A mold assembly includes an upper mold cavity, a base plate, and a flow element. The upper mold cavity defines a primary runner and a plurality of sub-runners on a top surface in communication with the upper mold cavity. A gate is defined in the primary runner for injecting liquefied plastic into the upper mold cavity. The base plate defines an outlet thereon corresponding to the gate for outputting the liquefied plastic. The upper mold cavity and the base plate corporately form a closed cavity therebetween. The flow element is received in the closed cavity and movable between a first position and a second position. In the first position, the flow element is engaged with the primary runner and plurality of sub-runners. In the second position, the flow element is separated with the primary runner and plurality of sub-runners.