GB1590479A - Process for injection moulding shotshell case - Google Patents

Description

(54) PROCESS FOR INJECTION MOULDING SHOTSHELL CASE (71) I, LARRY SORENSON, a New Zealand citizen, of 148 Wheatley Road, Ormond, in the State of Victoria, Commonwealth of Australia do hereby declare the invention, for which I pray that a patent may be granted to me and the method by which it is to be performed, to be particularly described in and by the following statement: This invention provides a process for injection moulding plastics shotshell casings.

It is already known to produce all plastic shotshell casings by injection moulding as can, for example, be seen from the disclosures of United States Patents 3,590,740; 3,591,898; 3,614,929; 3,673,965; 3,722,411; 3,722,412 and 3,752,434. However, the all plastic casings produced to date have not been strong enough to be reloaded repeatedly. The present invention enables injection moulding of all plastics shotshell casings which are significantly stronger than the prior art plastics casings and which in particular can be reloaded repeatedly.

Traditionally in the field of injection moulding it has been considered necessary to design the injection equipment and to control the process conditions to ensure a smooth, laminar flow of the injected material into the mould cavity. This practice has been followed when injection moulding all plastics shotshell casings. I have found that it is possible to injection mould all plastics shotshell casings of surprising strength by appropriate choice of injection material and by departing from the traditional practice of ensuring smooth flow of the injected material into the mould.

According to the invention there is provided a process for injection moulding a plastics shotshell case, comprising the steps of: forming a mould cavity having an elongate tubular cavity portion which is closed at one end and at the other end is contiguous with a disc shaped cavity portion extending across said other end of the cylindrical cavity portion; heating a thermoplastic material in an injection moulding machine; injecting the molten thermoplastic material under pressure into the mould cavity via a plurality of injection orifices disposed in a circumferentially spaced array at the central part of the disc shaped cavity portion so that the molten thermoplastic material is directed in a plurality of turbulent streams radiating outwardly of the disc shaped cavity portion toward the cylindrical cavity portion; continuing the injection of the thermoplastic material until the cylindrical and disc shaped portions of the mould cavity are filled to form a cylindrical wall and end disc wall of a cartridge case; and separating the thus formed cartridge case from the mould cavity.

Preferably the thermoplastic material is a polymer with a molecular weight of at least 1,250,000 and preferably greater than 1,500,000, the total effective area of said orifices is in the range 0.004 in2 (2.5 mm2) to 0.014 in2 (9 mm2), the pressure applied to the thermoplastic material is in the range 15,000 psi (1050 Kg/cm2) to 25,000 psi (1760 Kg/cm2) and the temperature of the thermoplastic material at injection into the mould cavity is in the range 210 C to 3600C.

Preferably the thermoplastic material enters the mould cavity through a tubular injection piece extending into the disc shaped portion of the mould cavity, said orifices being formed in the peripheral wall of said injection piece.

In the process according to the invention temperature at which the thermoplastic material is injected into the mould cavity is much higher than would normally be recommended according to traditional injection moulding practice. Moreover, the material is injected through a much smaller orifice area than would be chosen according to traditional theory and at a higher pressure. These departures from normal practice are aimed at introducing the thermoplastic material into the mould cavity in a plurality of turbulent streams in a manner which can be likened to spraying of material outwardly into the parts of the mould cavity in which the casing wall is to be formed. It has been found that if high molecular weight thermoplastic materials of a type usually used for blow moulding (but not for injection moulding) are injected into the mould cavity in this way the walls of the casings have markedly superior strength characteristics. While not wishing to be limited to any particular theory or explanation for such improved strength characteristics it is thought to be caused by the phenomenon of biaxial orientation causing the thermoplastic material to become locked in a rigid network instead of the usual laminated structure, resulting in substantial elimination of the "weld lines" which appear in casings produced by conventional injection techniques and which significantly weaken the burst strength of the casing wall. By injecting the thermoplastic material into the mould cavity so that it sprays outwardly in a turbulent manner the thermoplastic material entering the cylindrical part of the mould cavity has random motion which results in a random orientation of the molcules therein until the material solidifies in an unusually strong, stable network.

In order that the invention may be more fully explained one particular moulding apparatus and results obtained in moulding shotshell casings by means of that apparatus will be described in detail by way of example with reference to the accompanying drawings.

Figure I is a vertical cross-section through part of the apparatus illustrating the manner in which plastics material is injected into a mould cavity formed between a stationary mould component and a moving mould component; Figure 2 is a view similar to Figure 1 but showing the mould components separated after a moulding operation; Figure 3 is a perspective view to an enlarged scale of that part of the apparatus through which the plastics material is injected into the mould cavity; and Figures 4 to 10 are graphs showing test results obtained when moulding shotshell casings with the apparatus of Figures 1 to 3.

The illustrated apparatus comprises a mould 10 having a stationary part 11 and a moving part 12 which may be connected in conventional manner respectively to the stationary and moving platens of a standard moulding machine whereby the moving part 12 can be moved into abutment with the stationary part 13 as shown in Figure 1 for a moulding operation and can be withdrawn to the position shown in Figure 2 after the moulding operation.

Moving mould part 12 comprises a cavity block 13 having a central generally cylindrical opening 14 and a core pin 15 which projects into the central opening. When the mould is in the closed condition shown in Figure lit defines a closed mould cavity 16 having the shape of the shotshell casing which is to be produced. This mould cavity comprises an elongate tubular cavity portion 17 defined between the wall of opening 14 and the outer periphery of core pin 15 and a disc shaped cavity portion 18 which is contiguous with cavity portion 17 and extends across one end of that portion 17, being defined between the stationary mould part 11 and the adjacent end 16 of core pin 15 within the end of the cylindrical opening 14 of the cavity block. The other end of the tubular mould portion is permanently closed by interengagement of the core pin and the cavity block at that position.

As is conventional in injection moulds, cavity block 13 is provided with cooling water passages 20 so that it can be cooled to a constant temperature by circulating chilled water through those passages and so control the solidification of the thermoplastic material injected into the mould cavity.

Preferably the outer periphery of core pin 15 is tapered slightly so as to reduce in diameter toward its end 16 to assist stripping of the casing produced by the moulding operation. The tubular wall of the resulting casing will be tapered accordingly.

The end of the cylindrical opening 14 in cavity block 13 adjacent stationary mould part 11 is relieved or rebated at 19 to form the usual end rim or bead on the resulting shotshell casing.

Stationary mould part 11 is fitted with a sprue bushing 21 through which the thermoplastic moulding material is conducted to the mould cavity 16 from the usual injection nozzle 22 of an injection machine. Sprue bushing 21 is generally tubular, having a stepped outer periphery 23 and a central plastic delivery passage 24 which is convergent toward injection nozzle 22 so that removal of a completed shotshell casing from the mould will pull the sprue out easily and cause the injected material to break off at, or within the hot nozzle tip.

The outlet or delivery end of sprue bushing 21 is formed with a tip portion 25 which projects into the central part of the disc shaped portion 18 of mould cavity 16. The configuration of this tip portion is most clearly seen in Figure 3. It comprises a cylindrical spigot 26 the outer end 27 of which is broken by four radial slots 28 arranged at regular spacing around the circumference of the spigot. When the mould is closed the outer end 27 of spigot 26 is engaged by the end of core pin 15 which closes off the ends of slots 28 to form four rectangular orifices disposed in circumferentially spaced array at the central part of the disc shaped portion 18 of mould cavity 16 and through which the thermoplastic material is injected from the central delivery passage 24 of the sprue bushing outwardly into the mould cavity.

The shape of the shotshell case produced by the moulding process is best seen in Figure 2 in which the mould is shown in an open condition with the shotshell casing still held to the stationary component by a sprue extending into sprue bushing 21. It has a tubular wall 31 and a disc shaped end wall 32 with the usual peripheral end rib of bead 33. Because of the projection of sprue bushing tip 25 into the mould cavity the end wall 32 of the wall casing will have, after removal of the sprue, a central cylindrical opening to receive a primer cap.

The sprue bushing tip is formed with a circumferential rib 29 of saw tooth cross-section to form a primer cap retaining groove in the outer end of this primer cap opening.

Nozzle 22 is the injection nozzle of an injection cylinder system. This cylinder system may be of conventional type and is not shown in the drawings. It is preferably of the screw feed type, comprising a heated barrel housing a feed screw which feeds thermoplastic material from a feed hopper at one end of the barrel to the nozzle 22 at the other end of the barrel.

It has been found that shotshell casings having improved strength can be produced with the illustrated apparatus if appropriate plastics material is used and provided that the injection orifices defined by sprue bush slots 28 are of an appropriate size and the process conditions are controlled in accordance with the present invention.

In relation to the thermoplastic material to be used it has been found that in order to obtain a shotshell casing capable of multiple reloading, the thermoplastic material should have a high molecular weight, preferably coupled with a relatively low melt index. More specifically, the thermoplastic material should have a molecular weight of at least 1,250,000 and preferably in the range 1,500,000 to 2,000,000 which is the upper limit on materials presently available in commercial quantities. Low stressed high density polyethylenes having a molecular weight within this range are particularly suitable. A small percentage of copolymerisation may be tolerated such as 5% butylene. This can provide advantages because certain degree of elasticity is desirable for crimping the end of the shotshell casing during loading.

Examples of suitable materials are Union Carbide DMPL 1155 (an essentially linear molecular which is a copolymer of 95% ethylene and 5% butylene) and Union Carbide DMDL 3103 (also an essentially linear molecular of 95% ethylene and 5% butylene but having a higher melt index than DMPL 1155). Both DMDL 3103 and DMPL 1155 are high density materials commonly used in blow moulding operations but not for injection moulding. Another suitable material is a rubber modified homopolymer of high density polyethylene obtainable from Allied Chemicals under the name PLASKON shotshell grade 6/34. [Plaskon" is a Registered Trade Mark] Figures 4 to 10 show the results of experiments performed with apparatus of the type illustrated in Figures 1 to 3 mounted on a standard 2 ounce Johns injection moulding machine. The mould was formed to produce 12 gauge shotshell casings of 70 mm length, 20 mm diameter, 1.20 mm average tube wall thickness and 2.54 mm end wall thickness. These casings had an average weight of 5.8 grams. An extensive testing program has shown that optimum results are obtained with a sprue bushing having a tip of 0.120 inch outer diameter with four end slots each providing an orifice measuring 0.075 inch (1.9 mm) by 0.03 inch (0.76 mm) giving a total injection orifice area of 0.009 in2 (5.8 mm2) the thermoplastic material being applied through nozzle 22 at a pressure of 20,000 psi (1406 Kg/cm2) and a temperature of 300"C. In order to indicate the importance of proper choice of conditions in accordance with the present invention Figures 4 to 9 show the effects of changing one parameter or condition while other conditions ae held at the experimentally determined optimum. In each of these figures the strength of the shotshell casing obtained is indicated on the vertical ordinate as measured by the number of reloads which it was possible to obtain before the casing failed.

Figure 4 shows the results obtained with thermoplastic materials of differing molecular weights when operating the apparatus under otherwise optimum conditions. It will be seen that it is desirable to use a material of as high a molecular weight as possible. Using a capacity of six reloads as a minimum requirement it will be seen that it is necessary to use a thermoplastic material having a molecular weight of at least 1,000,000, and preferably of the order of 1,800,000 to 2,000,000.

Figure 5 plots the results obtained by changing the temperature of the injected material and indicates that in order to obtain at least six reloads this temperature should fall within the range 210 C to 3500C and that optimum results are obtained if this temperature is held between 250"C and 320"C. It is thought that if the temperature is too-low the required spray effect is not achieved at the injection orifices and that if the temperature is too high there is a loss of molecular weight of the thermoplastic material leading to weakness of the resulting shotshell casing.

Figure 6 shows the effect of changes in the pressure applied to the thermoplastic material as measured at the outlet of the injection barrel i.e. the pressure as applied to the inlet of nozzle 22. It will be seen that in order to obtain at least six reloads this pressure should be maintained within the range 12,000 psi (843 Kg/cm2) to 25,000 psi (1757 Kg/cm2) and that optimum results are obtained if the pressure is maintained very close to 20,000 psi (1406 Kg/cm2).

It was found that the injection time to fill the mould cavity at the optimum pressure was 7 seconds. At the lower end of the acceptable pressure range the injection time increased to 9 seconds giving an injection rate of 0.6 gm/sec and at the highest acceptable pressure the injection time was reduced to 5 seconds giving an injection rate of 1.2 gm/sec. Thus it may be said that in order to achieve the desired spray injection effect when moulding a shotshell case, the ptessure must be such to produce an injection rate of between 0.6 gm/sec and 1.2 gm/sec.

Figure 7 illustrates the effect of cooling of cavity block 12 and plots the results obtained for different measured temperatures of the cavity block. It has been found that the optimum cavity block temperature is dependent on the tubular wall thickness of the shotshell case being moulded. The cavity block temperature must be low enough to cause reasonably rapid solidification of the thermoplastic material and in all cases this temperature should be below about 18"C, preferably less than 14"C. As discussed below the tubular wall thickness may vary between about 0.74 mm and 1.8 mm. When moulding casings having a wall thickness at the lower end of this range it is found that the cavity block temperature should not be less than about 8"C, since too rapid cooling can result in clogging of the mould cavity before a full charge of material has been injected. However, when thicker walled casings are being moulded good results can be achieved with a cavity block temperature as low as 2"C.

Figures 8 and 9 show the effect of variations in the number and size of the injection orifices 28. It will be seen that there should be at least two orifices, preferably2, 4, 5 or 6, and that the total effective area of the orifices should be in the range 0.004 in (2.5 mm2) to 0.014 in2 C9.0 0 mm2) and that optimum results are achieved if the total area is around 0.01 in2 (6.45 mum ). In one preferred embodiment there are at least four orifices disposed in a circumferentially spaced array at a regular spacing and each having an effective area in the range 1.29 mm2 to 1.95 mm2. It is to be noted that between 4 and 7 grams of the thermoplastics material may be injected into the mould within an injection time in the range of 5 to 9 seconds.

Figure 10 shows the effect of changes in the time interval during which the moulded shotshell case is retained in the cooled mould before ejection. It will be seen that this.

cooling time should be at least 8 seconds and preferably around 13 seconds. If the cooling time is greater than about 18 seconds, variable results are obtained in production runs because the feed material needs to be retained in the injection barrel for longer periods, during which it is subjected to high temperatures and suffers a loss of molecular weight. In order to avoid such loss of molecular weight due to heating for too long a period in the injection barrel, it is also important that the injection barrel should not be charged with an excessive quantity of material. Specifically it has been found that provided the charge of material in the injection barrel is maintained below 200 grams high strength shell casings can be produced consistently, but if the barrel is charged beyond this limit variable results can occur.

The tubular wall thickness of the casing must be chosen so as to provide the combination of strength and flexibility required for crimping, firing and multiple reloading. Generally the casing will be crimped at a position spaced from its open end by a distance which is between 1/2 and 3/4 of the tube diameter and it has been found that the wall thickness in this region should be in the range 0.74 mm to 1.8 mm. To ensure adequate strength and a high muzzle velocity on firing the end wall of the casing should be at least 2 mm thick.

It will be appreciated that Figures 4 to 10 indicate very briefly the results of a large number of tests. In order to give a more complete indication of the results which can be achieved by means of the process of the present invention details of several specific test runs are given below. Each of these test runs was carried out on a Johns 2 ounce machine, using an injection screw 24 inches long. The sprue bushing tip has an outside diameter of 0.120 inches (3.05 mm) and was provided with four end slots each 0.075 inches long (1.9 mm), 0.030 inches wide (0.76 mm) and 0.035 inches deep (0.89 mm). Approximately 1 Kg of thermoplastic material was heated in the injection moulding machine and the weight of each casing obtained in each run was approximately 5.8 grams.

Run 1 Material : HX538 Temperature of injection chamber : Feed end 270"C Middle 280"C Injection end 285"C Temperature of injected material : 280"C Injector pressure : 20,000 psi (1757 Kg/cm2) Injection time : 7 seconds Cooling time : 8 seconds Temperature of mould cavity block : 2"C Number of reloads obtained : 27 Run 2 Material : HX538 Temperature of injection chamber : Feed end 290"C Middle 300"C Injection end 320"C Temperature of injected material : 300"C Injector pressure : 20,000 psi (1757 Kg/cm2) Injection time : 7 seconds Cooling time : 18 seconds Temperature of mould cavity block : 12"C Number of reloads obtained : 14 Run 3 Material: DMDL3103 Temperature of injection chamber : Feed end 280"C Middle 290"C Injection end 300"C Temperature of injected material : 290"C Injector pressure : 20,000 psi (1757 Kg/cm2) Injector time : 7 seconds Cooling time : 25 seconds Temperature of mould cavity block : 12"C Number of reloads obtained : 10 Run 4 Material : DMDL1155 Temperature of injection chamber : Feed end 290"C Middle 300"C Injection end 320"C Temperature of injected material : 300"C Injector pressure : 20,000 psi (1757 Kg/cm2) Injector time : 7 seconds Cooling time : 27 seconds Temperature of mould cavity block : 12"C Number of reloads obtained : 13 It will be seen that the above results indicate that it is possible to produce all plastic shotshell casings of surprising strength using thermoplastic material hitherto not used for injection moulding. Essentially these results are achieved by injecting the thermoplastic material through an array of small orifices under pressure and temperature conditions such that the material sprays outwardly into the barrel of the mould cavity. Although the test results shown herein have been obtained in the production of standard 12 gauge shotgun shell casings it has been found that there is no significant variation in the orifice, size, temperature and pressure requirements when producing shotshell casings of other sizes.

When producing larger or smaller shells the injection time to fill the mould cavity will be slightly increased or decreased accordingly but the process conditions which promote turbulence in the material entering the mould cavity remain the same.

The illustrated apparatus has been advanced by way of example only and it could be modified considerably. For example, it will readily be appreciated that the invention may be applied to multi-cavity machines in which case the parameters determined in accordance with the invention apply to the injection of material into each mould cavity. The manner of forming the mould cavity could be varied and the structure defining the injection orifices within that cavity could also be modified.

WHAT I CLAIM IS: 1. A process for injection moulding a plastics shotshell case, comprising the steps of forming a mould cavity having an elongate tubular cavity portion which is closed at one end and at the other end is contiguous with a disc shaped cavity portion extending across

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (16)

**WARNING** start of CLMS field may overlap end of DESC **. Run 3 Material: DMDL3103 Temperature of injection chamber : Feed end 280"C Middle 290"C Injection end 300"C Temperature of injected material : 290"C Injector pressure : 20,000 psi (1757 Kg/cm2) Injector time : 7 seconds Cooling time : 25 seconds Temperature of mould cavity block : 12"C Number of reloads obtained : 10 Run 4 Material : DMDL1155 Temperature of injection chamber : Feed end 290"C Middle 300"C Injection end 320"C Temperature of injected material : 300"C Injector pressure : 20,000 psi (1757 Kg/cm2) Injector time : 7 seconds Cooling time : 27 seconds Temperature of mould cavity block : 12"C Number of reloads obtained : 13 It will be seen that the above results indicate that it is possible to produce all plastic shotshell casings of surprising strength using thermoplastic material hitherto not used for injection moulding. Essentially these results are achieved by injecting the thermoplastic material through an array of small orifices under pressure and temperature conditions such that the material sprays outwardly into the barrel of the mould cavity. Although the test results shown herein have been obtained in the production of standard 12 gauge shotgun shell casings it has been found that there is no significant variation in the orifice, size, temperature and pressure requirements when producing shotshell casings of other sizes. When producing larger or smaller shells the injection time to fill the mould cavity will be slightly increased or decreased accordingly but the process conditions which promote turbulence in the material entering the mould cavity remain the same. The illustrated apparatus has been advanced by way of example only and it could be modified considerably. For example, it will readily be appreciated that the invention may be applied to multi-cavity machines in which case the parameters determined in accordance with the invention apply to the injection of material into each mould cavity. The manner of forming the mould cavity could be varied and the structure defining the injection orifices within that cavity could also be modified. WHAT I CLAIM IS:

1. A process for injection moulding a plastics shotshell case, comprising the steps of forming a mould cavity having an elongate tubular cavity portion which is closed at one end and at the other end is contiguous with a disc shaped cavity portion extending across

said other end of the cylindrical cavity portion; heating a thermoplastic material in an injection moulding machine; injecting the molten thermoplastic material under pressure into the mould cavity via a plurality of injection orifices disposed in a circumferentially spaced array at the central part of the disc shaped cavity portion so that the molten thermoplastic material is directed in a plurality of turbulent streams radiating outwardly of the disc shaped cavity portion toward the cylindrical cavity portion; continuing the injection of the thermoplastic material until the cylindrical and disc shaped portions of the mould cavity are filled to form a cylindrical wall and end disc wall of a cartridge case; and separating the thus formed cartridge case from the mould cavity.

2. A process for injection moulding a plastics shotshell case, comprising the steps of forming a mould cavity having an elongate tubular cavity portion which is closed at one end and at the other end is contiguous with a disc shaped cavity portion extending across said other end of the cylindrical cavity portion; heating a thermoplastic material in an injection moulding machine; injecting the molten thermoplastic material under pressure into the mould cavity via a plurality of injection orifices disposed in a circumferentially spaced array at the central part of the disc shaped cavity portion; continuing the injection of the thermoplastic material until the cylindrical and disc shaped portions of the mould cavity are filled to form a cylindrical wall and end disc wall of a cartridge case; and separating the thus formed cartridge case from the mould cavity, wherein the total effective area of said orifices is in the range 2.5 mm2 to 9 mm2 and the thermoplastic material is injected into the mould at a temperature in the range 210 C to 3600C at a rate of between 0.5 and 1.0 grams per second.

3. A process as claimed in claim 2, wherein between 4 and 7 grams of the thermoplastic material is injected into the mould within an injection time in the range 5 to 9 seconds.

4. A process as claimed in any one of the preceding claims, wherein the thermoplastic material is a polymer with a molecular weight of at least 1,250,000 and said injection moulding machine applies to that material a pressure in the range 1050 Kg/cm2 to 1760 Kg/cm2.

5. A process as claimed in any one of the preceding claims, wherein the thermoplastic material enters the mould cavity through a tubular injection piece extending into the disc shaped portion of the mould cavity along the central axis of the mould cavity, said orifices being formed in the peripheral wall of said injection piece.

6. A process as claimed in claim 5, wherein the mould cavity is formed by a mould having a central core portion and in the formation of the mould cavity the central core portion is brought into abutment with said injection piece.

7. A process as claimed in claim 6, wherein said orifices are open ended slots in the end of the tubular injection piece which abuts the core portion of the mould on formation of the mould cavity.

8. A process as claimed in any one of the preceding claims, wherein there are at least four of said orifices disposed in a circumferentially spaced array at regular spacing and each having an effective area in the range 1.29 mm2 to 1.95 mm

9. A process as claimed in any one of the preceding claims, wherein said thermoplastic material has a molecular weight in the range 1,500,000 and 2,000,000.

10. A process as claimed in any one of the preceding claims, wherein said thermoplastic material comprises at least 90% high density polyethylene.

11. A process as claimed in any one of the preceding claims, wherein the temperature of the thermoplastic material at injection into the mould cavity is in the range 250"C to 3200C.

12. A process as claimed in any one of the preceding claims, wherein the mould cavity is defined within a mould body which is cooled to a temperature of less than 18"C.

13. A process as claimed in claim 12, wherein the mould body is cooled to a temperature of less than 14"C.

14. A process as claimed in any one of the preceding claims, wherein the case is cooled in the mould cavity for at least 8 seconds before being separated therefrom.

15. A process as claimed in any one of the preceding claims, wherein the thickness of the tubular wall of the casing in the region spaced from its open end by a distance of about one half of the tube diameter is in the range 0.74 mm to 1.8 mm.

16. A process for injection moulding sustantially as herein described with reference to the accompanying drawings.