Sprues?runners and gatesfulfill the function of conveying the plastics melt from the nozzle of theinjection unit to the individual cavities. While it is true that they may bereused in the form of regrind, their presence nevertheless means a reduction inthe performance of the injection molding machine inasmuch as they must beplasticated in the barrel. With smaller parts?they may account for50 % or more of the actual shot weight.SprueThe sprue may be considered thecontinuation in the mold of the channel in the nozzle. Single-cavity moldswhere the sprue leads directly to the molded part are said to have direct spruegating.Very often, the performance of asingle-cavity injection mold is determined by the cooling time of this sprue.In addition to providing adequate cooling of the sprue bushing, the diameter ofthe smallest opening in the sprue bushing should be kept as small as possibleand permitted by proper filling of the cavity.
No universally applicable rulescan be given here?since filling of the cavity depends on very many factors.The sprue should have 1.5° of draft.Greater draft may simplify removal from the sprue bushing, but function of thelength of the runner?since it may be assumed that the pressure lost in a runner increasesat least proportionally with the length. In all likelihood it will probablyincrease more than proportionally?since the cross section is reduced by solidification of the meltalong the walls?and the more so the greater the distance from the sprue.
Because thesprue and runner system represent lost material and lost plasticating capacity,the runners should be designed to be as short as possible and with the smallestpossible cross section. The length of the runners is determined by the numberof cavities in the mold and the geometrical arrangement of the individualcavities.Shape of runner cross sectionThe smallest surface area, and thereforethe least amount of cooling in relation to the cross-sectional area?is providedby a full round runner. It should be thus employed whenever possible. The meltsolidifies last in the center of a round runner. Accordingly?the plasticsmelt will continue to flow the longest under holding pressure down the centerof a full round runner.
Hence gates (the locations where the runners enter thecavities) should be so designed that melt flows through them into the cavityvia a circular or rectangular cross section fed from the center of the runner.Friction of the plastic melt at the smallest cross section of the runnerprovides for local heating of the steel around the gate so that melt can beforced in under holding pressure a longer period of time to compensate forshrinkage before the gate freezes.Full round runners cannot be used when flatslide surfaces must move relative to the runners. In such cases, a recessedhalf-round runner may be employed (Fig.
2.1 /2). The advantage of this shape isthat it has to be machined on only one side of the mold plate. However, arecessed half-round runner with the same radius of curvature as a full roundrunner of identical diameter contains 12.5 % more material.