Injection molding plastic parts includes furnishing resin material to a screw barrel having a rotatable screw therewithin, rotating the screw to work the resin material into a molten state, furnishing additive such as liquid color or another additive to the screw barrel interior at a first rate during screw rotation and driving the screw longitudinally from a first position to a second position to force an additive-resin material mixture resulting from screw rotation into a mold while furnishing additive with resin to the screw barrel interior at a second additive addition rate.

A method for producing a foamed molded product includes: using a molding apparatus including a plasticizing cylinder having a high pressure kneading zone and a pressure reduction zone; and a screw; controlling a pressure of the pressure reduction zone to be a first pressure in a state that communication between the high pressure kneading zone and the pressure reduction zone is shut off, the first pressure being not less than an atmospheric pressure and not more than a maximum pressure of the high pressure kneading zone which is reached in a case that a molten resin is brought in contact and kneaded with a physical foaming agent; controlling back pressure of the screw to be a second pressure which is not less than the first pressure; and measuring a predetermined amount of the molten resin from which the gasified physical foaming agent has been separated.

A process of forming molded articles using an injection molding apparatus is provided. The process includes providing a thermoplastic material to the injection molding apparatus. The thermoplastic material is heated such that the thermoplastic material is in a molten state. The molten thermoplastic material is injected into at least one mold cavity of the injection molding apparatus using an injection element. Melt pressure of the thermoplastic material filling the at least one mold cavity is monitored using a sensor. The sensor provides a signal indicative of melt pressure to a controller. The controller controls introduction of a non-reactive additive to the thermoplastic material thereby changing a viscosity of the molten thermoplastic material based on the signal. A molded article is formed by reducing a mold temperature of the thermoplastic material within the at least one mold cavity.

A partial reflux injection function part 5 is provided according to which resin R at the front side of a screw head portion 4s is injected into the metal mold 2 to fill the metal mold 2 with the resin R in synchronization with the refluxing of a predetermined amount of the reflux resin Rm to be used as a part of the resin R to the screw body part 4m side at the rear side of the screw head portion 4s through the inner side of the screw head portion 4s and/or the outer periphery side of the screw head portion 4s. A control function is provided that sequentially performs a plasticization step Wi1 to rotate the screw 4 in the heating cylinder 3 to thereby subject the resin in the heating cylinder 3 to a plasticization processing; an injection step Wi2 to allow, after the completion of the plasticization step Wi1, the screw 4 to move forward to thereby inject, into the metal mold 2, a part of the plasticized resin R accumulated at the front side of the screw head portion 4s of the screw 4 to fill the metal mold 2 with the resin; and a plasticization promotion step Wi3 to allow, after the completion of the injection step Wi2, a part of the remaining part of the accumulated plasticized resin R to be refluxed as the reflux resin Rm to the rear side of the screw head portion 4s.