Coinjection of hollow articles and preforms

In an injection molding machine including a multi-cavity mold where each cavity is fed by a coinjection nozzle having a plurality of insulated flow paths, said machine having means for controlling and measuring material flowing in each flow path, a method of molding layers of different thermoplastic materials in each mold cavity, layers of each material being molded individually and simultaneously in all cavities, said layers being molded in sequence into a multi-layered article, and at least one material having a substantially different process temperature, comprising the steps of:

• providing individual sources of said different thermoplastic materials each properly and individually conditioned thermally for processing to avoid overheating or freezing,
• providing a mold with a plurality of cavities,
• selecting a number of individual and volumetrically precise quantities of a first material corresponding to the number of mold cavities,
• selecting a number of individual and volumetrically precise quantities of a second material corresponding to the number of mold cavities, the volumetric sum of the individual quantities of the first and second materials being less than the total volume of an individual cavity,
• feeding one precise quantity of said first material to each said mold cavity individually and simultaneously while said second material remains static,
• thereafter feeding one precise quantity of said second material to each said mold cavity individually and simultaneously while said first material remains static, and,
• thereafter feeding an additional quantity of said first material to each said mold cavity individually and simultaneously in order to pack or fill each mold cavity completely to create said multi-layered article, wherein each different material fed to each said mold cavity is measured and controlled individually.