operated so as to create a
vacuum in the air tight vessel of about 10 points indicated by the vacuum gauge.
The stop valve E is then opened gradually, by so doing the vacuum suction will
act through the duct E and the apertures 1 covered by the pattern P. The
exhaustion of the air in the pores and cavities of the pattern will cause the external
air to exercise a uniform pressure on the exterior surface of the rubber sheet laying
over the pattern and gradually draw or press the same snugly to the pattern.
The air pump is kept in operation so as to raise the vacuum to about 20 points
on the vacuum gauge, which will draw and hold the elastic rubber firmly in close
contact with the pattern.
The rubber sheet is rendered more pliable and yielding by pouring hot water
over the outside surface or applying a gentle jet of steam over it during the
described operation.
Should any small leakage occur during the operation of drawing or fitting the
rubber sheet to the pattern such leaks must be carefully stopped with small pieces
of plastic rubber.
When the rubber sheet is perfectly drawn down and lays in perfect contact with
the pattern the cover B is closed and firmly secured to the base A. Steam is then
gradually admitted to the interior of the vulcanizing vessel through the pipe G.
The exhaust valve H is somewhat opened to let part of the steam and water
formed by condensation escape.
The passage of the steam through the vessel creates a circulation in and heat-
ing of the apparatus. The air pump is kept in operation during the whole of the
time, until the steam pressure in the vulcanizer has reached a pressure of about one
atmosphere. Then the air exhaust pump is stopped. The pipe E2 is detached
beyond the stop valve E³ and that valve is then gradually opened.
The steam pressure acts now on the outside surface of the rubber over the pattern,
the heat softens the rubber and the steam pressure forces the plastic rubber into
every cavity of the pattern taking a perfect and sharp impression of the finest
details.
Through the aperture F leading to the duct E and pipe the valve E3 being
open, all gases or vapours created by the heat in the pattern can freely escape to

page_text: the open air, and cannot counteract the steam pressure on the rubber. The steam
pressure in the vulcanizer is now raised to about three atmospheres, and the heat
to 250° F., and is kept at that for say two hours, which will completely effect the
vulcanization of the rubber.
The vessel is then opened and a perfect rubber mould can be peeled off or
removed from the pattern. This rubber mould being too plastic or yielding is
placed in the desired shape and a rigid base of plaster is formed on its back for its
proper support while castings are taken from it.
So far the making of a rubber mould by this method and also the required
apparatus has been described. To this must be added a description of the manner
in which a plaster mould is obtained from an undercut object because if it is
wished to produce a rubber pattern which is in relief a rigid mould is required
in which a rubber pattern can be formed and vulcanized. This is done in the
following manner: Suppose one has a high relief made of metal or plaster of
Paris of which a fac-simile copy is required in elastic rubber, which is to serve
as a model from which moulds are to be taken into which metals are to be cast.
To make a copy of such an original model of india rubber a mould must first be
produced from it. This mould must be of rigid material such as plaster or metal.
For this purpose a glue mould is cast on the original model, white bone glue being
the most suitable and giving the best results.
When such a glue mould is made the surface is hardened by immersing the
mould for say, 20 minutes into a strong solution of alum in water. The mould is
then taken from this solution and dried on the surface by exposure to air for an
hour or so.
Into this glue model glue is again cast at a low temperature—just
sufficient to make the glue fluid enough to run. This glue cast when perfectly
cold can be removed from the glue mould and forms a relief in glue. Its surface
is hardened with alum as above stated. This hardening forms a surface on the
glue relief which will resist the action of the plaster of Paris on it, so that one and
sometimes two perfect plaster of Paris castings can be obtained from it. From
such a plaster of Paris casting-either in intaglio or relief-can be produced
either a rubber mould or a rubber relief. The rubber reliefs are used for forming
undercut moulds of a composition of plaster of Paris and asbestos pulp such moulds
being used for making metal castings.
Having fully described the method and apparatus for making such indiarubber
moulds and patterns, it should be stated that in this work as in many others the
operator must possess a certain amount of general and detail knowledge of working
and manipulating indiarubber and a knowledge and practical experience in making
plaster of Paris castings and must exercise a certain amount of care, preparing
and making the great varieties of moulds and patterns which can be produced. The
principle and method described is adoptable to an endless variety of forms which
cannot be fully described in a Patent Specification.
Having now particularly described and ascertained the nature of the said inven-
tion as communicated to me by my foreign correspondents and in what manner the
same is to be performed, I declare that what I claim is :-
1. In apparatus for vulcanizing rubber a base provided with a duct or hollow
space E passages from the hollow space to the upper surfaces of the base and an
outlet from the hollow space a stop valve connected thereto substantially as and for
the purposes described.
2. In combination with a vulcanizing chamber a vacuum pump or other
mechanical means for exhausting air and creating a vacuum or suction in the duct
or hollow space and in the chamber and through the object placed therein, substan-
tially as described.
3. The formation or production of an indiarubber mould or intaglio substantially
in the manner described.
4. The formation or production of an indiarubber relief or pattern substantially
in the manner and for the purposes described.

page_text: 5. The combination and arrangement of parts constituting the complete
apparatus for forming indiarubber moulds, patterns or models, substantially as
described or illustrated in the accompanying drawings.
'
1
1000
BOULT & WADE,
Agents for the Applicant.