A method of extrusion of pencil leads which write chalks or the like. And piston press for carrying out the method

摘要

A vacuum extrusion press is used for plastic substances which initially contain entrained air, esp. for the lead or chalk material intended for use in pencils, crayons or the like. The press comprises a barrel and compressing piston; the rear end of the barrel, remote from the extrusion mouthpiece, is in communication with a feed vessel, the communicating opening being situated between the dead-centre positions of the piston. This feed vessel is pref. mounted above the barrel, the bottom of the vessel being formed by the internal space of the barrel. The air evacuation line is pref. connected to the upper part of this vessel, its communicating opening being at a little distance from an internal baffle plate inside the vessel. The operations of precompressing and removing the air from the mixture, and also of finally compressing at high pressure and extruding the material are all carried out in a single apparatus. This enables the difficulties of synchronising the programme between two or three successive processing cylinders each having a different operating period to be avoided.