Method of molding a plastic article including injecting based upon a

Abstract

A method for injecting a plastic material into a mold cavity to precisely fill the mold cavity under low pressure and to avoid packing the mold cavity with plastic material. The plastic material is injected into the mold cavity at a rate sufficient to maintain an unbroken melt front and to avoid any spraying or splashing of the plastic material within the mold cavity. After an initial unbroken melt front is established, the rate at which the material is injected is increased in proportion to the size of the melt front. After a predetermined amount of the material has entered the mold cavity, the material is injected based upon a rate-dominated control algorithm which maintains the unbroken melt front. After detecting an indicia of a decrease in the surface area of the melt front, the material is injected based upon a pressure-dominated control algorithm where the injection rate is adjusted as needed so that an injection set limit is not exceeded in order to prevent overrunning the end point of the mold process and to eliminate splashing and associated backflow problems. The injection pressure is monitored and controlled until the mold cavity has been precisely filled, at which time the injection pressure is maintained until the plastic article has hardened within the mold cavity. By filling the mold cavity exactly, the prior art problems associated with packing, such as warping and flash, are eliminated in the present invention and a more uniform product is produced from less material.