The invention is an integrally blow-moulded bag-in-container and preform for making it. The bag-in-container has an inner layer forming the bag and an outer layer forming the container, and a mouth fluidly connecting the volume defined by the bag to the atmosphere. The container further has at least one interface vent fluidly connecting the interface between inner and outer layers to the atmosphere, wherein the at least one vent runs parallel to the interface between inner and outer layers and opens to the atmosphere at a location adjacent to, and oriented coaxially with the bag-in-container's mouth. Processes for manufacturing a preform and a bag-in-container as defined above are defined too.

For a preform, on an outer circumferential side of a base material layer, which is made of a first colored resin, a colored layer, which is made of a second colored resin having a color different from that of the first colored resin and which forms a gradation region in which the layer thickness gradually decreases upward, is formed. Furthermore, on the outer circumferential side of the colored layer, a coating layer, which is made of the first colored resin, is formed.

A method for manufacturing a container includes injecting a first material into a first mold to form a top portion of a preform. The first material and a second material are injected into the first mold to form a bottom portion of the preform. The preform is disposed in a second mold. The preform is blow molded into a finished container.

A method of producing a resin container includes injection-molding a resin intermediate molded body of bottomed cylindrical shape as a first injection-molding, injection-molding a resin material outside the intermediate molded body as a second injection-molding, to produce a multilayer preform in which a resin layer is layered on an outer peripheral side of the intermediate molded body, and blow-molding the preform in a state of containing residual heat from injection-molding to produce a resin container having a bottom portion thicker in wall thickness than a body portion. A portion of the inner peripheral side of the intermediate molded body, the portion reaching a bottom portion of the intermediate molded body, has a tapered region in which the wall thickness increases toward the bottom portion.

A manufacturing method of a resin container includes: a first injection molding process for injection-molding a first layer of a preform by using a first resin material, the preform having a bottomed cylindrical shape and including a groove that extends in an axis direction; a second injection molding process for injecting a second resin material into the groove of the first layer, the second resin material having a color that is different from a color of the first resin material, and laminating a second layer on an outer peripheral side or an inner peripheral side of the first layer; and a blow-molding process for blow-molding a preform that includes multiple layers and has been obtained in the second injection molding process in a state where residual heat at a time of injection molding is contained, and manufacturing the resin container having a vertical-striped color pattern formed by using the second layer.