U.S. patent application number 10/917857 was filed with the patent office on 2005-03-03 for recycle methods for water based powder injection molding compounds.
Invention is credited to Behi, Mohammad, LaSalle, Jerry C., Marsh, Gary, Stevenson, James F..
Application Number | 20050046062 10/917857 |
Document ID | / |
Family ID | 24333791 |
Filed Date | 2005-03-03 |
United States Patent
Application |
20050046062 |
Kind Code |
A1 |
Stevenson, James F. ; et
al. |
March 3, 2005 |
Recycle methods for water based powder injection molding
compounds
Abstract
A process for adjusting the level of water or water soluble
additives in aqueous-based powder injection molding compounds for
the purpose of recycling scrap material, controlling shrinkage or
rehydrating dry feedstock. Depending on the objective, the process
may require material granulation equipment, equipment for the
addition or removal of water and mixing equipment. The molding
compounds may be comprised of either recycled scrap material before
being heat-treated or dry, virgin feedstock material.
Inventors: |
Stevenson, James F.;
(Morristown, NJ) ; Marsh, Gary; (Pittstown,
NJ) ; LaSalle, Jerry C.; (Montclair, NJ) ;
Behi, Mohammad; (Lake Hiawatha, NJ) |
Correspondence
Address: |
Honeywell International, Inc.
101 Columbia Road
Morristown
NJ
07962
US
|
Family ID: |
24333791 |
Appl. No.: |
10/917857 |
Filed: |
August 13, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10917857 |
Aug 13, 2004 |
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09583604 |
May 31, 2000 |
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6776954 |
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Current U.S.
Class: |
264/40.1 ;
264/140; 264/328.1 |
Current CPC
Class: |
B22F 2998/00 20130101;
B28C 7/022 20130101; B22F 2999/00 20130101; B28C 7/0409 20130101;
B22F 3/225 20130101; Y02P 10/20 20151101; B22F 3/22 20130101; B22F
2998/00 20130101; B22F 3/225 20130101; B22F 2998/00 20130101; B22F
8/00 20130101; B22F 2999/00 20130101; B22F 2203/01 20130101; B22F
2201/05 20130101 |
Class at
Publication: |
264/040.1 ;
264/140; 264/328.1 |
International
Class: |
B29C 045/76 |
Claims
1. A process for adjusting the processing characteristics and
product properties of a water-based injection molding compound,
comprising: a) providing a metal or ceramic powder-based molding
compound that is at least one of green material or dry virgin
feedstock material; b) exposing the molding compound to water; and
c) measuring at least one of water content or additive content in
the molding compound to produce an optimum balance of injection
molding process characteristics and product properties.
2. The process of claim 1, wherein the molding compound is
comprised of recycled green scrap material.
3. The process of claim 1, wherein the molding compound is
comprised of dry, virgin feedstock materials.
4. (Cancel).
5. (Cancel).
6. (Cancel).
7. (Cancel).
8. (Cancel).
9. (Cancel).
10. The process of claim 1, wherein the injection molding process
characteristic is green strength.
11. The process of claim 1, wherein the injection molding process
characteristic is ease of flow.
12. The process of claim 1, wherein the product property is
shrinkage.
13. (Cancel).
14. (Cancel).
15. (Cancel).
16. (Cancel).
17. (Cancel).
18. A molding compound, comprising the product made by the process
of claim 1.
19. (Cancel).
20. The process of claim 1 wherein the step of exposing the molding
compound to water comprises exposing the molding compound to a
water-based rehydration solution until equilibrium is reached.
21. The process of claim 20 wherein the step of exposing the
molding compound to a water-based rehydration solution comprises
immersing the molding compound in the water-based rehydration
solution at a temperature of from 4 to 60.degree. C.
22. The process of claim 21 wherein the step of exposing the
molding compound to a water-based rehydration solution comprises
immersing the molding compound in the water-based rehydration
solution at room temperature.
23. The process of claim 1 wherein the step of exposing the molding
compound to water comprises spraying the molding compound with
water.
24. The process of claim 23 wherein the step of exposing the
molding compound to water comprises spraying the molding compound
with a water-based rehydration solution.
25. The process of claim 1 further comprising granulating the
molding compound.
26. The process of claim 25 wherein the molding compound is
granulated after it is exposed to water.
27. The process of claim 25 wherein the molding compound is
granulated before it is exposed to water.
28. The process of claim 20 wherein the water-based rehydration
solution comprises at least one of water-soluble additives or
biocides.
29. The process of claim 1 wherein the molding compound is a
ceramic powder-based molding compound.
30. The process of claim 1 comprising measuring the water
content.
31. The process of claim 1 comprising measuring the additive
content.
32. The process of claim 30 wherein the measured water content of
the molding compound is 8 wt % or less.
33. A process for adjusting the processing characteristics and
product properties of a water-based ceramic powder-based molding
compound, comprising: d) providing a ceramic powder-based molding
compound that is at least one of green material or dry virgin
feedstock material; e) exposing the molding compound to water; and
f) measuring at least one of water content or additive content in
the molding compound to produce a molding compound having a water
content of 8 wt % or less.
Description
FIELD OF THE INVENTION
[0001] This invention relates to a process for adjusting the level
of water or water soluble additives in aqueous-based powder
injection molding compounds for the purpose of recycling scrap
material, controlling shrinkage or rehydrating dry feedstock.
Depending on the objective, the process may require material
granulation equipment, equipment for the addition or removal of
water and mixing equipment.
BACKGROUND OF THE INVENTION
[0002] Currently, water-based metal and ceramic injection molding
compounds containing an agar binder are supplied to customers with
a very tight tolerance on solids level of .+-.0.20 wt %. Meeting
this specification requires shipping in waterproof containers and
maintaining temperatures in a narrow range during shipping and
storage. If these compounds were manufactured in pellet form and
could be dried after compounding and shipped dry, shipping and
storage could be greatly simplified. The pellets would then need to
be rehydrated to specification levels prior to use in an injection
molding process. Rehydrating the pellets at a user's facility would
require additional equipment. However, a sophisticated user would
be able to use water level or water-soluble additives to make minor
adjustments to improve processability or to control the dimensions
of the final heat-treated or sintered part.
[0003] The same process can be used to recycle scrap parts in the
form of runners, rejected parts, start-up scrap and purge scrap
generated from an injection molding process prior to sintering,
known as "green" material. There is a strong economic incentive to
recycle this green material, as it is fairly expensive. The green
scrap material is first granulated and then rehydrated to
specification levels, as discussed above with respect to the dry
pellets. The recycle process results in an increase in flow as
evidenced by the higher spiral flow results and a decrease in green
strength. This loss of green strength, which may compromise removal
from the mold, can be reversed by the addition of a water soluble
additive such as calcium borate.
[0004] Shrinkage levels for powder injection molding materials is
very high, typically about 16%. A common problem for injection
molding manufacturers is meeting close dimensional tolerances for
sintered parts. Some of the reasons for variation in shrinkage
include (i) variation in material composition, (ii) variation in
molding or sintering conditions, and (iii) improper design of the
mold. Manufacturers generally do not want to make expensive changes
to their molds, especially if they suspect that the variations may
be transient. Also, they may not be willing or may not know how to
make the required adjustments in molding and sintering conditions
to achieve dimensional control. A better and more cost effective
way to control shrinkage is to adjust the water level of the
compound.
[0005] The present invention solves the problem of water loss in
recycled parts or dry compound pellets by rehydration of the
granulated recycled parts or pellets to the appropriate water level
required to meet specification, in order to assure the proper
material solids level. The invention also solves the problem of
shrinkage control by allowing the water level to be adjusted either
up or down to yield the desired shrinkage.
BRIEF SUMMARY OF THE INVENTION
[0006] The present invention provides a process for adjusting the
water content of a water-based injection molding compound to
specified levels in order to produce desired processing
characteristics and product properties including the steps of
providing a metal or ceramic powder-based molding compound,
providing apparatus for the addition or subtraction of water to the
compound, and measuring the water content in the compound to
produce an optimum balance of injection molding process
characteristics and product properties.
[0007] The present invention also provides a process for adjusting
the level of water-soluble additives in a water-based injection
molding compound to specified levels in order to produce desired
processing characteristics and product properties including the
steps of providing a metal or ceramic powder-based molding
compound, providing apparatus for the addition or subtraction of
water-soluble additives to the compound, and measuring the content
of water-soluble additives in the compound to produce an optimum
balance of injection molding process characteristics and product
properties.
[0008] The molding compounds may be comprised of either recycled
scrap material before being sintered or dry, virgin feedstock
material.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The invention will be more fully understood and further
advantages will become apparent when reference is made to the
following detailed description and the accompanying drawings in
which:
[0010] FIG. 1 is a schematic representation of an embodiment of the
recycling process.
[0011] FIG. 2 is a graph showing that storage of scrap parts in a
"wet" condition results in water levels that are slightly below
specification.
[0012] FIG. 3 is a graph comparing the shrinkage for golf putters
made from recycled and virgin injection molding compound
material.
[0013] FIG. 4 is a graph showing the yield strength and tensile
strength of tensile bar specimens made from recycled and virgin
injection molding compound material after sintering.
[0014] FIG. 5 are graphs comparing the elongation and reduction in
area of tensile bar specimens made from recycled and virgin
injection molding compound material after sintering.
[0015] FIG. 6 is a graph comparing the spiral flow results for
recycled and virgin injection molding compound material.
[0016] FIGS. 7A and 7B are untouched optical micrographs of 17-4PH
stainless steel alloy as a virgin material (A) containing 15%
recycled material, and (B) after sintering in a pusher furnace; and
FIGS. 7C and 7D are etched optical micrographs of 17-4PH stainless
steel alloy (C) as a virgin material, and (D) containing 15%
recycled material.
[0017] FIG. 8 is a graph showing the mechanical properties for the
specimens of Example 2.
[0018] FIG. 9 is a graph showing the shrinkage for the specimens of
Example 2.
DETAILED DESCRIPTION OF THE INVENTION
[0019] This invention provides a process for adjustment of the
water level and the concentration of water-soluble additives of
water-based injection molding compounds. The process can be used
for scrap material before sintering, for dry virgin pellets and to
adjust shrinkage of feedstock. Optimally, the first step in the
process is exposing the material to a water-based rehydration
solution until equilibrium water and additive levels are
established between the material and the surrounding water. Trace
additives, e.g. calcium metaborate (a processing aid) or biocides,
are added to the rehydrating solution to give the desired
processing characteristics and bacterial resistance to the
equilibrated composition. In the case of scrap material, excess
surface water is removed from the parts and the parts are then
granulated using conventional equipment.
[0020] In actual experiments conducted using scrap parts from an
injection molding project, moisture equilibration data taken on a
variety of parts over temperatures ranging from about 4.degree. C.
to about 60.degree. C. showed a small increase in the equilibrium
moisture level with increasing temperature from 4.degree. C. to
about 40.degree. C. and then an accelerating increase over
temperatures ranging from about 40.degree. C. to about 60.degree.
C. At temperatures higher than 80.degree. C., the agar binder
material in the parts dissolves. Based on the aforementioned data
and the experience described in the Examples, it is preferred that
the rehydration process be carried out at room temperature for
several reasons. Carrying out the process at room temperature is
easier to accomplish and is more convenient; the equilibrium
moisture level is insensitive to temperature at or about room
temperature; and finally, the equilibrated moisture level is
generally slightly below specification at room temperature. The
latter point means that partial surface drying, which is easier to
accomplish than complete surface drying, will yield parts very
close to the specified water level.
[0021] This initial rehydration to equilibrium process has several
advantages. Scrap with varying degrees of drying and thickness can
be handled because, regardless of the initial water content or
thickness, the final water concentration will be at the equilibrium
value. FIG. 2 shows thick watch bezel parts and thin strands with
varying initial water content (lower, light-colored bars) all
equilibrated to the same approximate equilibrium value (higher,
dark-colored bars), which is slightly dependent on temperature. The
equilibrated material is softer and therefore easier to granulate
and produces fewer fine particles, which are undesirable for
injection molding because smaller particles tend to aggregate.
[0022] A preferred embodiment of the invention is a process
involving optimal initial rehydration, granulation of scrap parts,
and the addition of water in a mixer This equipment is shown in
FIG. 1. The scrap parts are granulated using conventional plastics
granulating equipment, and the granules are then tumbled to mix
them thoroughly. The average water concentration in the batch is
determined by sampling the mixture appropriately and averaging the
measured water concentrations. The amount of water required to
bring the batch to the specified water concentration level is
calculated and then added to the batch by spraying over the surface
of the granules. The granules are tumbled to distribute the water.
The advantages of this process are that only one piece of
equipment, a mixer, is required and the tumbling operation
agglomerates the fine particles to form small particles, which are
better suited for injection molding. Some disadvantages are that
careful sampling is required to determine the proper amount of
water to be added; extra fine particles are produced if dry
material is granulated; and water concentration must be uniform in
the final mixture.
[0023] Another embodiment involves granulating the scrap parts,
drying the granules in an oven to remove all moisture, and then
adding sufficient water to rehydrate the material to the specified
level. This procedure removes the sampling requirements from the
previous process, but requires the use of a drying oven, the
addition of a large amount of water, and care to insure that the
water addition and final concentration are uniform.
[0024] Batches having water levels that deviate from specification
for any of the processes described above can be blended to produce
an average water level that meets specification.
[0025] A third embodiment is the rehydration to equilibrium
process. This process starts as the initial rehydration process
described above, but special care is taken to produce rehydrated
material that meets the final water specification or is
sufficiently close so that it can be blended. A continuous process
using an underwater conveyor is one method that can be used. The
scrap parts are placed in the conveyor hopper, which is filled with
rehydrating solution. After a sufficient amount of time for the
parts to equilibrate, the conveyor is started and the low cleats
move a few parts at a time up the inclined conveyor. The conveyor
is enclosed, and fans operating at a controlled speed blow air over
the parts moving up the conveyor. The moisture level of the parts
leaving the conveyor is controlled by appropriately adjusting the
conveyor speed and air velocity. The parts are fed directly to a
granulator and the granules are collected and stored in closed
containers for use as a molding compound. The process is well
suited for automated operation and is appropriate for recycle rates
of thousands of pounds per day. This type of automated continuous
process can be fine-tuned to produce feedstock material within
tight tolerances similar to the operation of a cooling conveyor on
a conventional compounding line for water-based compounds.
[0026] An alternate version of the process can be used to rehydrate
dry feedstock pellets. The dry feedstock pellets are metered onto a
small conveyor at a location where the conveyor is underwater. The
pellets are rehydrated for an appropriate amount of time
(underwater distance along conveyor/conveyor speed) and then
partially surface dried before entering the feed system of an
injection molding machine. This type of feed system would be
appropriate for a large scale operation including several injection
molding machines.
[0027] Scrap parts can also be rehydrated in a batch process
analogous to the continuous process described above. In a batch
process, parts are exposed to the rehydrating solution, for
example, by soaking in a container and then being subjected to a
drying operation. To achieve uniform drying of the parts, the
drying operation requires air flow or tumbling or both. The parts
are then granulated in conventional equipment and stored in closed
containers.
[0028] When only small amounts of material are to be reprocessed,
for example, less than 40 pounds per day, a manual method for
rehydrating the material can be advatageously employed. This
process involves parts to be recycled in containers of rehydrating
solution and allowing them to equilibrate. The parts are then
placed on a screen and partially dried by air flow or other
appropriate means. They are then granulated. After mixing the
granulated material with water, the water content is measured. If
an adjustment is required, the granulated material is further dried
in an inclined rotating container exposed to moving air, or water
is added, and the material is tumbled on a roll mill.
EXAMPLE 1
[0029] Four hundred pounds of unsintered, scrap golf putter parts
made of water-atomized 17-4 stainless steel in an injection molding
machine using the preferred granulation and rehydration process
shown in FIG. 1 were used. The scrap parts were first sprayed with
water to yield a typical water level of 7.7 wt %. Parts that were
not water treated had water levels as low as 3.5 wt %. No
toughening agent was present in the water used in this example. The
parts were granulated in a conventional Cumberland plastics
granulator with a 1/4 inch screen. Granulate was stored in
water-tight buckets, each containing about 50 lbs.
[0030] Water addition was accomplished using a 1.7 cu. ft. double
cone mixer, the Rota-Cone REC-18 made by Paul O. Abbe Co. The mixer
contains a stationary internal nozzle for adding water in the form
of a mist. The mixer was charged with approximately 160 lbs. of
granulate (60% fill factor). The mixer was rotated for at least
five minutes at a speed of 23 RPM to thoroughly mix the charge. The
charge was then sampled at least once at four locations to
determine the average water content. The mixer continued rotating
during the 10 minutes required to measure the water content using
an Arizona Instruments analyzer. The amount of water necessary to
be added to bring the water level to 8 wt % was determined from the
measured samples and expected water levels based on a mean balance
for the charged materials. The required amount of water was added
intermittently (typical cycle: 15 sec. spray on, 45 sec. spray off)
through the mist nozzle using compressed air. The mixer was rotated
for at least 5 minutes and sampled for the first time. It was then
rotated 10 minutes and sampled again. The two measurements were in
close agreement for the three mixing trials. If the measured values
were within 0.15% of the specified amount of 8 wt %, the rehydrated
water was returned to the plastic buckets for molding. Otherwise
the process was repeated starting with the water addition step. The
rehydrated, recycled 17-4 compound was mixed with virgin 17-4
compound in designated proportions including 0% recycled material
designated DC 1-00 (virgin control), 15% recycled material
designated DC 1-15, 30% recycled material designated DC1-30, 45%
recycled material designated DC 1-45 and 100% recycled material
designated DC 1-100. "DC" indicates use of the Abbe double cone
mixer; "1" indicates recycling one time; and "XX%" indicates the
percentage of recycled material. In addition to the material
prepared on the DC equipment, data are also repeated for 17-4
compounds recycled two and three times following the same process
but using a closed container on a rolling mill to accomplish the
mixing. These compounds, which were 100% recycled material, are
designated RM2-100 and RM3-100.
[0031] Percent shrinkage measurements comparing the sintered
dimensions to the mold dimensions are shown in FIG. 3 for three
thickness, one width and one length measurement on an injection
molded golf putter made from the above compositions. The predicted
values were obtained from a model for shrinkage based on
conservation of mass. They show the same trends as the measured
average values. This data shows that there is no significant change
in shrinkage with recycled content.
[0032] Mechanical property data is shown in FIGS. 4 and 5 for
tensile bars molded from the material in this Example. The tensile
and yield strength data in FIG. 4 show that the recycled material
has slightly higher properties than the virgin material, and the
properties are above the minimum values given by Standard 35,
"Materials Standards for Injection Molded Parts", 1993-94, Metal
Powder Industries Federation, Princeton, N.J.
[0033] FIG. 5 shows that elongation and reduction in area values
are comparable to the virgin control group although there is
appreciable scatter.
[0034] Flow properties of virgin and recycled material can be
evaluated from the spiral flow data in FIG. 6. Spiral flow is an
ASTM D3123-94 standardized test in which materials injected at
180.degree. F. and 500, 1000 and 1500 psi flow until they stop
flowing off in a curved channel in a mold at 72.degree. F. This
flow distance is measured and shown in FIG. 6 for various recycled
compositions and several virgin control groups. It should be noted
that the distance from the RM1-30 (30% recycled material)
composition is similar to the virgin material, but that the 100%
recycled compositions show 50% longer flow length. An increase in
flow length is expected because the additional mechanical work
during the recycle process will tend to break down the gel binder.
There is also a possibility that calcium metaborate, a
strengthening agent, may have leaked out of the compound during the
recycle process.
[0035] Photo-micrographs at 100.times. magnification of
cross-sections of golf putters are shown in FIG. 7. All parts were
sintered under identical conditions in a hydrogen environment in a
pusher (continuous) furnace. FIGS. 7A (virgin) and 7B (15% recycled
material) show essentially the same levels of porosity and pore
size distribution, which indicates no porosity differences between
virgin and recycled material. FIGS. 7C and 7D, which are etched,
show a uniform martinsitic structure in both samples, which
indicates that these virgin and recycled materials have the same
chemical and mechanical properties.
EXAMPLE 2
[0036] Scrap parts [composed of clips (37 g.), runners (920 g.) and
watch cases (42 g.)] made from Honeywell International Inc.
PowderFlo.RTM. compound based on a 17-4 stainless steel air
atomized powder were collected and soaked in a rehydration solution
containing 0.27 wt % calcium metaborate until an equilibrium
condition existed. One batch (#42) was soaked at room temperature
for 29 hours, and another batch (#45) was soaked at 45.degree. C.
for 15 hours. Equilibrium may have actually been achieved in
shorter times.
[0037] Each rehydrated batch was surface dried in a closed
cylindrical wire basket, which was rotated manually to give all
parts equal exposure to an air jet moving along the outside surface
of the basket. The drying operation was stopped when the parts were
partially surface dried. Moisture measurements in an air oven and
an automated instrument showed 7.28 and 7.31 wt % water for batch
#42. The water levels for batch #45 were 7.18 and 7.28 wt %. The
target water level was 7.4.+-.0.2 wt %. The two batches were
granulated separately in a Cumberland 3-knife 8".times.10"
granulator, and the regrind was stored in waterproof
containers.
[0038] After one day, virgin material and the two regrind batches
were injection molded on a 40-ton Arburg machine to form short
tensile bars. The moldings were performed under standard conditions
other than the need for manually assisted feeding for the regrind
materials because of nonuniform granulate size. Stock temperatures
in the molding machine were 92-93.degree. C., which are in excess
of 85.degree. C., the temperature required to melt previously
gelled material.
[0039] The quality of the recycled materials is judged by
mechanical properties (tensile strength and yield strength) and by
dimensional variations (shrinkages). These measurements are graphed
as median (midpoint) values which are indicative of the typical
values of the data without giving undue weight to outlying points,
such as early failure due to a defect. The standard deviation, also
shown on the graph, indicates the variability of the data and is
very sensitive to outliers.
[0040] Following sintering without heat treatment, ten tensile bars
in each set (batches #42 and #45) and a control (virgin) part were
tested for yield strength and tensile strength and for dimensional
shrinkage. FIG. 8 shows that median values of yield strength and
tensile strength are comparable for the virgin and recycled
materials. Variability, as measured by standard deviation, was also
comparable for the virgin and recycled materials although the
standard deviation does show considerable variation. The shrinkage
data in FIG. 9 also shows that the virgin and recycled material
have similar shrinkage profiles, but the variation was somewhat
higher for the recycled material. This result is attributed to
variability in the injection molding process due to difficulties in
the feed process.
EXAMPLE 3
[0041] A 17-4 atomized stainless steel compound with 91.3 wt %
solids (8.7 wt % water) was dried in an open rotating inclined
chamber with vanes to make two batches of compound with 92.0 and
92.5 wt % solids. These compounds were molded to form tensile bars
and sintered under the same conditions. The average shrinkages were
determined. By removing water, the average shrinkage of each
compound was reduced as shown in Table 1 below.
1TABLE 1 Initial Solids % Sintered Density % Average Shrinkage 91.3
98.8 18.7 92.0 98.7 18.0 92.5 98.5 17.0
EXAMPLE 4
[0042] Dry feedstock was prepared by drying 17-4 feedstock pellets
in an air oven set at 75.degree. C. for 16 hours to achieve a water
level of approximately 0.1 wt %. The dry feedstock was rehydrated
by two methods using two solutions of different composition.
[0043] The first rehydration method consisted of immersion of the
dry pellets in deionized water for 16 hours, removing the excess
water using a sieve, and adjusting the water level to the specified
value by either (1) drying in an open rotating inclined chamber
with internal vanes, or (2) adding a small amount of water using
the method of Example 1.
[0044] The second method consisted of alternately spraying and
mixing the dry pellets with either (1) deionized water, or (2)
deionized water containing approximately 0.02% calcium metaborate
as a strengthing agent. The pellets were allowed to equilibrate for
one hour and then mixed to give a uniform composition.
[0045] The parts were molded in a conventional injection molding
machine, and the green strength of the parts was determined by a
creep tensile test immediately following molding. The molded part
was subjected to a stress of 23 psi and allowed to elongate until
it broke. The time it took to break was recorded. The number of
short shots (incomplete mold filling because the pressure is
insufficient) was also recorded for each set of conditions. The
results are summarized in the Table 2:
2 TABLE 2 Dried Feedstock Spraying Water w/ No Drying Immersion
Calcium Control Water Only Water Only Metaborate Time to Break 45.9
43.8 38.4 223.8 Avg. (sec.) Time to Break 26.0 7.5 7.0 117.0 Std.
Dev. (sec.) Short Shots/ 2/13 0/12 0/12 12/18 Total Shots
[0046] The data in the table above shows clearly that chemical
modification of the feedstock by the addition of calcium metaborate
in the rehydrating solution increases the green strength of the
molded part, and as an unexpected but undesirable consequence,
makes mold filling more difficult.
[0047] Having thus described the invention in rather full detail,
it will be understood that various changes and modifications may
suggest themselves to one skilled in the art, all falling within
the invention as defined by the subjoined claims.
* * * * *