Apparatus for molding plastic

DETAILED DESCRIPTION What we claim is: 1.
Apparatus for molding a plastic, comprising: an injection mold having a mold cavity; a pressure sensor for sensing the pressure of plastic injected into the mold cavity to generate a cavity internal pressure signal; inflection point detecting means for differentiating the cavity internal pressure signal from said pressure sensor by means of a differentiation circuit, to detect an inflection point corresponding to a gate sealing point of plactic with the mold cavity, and to generate an inflection point signal when said inflection point corresponding to a gate sealing point is detected; zero sensing means for sensing that the cavity internal pressure signal from said pressure sensor becomes one of zero and a predetermined value close to zero to generate a zero signal; and holding pressure removing means for removing a holding pressure which is exerted on plastic in the mold cavity, said holding pressure being removed in response to the zero signal from said zero sensing means when the inflection point corresponding to the gate sealing point is not detected by said inflection point detecting means, and for removing the holding pressure in response to the inflection point signal from said inflection point detecting means when the inflection point corresponding to the gate sealing point is detected by said inflection point detecting means.
2.
An apparatus according to claim 1, including means for setting said predetermined value close to zero in accordance with the levels of the holding pressure, the mold temperature and the plastic temperature.
3.
Apparatus for molding a plastic, comprising: an injection mold having a mold cavity; a pressure sensor for sensing the pressure of plastic injected into the mold cavity to generate a cavity internal pressure signal; inflection point detecting means for differentiating the cavity internal pressure signal from said pressure sensor by means of a differentiation circuit, to detect an inflection point corresponding to a gate sealing point of the plastic in the mold cavity; and means for removing a holding pressure which is exerted on plastic in the mold cavity, said holding pressure being removed when the inflection point corresponding to the gate sealing point is detected by said inflection point detecting means