Instantaneously Completely Combustible Cartridge Case Member Of Polyvinyl Acetal

Abstract

By first radially compression-molding a hollow cylindrical foam of a synthetic resin such as polyvinyl acetal, polyurethane and polystyrene in a mold utilizing a pressurized fluid and then by expanding the compressed foam with heat -- after moistening it -- in a heat-expansion mold, an instantaneously completely combustible cartridge case member is obtained. By the inclusion in the foam of at least one kind of reinforcement filler selected from two groups, i.e., fibers of the said resins and a cloth of said resins, prior to being compressed, the said foam can have an increased strength. By also including a binder of a synthetic resin after compression, further increased strength and hardness of the foam is obtained. The outer surface of the case member may be given an easily combustible thin film and also a damp-proof thin coating on top of the said film. This cartridge case member does not combust spontaneously during handling or during transportation since it combusts at a high temperature. This case member leaves no solid combustion residues at the time of combustion within a firearm. Also, the aforesaid cartridge case members are suitable for manufacture on a mass production basis and the cost of manufacture can be lowered greatly.

Description

BACKGROUND OF THE INVENTION

a) Field of the Invention

The present invention is concerned with a cartridge case for rounds of firearms, and more particularly, it relates to a hollow cartridge case member which will combust completely instantaneously when ignited by the propellant contained therein before the shot is discharged from the muzzle of the firearm, without leaving any solid residues of combustion. The present invention further relates to a method of manufacturing the cartridge case member of the type described.

B) Description of the Prior Art

The cartridges which are currently used are made of a metal such as brass or steel. These metallic cartridge cases can resist the high pressure produced in the cartridge chamber at the time of firing. Such a cartridge case is structured so that the highly pressurized gases produced within the cartridge chamber are prevented from leaking through the bottom of the chamber. Thus, these cartridge cases of the prior art function as they should. However, the metals with which the cartridge cases of the prior art are made are rather expensive and could be difficult to procure at any emergency time, and this is especially so with brass. Also, weapons such as tank and wheeled guns are designed so that firing is performed within a narrow space of the chamber or turret. In such an instance, a large number of hot empty metallic cartridge cases will be delivered one after another from the firearm and will scatter close to the marksman and they will hamper his otherwise normal fighting actions. In addition, a large volume of toxic gases is produced as the cartridge cases are emptied by firing, and these gases will endanger the physiological conditions of the marksmen. The foregoing facts will effect a marked lowering of the efficiency of fighting actions.

The aforesaid disadvantages and inconveniences of the metallic cartridge cases of the prior art may be solved by the provision of instantly combustible cartridge cases. However, a cartridge case which is merely combustible -- whether in whole on in a greater portion thereof -- is far from being practical for the reasons stated below. A practically useful cartridge case ought to satisfy at least the following requirements:

1) At the time of firing, it leaves no solid residues from combustion.

2) It is impact-proof in rough handling or during transportation.

3) It is moisture-proof.

4) When charged in the heated cartridge chamber of a firearm during successive firing, the cartridge case must never combust spontaneously.

5) It does not produce toxic gases at the time of firing.

6) Materials of the case must be availalbe easily and at low cost.

7) It can be manufactured easily and at lost cost.

The cartridge cases of the prior art invariably have failed to satisfy many of these requirements.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a new cartridge case member which is free of the aforesaid disadvantages and inconveniences and can satisfy substantially all of the above-listed requirements.

Another object of the present invention is to provide a new cartridge case member which satisfies substantially all of the aforesaid requirements by being constructed with an instantaneously completely combustible foam of a synthetic resin selected from the group consisting of polyvinyl acetal, polyurethane and polystyrene -- which become hard enough when molded into a cartridge case.

Still another object of the present invention is to provide a new cartridge case member of the type described, which has a highly enhanced mechanical strength and hardness by the inclusion therein of a reinforcement material which may be a reactive lower polymeric binder and/or polyvinyl acetal fibers and/or a coarse polyvinyl acetal cloth.

Yet another object of the present invention is to provide a new cartridge case member of the type described, which -- in addition to the features mentioned above -- is damp-proof due to the fact that the surface of the said case member is coated with a damp-proof material.

A further object of the present invention is to provide a simple and easy and low-cost method for producing the new cartridge case member which satisfies substantially all of the aforesaid requirements by being constructed with an instantaneously completely combustible foam of a synthetic resin of the type described -- which is hard enough when molded into a cartridge case member.

A still further object of the present invention is to provide a simple, easy and low-cost method for producing the new cartridge case member of the type described above which possesses an enhanced mechanical strength and hardness by the inclusion therein of a reinforcement material, i.e., a reactive lower polymeric binder and/or polyvinyl acetal fibers and/or a coarse polyvinyl acetal cloth.

A yet further object of the present invention is to provide a simple, easy and low-cost method for producing a new cartridge case member of the type described which -- in addition to the features mentioned above -- is moisture-proof by the fact that its surface is coated with a moisture-proof material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view, partly in section, showing an example of a cartridge case having an instantaneously completely combustible cartridge case member of the present invention.

FIG. 2 is a longitudinal sectional view of a compression-molding apparatus suitable for use in performing an excessive compression of a foamed material, representing a first step of the manufacturing method of the present invention.

FIG. 3 is a longitudinal sectional view of a heating apparatus suitable for use in performing heat-expansion of the compressed foamed material, representing a second step of the method of the present invention.

FIG. 4 is a chart showing the results of ignition point tests conducted on the cartridge case members produced acording to the method of the present invention as compared with that of the ignition point of the propellant.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention contemplates the provision of a new cartridge case member which is instantaneously completely combusted at the time of firing and which satisfies substantially all of the aforesaid requirements of a cartridge case. The present invention is also concerned with a method for producing this new cartridge case member. The present invention has been worked out based on the discovery that cartridge case members made of foams of various kinds of synthetic resins can satisfty the above-mentioned requirements to a certain degree.

Foams of synthetic resin may be divided roughly into two types, one of which has pores or cells which are sealed from each other and the other is such that the pores of cells intercommunicate with each other. Of these two types, foams having the intercommunicating pores are more desirable for use as the material of instantaneously completely combustible cartridge case member from the viewpoint of enhanced combustibility of same. However, foams having independent cells can be as equally effectively used as those of the intercommunicating type if they have a sufficiently large porosity and if the cell walls have an effectively small thickness. Foams having thin sealed cells will break easily into fine fragments upon being exposed to a high temperature and a high pressure within the cartridge chamber so that the surface area of the cells per unit weight will suddenly increase to a tremendous extent, enabling the whole of the foam to combust instantaneously.

For the reasons described above, it is possible to use foams of almost all kinds of synthetic resins and cellulose derivatives as the material of the instantly completely combustible cartridge case member of the present invention provided that they have a desirable condition of pores of cells. From the viewpoints of high mechanical strength, hardness, quick combustibility, easy moldability and easy processability which are important requirements of the material, however, the use of polyvinyl acetal, polyurethane and polystyrene has been found suitable. Of these three types of synthetic resins, polyvinyl acetal is found to be especially suitable for the aforesaid purpose. A major reason therefor may be explained as follows. Foam of polyvinyl acetal is of a structure which is characterized by the fact that fine fibrous polyvinyl acetal product which is produced from a reaction between polyvinyl alcohol and aldehyde in an aqueous solution forms a three dimensional reticular structure of molecules. Since polyvinyl acetal is thermoplastic by nature, it can be molded easily by subjecting same to compression-molding while being heated. On the other hand, polyvinyl acetal having an acetalization degree of the order of 65-86 percent tends to become pliant and swollen when heated with steam. This nature of polyvinyl acetal is utilized to give a desired configuration. This property of polyvinyl acetal to become swollen is considered to be due to the fact that the residual hydroxyl group in the molecules of polyvinyl acetal has affinity to water. Foam of polyvinyl acetal which is suitable for use as the material of cartridge case member of the present invention is of the following conditions, i.e., an average polymerization degree of 1,000-2,500, a polyvinyl alcohol to polyvinyl acetal intermolecular rearrangement ratio or acetalization degree of 65.0-86.0 percent, an apparent specific gravity of 0.06-0.12 and a porosity of 90.0-95.5 percent. Un-processed foam of polyvinyl acetal which is available as the material of the instantaneously completely combustible cartridge case member of the present invention, like foams of polyurethane and polystyrene, can have an enhanced mechanical strength and hardness by first subjecting it to a compression process and then blending therein a binder which may be either a polymer or a reactive intermediate product of a lower polymerization degree in an amount 30 percent or less, by weight, relative to the gross weight of polyvinyl acetal. The strength of this polyvinyl acetal foam can be improved further to suit for practical use by including, in the polyvinyl acetal, a reinforcement filler which is one or both of polyvinyl acetal fibers and a coarse polyvinyl acetal cloth prior to the compressing process.

Description will hereunder be made, by referring to FIG. 1, on the structure of an example of the instantaneously completely combustible cartridge case member manufactured according to the method of the present invention and having the optimum conditions.

In FIG. 1, reference numeral 1 represents generally a hollow cartridge case. Numeral 2 represents an instantaneously completely combustible cartridge case-constituting hollow member. This cartridge case member 2 is made of polyvinyl acetal foam. In order to enhance the mechanical strength of this member 2, the latter contains uniformly throughout therein polyvinyl acetal fibers 3, and in addition, the case member 2 has a coarse polyvinyl acetal cloth 4 which is embedded in the member 2 at a position close to the inner surface thereof in such a way that the surface of the cloth is substantially exposed to the external atmosphere and visible externally. Numeral 5 represents a bottom member made of a metal. Numeral 6 represents a hole formed through the bottom portion of said bottom member 5 for the attachment of a fuse. Numeral 7 represents a shoulder portion. Numeral 8 represents a shot or shell. Numeral 10 represents a layer of bonding agent with which the lower portion 9 of the said case member 2 is securely bonded to the upper portion 9a of the said base 5.

The instantaneously completely combustible hollow cartridge case member 2 having the aforesaid structure is made with a polyvinyl acetal foam having numerous very fine intercommunicating pores throughout the entirety thereof, and accordingly, the case member 2 is of a very large surface area. It should be understood that the length of time consumed from the time the propellant contained in the cartridge case starts to combust till the time the shot is discharged from the muzzle of a firearm is of the order of several milli-seconds. Therefore, at the time of firsing, the case member 2 made with the aforesaid materials and having the above-mentioned structure will be exposed -- during such a momentary short period of time -- to a very high temperature and a very high pressure produced from the combustion of the propellant, and will be combusted completely instantaneously, neither leaving any solid residues nor producing an objectionably large amount of toxid gases at the time of firing. Moreover, this case member 2 will not combust spontaneously when a round having a cartridge case made with this case member 2 is charged in the heated cartridge chamber of a firearm during successive firing. Furthermore, as will be discussed later, this case member 2 is of a mechanical strength necessary and sufficient for a satisfactory cartridge case member.

It should be understood that it is mainly from rough handling that cartridge cases sustain damages. Such damages caused during handling are not only due to the intensity of tension, compression or impact applied to the cartridge cases, but also -- and quite often actually -- due to dropping of rounds, in which instance the cartrdige cases are subjected to combined forces of tension, compression and impact. In the example of round shown in FIG. 1, the cartridge case member 2 is made of a basic material which is polyvinyl acetal foam. Accordingly, the case member 2 is of elasticity of an appropriate degree. Moreover, this case member 2 contains polyvinyl acetal fibers 3 distributed uniformly throughout its porous structure and also it has a coarse polyvinyl acetal cloth 4 embedded in the entire surface region of the polyvinyl acetal foam, and in addition, the shoulder portion of this case member 2 is of a greater thickness than the remaining portions. Therefore, the cartridge case member 2 is especially of an increased strength against impact. Cartridge case members made according to the present invention and having the aforesaid structure were subjected to a strength test, the result thereof is as shown in Table 1.

TABLE 1

Cartridge Tensile Anit-impact member strength strength structure (kg/cm.sup.2) (kg.cm/cm.sup.2) A approx. 10 approx. 3.0 A + B approx. 15 approx. 5.0 A + B + C approx. 22 approx. 6.0 wherein: A represents polyvinyl acetal foam; B represents polyvinyl acetal fibers; and C represents coarse polyvinyl acetal cloth.

Let us now assume that a round having a cartridge case made of brass is represented by I, and that a round having a cartridge case member made of a polyvinyl acetal foam containing polyvinyl acetal fibers distributed uniformly throughout thereof and also a coarse polyvinyl acetal cloth embedded in the entire surface region thereof is represented by II. The results of drop tests of these two kinds of rounds are compared as shown in Table 2.

TABLE 2

Type Number of rejectable Number of still of round rounds usable rounds Group I 7 3 Group II 1 9 wherein: Drop tests were conducted by dropping, from a height of 50 cm above a horizontal concrete floor, two rounds each of Group I and Group II held in the following five different positions: horizontal; vertical (shot facing downwardly); vertical (shot facing upwardly; at 45.degree. (shot inclined downwardly; and at 45.degree. (shot inclined upwardly).

From the results of this drop test, it may be said that those rounds in Group II are of a greater anti-impact strength so far as the dropping test alone is concerned.

As stated above, an aspect of the present invention concerns a method for manufacturing, in a reasonable manner, an instantaneously completely combustible hollow member of a cartridge case having the aforesaid features. More specifically, it concerns a method for maufacturing cartridge case members of the type described by the use of a hollow cylindrical polyvinyl acetal foam containing therein at least one of the following two kinds of reinforcement fillers, i.e., polyvinyl acetal fibers and a coarse polyvinyl cloth, and by first excessively compressing this cylindrical foam on a male mold member of a compression molding apparatus with a pressurized fluid applied onto the external circumference of the cylindrical foam radially against the surface of the mold member and then expanding, with heat, the resulting compressed cylinder -- which is then positioned in the space between an inner male mold member and an outer female mold member of a separate expansion molding apparatus -- after applying an appropriate amount of humidity to the foam, to thereby urge this foam tightly against the inner surface of the outer female mold member.

A modified method of the present invention comprises the use of a hollow cylindrical polyvinyl acetal foam containing therein at least one of the reinforcement fillers, i.e., polyvinyl acetal fibers and a coarse polyvinyl cloth, and by first excessively compressing this cylindrical foam on a male mold member of a compression mold with a fluid pressure applied onto the external circumference of the cylindrical foam radially against the surface of the mold member, then expanding, with heat, the resulting compressed cylindrical foam -- which is then positioned in the space between an inner male mold member and an outer female mold member of a separate mold -- after applying an appropriate amount of humidity to the foam, to thereby urge this foam tightly against the inner surface of the outer female mold member, thereafter treating the resulting molded cylindrical foam with a synthetic resin, and the forming an easily combustible thin film on the entire outer surface of the treated cylindrical foam by applying a thixotropic solution of a low concentration containing nitrocellulose and a thixotropic agent.

Description will hereunder be directed to an embodiment of the present invention by referring to the accompanying drawings.

FIG. 2 shows a longitudinal central sectional view of a compression molding apparatus utilizing a pressurized fluid. In this drawing, reference numeral 13 represents a male mold member having generally a cylindrical configuration. This male mold member has a central longitudinal gas-passage channel 11 communicating with the outside of this mold member 13 and a plurality of radial gas-passage channels 11a opening at one end to the circumferential surface of the mold member and communicating at the other end with the said central longitudinal gas-passage channel 11, and also has a closed region 12. Numeral 15 represents a pressure-resistant cylindrical frame having an inlet 18 for introducing a pressurized fluid -- a pressure medium -- into the space inside this pressure-resistant cylindrical frame 15. The fluid which may be used in the present invention includes water, oil and gas such as air. A hollow cylindrical foam piece 17 covered externally with an elastic cover 16 such as a rubber bag is mounted on the male mold member 13 and is set in the mold as shown in FIG. 2. Then, a pressurized fluid 14 is introduced into the space within the pressure-resistant cylindrical frame 15 of the mold through the pressurized fluid inlet 18 from an appropriate compression pump to compress the hollow cylindrical foam piece 17 radially against the circumferential surface of the male mold member 13. This hollow cylindrical foam piece 17 is the material for producing an instantaneously completely combustible hollow cartridge case member 2 shown in FIG. 1. This cylindrical foam piece 17 is made with a principal material which is polyvinyl acetal foam of an acetalization degree of 65-86 percent and having numerous intercommunicating fine pores throughout its structure and having polyvinyl acetal fibers 3 distributed uniformly throughout therein and also having a coarse polyvinyl acetal cloth 4 embedded close to one surface of the foam piece 17.

Let us now assume that, in FIG. 2, the pressure of the fluid 14 is introduced into the space inside the cylindrical frame 15 by an appropriate means. Whereupon, the elastic cover 16 will be caused to tightly contact the external circumferential surface of the hollow cylindrical foam piece 17 to compress this foam piece 17 evenly radially against the external circumferential surface of the male mold member 13. During this process of compression, the pressure of the fluid 14 is controlled so that the molded hollow cylindrical foam piece 17 will have an outer diameter appropriately smaller than the inner diameter of the outer mold member 20 shown in FIG. 3. The foam piece 17 which has been compression-molded in the aforesaid manner is indicated by a reference numeral 21 in FIG. 3. By molding the foam piece 17 only by utilizing a pressurized fluid without the application of heat in the first step, the pressure can be applied evenly to the entire outer surface of the hollow cylindrical foam piece 17. Thus, it is possible to mold the foam piece 17 so as to have a uniform desired thickness and a uniform desired density throughout the entirety thereof with no difficulty and without undesirable partial compression and density which would tend to take place in heat-compression molding as will be discussed below.

More specifically, the heat-compression molding comprises the steps of first inserting a hollow cylindrical polyvinyl acetal foam piece in a hollow cylindrical female mold member so that this foam piece is in tight contact with the inner face of the female mold member, thereafter supplying excessively heated steam having a temperature of 120-130.degree. C onto the tightly contacting surface of the foam piece externally of the female mold member through numerous small perforations formed through the wall of the female mold member to cause the steam to be absorbed in the porous structure to sufficiently soften same, thereafter inserting a male mold member heated to a temperature substantially the same as that of the steam into the hollow space of the foam piece up to a predetermined position by means of, for example, a hydraulic pressing device, then passing steam for 1-2 minutes to the foam piece to remove and correct any deformation of the foam piece, thereafter stopping the supply of the steam, and supplying dry gas such as air heated at 120-130.degree. C to dry the damp foam piece until the residual moisture in the foam structure drops to two percent or less. The said steam and the said dry air invariably pass through the porous structure and are discharged from a gas outlet provided in the end of each of these two mold members. When the residual moisture of the foam piece at the time the latter is taken out of the mold is two percent or less, there hardly occurs any restoration of the initial configuration of the foam piece. Since this heat-compression molding method uses a temperature of 120-130.degree. C for heating, it should be understood that if the foam piece is dried excessively by heating, polyvinyl acetal may be decomposed. Also, there is the fear tha only those portions of the surface of the foam piece which are exposed to heat may be compressed and that only the resulting compressed portions of the foam piece will have an increased desnity. If these phenomena occur, the foam piece will have a difference in density between its inner surface region and its outer surface region. Also, the surfaces of the foam piece and their adjacent regions will show an injury in the pore structure, which leads to a much lowered combustibility.

FIG. 3 is a longitudinal central sectional view of a heat-expansion molding apparatus. In this drawing, reference numeral 20 represents an outer female mold member having a generally cylindrical configuration and having a heater 19. A hollow cylindrical foam piece 21 which has been compression-molded by a pressurized fluid and which is mounted on a cylindrical inner male mold member 22 is inserted in an appropriate manner in the outer female mold member 20. Numeral 23 represents a central longitudinal channel provided in the inner male mold member 22 for the passage of gas such as air. Numeral 23a represents a radial channel communicating at one end to the outer circumferential surface of the inner male mold member and at the outer end with the central longitudinal gas channel 23. In the heat-expansion molding apparatus shown in FIG. 3, the foam piece 21 which has been compressed with a pressurized fluid is impregnated with moisture to an appropriate degree, for example about 10 percent, prior to being heat-molded in this apparatus. Along therewith, the outer female mold member 20 is heated at an appropriate temperature of 100.degree. C or higher, whereas the inner male mold member 22 is held at an appropriate temperature of 100.degree. C or lower. By keeping the said foam piece 21 in the mold under these conditions for an appropriate length of time, the hollow cylindrical foam piece 21 expands to tightly contact the inner circumferential surface of the outer female mold member 20. It should be noted that, at this stage of heat-expansion, the hollow cylindrical foam piece 21 is still impregnated with moisture, and that, if this foam piece 21 is taken out from the heat-expansion apparatus at the said stage, there is the fear that the foam piece 21 would tend to resume its initial size which it has prior to being compression-molded with a pressurized fluid. Accordingly, there arises the necessity for expelling the moisture from the porous structure. For this reason, a dry gas such as dry air is supplied onto the foam piece 21 through the gas channel 23 and accordingly through the radial gas channels 23a. After the moisture present in the porous structure has been removed substantially therefrom in this way, the heat-molded foam piece 21 is taken out from the heat-expansion molding apparatus. It should be noted also that the weight of the foam piece 21 before being subjected to the method of the present invention is regulated so that the foam piece after heat-molding will have a density of 0.25-0.30 g/cm.sup.3 which is a desirable condition of the foam piece suitable for use in the present invention. As stated previously, a polyvinyl acetal foam is of inter-communicating pores, and has a tremendously large surface area of pores and has an acetalization degree of 65 -86 percent and contains therein polyvinyl acetal fibers and a coarse polyvinyl acetal cloth. Therefore, the foam piece 21 not only satisfies the requirements of an instantaneously completely combustible cartridge case member, but also permits compression-molding utilizing a pressurized fluid as well as permits heat-expansion molding. More specifically, the fact that the foam piece of polyvinyl acetal has an acetalization degree of 65-86 percent means that is has 35-14 percent of residual hydroxyl groups. Accordingly, the moisture contained in the polyvinyl acetal porous structure will evaporate as it is brought into contact with the heated surface of the outer female mold member 20, and the resulting vapor of water will fill in the intercommunicating pores of the porous structure of stimulate the said residual hydroxyl group, thus rendering the foam piece soft and pliant. Until this evaporation takes place, the polyvinyl acetal foam piece has been in the compressed state which was produced previously in the first compression-molding step utilizing a pressurized fluid. Therefore, the foam piece 21 now gains the force to resume its initial configuration and size which it has prior to being compressed. As a result, the foam piece 21 will tightly contact the inner circumferential surface of the outer female mold member 20. The inner surface of this outer mold member 20 is of a temperature which is elevated to the decomposition point of the polyvinyl acetal foam. However, as moisture evaporates, the polyvinyl acetal foam is deprived of evaporation heat, and accordingly the polyvinyl acetal foam per se will never reach its decomposition point.

The aforesaid molding method of the present invention not only is advantageous from the economical point of view, but also contributes greatly to the easy combustibility of the cartridge case member. More specifically, if the case member is produced by relying on a heat-compression method, only those facial portions of the foam piece exposed to heat during the heat-compression process will be compressed, and only such portions will have an increased density as compared with the remaining portions not having been heated nor compressed sufficiently. In other words, according to the heat-compression method, the molded porous structure will have a difference in density between its inner facial region and its outer facial region, and also lose its porous structure in the vicinity of its surfaces, developing blocked pores and accordingly having a substantially decreased surface ara of pores. When a round having such a cartridge case member is fired, there will remain uncombusted surface portions of the foam like a crust of bread. Accordingly to the aforesaid method of the present invention, however, the foam piece obtained is of a uniform density and is free of the aforesaid disadvantage and inconvenience which are encountered in the products of heat-compression method, and moreover, the cartridge case member made with such a foam piece of the present invention will never give rise to the development of solid combustion residues which are encountered, at the time of firing, in cartridge case members produced by heat-compression method.

The aforesaid polyvinyl acetal foam which is molded according to the method of the present invention may have an increased strength and also may be kept from resuming its initial configuration and size after being molded, by treating same with a synthetic resin as will be discussed below.

An increase in the strength of polyvinyl acetal foam is attained, as stated previously, by including in a polyvinyl acetal foam at least one, preferably both, of two kinds of reinforcement fillers, i.e., polyvinyl acetal fibers and a coarse polyvinyl acetal cloth. By doing so, the tensile strength and the anti-impact strength of the foam as a cartridge case member are both markedly increased as shown in Table 1. More specifically, a polyvinyl acetal foam not containing any reinforcement filler exhibits a tensile strength of about 10 kg/cm.sup.2 and an anti-impact strength of about 3.0 kg/cm/cm.sup.2, as compared with the foam pieces containing one or both of the reinforcement materials, which exhibit a tensile strength of about 15-22 kg/cm.sup.2 and an anit-impact strength of about 5.0-6.0 kg cm/cm.sup.2, indicating an increase as great as about two times. These reinforced foam pieces permit the rounds having a cartridge case made of such a case member to have a strength sufficient for use even when the rounds are dropped from a height of 50 cm onto a concrete floor. Also, by impregnating the compressed polyvinyl acetal foam with a reactive resinous intermediate product which is capable of combining with the residual hydroxyl group contained in the polyvinyl acetal foam to form a three dimensional molecular structure, the resulting foam not only will have an improved mechanical strength but also will hardly develop any deformation of its configuration after being compressed, and furthermore will have an enhanced water-proofness since hydrophilic hydroxyl groups contained in the foam are destroyed. This treatment of polyvinyl foam with the aforesaid synthetic resin provides such a number of advantages as stated above An example of the blended resin composition which satisfies these requirements is:

polyurethane ?isocyanate group (mol)/hydroxyl group (mol) >1! -- 5 parts (including hardening agent)

penetrant -- 1 part

moisture repellant -- 1 part

solvent (a mixture of xylene, methylisobutylketone and butyl acetate) -- 40 parts.

The polyvinyl acetal foam after being heat-expanded is impregnated with a solution having the aforesaid composition. By arranging the isocyanate component of polyurethane to be present therein in an excessive amount as mentioned above, it is possible to cause the residual hydroxyl group contained in the polyvinyl acetal foam to be united with the said isocyanate group in an appropriate manner and to thereby obtain an instantly completely combustible cartridge case member having a three dimensional molecular structure.

The polyvinyl acetal foam which has been treated in the manner stated above has surfaces having numerous pores with a diameter of the order of 100 microns, and are of good gas-permeability.

However, unless these surface pores are blocked, there will arise the possibility for the instrusion of moisture into the interior of the cartridge case. It is considered that, by forming a thick film on the external surface of the case member, the case member will have an improved water-proofness. On the other hand, however, residues from combustion at the time of firing will undesirably tend to develop easily. A film which is considered to be suitable for the aforesaid purpose in contributing to easy combustibility is such that it contains an explosive component such as nitrocellulose, or such that a thin film is formed only at the surface of the foam without the resin penetrating into the porous structure through the pores present in the surface thereof. It should be noted, however, that in case a film is formed on the surface of the foam by the use of only an explosive component such as nitrocellulose, there will arise the problem of a difficulty to maintain its chemical stability and also of the danger of being easily ignited when a round having such a cartridge case member is charged in the cartridge chamber of a firearm in case the cartridge chamber has been heated to a high temperature from successive firing. It should be understood also that the formation of a thin film will require that the solution is of a low concentration. However, a solution of a low concentration is, in general, of a low viscosity which, in turn, will be insufficient for blocking the surface pores of the porous structure. Not only that, the thin solution will easily penetrate into the interior of the porous structure. Accordingly, it will become impossible to have the porous structure keep satisfactory water-proofness or damp-proofness. On the other hand, a solution having a low concentration and a high viscosity will not penetrate into the interior of the porous structure, but such a solution will deteriorate workability in such a way that, when the solution is applied onto the surface of the surface of the porous structure, it will be deposited as a thick and plump layer.

It is therefore proper to use an under-coating consisting of a thixotropic solution of a low concentration, and to apply a water-proof coating on top of this under-coating. An example of the under-coating composition is:

nitrocellulose -- 2 parts

nitrocellulose stabilizer -- 0.02 parts

pigment -- 1 part

thixotropic agent -- 2 parts

solvent (a mixture of xylene, methylisobutylketone and butyl acetate) -- 50 parts.

An under-coating solution having the aforesaid composition will exhibit thixotropy when this solution having a concentration of about five percent is heated and then quenched while being stirred. This solution under this condition is creamy and is not tacky but is of a good ductility, so that it can be applied lightly on the surface of the porous structure without penetrating into the interior thereof. This solution is applied with a brush to the surface of the porous structure. After being dried, a film of 10 micron or less is obtained. This film, however, is of a poor water-proofness. Accordingly, after the surface pores of the porous structure have been blocked substantially by the application of the said under-coating solution thereto, a water-proof coating is given on top of the said under-coating. As a water-proof coating, the use of a vinylidene solution is suitable. Furthermore, a pigment consisting of flaky aluminum powder will be effective in improving water-proofness. As a result of these treatments, the resulting porous structure can stand its exposure to a rainfall of not only the order of 4 .+-. 1 inches/hour, but also of the order of several times as great as that.

Cartridge cases having instantaneously completely combustible case members which were prepared in the manner described above were subjected to a number of different tests. The result of these tests is as follows.

1. Firing fitness:

No residues of combustion were left. No gas leakage occurred. Pressure in the cartridge chamber and initial velocity of shot were both found satisfactory.

2. Tensile strength:

Test samples were tested by Amsler tester according to JIS-Z-2112. The tensile strength of the case members was noted to be about 45 kg/cm.sup.2.

3. Compression strength:

Pieces cut out from the case members were tested on Amsler tension tester. Their compression strength was found to be about 20 kg/cm.sup.2.

4. Anti-impact strength:

Test samples were tested by Sharpi impact tester according to JIS-Z-2202, with the result that their anti-impact strength was about 6 kg.cm/cm.sup.2.

5. Water-proofness:

Samples were left in water for 48 hours at the depth of 300 mm below the water surface without any loss of water-proofness.

6. Shot holding strength:

Complete rounds having the said cartridge cases were tested by Amsler tension tester. The strength required for pulling out shots from case members was noted to be about 650 kg.

7. Vibration test:

Samples were subjected to vibration at: amplitude 3.0 mm, frequency 1,500 cpm and acceleration 3.9 G., without being affected adversely.

8. Drop test:

The result is as per Table 2. The result showed only an appreciable degree od deformation in case members, with very few being found rejectable.

9. Stability test:

Samples were subjected to Abbel heat-resistance test according to NDS-XK4811 for 50 minutes with no appreciable change; Samples subjected to heat-resistance test at 134.5.degree. C for 60 minutes neither showed any appreciable change (NDS XK 4811).

10. Ignition point test:

The result is as shown in FIG. 4. In this drawing, vertical axis represents the temperature (.degree.C) at which samples were heated; horizontal axis represents the lapse of time (sec.) consumed till samples began to develop smoke. Curve A shows the result of test conducted on the instantaneously completely combustible cartridge case members of the present invention, whereas Curve B shows the result of test conducted on propellant. As is clear from FIG. 4, these case members develop smoke at the end of 50 seconds at 250.degree. C. Whereas, the propellant is ignited at the end of 10 seconds at the same temperature. When the quick heat-transmitting property of brass cartridge cases is taken into account, the cartridge cases made of the case members of the present invention are noted to be much safer.

11. Analysis of gases produced from combustion:

The result is as per Table 3. No substantial difference from the gases produced from propellant contained in brass cartridge cases is noted. ##SPC1##

As has been stated above, the cartridge case member of the present invention is made with a polyvinyl acetal foam which is the principal material and in which is contained at least one of the two kinds of reinforcement fillers, i.e., polyvinyl acetal fibers and a coarse polyvinyl acetal cloth. Therefore, this case member can satisfy most of the requirements of an instantaneously completely combustible cartridge case. Also, the said reinforced foam is prepared in a hollow cylindrical shape which is mounted on a male mold member of a compression-molding apparatus to be diametrically compressed excessively by utilizing a pressurized fluid in the first step, and then the resulting compressed hollow cylindrical foam is positioned between an inner male mold member and an outer female mold member of separate heat-expansion molding apparatus to be subjected -- after being moistened to an apropriate degree -- to expansion, by heating, to become in tight contact with the said outer female mold member, thus being molded into the configuration of a case member. Therefore, unlike the molding by a heat-compression molding, the case member of the present invention can be molded quite reasonably so as to have a uniform density and uniform desired thickness. As a final product, the cartridge case member of the present invention leaves no appreciable residue at the time of firing.

The molded case member can have an enhanced strength and hardness by being treated with a synthetic resin. Moreover, it can have a very desirable water-proofness and combustiblity by being given -- at its surface -- a thin under-coating with a solution containing nitrocellulose and a thixotropic agent and being capable of exhibiting thixotropicity at a low concentration, and thereafter by forming a water-proof thin film on top of the said under-coating. Besides, the applications of these solutions is easy and needs no well-trained skill.

It is to be understood that the currently used firearms are not designed so as to have a gas-enclosing function in the bottom portion of their cartridge chambers. In order to suit for these current firearams, the cartridge case member 2 of the present invention desirably is provided -- at its bottom -- with a metal bottom member 5 having a small longitudinal length which is bonded by a layer of adhesive 10 at its portion of juncture 9a to the corresponding portion of juncture 9 of the case member 2, in a manner as shown in FIG. 1, for establishing the enclosure of the gases produced at the time of firing. The adhesive which is most suitable for use in these portions of juncture is an epoxy resin.

Claims

We claim:

1. An instantaneously and completely combustible cartridge casing, comprising: a hollow cylindrical casing made of thermoplastic polyvinyl acetal foam having uniformly distributed, intercommunicating fine pores and having a substantially uniform density in the range of from about 0.25 to about 0.30 g/cm.sup.3, the casing being made from polyvinyl acetal foam having an average polymerization degree of from 1,000 to 2,500, an acetalization degree of from 65 to 86 percent, a porosity of from 90 to 95.5 percent and an apparent specific gravity of from 0.06 to 0.12, by compressing a hollow cylinder of said polyvinyl acetal foam on a male mold member by applying a uniform radially inwardly directed pressure on the entire external surface of the cylinder, then wetting the cylinder with water, heating the cylinder to expand same into contact with a female mold member and then drying the cylinder before removing it from the female mold member.

2. An instantaneously and completely combustible cartridge casing according to claim 1, in which said foam contains therein at least one kind of reinforcement filler selected from the group consisting of polyvinyl acetal fibers and polyvinyl acetal cloth.

3. An instantaneously and completely combustible cartridge casing according to claim 1, in which said foam contains polyvinyl acetal fibers mixed therein and also contains a layer of polyvinyl acetal coth embedded therein.

4. An instantaneously and completely combustible cartridge casing according to claim 1, in which said foam contains distributed therein synthetic resin reacted with the residual hydroxyl groups contained in the said polyvinyl acetal foam to form a cross-linked structure.

5. An instantaneously and completely combustible cartridge casing accoridng to claim 4, in which said casing has on its external surface an easily combustible thin film comprising nitrocellulose and a thixotropic agent.

6. An instantaneously and completely combustible cartridge casing according to claim 5, in which said easily combustible thin film is covered with a water-proof thin vinylidene chloride coating.

7. An instantaneously and completely combustible cartridge casing according to claim 1, in which the said casing has a metal bottom member securely bonded to the bottom thereof.