Injection molding of shotgun shells

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Virginia Tech


In this investigation, the possibility of producing injection molded shotgun shells was explored. Also, the molding cycle and the effect of molding conditions on the mechanical properties of molded parts were investigated.

The polymer obtained from a commercial brand of shotgun shells was analyzed using infrared spectroscopy and differential thermal analysis. The results from the analyses revealed that polyethylene was the main component used in the manufacture of commercial shotgun shells.

An injection mold with automatic ejector mechanism was designed. Before attempting to construct the mold a computer simulation was developed to predict if the "most difficult to fill" cavity, i.e., the shotgun shell cavity, could be completely filled with the injection molding equipment at hand. The simulation predicted the axial distance filled in the shotgun shell cavity within 1%.

The injection molder used in this investigation was interfaced with a PDP 8/e minicomputer; as a result the pressure and temperature of the polymer in the mold were monitored and the minimum molding cycle could be determined.

Tensile, compressive and bursting strength tests were performed on injection molded specimens of polyethylene and the commercial polymer. From these tests it was established that for a molding cycle in which the plunger is kept forward until the mold is opened and "steady-state" cycle conditions have been established, the tensile modulus and tensile strength at yield increase as injection pressure and/or temperature are decreased; the opposite was true for the same properties measured in compression. Finally, polyethylene was compounded with additives to improve its mechanical properties.