Elements of the cluster
Patterns are attached to a wax sprue either directly through their gates or through runner bars. In most industrial clusters the central sprue is both the distribution channel and the reservoir of feed metal. Cluster design determines how many parts are poured per mould and therefore has a strong influence on cost.
Filling
Metal should enter the cavity quickly enough to fill thin sections before freezing, but without turbulence that entrains air and oxide films. Design measures include gating into heavier sections, avoiding free-falling streams where the alloy forms strong oxide films, sizing the pouring cup and sprue so that they remain full, and using filters where appropriate. Shell permeability allows air to escape; blind features that could trap air may need vents.
Feeding and directional solidification
Most alloys contract in volume when they change from liquid to solid. Unless liquid metal can flow in to compensate, the deficit appears as shrinkage cavities or dispersed porosity. The governing principle is directional solidification: freezing should begin in the regions furthest from the feed source and progress steadily towards the gate, so that a liquid path stays open until the casting is solid.
In practice this means:
- gates are attached to the heaviest section of the part;
- the gate and sprue must stay liquid longer than the section they feed, which is assessed with the section modulus (volume divided by cooling surface);
- isolated heavy sections separated from the gate by thin walls are the classic site of shrinkage and should be avoided in design or fed with an additional gate;
- shell insulation or local cooling can be used to steer the temperature gradient.
Alloys with a long freezing range, such as many nickel alloys and some bronzes, tend to form fine dispersed micro-porosity rather than a concentrated cavity and are more demanding to feed.
Solidification structure and cooling rate
The hot ceramic shell extracts heat slowly compared with a metal die. Investment castings therefore solidify relatively slowly, which favours complete filling but produces a coarser grain than chill casting. Grain size, dendrite spacing and segregation depend on local section thickness, shell temperature and pouring temperature. Where a fine equiaxed grain is required, particularly in nickel and cobalt alloys, nucleating additions in the first shell coat and low superheat are used. Cooling after solidification affects residual stress and, in hardenable steels, the as-cast microstructure.
Simulation
Mould-filling and solidification simulation is used to compare gating variants before tooling is finalised, to locate the last regions to freeze and to estimate the risk of porosity. Simulation supports, but does not replace, sectioning and radiography of first samples.
Related defect descriptions: shrinkage porosity, misruns, hot tears.