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Description  |
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BACKGROUND OF THE INVENTION
A. Field of the Invention
The present invention relates to the combination of an injection molding
device with a microprocessor, and a method to produce capsules of a
moldable hydrophilic polymer composition, preferably gelatin. The present
invention utilizes gelatin, preferably made from various types of gelatin
including acid or alkaline processed ossein, acid processed pigskin, or
alkaline processed cattle hide. Said types of various gelatin have a
molecular mass range of 10,000 to 2,000,000 Dalton or a molecular mass
range of 10,000 to 2,000,000 and 10,000,000 to 20,000,000 Dalton. The
method for determination of the molecular mass distribution of the various
types of gelatin used in the present invention is performed as described
in the following references:
I. Tomka, Chimia. 30, 534-540 (1976)
I. Tomka, et al, Phot. Sci. 23, 97 (1975)
Gelatin having a molecular mass range between 10,000 and 2,000,000 Dalton,
was found to give less deformation of capsule parts after ejection from a
capsule mold,
When in the following description the term "gelatin" is used, other
hydrophilic polymers whose properties were acceptable as capsule materials
are also included. Hydrophilic polymers are polymers with molecular masses
from approximately 10.sup.3 to 10.sup.7 Dalton carrying molecular groups
in their backbone and/or in their side chains and capable of forming
and/or participating in hydrogen bridges. Such hydrophilic polymers
exhibit in their water adsorption isotherm (in the temperature range
between approximately 0.degree. to 200.degree. C.) an inflection point
close to the water activity point at 0.5. Hydrophilic polymers are
distinguished from the group called hydrocolloids by their molecular
dispersity. For the maintenance of the molecular dispersity of said
hydrophilic polymers a fraction of water-according to the working range of
the present invention-of 5 to 25% by weight of said hydrophilic polymers
must be included provided that the temperature of said hydrophilic
polymers is in the working range between 50.degree. C. and 190.degree. C.
of the present invention.
There are other hydrocolloids, not hydrophilic polymers in the sense of
this definition, which contain more or less spherical or fibrous particles
whereby those particles are composed of several macromolecules of a
hydrophilic polymer within the molecular mass range of 10.sup.3 -10.sup.7
Dalton giving rise to particle sizes between 0.01-10 microns which is the
typical range of colloidal particles. It is a primary object of the
present invention to utilize hydrophilic polymers in the production of
capsules.
B. Reference to Copending Patent Applications
Concurrently with this application, please also refer to patent application
U.S. Ser. No. 362,177 filed Mar. 26, 1982 and to patent application U.S.
Ser. No. 362,430 filed Mar. 26, 1982, both of which are copending with
this application.
C. Description of the Prior Art
Capsule-making machines have been developed to utilize dip-molding
technology. Such technology involves the dipping of capsule-shaped pins
into a gelatin solution, removing the pins from the solution, drying of
the gelatin upon the pins, stripping off the gelatin capsule parts from
the pins, adjusting for length, cutting, joining and ejecting the
capsules. Prior art capsule-making machines have utilized the combination
of mechanical and pneumatic elements to perform these functions at speeds
up to about 1,200 size 0 capsules per minute. While the above described
apparatus are in general suitable for the intended purposes, it is
desirable to produce capsules at considerably higher speed, over 15,000
size 0 capsules per minute, while at the same time precisely controlling
the properties of the gelatin in order to produce the capsules
hygienically and with minimum dimensional deviations so that the capsules
can be filled on high speed equipment.
Shirai et al. in U.S. Pat. No. 4,216,240 describes an injection moldng
process to produce an oriented fibrous protein product. The fibrous
product as obtained by this process differs fundamentally from the
transparent glasslike material of the capsules obtained from the present
invention. Furthermore to obtain a flowable mass for the molding process,
the protein mixtures used by Shirai et al. have to be denatured and thus
lose their capacity to undergo dissolution.
Nakatsuka et al. in U.S. Pat. No. 4,076,846 uses binary mixtures of starch
with salts of protein materials to obtain an edible shaped article by an
injection molding process. With the present invention shaped articles from
protein materials, preferably gelatin and other hydrophilic polymers can
be produced without the addition of starch.
Heusdens et al. in U.S. Pat. No. 3,911,159 discloses the formation of
filamentous protein structures to obtain edible products of improved
tenderness. With the present invention shaped articles are produced
without a filamentous protein structure.
The use of an injection molding device for producing capsules of gelatin
and other moldable hydrophilic polymers with similar properties is new and
has not been suggested in the technical literature. A prerequisite for any
material to be moldable by an injection process is its ability to pass a
glass transition point at a temperature compatible with the thermal
stability of the material and the technical possibilities of an injection
molding device.
The present invention provides an improved injection molding method adapted
to be automatically controlled by microprocessing techniques and utilizing
molding compositions in the form of hydrophilic polymers, such as gelatin,
possessing a dissolution point within a temperature range usable for an
injection molding process, and which permit the water content to be
controlled within a predetermined range to avoid the need for additional
steps of either drying or humidifying of the molded product formed by the
injection molding device.
SUMMARY OF THE INVENTION
The present invention covers an improved automatic injection molding device
having, a hopper unit for receiving, storing, maintaining and feeding
gelatin, and having a discharge outlet; an injection unit including, a
cylindrical means having an inlet end connected to the outlet for said
hopper unit to receive gelatin therefrom, and a transport and ram member
rotatably and slidably mounted in said cylindrical means for moving and to
permit plasticizing of the gelatin therein, a molding unit connected to
the end of the cylindrical means remote from the inlet end including,
capsule part mole means, and means for ejecting molded capsule parts. A
normally closed valve means is connected between the cylindrical means and
the molding unit whereby on movement of the valve means to open position a
predetermined quantity of plasticized gelatin will be delivered to said
capsule part mold means in the molding. Also, a microprocessor having
parameters stored in a memory therein to define a plurality of time
settings is used to provide a desired work cycle for said injection unit
and said molding unit and the optimum pressure, temperature and water
content for the gelatin in the hopper unit. Sensing means sense and signal
the actual operating times of the injection unit and the molding unit and
the temperatures, pressure and water content of the gelatin in the hopper
unit. Means are connected to the sensory means and the microprocessor for
detecting the deviation between the actual conditions sensed by said
sensory means and the stored parameters in the microprocessor. This
includes means for signaling said deviation and actuating means connected
to said deviation signaling means for adjusting the operation of the
hopper, the injection unit and the molding unit to maintain optimum
operation of the automatic capsule molding device.
Additionally, the present invention also covers methods for molding capsule
parts under controlled conditions of time, temperature, pressure and water
content of gelatin comprising the steps of:
a. melting the gelatin by heating,
b. dissolving in water the molten gelatin,
c. plasticizing (plasticating) the molten gelatin by the heating along with
mechanical working thereof,
d. moldng the plasticized (plasticating) gelatin by cooling in a closed
capsule parts mold, and
e. ejecting capsule parts of cooled gelatin from the capsule parts mold.
It is therefore a primary object of the present invention to provide a new
and improved injection molding-microprocessor apparatus and a method for
molding gelatin, which alleviates one or more of the above described
disadvantages of the prior art arrangements.
It is another object of the present invention to provide a new and improved
injection molding-microprocessor apparatus and a method of molding gelatin
capsules by continuous monitoring and control of the pertinent gelatin
parameters in order to prevent degradation of the gelatin and deformation
of the capsule parts.
It is a further object of the present invention to provide an injection
molding-microprocessor apparatus and a method of molding gelatin capsules
at high speed and with precision in order to use the gelatin capsule with
high speed filling equipment.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention both as to its organization and method of operation together
with further objects and advantages thereof will best be understood by
reference to the following specifications and taken in conjunction with
the accompanying drawings.
FIG. 1 is a layout of the reciprocating screw injection molding device for
making gelatin capsule parts;
FIG. 2 is a schematic of the injection molding work cycle for making
gelatin capsule parts;
FIGS. 3 is a schematic of the combined injection device-microprocessor
apparatus for gelatin capsule parts;
FIG. 4 is an expanded schematic of the exit end of the injection molding
device;
FIG. 5 is the diagram of dependence of shear viscosity of gelatin within
the pertinent ranges of the shear rate in the present invention;
FIG. 6 is the diagram of molding area for gelatin within the ranges of
time, temperature, pressure and water content of gelatin for the present
invention;
FIG. 7 is the diagram of dependence of glass transition temperature range
and melting temperature range for the pertinent water content ranges of
the gelatin;
FIG. 8 is the diagram of dependence of differential calorimeter scan in
which the heat consumption rate of the gelatin is plotted for the
pertinent temperature range of the present invention;
FIG. 9 is a diagram of dependence of the logarithmic bulk elastic storage
module of the gelatin for the pertinent temperature range of the present
invention;
FIG. 10 is a diagram of dependence of equilibrium water content of the
gelatin in the entire water activity range; and
FIG. 11 is a diagram of dependence of differential heat of water adsorption
in the pertinent range of water content of the gelatin of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIG. 1 the injection molding device 27 generally consists
of three units: a hopper unit 5, an injection unit 1 and a molding unit 2.
The function of the hopper unit 5 is receiving, storing, maintaining and
feeding gelatin 4 at a constant temperature and at a constant water
content. The hopper unit 5 comprises a vertical cylinder 30 having a
closed top 31 with an inlet 32 therein to receive gelatin 4. At the bottom
of the vertical cylinder 30 is a closed conical funnel 33 and a discharge
outlet 34 to feed gelatin 4 into the injection unit 1. There is an air
duct 35 communicating between the closed top 31 and the conical funnel 33
wherein air is circulated by a blower 36, the air temperature is
controlled by a thyristor 37 and the air relative humidity is controlled
by a steam injector 38.
The function of the injection unit 1 is melting, dissolving in water and
plasticizing or plasticating in the extruder barrel 17 the particulate
gelatin 4 fed from the hopper unit 5 into the extruder inlet 54 and
injecting the plasticized gelatin 14 into the molding unit 2.
The function of the molding unit 2 is automatically holding, opening and
closing the mold 6 having capsule-shaped cavities 19 therein, and ejecting
the capsule parts 7 therefrom.
Within the injection unit 1 the screw 8 both rotates and undergoes axial
reciprocal motion. When the screw 8 rotates, it performs the functions of
melting, dissolving in water and plasticizing the gelatin 4. When the
screw 8 moves axially, it performs the function of injecting by
transporting and ramming the plasticized gelatin 14 into the mold 6. The
screw 8 is rotated by a variable speed hydraulic motor and drive 10, and
its axial motion is reciprocated by a duplex hydraulic cylinder 9.
Compression of the plasticized gelatin 14 in front of the rotating screw 8
forces back the screw assembly 20 containing the screw 8, the drive 10 and
the cylinder 9. When the screw assembly 20 reaches a pre-setback position
a limit switch 12 is contacted. When a defined time has elapsed during
which the gelatin 4 becomes fully plasticized gelatin 14 the hydraulic
cylinder 11 brings the screw assembly 20 forward and uses the screw 8 as a
ram for the plasticized gelatin 14 to be injected through a valve body
assembly 50 (FIG. 4) including a one-way valve 15, a needle valve 23, a
nozzle 22 and an outlet port 21 into the molding unit 2. The one-way valve
15 prevents the plasticized gelatin 14 from going back over the helical
flutes 16 of the screw 8. The extruder barrel 17 has steam heating coils
18 to heat the gelatin 4 while it is being compressed by the screw 8 into
plasticized gelatin 14. It is desirable for the plasticized gelatin 14 to
be heated at the lowest possible temperature and to be transported with
the lowest possible speed of the screw 8. The speed of the screw 8 and the
heating of the plasticized gelatin 14 within the extruder barrel 17 by the
steam heating coils 18 control the quality and the output rate of the
plasticized gelatin 14 injected into the molding unit 2. The molding unit
2 holds the mold 6 having capsule-shaped cavities 19 therein into which
the plasticized gelatin 14 is injected and maintained under pressure.
Refrigerant cooling conduits 24 encircle the mold 6 so that when the
plasticized gelatin 14 in the mold 6 has cooled and sufficiently
solidified, the molding unit 2 opens, the mold 6 separates and the capsule
parts 7 are ejected.
Referring now to FIG. 1 and also to FIG. 2 which depicts the injection
molding work cycle for gelatin 4 (containing approximately 17% water, by
weight) plotted against time. In general the work cycle of gelatin 4 is as
follows in the injection molding device 27 of the present invention:
a. particulate gelatin 4 is fed into the hopper unit 5 where it is
received, stored and maintained under controlled conditions of temperature
ranging from ambient to 100.degree. C., pressure ranging from
1-5.times.10.sup.5 Newtons per square meter (N.times.m.sup.-2) and water
content ranging from 5 to 25% by weight of gelatin,
b. the gelatin is melted and mechanically worked under controlled condition
of temperature ranging from 50.degree. to 190.degree. C., water content
ranging from 5 to 25% by weight of gelatin and pressure ranging from 600
to 3000.times.10.sup.5 N.times.m.sup.-2 to form a melt,
c. the molten gelatin melt is dissolved in water under controlled
conditions of temperature ranging from 50.degree. to 190.degree. C.,
pressures ranging from 600 to 3000.times.10.sup.5 N.times.m.sup.-2 and
water content ranging from 5 to 25% by weight of gelatin,
d. the heating and mechanical working of the gelatin melt plasticizes
(plasticates) the mixture and this is done under the controlled conditions
of temperature ranging from 50.degree. to 190.degree. C., pressure ranging
from 600 to 3000.times.10.sup.5 N.times.m.sup.-2 and water content ranging
from 5 to 25% by weight of gelatin,
e. the plasticized (plasticated) gelatin melt is injected into a
capsule-shaped part mold 6 under controlled conditions of temperature
below 50.degree. C., injection pressure ranging from 600 to
3000.times.10.sup.5 N.times.m.sup.-2 and a clamping force of the mold 6
below approximately 600,000 Newton, and
f. The capsule-shaped parts 7 are ejected from the molded gelatin within
the capsule mold 6.
Beginning at Point A of FIG. 2 the screw 8 moves forward and fills the mold
6 with plasticized gelatin 14 until Point B and maintains the injected
plasticized gelatin 14 under high pressure, during what is called the hold
time from Point B until Point C of FIG. 2. At Point A the one-way valve 15
near the end of the screw 8 prevents the plasticized gelatin 14 from
flowing back from the nozzle 22 onto the screw 8. During hold time,
additional plasticized gelatin 14 is injected, offsetting contraction due
to cooling and soldification of the plasticized gelatin 14. Later, the
outlet port 21, which is a narrow entrance to the molding unit 2 closes,
thus isolating the molding unit 2 from the injection unit 1. The
plasticized gelatin 14 within the mold 6 is still at high pressure. As the
plasticized gelatin 14 cools and solidifies, pressure drops to a level
that is high enough to ensure the absence of sinkmarks, but not so high
that it becomes difficult to remove the capsule parts 7 from the
capsule-shaped cavities 19 within the mold 6. After the outlet port 21
closes at Point C, screw 8 rotation commences. The plasticized gelatin 14
is accommodated in the increased cylindrical space in front of the screw 8
created by its backward axial motion until Point D. The flow rate of the
plasticized gelatin 14 is controlled by the speed of the screw 8 and the
pressure is controlled by the back pressure (i.e., the hydraulic pressure
exerted on the screw assembly 20) which in turn determines the pressure of
the plasticized gelatin 14 at the nozzle 22 in front of the screw 8. After
plasticized gelatin 14 generation for the next shot into the mold 6, the
screw 8 rotation ceases at Point D. The gelatin 4 on the stationary screw
8 continues to melt, Points D to E, by heat conduction from the steam
heating coils 18 on the extruder barrel 17. This period is called soak
time. Meanwhile, the solidified capsule parts 7 are ejected from the mold
6 Thereafter the mold 6 closes to accept the next shot of plasticized
gelatin 14. All of these operations are automated and controlled by a
microprocessor as hereinafter described.
Referring now to FIG. 2 and and also to FIG. 3. The injection molding work
cycle of FIG. 2 is accomplished on the injection molding device 27 of FIG.
3 by hydraulic and electrical components and the corresponding circuits
controlled by the microprocessor 28 of FIG. 3.
Through the use of solid-state circuitry and speed temperature, limit and
pressure switches for the electric and hydraulic systems, the
microprocessor 28 of the present invention utilized command signals in its
memory for the parameters of time, temperature and pressure conditions of
Table 1 below for the injection molding work cycle of FIG. 2 to be
accomplished by the injection molding device 27 of FIG. 3 in producing
gelatin capsule parts 7.
TABLE 1
__________________________________________________________________________
Ranges of Time, Temperature and Pressure for the
Injection Molding Work Cycle of FIG. 2:
POINTS
A B C D E
__________________________________________________________________________
Time 10.sup.-2 - 1
10.sup.-2 - 1
10.sup.-2 - 1
10.sup.-2 - 1
10.sup.-2 - 1
(seconds)
Temperature ambient-100
50-190
50-190
50-190
50-190
(.degree.Celsius)
Pressure 1-5 600-3000
600-3000
0-3000
600-3000
(10.sup.5 .times. N .times. m.sup.-2)
(Newtons per square meter)
__________________________________________________________________________
Referring now to FIG. 3 illustrating the combined injection molding device
27 and microprocessor 28 utilizing the method of present invention.
The combined injection molding device 27 and microprocessor 28 comprises
six control circuits of which five are closed-loop, fully analog, and one
is on-off. Starting at molding cycle Point A in FIG. 2, the injection
molding work cycle operates as follows:
When sufficient plasticized gelatin 14 has accumulated in front of the
screw 8 (microprocessor time controlled) and also when the screw assembly
20 carrying the screw 8, drive 10 and hydraulic motor 9 has been pushed
far enough backwards against a constant back-pressure as controlled by
control circuit 3, limit switch 12 will be actuated by sensing circuit
I.sub.4. Upon these two conditions control circuit 4 is actuated causing
the hydraulic fluid to flow into the forward portion of the hydraulic
cylinder 9. This rams the screw assembly 20 forward, thus injecting the
plasticized gelatin 14 into the mold 6 as molding cycle Point B of FIG. 2
is reached, and, as controlled by the microprocessor 28, the screw 8
remains stationary in this forward position under high pressure for a
certain period of time until Point C.
From molding cycle Point B of FIG. 2 onwards the plasticized gelatin 14
cools down in the mold 6 and the port 21 closes at molding cycle Point C
of FIG. 2.
At molding cycle Point C of FIG. 2 the screw 8 starts to rotate again and
the hydraulic pressure reduced from the forward portion of the hydraulic
cylinder 9 to a pressure slightly less than the pressure set for the
backward portion of the hydraulic cylinder 9.
The barrel 17 is kept under constant pressure towards the mold 6 by the
pressure in the back position of the hydraulic cylinder 11. This is
achieved by means of the control circuit 2 where a proportional hydraulic
valve is controlled by a pressure circuit sensor I.sub.2.
As the screw 8 rotates a recharge of gelatin 4 is made from the hopper 5.
During a certain time period and at a defined rotating speed of the screw
8, controlled by control circuit 3, a precise amount of gelatin 4 is fed
into the extruder barrel 17. Control circuit 3 is actuated by a speed
sensor circuit I.sub.3 measuring the rotating speed of the screw 8 and
sensing back to a hydraulic proportional flow control valve O.sub.3,
controlled by control circuit 3, thus assuring a constant rotating speed
of the hydraulic motor 10, irrespective of the changing torque resulting
from introduction of the gelatin 4 recharge.
When the load time is completed, the screw 8 rotation is stopped and
molding cycle Point D of FIG. 2 is reached. The soak time from molding
cycle Points D to A of FIG. 2 allows for the gelatin 14 to plasticize
completely under controlled temperature conditions as controlled by
control circuit 1.
A temperature sensor circuit I.sub.1 senses a thyristor heat regulator
O.sub.1 heating the extruder barrel 17 as directed by control circuit 1.
During the time interval from molding cycle Points B to E on FIG. 2, the
mold 6 has cooled down sufficiently so that the finished capsule parts 7
can be ejected from the mold 6.
After ejection of the capsule parts 7, the work cycle returns to Point A of
FIG. 2 where a certain volume of plasticized gelatin 14 has accumulated in
front of the screw 8 (sensory circuit I.sub.4 is actuated and time has
elapsed) so that the work cycle of FIG. 2 can be repeated.
It is important to note the temperature and humidity control loops 5 and 6,
for the maintenance of precise water content of the gelatin in the hopper
5, which is essential for proper operation at the desired speeds.
The microprocessor 28 includes a memory section 51 to store the desired
operating parameters; a sensing and signalling section 52 to receive the
sensing signals of actual operating conditions, to detect the deviation
between the desired and actual operating conditions, and to send signals
for adjustment through the actuating section 53 to the thyristors and
valves.
Referring now to FIG. 4 there is shown the valve body assembly 50 including
the outlet port 21, the nozzle 22, the needle valve 23, and the one-way
valve 15 These elements operate as follows:
At Point A in FIG. 2 the needle valve 23 is retracted from the outlet port
21 and the one-way valve 15 is retracted from the body of the nozzle 22 so
as to form an inlet opening 55 for plasticized gelatin 14 into the nozzle
22 which defines a charging chamber for plasticized gelatin 14. The
plasticized gelatin 14 is injected through the nozzle 22 and into the mold
6 during the mold-filling time between Points A and B in FIG. 2. At Point
C in FIG. 2 the needle valve 23 is pushed forward so as to close the
outlet port 21 during which time, between Points C and E in FIG. 2, the
mold 6 is closed and the capsule part 7 in the mold 6 is cooling. The
needle valve 23 remains closed between Point E and A in FIG. 2 during
which time the capsule part 7 is ejected from the mold 6. The total time
period between Point B and A in FIG. 2 must be less than 5 seconds so that
the plasticized gelatin 14 does not solidify in the nozzle 22. This is an
important aspect of the present invention because:
(a) faster production times are made possible in order to achieve greater
output;
(b) there is no loss of plasticized gelatin 14 in the production cycle due
to solidification in the nozzle 22 and the mold 6; and
(c) there is a minimum risk of degradation of the plasticized gelatin
because it remains in the production cycle for a short time and is only
utilized once in each production cycle because the plasticized gelatin 14
is solidified only once in the capsule-shaped cavities 19 and not in the
nozzle 22.
The one-way valve 15 and the needle valve 23 are actuated by a
spring-tensioned lever 25 which normally closes both the outlet port 21
and the nozzle 22 until the lever 25 is cam-actuated pursuant to signals
from the microprocessor 28.
The thermomechanical properties of gelatin, i.e. storage and loss shear
modules at different temperatures, are strongly dependent on its water
content. The capsule molding process of the present invention can be used
for gelatin with a water content preferably within a range of 5 to 25%.
The lower limit is defined by the maximum processing temperature of
190.degree. C., which in turn cannot be exceeded in order to avoid
degradation. The upper limit is determined by the stickiness of the
finished capsules. The abbreviations in Table 2 below will be used
hereinafter in this application.
TABLE 2
______________________________________
Abbreviations of Used Physical Parameters
ABBRE-
VIATION UNIT DESCRIPTION
______________________________________
T.sub.a,P.sub.a
Degree C., N .times. m.sup.-2
Ambient temperature
and pressure.
H(T,P) KJoule .times. Kg.sup.-1
Enthalpy of the hydro-
philic polymer-water
system at a given
pressure and
temperature.
(T,P) N.sup.-1 .times. m.sup.2
Compressibility of
the hydrophilic polymer
at a given temperature
and pressure. Its
numerical value is the
relative volume change
due to change of
pressure by a unit amount.
.alpha.(T,P)
(Degree C.).sup.-1
Volumetric thermal expan-
sion coefficient of the
hydrophilic polymer at
a given temperature and
pressure. Its numerical
value is the relative
volume change due to
change of temperature
by a unit amount.
V(q,T,P)
Kg .times. sec.sup.-1
is the flow rate of the
hydrophilic polymer at
a given temperature and
shear deformation rate
and pressure. Its
numerical value is the
volume of a melt leaving
the exit crosssectional
area of an injection
molding device in unit
time due to the applied
shear deformation rate.
T.sub.G1;
Deg C. The temperature range
T.sub.G2 (X) of the glass-transition
of the hydrophilic polymer.
T.sub.M1;
Deg C. The temperature range
I.sub.M2 (X) of the melting of the
partially crystalline
hydrophilic polymer.
T.sub.E (t)
Deg C. The temperature of the
hydrophilic polymer in
the nozzle area of the
injection unit.
T.sub.M (t)
Deg C. The temperature of the
hydrophilic polymer
in the mold.
P.sub.M N .times. m.sup.-2
The pressure of the
hydrophilic polymer
in the mold.
P.sub.E N .times. m.sup.-2
The pressure in the
nozzle area of the
hydrophilic polymer.
X The water content of the
hydrophilic polymer,
expressed as the weight
fraction of the water -
hydrophilic polymer system.
______________________________________
For the control and regulation of the injection molding process (IMP) we
need the knowledge of the
(1) heat consumption of the melting process:
H(T.sub.E,P.sub.E)-H(T.sub.a,P.sub.a)
(2) the heating rates of the hydrophilic polymers in the injection molding
device. To calculate this we need the heat conduction number of the
hydrophilic polymer and the heat transfer number of the hydrophilic
polymer and the specific material of construction of the barrel which is
in contact with the hydrophilic polymer. The heating rate and the heat
consumption of the hydrophilic polymer give the minimum time interval
necessary to make the hydrophilic polymer ready to inject and the
necessary heating power of the injection molding device.
(3) the T.sub.E depends on X of the hydrophilic polymers. If the water
content of the hydrophilic polymer in the mold is too low the resulting
T.sub.E will be too high and cause degradation. A minimum water content of
5% by weight is required to keep T.sub.E below 190.degree. C.
(4) the flow rate V(g,T,P) is as well strongly dependent on the water
content of the hydrophilic polymer. To speed up the IMP we need a high
flow rate V(g,T,P) which can be achieved by a higher water content.
The upper limit of the water content is defined by the stickiness and
mechanical failure of the capsules; a water content of 25% (0.25) by
weight cannot be generally exceeded. The range within which capsules can
be molded by the method of the present invention is therefore within 0.05
to 0.25 of water content. Better capsules are made with a water content in
the range between 0.10 and 0.20; the best capsules were made with the
water content in the range between 0.12 and 0.18.
The hydrophilic polymer in the mold will reduce its volume due to the
temperature change T.sub.M -T.sub.a. This would result in voids and
diminution of size of the capsule, which therefore would be of
unacceptable quality. It is an absolute requirement in capsule making that
the dimensional deviations are less than 1%. To compensate for shrinking
by the temperature change the mold must be filled at a distinct pressure
P.sub.M. This filling pressure is determined by the quantities
.alpha.(T,P) and (T,P). The injection pressure (P.sub.E) depends again on
T.sub.E, which as was shown already is in turn strongly dependent on X.
Referring now to FIG. 5, the shear rate dependent shear viscosity of
gelatin at 90.degree. C. is shown for gelatin with a water content X of
0.17. The capillary has a diameter of d=1.05 mm, and a length of 5.0 mm;
the ratio of length to diameter is therefore L/d=4.75.
Referring now to FIG. 6, the molding area diagram for gelatin with water
content of 0.17. During injection molding the plasticized gelatin is
discontinuously extruded and immediately cooled in a mold of the desired
shape of the capsule part. Moldability depends on the gelatin properties
and the process conditions, of which the thermomechanical properties of
the gelatin as well as the geometry and the temperature and pressure
conditions of the mold are the most important. In the molding area diagram
of FIG. 6 the limits of pressure and temperature are indicated for the
processing of gelatin in the combined injection molder-microprocessor of
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