Sprues?runners and gates fulfill the function of con¬veying the plastics melt from the nozzle of the injec¬tion unit to the individual cavities.
While it is true that they may be reused in the form of regrind, their pres¬ence nevertheless means a reduction in the perform¬ance of the injection molding machine inasmuch as they must be plasticated in the barrel. With smaller parts?they may account for 50 % or more of the actual shot weight.SprueThe sprue may be considered the continuation in the mold of the channel in the nozzle. Single-cavity molds where the sprue leads directly to the molded part are said to have direct sprue gating.
Very often, the performance of a single-cavity injec¬tion mold is determined by the cooling time of this sprue. In addition to providing adequate cooling of the sprue bushing, the diameter of the smallest open¬ing in the sprue bushing should be kept as small as pos¬sible and permitted by proper filling of the cavity. No universally applicable rules can 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 with a longer sprue results in a greater diameter and thus longer cooling time. The nozzle orifice should be about 0.
5 mm smaller in diameter than the smallest opening in the sprue bushing (Fig. 2.1/1) so that there is no undercut at the end of the sprue to hinder re¬moval.RunnersIn multiple-cavity molds?the plastics melt must flow to the individual cavities through runners in the mold parting line. The same basic rules that apply to the sprue apply also to the cross section of these run¬ners. An additional factor that must be considered is that the cross section is also a function of the length of the runner?since it may be assumed that the pressure lost in a runner increases at least proportionally with the length.
In all likelihood it will probably increase more than proportionally?since the cross section is re¬duced by solidification of the melt along the walls?and the more so the greater the distance from the sprue. Be¬cause the sprue and runner system represent lost mate¬rial and lost plasticating capacity, the runners should be designed to be as short as possible and with the smallest possible cross section. The length of the run¬ners is determined by the number of cavities in the mold and the geometrical arrangement of the individ¬ual cavities.Shape of runner cross sectionThe smallest surface area, and therefore the least amount of cooling in relation to the cross-sectional area?is provided by a full round runner. It should be thus employed whenever possible. The melt solidifies last in the center of a round runner. Accordingly?the plastics melt will continue to flow the longest under holding pressure down the center of a full round run¬ner.
Hence gates (the locations where the runners enter the cavities) should be so designed that melt flows through them into the cavity via a circular or rectangu¬lar cross section fed from the center of the runner. Friction of the plastic melt at the smallest cross section of the runner provides for local heating of the steel around the gate so that melt can be forced in under holding pressure a longer period of time to compen¬sate for shrinkage before the gate freezes.Full round runners cannot be used when flat slide surfaces must move relative to the runners. In such cases, a recessed half-round runner may be employed (Fig. 2.1 /2). The advantage of this shape is that it has to be machined on only one side of the mold plate.
How¬ever, a recessed half-round runner with the same radi¬us of curvature as a full round runner of identical di¬ameter contains 12.5 % more material.