Note: Descriptions are shown in the official language in which they were submitted.
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PRESSURE MOLDING PROCESS USING
SALT CORPS AND A COMPOSITION
FOR MAKING CORES
. . _ . .
Field of the Invention
This invention relates to pressure molding
process and particularly to a molding process utilizing
a salt core or molding core made from a mixture of
particles of inert material and relatively low melting
temperature salt wherein the core is removed from the
finished article by melting the core from the article
with the core intact therein into a molten bath of the
core material.
Prior Art and Background of the Invention
United States Patent No. 3,692,551, issued
15 September 18, 1972 to W. R. Weaver, discloses a pressure
molding process utilizing a non-detrudable molding core
-- which is made by casting a molten salt and sand mixture.
As disclosed in the patent, the use of salt mixtures having
a relatively low melting point produces a core having
extremely smooth procelain-like surface characteristics
which are highly desirable in pressure molding processes.
In addition, the use of sand as part of the core compost-
lion results in the core having high compressive strength,
a requirement for die casting and for molding articles
out of plastics.
According to the patent, the cores are made by
dip-coating or surfa~-coating solid sand cores in a molt
ten salt bath or, alternatively, by mixing sand into a
molten salt bath and casting the resulting liquid mixture
into core form.
he patent discloses top removal of the core
from the finish article by dissolving, flushing, and
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leaching out the core as a solute in water or in weak
acid solution or by mechallical vibratioll.
SUMMARY OF THE INVENTION
S In one aspect, our invention resides in the
process for making molded articles by forming a
moldable material around solidified salt cores
comprising the steps of a) preparing a homogeneous
molten salt mixture of one or more water soluble
salts having low melting temperatures, typically
sodium nitrate, potassium nitrate, and optionally
sodium nitrate; b) forming a mold core of qivell shape
from a portion of the molten salt mixture, and
cooling and solidifying the formed core; c) coating
the solidified mold core with a wet film of
hydrophobic lubricating material, and drying the film
on the core; and d) pressure molding a moldable
material such as plastic resin around the resulting
solid and coated core to cause the moldable material
to solidify and thus obtain a solid structurally
rigid molded article containincl the core. The
process preferably includes the further step of
reclaimincJ the core material, treating as necessary,
and returnitlc1 it to the mixture.
The homogeneous molten salt mixture in most
cases will comprise about forty percent to sixty-nine
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percent by weight sand. respite the presence of such
granular or crystal in material, it has been found
that the molten salt cores nevertheless exhibit a
smooth porcelain-like finish which results in an
extremely smooth and friction less surface on the
interior of the molded plastic article. This may be
of considerable importance where the article is used
as a conduit for fluid such as cooling water.
According to another aspect of the invention, a
dry, packaged premix composition for producing mold
cores for use in the aforementioned process comprises
an unfused blended dry mixture of inert material and
granulated mold core salt, said salt being of the
water soluble, low melting temperature type.
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Preferably a-t least one of the inert materials
is ox such physical character as to add -tensile strength
and legibility to -the cores. The preferred inert
materials are essentially two-dimensional in character;
i.e., in -the form of flakes, needles, or strands. Suitable
materials are mica flakes, glass fibers, carbon fibers,
plastic fibers, metal needles, and calcium silicate. Such
materials are preferably present in the bath in quantities
comprising about forty percent to about sixty percent
by weight.
According to another aspect of the invention,
it has been found preferable for reasons of economy and
otherwise to recover as much of the core material from
the molded plastic articles as is possible, and returning
such material to active use. In some cases, this is
accomplished by washing the core materials out of the
molded plastic articles, filtering the resulting soul-
tion/mixture to remove and preferably recover -the inert
particles, and thereafter boiling the water of of -the
mixture to recover the salt. The salt can then be reused
as desired.
In other arrangements, particularly those using
thermal setting resins for -the molded plastic articles,
the molded articles, complete with cores, may be placed
25 in or over the bath and heated so as to cause the core
materials to melt and drain from the articles into either
an intermediate container for treatment purposes or dir-
easily back into the core forming bath. To our surprise,
it has been found possible to actually immerse articles
made of thermoplastic resin in-to the molten core material
bath thereby to make direct recovery of the core materials.
In other cases, it is desirable to heat the article out-
side of the bath such as by microwave or radiant energy
heating to melt out the core.
According to a final aspect of our invention,
a premix of salt and inert materials in the proper qua-
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lilies is prepared and bagged for shipment to molding
sites where the premixed materials may be transferred
into a furnace or bath directly and in sufficient qua-
lilies to make up the bath of appropriate size.
Brief Description of the Drawing
The single figure is a flow chart in block
diagram form illustrating a preferred embodiment of the
pressure molding process of the present invention.
Detailed Description
Referring to the drawing, the present process
for making core-free (hollow) molded articles by form-
in a moldable material around solidified salt cores or
mold cores made of particles of inert material intimately
- mixed and covered with used salt comprising the steps
of:
A. preparing a bath by means of mixing and
heating (step 10) the particles and salt (step 12~ to
obtain a homogeneous molten bath mixture;
B. forming a mold core step 14) of given
shape from a portion of the molten bath mixture and
cooling and solidifying the formed core;
C. pressure molding a moldable material
(step 16~ around the resulting solid core to cause the
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moldable material to solidify and thus obtain a solid
structurally rigid molded article containing the core;
and
D. thereafter heating the molded article
(step 18) positioned in or over the molten bath under
temperature conditions sufficient to melt and recover
the core from the article into the bath (by gravity flow
with agitation if necessary) but insufficient to
adversely affect the article thus freed of the core; and
optionally washing (step 20) the article to remove
residual salt and/or sand.
For purposes of the invention, as indicated,
one uses sand as a core material. In a preferred
embodiment, one uses washed foundry sand that, with
respect to particle size, is sufficiently fine for
molding, preferably in the range of mesh sizes from 40
to 500, and more preferably from 80 to 1~0. Also one
uses a low melting salt mixture, which may comprise
alkali metal nitrates, nitrites, chlorides, carbonates,
sulfates and phosphates. Preferably, one uses a mixture
comprising alkali metal nitrates and/or nitrites such as
sodium and potassium nitrates and nitrites, optionally
with one or more alkali metal chlorides. Eutectic salt
mixtures are preferred. Commercially available grades
are suitable, especially granulated salt mixtures forum-
fated to melt at a specified temperature in a range of
graded temperatures. In a preferred embodiment, the
core material comprises one or more alkali~ing salts or
agents for strengthening purposes, not more than about
5% by weight, such as an alkali metal or alkaline earth
metal hydroxide or carbonate, preferably sodium or
calcium hydroxide. It is thought that the alcoholizing
agent serves to provide an advantageously strengthening
chemical bond between the inert material and the fusible
salt. The core material may be an hydrous or may contain
water in small amounts as, for example, ranging from 1
or 2% up to about 10% or more by weight. For purposes
of the invention, the term "low melting" refers to salt
mixtures selected such that, when constituted as intact
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mold cores in articles ready for melt-out to the molten
bath, they can be melted out ox the articles to recover
the core material without adversely affecting the no
suiting core-free molded articles. Preferred sand/salt
bath mixtures are those melting in the range from about
250 degrees to about 600 degrees F. or more. The inert
material and salt mixture may vary in its composition,
preferably containing about 40 to about I by weight
inert material. The mixture, as is hereinafter desk
cried in greater detail, is placed in a suitable heating vessel such as a commercial heat treating fur-
nice and brought to a molten state for casting purposes.
The mixture is maintained in a relatively homogeneous
condition by means of a mechanical agitator which may be
selected from any of various types which are commercially
available.
From the melt, material is taken as needed
and used to form cores of the desired size and con fig-
unction. In accordance with the teaching of the Weaver
Patent 3,692,551, such cores are preferably made by
casting the molten salt/sand mix using a hinged mold.
Thereafter, the cores are used to pressure mold a mold-
able material, such as a metal, or a thermoplastic or
thermosetting filled or unwilled plastic, in a compress
soon molding or injection molding machine (or a discus-
soon of suitable plastics and compression molding apt
pyrites, see Plastics, Thea Ed., Van Nostrand, New York,
1981; Modern Plastics Encyclopedia, 1982-1983, McGraw-
Hill, 1982). In some instances, to facilitate the
molding, it is desirable to use a barrier coating or
slip aid to coat the salt core prior to molding. For
this purpose, the salt core is covered with a lubrica-
tying coating which will make for easier injection mold-
in and a smoother surface and will prevent attack on
the salt core. Any ox various coating materials are
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suitable such as silicones, paraffin waxes, or heavy
oils dissolved (e.g. 3-5% by weight) in mineral spirits
which after applying in a wet film and air drying leave
a smooth hard dry hydrophobic surface. The
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coating serves not only to provide a smoother better
interior surface for the molded part but also to result
in lower core breakage. The cores may be used in a
variety of forms of metal casting and plastic molding
involving a wide range of temperature and pressure con-
Dennis. For plastic molding especially, one uses a
core that has a melt-out temperature sufficiently lower
than the critical temperature, i.e. the softening point
or degradation point, of the particular plastic being
molded so that under the melt-out conditions employed
the molded plastic article is not adversely affected.
The choice of core will vary from case to case but in
general, for a typical melt-out cycle time, e.g. 2 5
minutes, will be such what the melt-out temperature is
lower by at least about 50 degrees F. than the critical
temperature of the plastic article undergoing core no-
moral. During the pressure molding step, the core is
suitably supported within the mold so that the mold
material under compression essentially surrounds and
captures the core and thereafter becomes structurally
rigid. In accordance with one embodiment of the invent
lion, the thus finished article with the core intact
therein is immersed into, i.e., positioned within, the
core source bath at step 18 for a time sufficient to
effectively melt the core material out of the article
and thus directly make up or recover said material back
into the bath without the need for leaching or
mechanical vibration or other recovery procedure. In
another embodiment, the finished molded article with the
core intact therein is positioned, no, within the bath,
but over the bath such that when the core is melted (by
means presently to be descried), the melted core ma-
tonal is allowed Lo flow and drop by gravity into the
bath, thus recovering the core material for further
ongoing operation. In this embodiment, heating of the
article for melt-out of the core is done by radiant
energy, preferably microwave heating in the zone above
the bath. As indicated, during the molding step 16 and
prior to the melt-out step 18, the molded article be-
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comes structurally rigid (that is, sets up or is
hardened or cured or partially cured). The molten bath
(step 18), as indicated, or the mentioned microwave zone
or its equivalent melt-out zone, is kept below the
temperature which would unduly soften or melt or other-
wise adversely affect the molded article. The article
is thereafter washed or rinsed, if necessary, at step 20
to remove all traces of salt or sand therefrom A small
amount of sand/salt mix may be lost in the course of the
lo process, although this amount is far less than would be
lost or wasted if the cores were leached out or broken
out and discarded. For example/ there may be losses
both in casting the core and by drag out between the
immersion melt-out step 18 and rinse or wash step 20.
In a preferred embodiment at step 10, to insure
uniformity of core composition and to eliminate the need
for weighing and mixing separate quantities of sand and
salt, we mix dry sand and granulated salt and bulk-
package the mixture, preferably in small (e.g., 501b.~
bugs for shipment and storage. Thus, at the work site,
one or more bags or partial bags of the premix ad van-
tageously may be added directly to the bath (step 12)
for make up purposes or used to originally constitute
the bath at start-up.
The invention and best mode of practicing the
same are illustrated by the following examples.
Example 1. A dry premix is made by blending
washed foundry sand (by weight 55 parts, 80-100 mesh,
Wed Ron C-3~ sand) and granulated salt (27 parts sodium
nitrate, 18 parts potassium nitrate). The resulting dry
blended premix, packaged in shipper bags (50 lb.
capacity), is suitable for shipment and long term
storage, without appreciable particle desegregation.
The premix melting at about 430 degrees F. can be used
fox making a molten salt bath operating at a useful
working temperature of about 470-480 degrees I. or
higher. To a bath apparatus (jacketed resistance band
heated steel pot, 15 inches deep by 15 inches in
diameter) are added 100 pounds of the sand/salt premix.
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The bath is heated to melt the premix and the molten
bath mixture is maintained at a temperature of about ~80
degrees F. and stirred with a heavy duty air motor
agitator that keeps the sand uniformly suspended. For
core molding, a split mold of rectilinear shape is used.
The mold, of heavy aluminum, is of a design that can be
quickly opened and closed by means of spring loaded
quick release hinges. The mold is jacketed with
circulating pressurized hot (230 degrees F.) water. For
lo core molding, the molten bath mixture is gravity fed to
the mold by pouring from a preheated ladle into an
opening (spruce hole) in the top of the mold. The thus
filled mold is allowed to cool for 20-25 seconds and is
then opened and the molded core part removed and held
for transfer to compression molding apparatus.
Typically, the molded core parts are uniform and very
smooth. The compressive strengths at room temperature
and 330 degrees F. are greater than 2~,000 and 6,000
psi, respectively. The cores can be remelted in 2 to 5
minutes at 475-480 degrees F. A molded core part,
prepared as described and maintained at 330 degrees F.
is transferred from the core mold to the mold cavity of
an injection molding machine. Thermoses finlike resin
bead material (Resin RX6~0, Rogers, Co., Manchester,
Corn.) at a temperature of 330 degrees F. and a pressure
of 15,000 psi is then injection molded into the mold
cavity and around the molded salt core. After I
seconds, the resulting finlike resin molded article
cools in the mold sufficiently to polymerize and
solidity and become structurally rigid. The molded
plastic article with the salt core intact is then
removed and immersed for 2 to 5 minutes in the salt bath
mixture maintained at 480 degrees F. While immersed,
the article is purposely held in a position allowing for
good drainage of liquid from cavities of the article,
especially for good drainage just as the article is
being removed from the bath. During the melt-out
period, the salt core melts completely so that as the
plastic article is being removed from the salt bath
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mixture, any remaining melted salt core material quickly
drains from the article into the bath for replenishment
of the bath. Any traces of salt or sand remaining on
the molded article are washed off with water.
Example 2. A dry premix is made by blending
50% by weight of washed foundry sand (80-100 mesh,
Wed Ron C-30 sand), 48~ of granulated salt (sodium
nitrate/sodium nitrite/potassium nitrate, 30:20:50), and
2% of chopped glass fiber. The resulting dry blended
lo premix, packaged in shipper bags t50 lb. capacity), is
suitable for shipment and long term storage, without
appreciable particle desegregation. The premix melting
at about 305 degrees F. can be used for making a molten
salt bath operating at a useful working temperature of
about 350-380 degrees I or higher. To a bath apparatus
(jacketed resistance band heated steel pot, 15 inches
deep by 15 inches in diameter) are added 100 pounds of
the sand/salt premix. The bath is heated to melt the
premix and the molten bath mixture is maintained at a
temperature of about 350 degrees F. and stirred to keep
the sand uniformly suspended. For core molding, a mold
such as the mold described above is wised. The molten
bath mixture is gravity fed to the mold by pouring from
a preheated ladle into an opening (spruce hold) in the
top of the mold. The thus filled mold is allowed to
cool for 20-25 seconds and is then opened and the molded
core part removed and held for transfer to compression
molding apparatus. Typically, the molded core parts are
uniform and very smooth. The compressive strengths at
room temperature and 250 degrees F. are greater than
20,000 and ~,000 psi, respectively. The cores can be
remelted in 20-30 seconds at 350-380 degrees F. A
molded core part, prepared as described and maintained
at 250 degrees F. is transferred from the core mold to
the mold cavity of an injection molding machine. A
commercially available polyester resin bulk molding
compound (BMC) at a temperature of 2~0 degrees F. and a
pressure of 15,000 psi is then injection molded into the
mold cavity and around the molded salt core. After
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setting, the molded article polymerizes in the mold surf-
ficiently to solidify and become structurally rigid.
The article with the salt core intact is then removed
and immersed for 2 to 5 minutes in the salt bath mixture
maintained at 350 degrees F. to melt the core. While
immersed, the article is positioned to allow the core
liquid to drain from cavities of the article, and the
article is washed and dried.
Further examples which follow the foregoing
procedure according to the invention, at different
temperatures with appropriate moldable materials, are as
follows:
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