Title of Invention	"A METHOD AND AN APPARATUS FOR PROCESSING THIN WEB CASE BONDED COMPOSITE PROPELLANT GRAINS"
Abstract	The present invention relates to an apparatus and method for casting of composite propellant material/slurry to obtain thin web case bonded composite propellant grains having diameter in range of 200mm to 110mm with the composite propellant slurry having viscosity in the range of 12,000 poises to 30,000 poises by using pressure/bottom casing technique. Bottom casting set-up (as shown in fig.) comprises of a mass holder (3), a piston (2), a ball valve (6) at the bottom of mass holder followed by a manifold (5) with two outlets having ball valve (6) in each line. These valves are connected with flexible branded polythene hoses (7). These hoses are tightened on the nipples. Other ends of hoses are connected to the cones (8) of mould assembly fixture. Mandrels (12) are positioned at the centre of each mould (10) at the bottom and over the cone. Then paper sleeves (inhibition sleeves) (11) are inserted in the moulds which get fitted in the slot given on the bottom centering ring (9). The mandrels are then held at the top with a top centering ring (13). Finally the top is closed with a metallic cup (14) with provision on it for application of vacuum (15). For multiple grain casting manifold is provided with suitable no. of outlets. The propellant slurry is filled up in the mass holder under vacuum and then injected into the moulds under the pressure of the piston. The moulds are then pressurized at a predetermined pressure for a required time to consolidate the slurry and remove air voids. The consolidated slurry is then cured at a temperature in the range of 40°C to 70°C for 1 to 5 days. The cured grains are finally removed for the moulds as free grains and obtaining the paper insulated case bonded pyrogen igniter grains.

Title of Invention

"A METHOD AND AN APPARATUS FOR PROCESSING THIN WEB CASE BONDED COMPOSITE PROPELLANT GRAINS"

Abstract

The present invention relates to an apparatus and method for casting of composite propellant material/slurry to obtain thin web case bonded composite propellant grains having diameter in range of 200mm to 110mm with the composite propellant slurry having viscosity in the range of 12,000 poises to 30,000 poises by using pressure/bottom casing technique. Bottom casting set-up (as shown in fig.) comprises of a mass holder (3), a piston (2), a ball valve (6) at the bottom of mass holder followed by a manifold (5) with two outlets having ball valve (6) in each line. These valves are connected with flexible branded polythene hoses (7). These hoses are tightened on the nipples. Other ends of hoses are connected to the cones (8) of mould assembly fixture. Mandrels (12) are positioned at the centre of each mould (10) at the bottom and over the cone. Then paper sleeves (inhibition sleeves) (11) are inserted in the moulds which get fitted in the slot given on the bottom centering ring (9). The mandrels are then held at the top with a top centering ring (13). Finally the top is closed with a metallic cup (14) with provision on it for application of vacuum (15). For multiple grain casting manifold is provided with suitable no. of outlets. The propellant slurry is filled up in the mass holder under vacuum and then injected into the moulds under the pressure of the piston. The moulds are then pressurized at a predetermined pressure for a required time to consolidate the slurry and remove air voids. The consolidated slurry is then cured at a temperature in the range of 40°C to 70°C for 1 to 5 days. The cured grains are finally removed for the moulds as free grains and obtaining the paper insulated case bonded pyrogen igniter grains.

Full Text	'FIELD OF INVENTION This invention relates to an apparatus and method for casting of composite propellant material/slurry to obtain thin web case bonded composite propellant grains having diameter in the range of 200 mm to 110 mm with the composite propellant slurry having viscosity in the range of 12,000 poises to 30,000 poises by using pressure/bottom casting technique. DESCRIPTION OF THE PRIOR ART Composite propellants are heterogeneous mixture of inorganic oxidizer and fuel held in continuous matrix of polymeric binder. The major ingredients of composite propellant are oxidizer (generally ammonium perchlorate), metallic fuel (generally aluminium powder) and binder (presently Hydroxyl Terminated Poly Butadiene prepolymer & toluene Di-isocyanate curator) Composite solid rocket propellant grains are cast by vacuum casting method (top casting). In this method composite propellant slurry having viscosity between 5000 poises to 12,000 poises is cast by keeping rocket motor in a chamber and applying vacuum to the chamber. This method is also called as the gravity casting method. The draw backs of this method are, a low web thickness of the grain (around 10mm). and high viscous slurry (above 12000 poise at 40°c), high solid loading (above 86%) results in defects like porosity, voids, discontinuities in flow of slurry, etc, thereby reducing the yield of finished product. Another technique, under "die-casting" principle wherein slurry is injected by force in moulds have been developed (referred in "solid rocket propulsion technology" Ed. Alien Davenas. 1993. Pergamon Press Ltd.. page no. 445 to 448). In another method the viscous slurry is placed in flexible pouch, which is connected below mould and then the pouch is deformed by gas pressure and the propellant moves vertically up in the mould. There are some other methods called as spring casting, stamping and pressure casting using specially built mixer-extruder (Archimedes screw) force the slurry in mould (under vacuum) from bottom have been reported. The draw backs of these methods are that, the entire set up like valves, pistons, etc. are cured in oven along with grain needing huge effort for cleaning of moulds, valves, pistons and slurry holders. Other methods used Prior to the present invention, existing technique (vacuum casting/gravity casting technique) for casting the propellant slurry in a mould having greater diameter than required i.e. 220 mm dia/130 mm dia (increasing the web thickness to facilitate casting) was used. In this method these large diameter grains needed to be machined to the required dimensions after curing, on lathe machine with mandrel inside. This involves risk during propellant machining and is a time consuming process. 50-60% propellant material is wasted in machining to desired diameter. This method is not suitable for large scale processing. In this method the grain after decoring is inserted in the paper sleeve creating a weak interface between the propellant and the sleeve due to the presence of air layer during bonding. The drawbacks of this method are 1. Hazardous operation of machining- this also increased process time. 2. Weak interface bond. Another technique referred in Propellant & Explosives technology - course notes of the professional development short term course conducted at IIT (Madras) Chennai, India December 6 & 7,1998 page 139 and in VSSC. Where the propellant is fed through from the bottom by the application of air / inert gas pressure into the moulds. This technique find application for casting moderately viscous material but also suffers from the drawback that propellant grain contain air voids, thereby reducing the yield. The yield is only of the order of 50%. Need for present invention: There is a need of a casting of grains having diameter in the range of 200 mm to 110 mm of composite propellant having viscosity in the range of 12,000 poises to 30,000 poises using pressure/bottom casting technique especially for pyrogen igniters. The technique shell process the case bonded /free grains in production mode. OBJECTS OF THE INVENTION The main objective of this invention is to establish a composite propellant casting technique for high viscosity slurry suitable for case bonded / free grains having low web thickness in the diameter range of 200 mm and 110 mm. Yet another objective of the invention is to establish a multiple grain casting technique for production of defect free grains with higher percentage yield as compared to the conventional vacuum casting technique Yet another objective of the invention is to establish a casting technique for high solid loading (88- 90%) propellant slurry for high performance motors. Yet another objective is to process grains with complicated web geometry without propellant machining thus enhancing the plant safety. SUMMARY OF THE INVENTION Composite propellant used in this development work were processed by mixing the ingredients in vertical planetary mixer. Propellant slurry from the mixer bowl is first cast in mass holder by vacuum casting technique to de-aerate the slurry. This technique was developed for casting pyrogen igniter grains having low web thickness of 10-35 mm. For casting the pyrogen igniter grains of 200 mm/110 mm dia, paper sleeve is inserted in the mould of bottom casting assembly. For processing the case bonded high performance motor the rubber line motor casing acts as mould. The necessary port geometry is provided by positioning the mandrel inside mould/motor and centered with specially designed bottom and top rings. Specially designed cone for smooth flow slurry is attached below mould and is connected to the mass holder by means of flexible polythene hose of suitable diameter. Pressure of 4kg/cm2 to 15kg/cm2 is applied to the piston positioned over propellant surface in the mass holder. Piston is designed for proper air vent and non-sparking conditions during movements. When pressure is applied to the slurry, it flows through polythene hose and finally fills the web of mould/motor. When slurry reaches at the top, it comes out in the transparent tube provided at the top cup on the mould/motor for vacuum application. Casting is terminated by closing the valve provided in a manifold and the casting assembly is detached from the mass holder. The hose is tied to the top of the mould assembly to avoid back flow of slurry from bottom. Then entire assembly with stand and hose tied to top is put in the pit and air pressure of 2kg/cm2 -5kg/cm2 is applied in the pit for 8 - 48hrs to consolidate the slurry in mould/motor. Then after releasing the pressure, the assembly is removed from the pit and put in an oven for curing. Propellant curing is done at 40°c to 70°c for 1 to 5 days based on the formulation and web thickness. The casting assembly is designed taking in to consideration (a) the type of grain (free standing / case bonded) (b) port geometry / web thickness (c) motor designing specially the ends. Special sealing arrangement is needed to prevent slurry flow between sleeve and mould wall while casting pyrogen igniter grain of 200/110 mm dia. This unique casting process involves de-aeration of slurry during casting in mass, holder by conventional vacuum casting process. Further, the slurry is bottom cast in low web mould / motor by forcing the slurry from mass holder under piston pressure. To facilitate the slurry flow and remove air pockets, the mould / motor is maintained under high vacuum level during pressure casting. After casting the mould / motor is kept under pressure for slurry consolidation to achieve the defect free grain. The entire process is safe and production rate is high with minimum propellant wastage. After processing cleaning is also less compared to other methods. DETAILED DESCRIPTION Bottom casting set-up (as shown in fig.) comprises of a mass holder (3), a piston (2), a ball valve (6) at the bottom of mass holder followed by a manifold (5) with two outlets having ball valve (6) in each line. These valves are connected with flexible branded polythene hoses (7). These hoses are tightened on the nipples. Other ends of the hoses are connected to the cones (8) of mould assembly fixture. Mandrels (12) are positioned at the centre of the each mould (10) at the bottom and over the cone. Then paper sleeves (inhibition sleeves) (11) are inserted in the moulds which get fitted in the slot given on the bottom centering ring (9). The mandrels are then held at the top with a top centering ring (13). Finally the top is closed with a metallic cup (14) with provision on it for application of vacuum (15). For multiple grain casting manifold is provided with suitable no. of outlets. The volume of mass holder is decided based on the no. of grains to be cast in single batch. Stages of bottom casting Processing of solid propellant grains by bottom casting technique involves following stages: a) Propellant formulation - For pyrogen igniter grains, a composite propellant composition based on HTPB low aluminium content and trimodal ammonium perchlorate have been developed and used for bottom casting. Similarly for high performance motor, compositions were developed to meet the propellant burning rate of 8-25 mm/sec at 70 kg/cm2 and desired mechanical properties b) Mixing of ingredients - Propellant ingredients are mixed in vertical planetary mixer with fixed cycle time and ingredient addition sequence. The end of mix (EOM) viscosity (Brooks field) achieved is around 12000 -25000 poise at 40°c. c) Vacuum casting In mass holder - The propellant slurry is first cast in a mass holder under vacuum. Vacuum level throughout this process was kept 5-10 torr. Flow of slurry is controlled by ball valve. d) Bottom casting - After vacuum casting, vacuum is released from the mass holder. A freely sliding piston with rubber 'O' ring on the circumference is inserted in mass holder & positioned over de-aerated propellant slurry. Ball valve at the bottom of mass holder is opened. Two ball valves attached to the manifold are also opened and propellant slurry is allowed to move down in flexible hoses. Hydraulic plunger is lowered so that it presses down the piston at the rate of 1 to 5mm/ sec. Pressure of the hydraulic plunger is kept 4kg/cm2 -15kg/cm2. Flow of slurry is monitored by reduction of slurry level in mass holder and flow can be seen through flexible transparent hoses. When slurry comes to the cone level then vacuum is applied to both the moulds / motor from the top cups. Vacuum level is maintained at 2 to 10 torr. The piston pushes the propellant slurry through hoses that gets distributed in both the sleeves, in the moulds. After sometime propellant slurry can be seen coming in transparent vacuum tube (vacuum and sight tube) connected at the top of each of the moulds. This indicates that sleeves in the moulds have been filled with the propellant slurry. Then ball valve of the respective line is closed and slurry is allowed to flow in the other mould. After filling both the moulds, plunger pressure is stopped. Bait valve at the bottom of the mass holder is closed. Polythene hoses are removed from the manifold, folded and tied at the top of the respective mould so that propellant slurry does not flow back. Then the entire set up with stand are sent for pressurization. e) Pressure Consolidation/curing- Moulds with polythene hoses tied at the top are put in a pit with stand. This pit is then pressurized to 2kg/cm2 - 10kg/cm2. These moulds are kept in pressurized condition for 8 to 48hrs for consolidation of slurry. After removing from pit, these moulds are kept in Air / water oven where curing is done at 40°c to 70° c for 1 to 5 days. f) Demoulding&decoring- After curing, polythene hoses are detached from the mould assembly. The cured grains are then vertically removed from the moulds assembly as free grains or paper insulated case bonded pyrogen igniter grains. Similarly, mandrels are removed from the grains. WORKING EXAMPLES Stages of bottom casting Processing of solid propellant grains by bottom casting technique involves following stages: a) Propellant formulation - For pyrogen igniter grains, a composite propellant composition based on HTPB low aluminium content and trimodal ammonium perchlorate have been developed and used for bottom casting. Similarly for high performance motor, compositions were developed to meet the propellant burning rate of 8 mm/sec at 70 kg/cm2 and desired mechanical properties for eg. 1 and 16 mm/sec at 70 kg/cm2 and desired mechanical properties for eg. 2. b) Mixing of ingredients - Propellant ingredients are mixed in vertical planetary mixer with fixed cycle time and ingredient addition sequence. The end of mix (EOM) viscosity (Brooks field) achieved is around 14000 poise at 40°c for eg. 1 and 19000 for eg. 2.. c) Vacuum casting in mass holder - The propellant slurry is first cast in a mass holder under vacuum. Vacuum level throughout this process was kept 5-6 torr. Flow of slurry is controlled by ball valve. d) Bottom casting (eg. 1) - After vacuum casting, vacuum is released from the mass holder. A freely sliding piston with rubber 'O' ring on the circumference is inserted in mass holder & positioned over de-aerated propellant slurry. Ball valve at the bottom of mass holder is opened. Two ball valves attached to the manifold are also opened and propellant slurry is allowed to move down in flexible hoses. Hydraulic plunger is lowered so that it presses down the piston at the rate of 2 mm/sec. Pressure of the hydraulic plunger is kept 4.5 kg/cm2. Flow Of viurry is monitored by reduction of slurry level in mass holder and flow can be seen through flexible transparent hoses. When slurry comes to the cone level then vacuum is applied to both the moulds / motor from the top cups. Vacuum level is maintained at 3 to 5 torr. The piston pushes the propellant slurry through hoses that gets distributed in both the sleeves, in the moulds. After sometime (20 - 25 min) propellant slurry can be seen coming in transparent vacuum tube (vacuum and sight tube) connected at the top of each moulds. This indicates that sleeves in the moulds have been filled with the propellant slurry. Then ball valve of the respective line is closed and slurry is allowed to flow in the other mould. After filling both the moulds, plunger pressure is stopped. Ball valve at the bottom of the mass holder is closed. Polythene hoses are removed from the manifold, folded and tied at the top of the respective mould so that propeflant slurry should not flow back. Then the entire set up with stand are sent for pressurization. e) Pressure Consolidation/curing - Moulds with polythene hoses tied at the top are put in a pit with stand. This pit is then pressurized to 1.8 kg/cm2. These moulds are kept in pressurized condition for 16 hrs for consolidation of slurry. After removing from pit, these moulds are kept in Air / water oven where curing is done at 60°c for 3 days. f) Demoulding&decoring- After curing, polythene hoses are detached from the mould assembly. The cured grains are then vertically removed from the moulds assembly as free grains or paper insulated case bonded pyrogen igniter grains. Similarly, mandrels are removed from the grains. Bottom casting (eg. 2) - After vacuum casting; vacuum is released from the mass holder. A freely sliding piston with rubber 'O' ring on the circumference is inserted in mass holder & positioned over de-aerated propellant slurry. Ball valve at the bottom of mass holder is opened. Two ball valves attached to the manifold are also opened and propellant slurry is allowed to move down in flexible hoses. Hydraulic plunger is lowered so that it presses down the piston at the rate of 5 mm/sec. Pressure of the hydraulic plunger is kept 6.5 kg/cm2. Row of slurry is monitored by reduction of slurry level in mass holder and flow can be seen through flexible transparent hoses. When slurry comes to the cone level then vacuum is applied to both the moulds / motor from the top cups. Vacuum level is maintained at 6 to 8 torr. The piston pushes the propellant slurry through Hoses that gets distributed in both the sleeves, in the moulds. After sometime (20 - 25 min) propellant slurry can be seen coming in transparent vacuum tube (vacuum and sight tube) connected at the top of each moulds. This indicates that sleeves in the moulds have been filled with the propellant slurry. Then ball valve of the respective line is closed and slurry is allowed to flow in the other mould. After filling both the moulds, plunger pressure is stopped. Ball valve at the bottom of the mass holder is closed. Polythene hoses are removed from the manifold, folded and tied at the top of the respective mould so that propellant slurry should not flow back. Then the entire set up with stand are sent for pressurization. e) Pressure Consolidation/curing - Moulds with polythene hoses tied at the top are put in a pit with stand. This pit is then pressurized to 2 kg/cm2. These moulds are kept in pressurized condition for 24 hrs for consolidation of slurry. After removing from pit, these moulds are kept in Air/ water oven where curing is done at 50°c for 5 days. f) Demoulding & decoring - After curing, polythene hoses are detached from the mould assembly. The cured grains are then vertically removed from the moulds assembly as free grains or paper insulated case bonded pyrogen igniter grains. Similarly, mandrels are removed from the grains. Testing- grains are subjected to radiographic inspection and down line operations are carried out as per the requirement. One grain per batch is sent for static test for ballistic properties evaluation. Propellant from the cone is sent for mechanical properties evaluation. It is to be understood that the process of the present invention is susceptible to modifications, changes and adaptations by those skilled in the art. Such modifications, changes, adaptations are intended to be within the scope of the present invention which is further set forth under the following claims: Advantages of the present invention 1. The bottom casting technology is ideal for casting propellant grains of lower web thickness of 10mm to 25mm and diameter 200/110 mm. 2. The bottom casting technology is ideal for casting propellant grains having complicated geometry. 3. By bottom casting technology multiple grain casting is possible. 4. By bottom casting technology propellant slurry having high viscosity of 12000 poise to 30000 poise can be cast. 5. By bottom casting technology propellant slurry having high sotid loading can be cast. 6. The technology is ideal for casting small motors (upto 2 m length) with high viscosity slurry. We Claim: 1. A method for processing thin web case bonded composite propellant grains with the said propellant grains have diameter in the range of 100 to 200mm, said method comprising the steps of: a) obtaining a propellant slurry with a required end of mix (EOM) viscosity and storing it in the propellant slurry storing and injecting means (3); b) injecting the propellant slurry from the said propellant slurry storing and injecting means (3) into one or more moulds (16) with the said moulds being maintained under vacuum at a predetermined level; c) pressurizing the moulds at a predetermined pressure for a required time to consolidate the slurry and remove air voids; d) curing the consolidated slurry at a required temperature for 1 to 5 days; and e) removing the cured grains from the mould assemblies as free grains and obtaining the paper insulated case bonded pyrogen igniter grains. 2. The method as claimed in claim 1, wherein the end of mix (EOM) viscosity is around 12000 to 25000 poise. 3. The method as claimed in claim 1, wherein the propellant slurry is stored in the mass holder at a vacuum in the range of 5 to 10 torr. 4. The method as claimed in claim 1, wherein, the vacuum in the moulds is kept in the range of 2 to 10 torrs. 5. The method as claimed in claim 1, wherein the moulds are pressurized at a pressure in the range of 2Kg/cm2 to 10Kg/cm2 for a time in the range of 8 to 48 hrs. 6. The method as claimed in claim 1, wherein the curing of the consolidated slurry is done in air or water over. 7. The method as claimed in claim 1, wherein the curing is done at a temperature of 40°C to 70°C. 8. An apparatus for performing the method of processing thin web case bonded composite propellant grains, wherein the propellant grains have diameter in the range of 100 to 200 mm as claimed in claim 1, said apparatus comprising: a) a propellant slurry holding and injecting means (3) for storing and injecting a propellant slurry having a viscosity in the range of 12,000 to 30,000 poises; b) one or more moulds (16) being connected to the propellant slurry injecting means at its bottom end through a connecting means (17), and to vacuum creating means (15) at its top end for creating negative pressure inside the mould; characterized in that each of the said moulds comprise of a cylindrical tube (10) provided with a slurry distributing cone (8) at a bottom end and a metallic cup (14) at a top end, the said cylindrical tube housing a mandrel (12) between the top and the bottom ends and being further provided with a paper sleeve/inhibition sleeve (11) placed between the mandrel (12) and the walls of the said cylindrical tube extending from top to bottom end of the cylindrical tube. 9. The apparatus as claimed in claim 8, wherein the connecting means consists of a manifold (5) connected to the injecting means at one end and to one or more flexible branded plastic hoses (7) at its other end. 10. The apparatus as claimed in claim 9, wherein the manifold consists of ball valves (6) at both ends for regulating the flow of propellant slurry. 11. The apparatus as claimed in claim 8, wherein the mandrels are held at the top of the moulds with a top centering ring (13). 12. The apparatus as claimed in claim 8, wherein the paper sleeve is fixed at the bottom end of the mould assemblies in a slot given on a bottom centering ring (9). 13. The apparatus as claimed in claim 8, wherein the injecting means comprises of a freely sliding piston (2) placed inside the said injecting means over the propellant slurry with the said piston connected at a top end to a hydraulic plunger to provide pressure to press down the piston and push the propellant slurry. 14. An apparatus for processing thin web case bonded composite propellant grains substantially as herein described with reference to the accompanying drawings.

Full Text

'FIELD OF INVENTION
This invention relates to an apparatus and method for casting of composite propellant material/slurry to obtain thin web case bonded composite propellant grains having diameter in the range of 200 mm to 110 mm with the composite propellant slurry having viscosity in the range of 12,000 poises to 30,000 poises by using pressure/bottom casting technique.
DESCRIPTION OF THE PRIOR ART
Composite propellants are heterogeneous mixture of inorganic oxidizer and fuel held in continuous matrix of polymeric binder.
The major ingredients of composite propellant are oxidizer (generally ammonium perchlorate), metallic fuel (generally aluminium powder) and binder (presently Hydroxyl Terminated Poly Butadiene prepolymer & toluene Di-isocyanate curator)
Composite solid rocket propellant grains are cast by vacuum casting method (top casting). In this method composite propellant slurry having viscosity between 5000 poises to 12,000 poises is cast by keeping rocket motor in a chamber and applying vacuum to the chamber. This method is also called as the gravity casting method.
The draw backs of this method are, a low web thickness of the grain (around 10mm). and high viscous slurry (above 12000 poise at 40°c), high solid loading (above 86%) results in defects like porosity, voids, discontinuities in flow of slurry, etc, thereby reducing the yield of finished product.
Another technique, under "die-casting" principle wherein slurry is injected by force in moulds have been developed (referred in "solid rocket propulsion technology" Ed. Alien Davenas. 1993. Pergamon Press Ltd.. page no. 445 to 448).
In another method the viscous slurry is placed in flexible pouch, which is connected below mould and then the pouch is deformed by gas pressure and the propellant moves vertically up in the mould. There are some other methods called as spring casting, stamping and pressure casting using specially built mixer-extruder (Archimedes screw) force the slurry in mould (under vacuum) from bottom have been reported. The draw backs of these methods are that, the entire set up like valves, pistons, etc. are cured in oven along with grain needing huge effort for cleaning of moulds, valves, pistons and slurry holders.
Other methods used
Prior to the present invention, existing technique (vacuum casting/gravity casting
technique) for casting the propellant slurry in a mould having greater diameter
than required i.e. 220 mm dia/130 mm dia (increasing the web thickness to facilitate casting) was used.
In this method these large diameter grains needed to be machined to the required dimensions
after curing, on lathe machine with mandrel inside. This involves risk during propellant machining
and is a time consuming process. 50-60% propellant
material is wasted in machining to desired diameter. This method is not suitable for large scale processing. In this method the grain after decoring is inserted in the paper sleeve creating a weak interface between the propellant and the sleeve due to the presence of air layer during bonding.
The drawbacks of this method are
1. Hazardous operation of machining- this also increased process time.
2. Weak interface bond.
Another technique referred in Propellant & Explosives technology - course notes of the professional development short term course conducted at IIT (Madras) Chennai, India December 6 & 7,1998 page 139 and in VSSC.
Where the propellant is fed through from the bottom by the application of air / inert gas pressure into the moulds. This technique find application for casting moderately viscous material but also suffers from the drawback that propellant grain contain air voids, thereby reducing the yield. The yield is only of the order of 50%.
Need for present invention:
There is a need of a casting of grains having diameter in the range of 200 mm to 110 mm of composite propellant having viscosity in the range of 12,000 poises to 30,000 poises using pressure/bottom casting technique especially for pyrogen igniters. The technique shell process the case bonded /free grains in production mode.
OBJECTS OF THE INVENTION
The main objective of this invention is to establish a composite propellant casting technique for high
viscosity slurry suitable for case bonded / free grains having low web thickness in the diameter
range of 200 mm and 110 mm.
Yet another objective of the invention is to establish a multiple grain casting technique for production
of defect free grains with higher percentage yield as compared to the conventional vacuum casting
technique
Yet another objective of the invention is to establish a casting technique for high solid loading (88-
90%) propellant slurry for high performance motors.
Yet another objective is to process grains with complicated web geometry without propellant
machining thus enhancing the plant safety.
SUMMARY OF THE INVENTION
Composite propellant used in this development work were processed by mixing the ingredients in vertical planetary mixer.
Propellant slurry from the mixer bowl is first cast in mass holder by vacuum casting technique to de-aerate the slurry.
This technique was developed for casting pyrogen igniter grains having low web thickness of 10-35 mm.
For casting the pyrogen igniter grains of 200 mm/110 mm dia, paper sleeve is inserted in the mould of bottom casting assembly. For processing the case bonded high performance motor the rubber line motor casing acts as mould. The necessary port geometry is provided by positioning the mandrel inside mould/motor and centered with specially designed bottom and top rings. Specially designed cone for smooth flow slurry is attached below mould and is connected to the mass holder by means of flexible polythene hose of suitable diameter. Pressure of 4kg/cm2 to 15kg/cm2 is applied to the piston positioned over propellant surface in the mass holder. Piston is designed for proper air vent and non-sparking conditions during movements. When pressure is applied to the slurry, it flows through polythene hose and finally fills the web of mould/motor. When slurry reaches at the top, it
comes out in the transparent tube provided at the top cup on the mould/motor for vacuum application. Casting is terminated by closing the valve provided in a manifold and the casting assembly is detached from the mass holder. The hose is tied to the top of the mould assembly to avoid back flow of slurry from bottom. Then entire assembly with stand and hose tied to top is put in the pit and air pressure of 2kg/cm2 -5kg/cm2 is applied in the pit for 8 - 48hrs to consolidate the slurry in mould/motor. Then after releasing the pressure, the assembly is removed from the pit and put in an oven for curing. Propellant curing is done at 40°c to 70°c for 1 to 5 days based on the formulation and web thickness.
The casting assembly is designed taking in to consideration (a) the type of grain (free standing / case bonded) (b) port geometry / web thickness (c) motor designing specially the ends. Special sealing arrangement is needed to prevent slurry flow between sleeve and mould wall while casting pyrogen igniter grain of 200/110 mm dia.
This unique casting process involves de-aeration of slurry during casting in mass, holder by conventional vacuum casting process. Further, the slurry is bottom cast in low web mould / motor by forcing the slurry from mass holder under piston pressure. To facilitate the slurry flow and remove air pockets, the mould / motor is maintained under high vacuum level during pressure casting. After casting the mould / motor is kept under pressure for slurry consolidation to achieve the defect free grain. The entire process is safe and production rate is high with minimum propellant wastage. After processing cleaning is also less compared to other methods.
DETAILED DESCRIPTION
Bottom casting set-up (as shown in fig.) comprises of a mass holder (3), a piston (2), a ball valve (6) at the bottom of mass holder followed by a manifold (5) with two outlets having ball valve (6) in each line. These valves are connected with flexible branded polythene hoses (7). These hoses are tightened on the nipples. Other ends of the hoses are connected to the cones (8) of mould assembly fixture. Mandrels (12) are positioned at the centre of the each mould (10) at the bottom and over the cone. Then paper sleeves (inhibition sleeves) (11) are inserted in the moulds which get fitted in the slot given on the bottom centering ring (9). The mandrels are then held at the top with a top centering ring (13). Finally the top is closed with a metallic cup (14) with provision on it for application of vacuum (15). For multiple grain casting manifold is provided with suitable no. of outlets. The volume of mass holder is decided based on the no. of grains to be cast in single batch.
Stages of bottom casting
Processing of solid propellant grains by bottom casting technique involves following stages:
a) Propellant formulation -
For pyrogen igniter grains, a composite propellant composition based on HTPB low aluminium content and trimodal ammonium perchlorate have been developed and used for bottom casting. Similarly for high performance motor, compositions were developed to meet the propellant burning rate of 8-25 mm/sec at 70 kg/cm2 and desired mechanical properties
b) Mixing of ingredients -
Propellant ingredients are mixed in vertical planetary mixer with fixed cycle time and ingredient addition sequence. The end of mix (EOM) viscosity (Brooks field) achieved is around 12000 -25000 poise at 40°c.
c) Vacuum casting In mass holder -
The propellant slurry is first cast in a mass holder under vacuum. Vacuum level throughout this process was kept 5-10 torr. Flow of slurry is controlled by ball valve.
d) Bottom casting -
After vacuum casting, vacuum is released from the mass holder. A freely sliding piston with rubber 'O' ring on the circumference is inserted in mass holder & positioned over de-aerated propellant slurry. Ball valve at the bottom of mass holder is opened. Two ball valves attached to the manifold are also opened and propellant slurry is allowed to move down in flexible hoses. Hydraulic plunger is lowered so that it presses down the piston at the rate of 1 to 5mm/ sec. Pressure of the hydraulic plunger is kept 4kg/cm2 -15kg/cm2. Flow of slurry is monitored by reduction of slurry level in mass holder and flow can be seen through flexible transparent hoses.
When slurry comes to the cone level then vacuum is applied to both the moulds / motor from the top cups. Vacuum level is maintained at 2 to 10 torr. The piston pushes the propellant slurry through hoses that gets distributed in both the sleeves, in the moulds. After sometime propellant slurry can be seen coming in transparent vacuum tube (vacuum and sight
tube) connected at the top of each of the moulds. This indicates that sleeves in the moulds have been filled with the propellant slurry. Then ball valve of the respective line is closed and slurry is allowed to flow in the other mould.
After filling both the moulds, plunger pressure is stopped. Bait valve at the bottom of the mass holder is closed. Polythene hoses are removed from the manifold, folded and tied at the top of the respective mould so that propellant slurry does not flow back. Then the entire set up with stand are sent for pressurization.
e) Pressure Consolidation/curing-
Moulds with polythene hoses tied at the top are put in a pit with stand. This pit is then pressurized to 2kg/cm2 - 10kg/cm2. These moulds are kept in pressurized condition for 8 to 48hrs for consolidation of slurry. After removing from pit, these moulds are kept in Air / water oven where curing is done at 40°c to 70° c for 1 to 5 days.
f) Demoulding&decoring-
After curing, polythene hoses are detached from the mould assembly. The cured grains are then vertically removed from the moulds assembly as free grains or paper insulated case bonded pyrogen igniter grains. Similarly, mandrels are removed from the grains.

WORKING EXAMPLES
Stages of bottom casting
Processing of solid propellant grains by bottom casting technique involves following
stages:
a) Propellant formulation -
For pyrogen igniter grains, a composite propellant composition based on HTPB low aluminium content and trimodal ammonium perchlorate have been developed and used for bottom casting. Similarly for high performance motor, compositions were developed to meet the propellant burning rate of 8 mm/sec at 70 kg/cm2 and desired mechanical properties for eg. 1 and 16 mm/sec at 70 kg/cm2 and desired mechanical properties for eg. 2.
b) Mixing of ingredients -
Propellant ingredients are mixed in vertical planetary mixer with fixed cycle time and ingredient addition sequence. The end of mix (EOM) viscosity (Brooks field) achieved is around 14000 poise at 40°c for eg. 1 and 19000 for eg. 2..
c) Vacuum casting in mass holder -
The propellant slurry is first cast in a mass holder under vacuum. Vacuum level throughout this process was kept 5-6 torr. Flow of slurry is controlled by ball valve.
d) Bottom casting (eg. 1) -
After vacuum casting, vacuum is released from the mass holder. A freely sliding piston with rubber 'O' ring on the circumference is inserted in mass holder & positioned over de-aerated propellant slurry. Ball valve at the bottom of mass holder is opened. Two ball valves attached to the manifold are also opened and propellant slurry is allowed to move down in flexible hoses. Hydraulic plunger is lowered so that it presses down the piston at the rate of 2 mm/sec. Pressure of the hydraulic plunger is kept 4.5 kg/cm2. Flow
Of viurry is monitored by reduction of slurry level in mass holder and flow can be seen through flexible transparent hoses.
When slurry comes to the cone level then vacuum is applied to both the moulds / motor from the top cups. Vacuum level is maintained at 3 to 5 torr. The piston pushes the propellant slurry through hoses that gets distributed in both the sleeves, in the moulds.
After sometime (20 - 25 min) propellant slurry can be seen coming in transparent vacuum tube (vacuum and sight tube) connected at the top of each moulds. This indicates that sleeves in the moulds have been filled with the propellant slurry. Then ball valve of the respective line is closed and slurry is allowed to flow in the other mould.
After filling both the moulds, plunger pressure is stopped. Ball valve at the bottom of the mass holder is closed. Polythene hoses are removed from the manifold, folded and tied at the top of the respective mould so that propeflant slurry should not flow back. Then the entire set up with stand are sent for pressurization.
e) Pressure Consolidation/curing -
Moulds with polythene hoses tied at the top are put in a pit with stand. This pit is then pressurized to 1.8 kg/cm2. These moulds are kept in pressurized condition for 16 hrs for consolidation of slurry. After removing from pit, these moulds are kept in Air / water oven where curing is done at 60°c for 3 days.
f) Demoulding&decoring-
After curing, polythene hoses are detached from the mould assembly. The cured grains are then vertically removed from the moulds assembly as free grains or paper insulated case bonded pyrogen igniter grains. Similarly, mandrels are removed from the grains.
Bottom casting (eg. 2) -
After vacuum casting; vacuum is released from the mass holder. A freely sliding piston with rubber 'O' ring on the circumference is inserted in mass holder & positioned over de-aerated propellant slurry. Ball valve at the bottom of mass holder is opened. Two ball valves attached to the manifold are also opened and propellant slurry is allowed to move down in flexible hoses. Hydraulic plunger is lowered so that it presses down the piston at the rate of 5 mm/sec. Pressure of the hydraulic plunger is kept 6.5 kg/cm2. Row of slurry is monitored by reduction of slurry level in mass holder and flow can be seen through flexible transparent hoses.
When slurry comes to the cone level then vacuum is applied to both the moulds / motor from the top cups. Vacuum level is maintained at 6 to 8 torr. The piston pushes the propellant slurry through Hoses that gets distributed in both the sleeves, in the moulds.
After sometime (20 - 25 min) propellant slurry can be seen coming in transparent vacuum tube (vacuum and sight tube) connected at the top of each moulds. This indicates that sleeves in the moulds have been filled with the propellant slurry. Then ball valve of the respective line is closed and slurry is allowed to flow in the other mould.
After filling both the moulds, plunger pressure is stopped. Ball valve at the bottom of the mass holder is closed. Polythene hoses are removed from the manifold, folded and tied at the top of the respective mould so that propellant slurry should not flow back. Then the entire set up with stand are sent for pressurization.
e) Pressure Consolidation/curing -
Moulds with polythene hoses tied at the top are put in a pit with stand. This pit is then pressurized to 2 kg/cm2. These moulds are kept in pressurized condition for 24 hrs for consolidation of slurry. After removing from pit, these moulds are kept in Air/ water oven where curing is done at 50°c for 5 days.
f) Demoulding & decoring -
After curing, polythene hoses are detached from the mould assembly. The cured grains are then vertically removed from the moulds assembly as free grains or paper insulated case bonded pyrogen igniter grains. Similarly, mandrels are removed from the grains.

Testing-
grains are subjected to radiographic inspection and down line operations are carried out as per the requirement. One grain per batch is sent for static test for ballistic properties evaluation. Propellant from the cone is sent for mechanical properties evaluation.
It is to be understood that the process of the present invention is susceptible to modifications, changes and adaptations by those skilled in the art. Such modifications, changes, adaptations are intended to be within the scope of the present invention which is further set forth under the following claims:
Advantages of the present invention
1. The bottom casting technology is ideal for casting propellant grains of lower web thickness of 10mm to 25mm and diameter 200/110 mm.
2. The bottom casting technology is ideal for casting propellant grains having complicated geometry.
3. By bottom casting technology multiple grain casting is possible.
4. By bottom casting technology propellant slurry having high viscosity of 12000 poise to 30000 poise can be cast.
5. By bottom casting technology propellant slurry having high sotid loading can be cast.
6. The technology is ideal for casting small motors (upto 2 m length) with high viscosity slurry.

We Claim:
1. A method for processing thin web case bonded composite propellant grains with
the said propellant grains have diameter in the range of 100 to 200mm, said method
comprising the steps of:
a) obtaining a propellant slurry with a required end of mix (EOM) viscosity and storing it in the propellant slurry storing and injecting means (3);
b) injecting the propellant slurry from the said propellant slurry storing and injecting means (3) into one or more moulds (16) with the said moulds being maintained under vacuum at a predetermined level;
c) pressurizing the moulds at a predetermined pressure for a required time to consolidate the slurry and remove air voids;
d) curing the consolidated slurry at a required temperature for 1 to 5 days; and
e) removing the cured grains from the mould assemblies as free grains and obtaining the paper insulated case bonded pyrogen igniter grains.

2. The method as claimed in claim 1, wherein the end of mix (EOM) viscosity is around 12000 to 25000 poise.
3. The method as claimed in claim 1, wherein the propellant slurry is stored in the mass holder at a vacuum in the range of 5 to 10 torr.
4. The method as claimed in claim 1, wherein, the vacuum in the moulds is kept in the range of 2 to 10 torrs.
5. The method as claimed in claim 1, wherein the moulds are pressurized at a pressure in the range of 2Kg/cm2 to 10Kg/cm2 for a time in the range of 8 to 48 hrs.
6. The method as claimed in claim 1, wherein the curing of the consolidated slurry is done in air or water over.

7. The method as claimed in claim 1, wherein the curing is done at a temperature of 40°C to 70°C.
8. An apparatus for performing the method of processing thin web case bonded composite propellant grains, wherein the propellant grains have diameter in the range of 100 to 200 mm as claimed in claim 1, said apparatus comprising:

a) a propellant slurry holding and injecting means (3) for storing and injecting a propellant slurry having a viscosity in the range of 12,000 to 30,000 poises;
b) one or more moulds (16) being connected to the propellant slurry injecting means at its bottom end through a connecting means (17), and to vacuum creating means (15) at its top end for creating negative pressure inside the mould;
characterized in that each of the said moulds comprise of a cylindrical tube (10) provided with a slurry distributing cone (8) at a bottom end and a metallic cup (14) at a top end, the said cylindrical tube housing a mandrel (12) between the top and the bottom ends and being further provided with a paper sleeve/inhibition sleeve (11) placed between the mandrel (12) and the walls of the said cylindrical tube extending from top to bottom end of the cylindrical tube.
9. The apparatus as claimed in claim 8, wherein the connecting means consists of a manifold (5) connected to the injecting means at one end and to one or more flexible branded plastic hoses (7) at its other end.
10. The apparatus as claimed in claim 9, wherein the manifold consists of ball valves (6) at both ends for regulating the flow of propellant slurry.
11. The apparatus as claimed in claim 8, wherein the mandrels are held at the top of the moulds with a top centering ring (13).
12. The apparatus as claimed in claim 8, wherein the paper sleeve is fixed at the bottom end of the mould assemblies in a slot given on a bottom centering ring (9).

13. The apparatus as claimed in claim 8, wherein the injecting means comprises of a
freely sliding piston (2) placed inside the said injecting means over the propellant slurry
with the said piston connected at a top end to a hydraulic plunger to provide pressure to
press down the piston and push the propellant slurry.
14. An apparatus for processing thin web case bonded composite propellant grains substantially as herein described with reference to the accompanying drawings.

Documents:

http://ipindiaonline.gov.in/patentsearch/GrantedSearch/viewdoc.aspx?id=2whN9Z5nuntTqKedQe9acQ==&loc=+mN2fYxnTC4l0fUd8W4CAA==

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Patent Number

272948

Indian Patent Application Number

224/DEL/2005

PG Journal Number

19/2016

Publication Date

06-May-2016

Grant Date

04-May-2016

Date of Filing

03-Feb-2005

Name of Patentee

DIRECTOR GENERAL, DEFENCE RESEARCH & DEVELOPMENT ORGANISATION (DRDO)

Applicant Address

MINISTRY OF DEFENCE, GOVT OF INDIA, WEST BLOCK-VIII WING 1, SEC-1, RK PURAM, NEW DELHI-110066, INDIA.

Inventors:

#	Inventor's Name	Inventor's Address
1	BIKASH BHATTACHARYA	HEMRL, SUTARWADI, PUNE-411 021, INDIA.
2	SATISH KRISHNAJI PACHANKAR	HEMRL, SUTARWADI, PUNE-411 021, INDIA.
3	NAGENDRA KUMAR YADAVA	HEMRL, SUTARWADI, PUNE-411 021, INDIA.
4	AVIJIT NAG	HEMRL, SUTARWADI, PUNE-411 021, INDIA.
5	RAKESH KUMAR VERMA	HEMRL, SUTARWADI, PUNE-411 021, INDIA.
6	PRAVIN PRAKASH SINGH	HEMRL, SUTARWADI, PUNE-411 021, INDIA.
7	SURYAKANT PANDURANG MANE	HEMRL, SUTARWADI, PUNE-411 021, INDIA.
8	PRABHAKAR SHANKAR KUMBHAR	HEMRL, SUTARWADI, PUNE-411 021, INDIA.

PCT International Classification Number

G03D 5/00

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA