An injection device which injects a molding material into a mold, the injection device comprising:
- an injection cylinder formed with an injection chamber accommodating the molding material;
- a supply device supplying the molding material into the injection chamber;
- a gas supply device composed of a high-pressure gas cylinder and a pressure reducing valve to supply into the injection chamber a gas of pressure lower than a pressure capable of dissolving in the molding material;
- a gas integrated flow measuring device measuring an integrated flow of the gas supplied into the injection chamber;
- an injection shaft reciprocating in a cylinder axis direction in the injection chamber;
- an injection driving device causing the injection shaft to reciprocate in the cylinder axis direction;
- and
- an injection control device configured to control the injection driving device to move the injection shaft backward and forward in the injection chamber to expand and reduce the injection chamber before supplying the molding material from the supply device into the injection chamber, such that when compressing and dissolving a predetermined amount of the gas in the molding material remaining in the injection chamber, a cycle from moving the injection shaft backward to supply the gas of an amount less than the predetermined amount into the injection chamber to moving the injection shaft forward to compress and dissolve the gas in the injection chamber into the molding material in the injection chamber is performed for a plurality of times until the integrated flow measured by the gas integrated flow measuring device reaches the predetermined amount,
- wherein the injection control device is configured to stop backward movement of the injection shaft and move the injection shaft forward to an initial position when the integrated flow measured by the gas integrated flow measuring device reaches the predetermined amount, and then, as the injection shaft is pushed back by the molding material supplied into the injection chamber from the supply device, stop the injection shaft at a position where an amount of the molding material in the injection chamber reaches a predetermined amount, thereafter move the injection shaft forward at a predetermined speed for a predetermined distance to inject the molding material into the mold, and thereafter to perform a pressure holding process in which the injection driving device moves the injection shaft forward at the predetermined pressure for a predetermined time, wherein the initial position is a position at a time when a previous pressure holding process was completed.