it is therefore possible to position the pin 121 or form the fluid inlet 10 in any other part of the mold having a complicated shape it is possible to position the pin 121 in any appropriate location the compressed fluid which has been used for forming the hollow interior of the molded product is easy to collect for reuse as is obvious from the foregoing another mold for hollow injection molding embodying this invention is shown in fig4 the mold is provided with a coil compression spring 7 in the control chamber 112 for causing the pin 121 to project and close the fluid inlet 10 instead of using a control fluid 50 as in example 1 in all other respects the mold is identical to that of example 1 the following is a description of the mode in which the mold is used and of the advantages which it affords the spring 7 urges the plunger 12 to advance when the synthetic resin is going to be injected as a result the pin 121 projects through and closes the fluid inlet 10 when the fluid 930 is forced through the fluid admitting nozzle 4 it increases the pressure in the fluid admitting chamber 111 sufficiently to overcome the closing force of the spring 7 and retract the plunger 12 as a result the pin 121 opens the fluid inlet 10 while.