Sprue, runner, and gate

The feed system is the flow melt passage in the mold, between the nozzle of the IMM and the mold cavity (Figure 17.1). This feature has a considerable effect on both the quality and economy of the molding process. The feed system must conduct the plastics melt to the cavity via a sprue, runner and gate at the correct temperature/pressure/time period, must not impose an excessive pressure drop or shear input, and should not result in non-uniform conditions at the cavities of multiimpression molds. 

The two most widely used methods for molding thermoset are compression and transfer molding. Thermosets, you will recall, undergo a permanent set, i.e., become essentially insoluble and infusible under the action of heat. Consequently, techniques for fabricating thermosets must take due cognizance of and make allowance for the fact that sprues, runners, and gates are not reusable and therefore constitute rejects. 

The main limitation of the process is the loss of material in the sprue, runners and gates - as thermosetting materials cure during moulding, this cannot be reused. Moulding tools are usually more complex and therefore more costly than compression-moulding tools. 

The sprues, runners, and gates that lead the material from the nozzle of an injection machine to the mold cavity. 

Runner system n. This term is sometimes used for the entire mold feeding system, including sprue, runners, and gates, in injection or transfer molding. 

Similar to die casting of metals. A thermoplastic molding compound is heated to plasticity in a cylinder at a conti-olled temperature and forced under pressure through sprues, runners and gates into a mold the resin solidifies rapidly, the mold is opened, and the parts ejected with certain modifications, thermosetting materials can be used for small parts. 

Similar to compression molding except that mold is closed empty and rubber compound is forced into it through sprues, runners and gates. 

Thermoset molding compound is fed from hopper into a transfer chamber where it is heated to plasticity. It is then fed by means of a plunger through sprues, runners, and gates of closed mold into mold cavity. 

The polymer melt injected from the nozzle of the injection molding machine flows through the sprue, runner and gate, and finally enters the cavity. The, feed system is a generic term for the sprue, runner and gate. The feed system is usually included in the flow analysis as part of the flow channel upstream to the cavity (Fig. 1.2). 

As shown in Fig. 8-2, the plastic to be molded successively flows through a nozzle, sprue, runner, and gate on its way to the cavity (mold). 

The feed system to moulds has three principal components sprue, runner and gate. The sprue is a tapered bore in line with the axis of the injection unit that conducts the melt to the parting line of the mould. The runner is a channel cut in the parting line of the mould to conduct material flow from the sprue to a point very close to the melt cavity. The gate is a relatively small and short channel which connects the runner to the mould cavity. Runners may be cold or hot, indicating the temperature at which the runner is maintained in comparison to the melt temperature. 

Flow behaviour High melt viscosity for unplasticised PVC. Injection speed not too high to avoid overheating through excessive shear rate. Sprue runners and gates as wide and short as possible. Avoid pin-point gates and sharp turns. 

Mould design (Sprue runner and gate geometries)... 

To prolong pressure transmission to the material in the cavity, the sprue, runners and gate should be as large as possible and temperature dependence of viscosity should be low. Mhen the largest moulded section is much larger than the sprue, other methods are needed to keep the sprue molten and these are discussed below. 

---