INDIA RUBBER STAMP MAKING.
We have seen that india rubber cannot be cast in moulds. Except in
special cases deposition from solution is not available. It has to
be shaped by a combination of heat and pressure. When gently heated
it softens and can be pressed in a mould. As it cools it retains the
shape thus given and is moulded. This applies to all unvulcanized india
rubber. If mixed rubber is moulded and heated to a higher temperature
without removal from the mould the curing process is brought about and
the rubber may be not only moulded but cured and the product is moulded
vulcanized india rubber. The mixed sheet whose manufacture is described
in chapter IV. (page 42) is the starting point in rubber stamp making.
It is made for this purpose by the manufacturers.
When the material is examined it looks like ordinary white india
rubber, being firm in texture and quite strong. On heating to 280° F.
to 290° F. (137° C. to 143° C.) it begins to become “cured,” and if in
a thin sheet one to ten minutes are sufficient for the process. As the
heat is applied the india rubber first softens and becomes much like
putty. It can now be pressed through the smallest orifice and will fill
up the finest details of anything it is pressed against. It is at this
point that pressure must be applied to drive it into the interstices of
the mould.
As the heat continues it begins to lose its doughy or putty-like
consistency. This marks the reaction of the vulcanizing materials.
They gradually combine with and change the nature of the caoutchouc.
The rubber while still quite soft is elastic. If pressed by the point
of a knife it yields, but springs back to its shape when released from
pressure. The india rubber is vulcanized.
On removal from the mould it will be found to reproduce its smallest
detail. The color and appearance have not changed much, but its nature
and properties are now those of vulcanized rubber. It is unaffected by
heat or cold within ordinary ranges of temperature, and if the india
rubber is of good quality and made by a proper formula it will last for
years.
[Illustration: SIMPLE VULCANIZING PRESS FOR RUBBER STAMPS.]
The first thing to be described is the mould, which includes the
arrangements for pressing the sheet of india rubber while heated. A
small press is needed for this purpose. It may be of the simplest
description, and as an example of a home-made but perfectly efficient
one the illustration may be referred to. The base of the press is a
piece of iron, if heat is to be directly employed. Where a chamber
vulcanizer is used both base and platen may be of wood. But from every
point of view iron is the best. It lasts forever, admits of direct
heating, and does not split, warp, or char. Through two holes drilled
near its opposite sides two ordinary bolts are thrust. It is best to
use flat headed bolts, and to countersink a recess for the heads in
order to keep the bottom level. The heads may need to be filed off
so as to reduce their thickness, in order to secure this object. The
bolts may be soldered in place. One thing should be carefully watched
for--the bolts should be set true so as to rise vertically from the
plane of the base.
The platen is best made of iron, cut of the shape shown. This is an
excellent disposition of the screw-bolt slots, as by swinging the
right end of the platen back it can be taken off without removing the
nuts and lifting it over the ends of the screws. Besides the two nuts
fitting the thread of the screws it is well to have half a dozen extra
ones larger than the others, which will slip easily over the bolts,
so as to act as washers. The object of these is to adapt the press to
objects of different thickness. The thread upon an ordinary bolt does
not extend clear to the head, but by slipping on some loose nuts the
plates can be forced together if desired.
This press can be simplified. Both base and platen can be made of wood,
the platen being simply bored for the bolts, and the latter driven
tightly through the holes in the base so as to retain their place.
Even this can be improved on as regards simplicity. Two blocks of wood
screwed together by two or more long wood screws may be made to do
efficient work.
One trouble is apparent with all these devices, and that is the want of
parallelism of the opposed planes. The base and platen may be true and
parallel or they may not. Perhaps the simplest way of securing this is
the best. It consists in placing across the base two distance pieces,
which may be slips of wood. These must have perfectly parallel faces.
As the press is screwed up they will be gripped between the platen and
base and will not only ensure their parallelism but will keep them at
an exact distance apart. Such distance pieces are shown in the same
cut. Pieces of printers’ “furniture,” spaces, or “quads,” may be used
for this purpose. They should not be fastened in place if there is need
to adapt the press to more than one thickness of material and matrix.
The above described apparatus is a vulcanizing press. A further
improvement in it may be effected by the use of spring pressure. Two
strong spiral springs may be dropped over the bolts, the nuts being
screwed on above them, or a powerful spring of flat brass or steel
ribband bent into the shape of a shallow letter V may intervene between
nuts and platen, the centre of the bend bearing against the centre of
the platen.
As regards the strength of the springs there is this to be said. The
distance pieces will prevent a spring that would ordinarily be too
powerful from doing any harm. Such distance pieces should be used, as
the springs must be based upon giving a pressure of many pounds per
square inch of surface to be acted on. They should have a range of an
eighth of an inch or more. The greater the range the more evenly will
they work.
The next cut shows an excellent little screw press, that is made for
the purpose of pressing vulcanizing flasks. This is so simple that it
will suggest to the mechanical reader how he can make a single-screw
press, which is by far the most convenient to use. In the stationery
stores very small model cast iron copying presses designed for use as
paper weights are sold. They are excellent for a limited amount of
small sized work.
[Illustration: VULCANIZING FLASK CLAMP.]
A large sized gas heated press, such as made for the purpose of
manufacturing rubber stamps, is shown in the next cut, p. 53. Its
construction is obvious. It is termed by the trade a vulcanizer. Its
manipulation will be given further on.
Type are generally the object to be copied. These are best set up with
high quads and spaces. Naturally rather a large type is chosen, with
extra wide spaces between the letters. Some advise rubbing the type
faces full of hard soap, afterwards brushing off the face, leaving the
hollows filled. Sometimes wax is recommended for the same purpose.
This prevents the plaster of the matrix entering so deeply into the
cavities of the letters.
[Illustration: GAS-HEATED STAMP VULCANIZER.]
The type forming the model to be reproduced, is locked in a frame. Two
pieces of printers’ furniture or other wooden strips screwed together
by wood screws at their ends will answer for a locking frame for small
inscriptions.
The model to be copied need not be type, but any desired relief may be
used, such as an electrotype, a stereotype, an engraving or another
rubber stamp. In any case it is to be placed upon a flat surface, best
an “imposing stone” or piece of marble, with the inscription upwards.
On each side of it distance pieces reaching about one-eighth inch above
its upper surface are to be placed.
The next shaping appliance is the matrix or mould, or reverse of the
model which is to be copied. This in the case of rubber stamps is
properly called the matrix. Those who have witnessed the stereotyping
of a large daily newspaper have seen the matrices of the type made of
paper and paste, the whole mixture being termed “flong.” Such a matrix
is required for rubber type, but paper is rather too susceptible to
heat although good work can be done with it. It also does not enter
as deeply into the cavities of the type as is desirable. As a rule a
fine quality of plaster of paris is to be recommended. What is sold as
dental plaster is the best, but common plaster can be used. It is mixed
with water or with a solution of gum arabic or dextrine in water. For
the latter enough gum should be added to make the mixing solution as
thick as thin syrup.
A piece of iron, perfectly flat and true, is now to be taken, large
enough to more than cover the inscription to be copied. Upon its
surface a putty made of the plaster and the liquid used in mixing is to
be spread. This should be rather stiff. The surface of the iron should
not be too smooth as it is desirable that the plaster should adhere
well on setting. The plaster should be smoothly spread to a depth of
three-sixteenths or a quarter of an inch. It is best applied with a
palette knife or trowel, although a table knife will answer perfectly.
If its surface does not become smooth it can be made so by applying a
little of the solution with the knife or trowel.
Before this has been done the model must be oiled. Olive oil or other
clear oil is applied to all parts of the type faces, and the excess
is then wiped off and cleared out of the interstices with a piece of
blotting paper.
Next the plate with the plaster is inverted and is pressed steadily
down upon the model until it strikes the distance pieces. It is left to
set. In about ten minutes it can be raised, when it will be found to
give a beautiful impression true to the smallest detail of each letter.
It has been said that water may be used as the mixing fluid. If this
is done it is well to strengthen the mould by saturating it with an
alcoholic solution of shellac, after it has dried thoroughly, best
for a few hours in an oven. This operates to strengthen the small
projections that are liable to crumble or to break off in use.
The dealers in rubber stamp supplies sell a lever press for conducting
the operation of producing the matrix. The type is locked in a special
chase, which is carried on a bed that travels under and out from under
the platen of the press upon rollers. From each corner of the chase in
which the type model is locked, a pin rises which is encircled by a
spiral spring. A square frame of flat iron with holes at the corners
for the pins to pass through, rests upon these springs well above the
type. The pins pass through holes in its corners. The matrix plate
with its coating of plaster is placed upon this frame, which supports
it above and not touching the type. The whole is now rolled under the
press and the lever pulled to produce the impression. As the pressure
is released the frame with the matrix is raised from the type by the
action of the springs. This can be done immediately, and before the
plaster has set. It is almost impossible to raise it by hand with the
requisite steadiness. The same chase with corner pins and springs can
be used in a screw press, the one press answering for making the matrix
and for moulding and curing the stamps. The plaster matrix can also
be made by casting from a thinner mixture of plaster and water. After
the type has been set up, or the model has been selected and placed
face up and horizontal, a little ridge or projection must be made all
around it. Paper can be pasted around it, and wound with thread for
this purpose. It is oiled and wiped off as before. The plaster is now
mixed with water to the consistency of cream, and is poured upon the
model until it lies even with the projecting ledges or paper border. In
an hour or less it can be removed. If water is used the mould should
before use be treated with shellac solution as already described. The
plaster may also be mixed with gum arabic solution, or with three
to ten per cent. of powdered marshmallow root. This increases its
toughness.
What is known as the oxychloride of zinc cement appears to the author
to be far preferable to common plaster of paris. It is a trifle more
expensive, but it costs so little that it is well worth trying. It
is made by mixing oxide of zinc with a solution of zinc chloride. No
particular strength of solution or proportions are prescribed; the zinc
chloride solution should be a strong one, and the mixture should be of
about the consistency of soft putty.
Zinc chloride may be bought as a solid substance or in strong solution.
The latter answers for the mixing directly. It may also be simply
made by dissolving metallic zinc in strong hydrochloric acid. The
manipulation is exactly the same as with plaster of paris.
The manufacture of papier maché and of other matrices is given in
a special chapter. For all ordinary purposes the plaster or cement
matrices are ample.
The stamp is made from the mixed uncured sheet rubber, whose
preparation in the factory, including the operation of calendering it
into sheets, has already been described. The best advice the reader can
be given is not to attempt to make it except as a matter of interest
and experiment. It can be purchased especially prepared for stamps from
the dealers in india rubber.
A piece is cut from the sheet large enough to cover the face of the
matrix. It should have a perfectly smooth surface, without cloth
wrapper marks sometimes found impressed on it. The sheet as received
from the maker is about one-eighth of an inch thick. It is thrown into
a box of powdered soapstone or talc to secure a coating of the same on
both sides. A little is dusted over the matrix and the excess is blown
off. The matrix is now placed upon the base of the press, and heat is
applied.
To carry out the process most simply the press if of metal may be
placed upon a support over a gas burner or kerosene lamp, or even on a
kitchen range or stove. It will in a few minutes become warm. The sheet
of india rubber is now dusted off and is placed in the press upon the
matrix. The platen of the press is screwed down upon it.
As the india rubber becomes hot it begins to soften and flow. By the
action of the screw of the press it must be forced down from time to
time as it softens. This drives the putty-like material into all the
interstices of the mould. The excess escapes from the sides of the
tympan in cases where the latter is of restricted area. The press
theoretically should be heated to the vulcanizing temperature, which
is 284° F. (140° C.). In practice the heat is not determined with a
thermometer. The operator learns by experience how much heat to apply.
The regulation type of gas heated press or stamp vulcanizer is shown in
the illustration on page 53.
As some of the india rubber is sure to protrude, the progress of the
work can be watched from its action. By pressing the point of a knife
against it the period of vulcanization can be told. Before the material
is heated it is elastic and resists the pressure of the knife; as heat
is applied it becomes soft like putty; as the heat increases it again
stiffens and becomes quite elastic. At this point the press can be
opened and the sheet and matrix can be taken out or the platen swung
aside. On pulling or stripping the sheet from the matrix it will be
found to reproduce the model in elastic india rubber to the minutest
detail.
[Illustration: OIL STOVE FOR HEATING VULCANIZERS.]
As regards the minor details there is something to be said. Distance
pieces to gauge the thickness have been recommended for the home-made
press, page 48. Care must be taken to have these low enough to provide
for enough excess of material to produce a good impression. For
ordinary stamp work they should allow about one-sixteenth of an inch
for the “squeeze.” It will be seen that by using the distance or gauge
pieces both for making the matrix and for moulding and curing the
stamp, absolute parallelism of surfaces will be secured.
The reader will have noticed in the description and will find at once
in practice that the press has to be screwed up as the rubber softens.
Where heavy iron presses are used the large mass of heated iron
comprised in the platen of the press instantly heats the upper surface
of the india rubber sheet and the heat immediately penetrates into it,
while the heated matrix heats it from below. Thus it softens at once,
and the press is directly turned down and the india rubber is driven
into the mould and curing at once begins. But where small presses are
used this manipulation is not so easy. For such the springs mentioned
on page 51, are highly to be recommended. The matrix and india rubber
can be put into the cold press, and the tympan with intervening springs
can be screwed down so as to compress them. Then on applying heat the
moulding takes place automatically.
With a hot press and good sheet a period of three to ten minutes is
ample for moulding and curing.
Instead of sprinkling with talc the matrix may be oiled and sprinkled
with plumbago and afterwards polished with a brush. This is not so
clean a material as talc and is not to be recommended for general use,
especially as oil is a bad substance to bring in contact with rubber.
The distance or gauge pieces whose use has been recommended are not
necessary where presses working truly parallel as regards their
opposing faces are used. But where home-made apparatus is used they
will be found a valuable addition.
In describing the simple press it was said that it could be made of
wood. It is evident that a wooden press could not be used for direct
heating. Such a press must be used in a hot chamber or vulcanizer,
properly so called. Originally rubber stamps were generally made in
chamber vulcanizers.
The next cut shows a combined matrix making, moulding and vulcanizing
apparatus of very convenient and compact form and adapted for rapid
work. As the press stands in the cut the matrix press is seen in front.
A box or chase is carried under its platen by two trunnions, so as to
be free to oscillate to a limited extent. The type model is secured in
this box. Above this box or chase is a cross-bar with screw and platen
attached, connected at will to two standards or pillars, so as to
constitute the matrix press.
A matrix plate swings on a hinge joint between the two presses. The
hinge-pin is removable. Its ends can be seen projecting to right and
left of the press columns. The hinge is at such a height that when the
matrix plate is swung forward over the type box it will rest upon it in
a nearly horizontal position. The pivoted box will adjust itself so as
to come into parallelism with the plate.
[Illustration: MATRIX MAKING, MOULDING AND VULCANIZING APPARATUS.]
When the matrix plate is swung back it falls upon the base plate of the
vulcanizing press seen in the rear.
In use the composition used for the matrix is spread upon the matrix
plate, which may for this purpose be removed from the apparatus. It
is replaced and the hinge-pin is pushed home. This is done with the
composition coated side facing the front of the apparatus as it stands
in the cut. The plate is then swung forwards, the platen of the matrix
press being turned forward out of the way, and is pressed down upon the
type or other model that rests in the type box. If desired the press is
used to force it home. The cross-bars of both the presses are arranged
to swing each one on one of the pillars, so that the platens are turned
to one side out of the way of the matrix plate as it is swung back and
forth.
The pressure is released and the platens are turned aside. The matrix
plate is swung over to the rear upon the bed-plate of the vulcanizing
press. Here it lies with the composition-matrix upwards.
A lighted lamp, either alcohol or gas, is placed beneath the bed-plate
of the vulcanizing press on which the matrix rests. This quickly dries
it and brings it to a good curing temperature. The cross-bar and platen
may be swung over it during the heating so as to be heated at the same
time. The matrix is talced when dry and hot; the mixed sheet itself
talced, is placed upon the matrix, the platen is screwed down upon it,
and in a minute or two the moulding and curing is completed.
[Illustration: RUBBER STAMP VULCANIZER.]
A vulcanizer, properly speaking, is a vessel arranged to heat to a
definite degree any desired articles which are to be cured. The
favorite type have been the steam vulcanizers. If steam is generated
from water at a constant pressure, other things being equal a constant
temperature will be produced. By raising or lowering the pressure
the temperature can be made to rise or fall. A steam vulcanizer is a
tightly sealed vessel which contains water and which is provided with a
thermometer or a pressure gauge as well as a safety-valve, safety disc
or safety plug. By keeping the gauge at constant pressure or by keeping
the thermometer constant the temperature can be limited and kept
steady. The following table gives some pressure in pounds per square
inch with temperatures corresponding to steam of such pressures:
Lbs. per square inch. Temp. Fahr. Temp. Cent.
45.512 275° 135°
52.548 284° 140°
60.442 293° 145°
67.408 300.2° 149°
The illustration, p. 64, shows a vulcanizer of modern type made for
rubber stamp work. In some recent vulcanizers the water and steam are
excluded from the vulcanizing chamber, being contained within double
walls forming a steam jacket and maintaining a constant heat within the
chamber. These illustrate a point that has been much misapprehended,
namely that curing is independent of pressure or atmosphere. Because
vulcanizers have generally been filled with steam at high pressure
many have supposed that the steam or pressure had something to do with
their action. The fact is that it is only the heat due to the steam at
such pressure that is instrumental. Steam is a very powerful radiator
and absorber of so called radiant heat. For this reason an atmosphere
of steam maintains all parts of the vulcanizer at an even temperature
and is to that extent advantageous. Its presence and the pressure it
generates are not by any means required for vulcanizing. Its pressure
is entirely without effect.
[Illustration: STEAM JACKET VULCANIZER.]
To use a steam vulcanizer, water is introduced, the article in the
press or mould is placed in it, and the top is secured. Heat is then
applied, best if on the small scale, from a Bunsen gas burner gas, or
oil stove. Either the pressure gauge or thermometer may be watched, and
the flame turned up or down to keep it at the proper temperature.
Moulding cannot be executed in the ordinary closed chambers. The press
must first be heated to the temperature of boiling water or thereabouts
and the moulding is then effected by screwing down the mould screw,
upon the sheet and matrix. It is then placed in the vulcanizer and
cured.
The manufacturers supply gas regulators which automatically regulate
the gas supply. These are worked by the steam pressure. If any one
wishes to study the practical manipulation of small steam vulcanizers
he can see them in use at any dentist’s office.
There is no need of a steam vulcanizer for ordinary stamp work. The hot
press system already described answers every purpose and is in use by
the most advanced manufacturers for thin sheet work. But if a wooden
moulding press is used then it must be heated in a vulcanizer or some
kind of oven or hot chamber.
A very simple and reasonably satisfactory oven or air bath can be
made from a flower pot and a couple of tin plates. A plate larger
in diameter than the mouth of the flower pot forms the base of the
apparatus. This is supported on a stand over the gas lamp or other
source of heat. A smokeless flame or one depositing no lampblack should
be used. Alcohol or a kerosene oil stove illustrated on page 59 are
excellent. On this plate a smaller plate is inverted, which latter must
be so small as to be surrounded by the flower pot and to be included
within it when the pot is placed over it like an extinguisher.
[Illustration: FLOWER POT VULCANIZER ON STAND.]
A chemical or round stemmed thermometer is arranged to go through the
aperture in the upturned bottom of the pot. This may be hung from a
support or it may be secured by passing through a hole in a cork or
block of wood. Its bulb should be near the part of the chamber to be
occupied by the mould or press.
The press with the article to be cured is placed upon the inner plate.
The temperature is maintained at the proper point by regulating the
heat, and all the conditions for excellent work are supplied. The
disposition of the apparatus is shown in the cuts.
[Illustration: INTERIOR OF FLOWER POT VULCANIZER.]
Another arrangement equally simple is given in the next cut. An iron
kettle has a layer of type metal or lead poured an inch thick cast
within it upon its bottom. A thermometer passing through a hole in the
cover enters a cup of glycerine that stands upon the bottom. This gives
the temperature.
The object of having a thick or a double bottom is to prevent excessive
radiation of heat from any one part. The essential condition for good
operation is to maintain an even temperature throughout the chamber.
[Illustration: FISH KETTLE VULCANIZER.]
The thermometer is not an absolute necessity. By removing the press
from time to time and inspecting the overflow of india rubber the
progress of the operation can be watched. An extra piece of india
rubber may be placed on a piece of wood by the side of or upon the
wooden portion of the press, and its condition can be taken as the
criterion. Pressure with the point of a knife will tell the vulcanizing
point.
By the press system of curing, a heat far above the vulcanizing
temperature may be made to do good work by a very short application.
There is however danger of burning the work if left in too long. If
the air-bath with thermometer or the steam vulcanizer is used, and
the heat is kept down to the proper curing temperature, there is no
danger of burning the india rubber even if the curing is considerably
prolonged.
As the flower pot has often to be lifted off for introduction or
removal of the press, and as it gets quite hot, a holder of some kind
is requisite. A piece of heavy blotting paper is very convenient for
this purpose.
The flower pot system with thermometer can be further simplified by
being used on a stove or range. A china saucer inverted, or some
similar support, should be placed under the pot. A part of the stove at
very low heat will suffice. The kettle vulcanizer, can also be placed
on a stove so as to dispense with gas or oil.
Finally, as the last step in simplifying the work, a stamp can be made
without any special apparatus beyond a hot flat iron. The matrix may be
placed on a stove where the heat is rather low, the talc-coated mixed
rubber sheet placed upon it, and on this a hot flat iron. In a few
minutes if the heat is sufficient the stamp will be finished.
A few words may be said about the type. High spaces and quads between
the letters should be used, such as will come up to the shoulder of the
type, as has been said. But a very nice effect is produced by using
low quads between words. This leaves each word elevated by itself,
producing a good appearance.
Autograph stamps are made from a model cut in wood by a wood engraver.
The autograph is written in some form of copying ink upon a piece of
paper, and is transferred by moistening and pressure to a block of
wood. With an engraver’s tool the wood is cut away from the lines, as
the block is routed after the inscription has been “outlined.” The
woodcut is used as a model for making a matrix.
It is evident that an autograph of fair quality could be obtained from
a chalk plate. But in rubber stamp work to get good results certain
essential parts should be of the best. These parts include the mixed
rubber, model and matrix. A departure from excellence in any of these
tends to the production of an inferior stamp. What is known as a
“healthy cure” is above all essential to the appearance of the product.
The stamp thus made is attached to a wooden handle by common glue or by
one of the rubber cements given in chapter XVI.