Preform and bottle

1. A method for manufacturing a synthetic resin preform, the preform comprising: (i) a base material layer made of a first colored resin, (ii) a colored layer that is made of a second colored resin having a color different from that of the first colored resin and forms a gradation region on an outer circumferential side of the base material layer, wherein a layer thickness of the gradation region gradually decreases upward, and (iii) a coating layer, made of the first colored resin, on an outer circumferential side of the colored layer, the method comprising:

filling the first colored resin in a gate disposed in correspondence with a middle of a bottom of the preform,

injecting into a cavity of a mold through the gate: (i) the first colored resin for a first predetermined time period, and (ii) the second colored resin material, having a color different from that of the first colored resin, for a second predetermined time period,

wherein:

(i) a starting time point of the first predetermined time period is set to be between a starting time point of the second predetermined time period and a finishing time point of the second predetermined time period,

(ii) a finishing time point of the first predetermined time period is set to be after the finishing time point of the second predetermined time period, and

(iii) an injection rate of the second colored resin is set to be faster than an injection rate of the first colored resin, so that:

(i) the second colored resin penetrates a center portion of the first colored resin under a state in which a region on an outer circumferential surface of the cavity is coated with the first colored resin, where the region corresponds to the gradation region, and then

(ii) the first colored resin penetrates a center portion of the second colored resin located inside the cavity of the mold, such that the first colored resin is caused to flow while stretching the second color resin, so as to form: (a) the colored layer on the outer circumferential side of the base material layer, and (b) the coating layer on the outer circumferential side of the colored layer.

2. The method for manufacturing the preform according to claim 1 , wherein the first colored resin has an IV in the range from 0.5 to 0.63.

3. A method for manufacturing a bottle, comprising:

biaxially stretch blow molding the preform manufactured by the method according to claim 1 , and

decorating with gradations associated with color density that are caused to appear by the colored layer.

4. A method for manufacturing a bottle, comprising:

biaxially stretch blow molding the preform manufactured by the method according to claim 2 , and

decorating with gradations associated with color density that are caused to appear by the colored layer.