Casting defect (surface defect) that may occur in all casting materials, regardless of molding processes and/or casting processes.
This is a defect that is visible to the unaided eye and frequently causes reject of the casting. In pressure die casting, the defect is referred to as cold lap point. It may also form inside the material as cold lap inclusion with its own casting skin so that it only becomes visible during machining.
Cold shuts preferably occur at level surfaces with relatively large thicknesses. The result may be separation of coherent material so that holes, unfilled areas, as well as rounded edges and overlaps remain. In pressure die castingcold shuts (cold laps) can be identified by very fine and thin foliation cleavages (Figures 1 to 4).
Cold shuts are caused by premature solidification of the melt during casting, e.g. caused by insufficient casting or mold temperatures or insufficient casting height or casting rate (see Mold filling time, Mold filling time in pressure die casting). Moreover, insufficient permeability to gas may result in overpressure within the mold retarding or decelerating the metal flow. Incorrect casting properties of the alloy may also cause cold shuts.
The major causes for this defect in gravity die casting may be insufficient ingot temperatures, too slow mold filling with insufficient venting and non-metal contaminations in the melt.
With pressure die casting the most frequent causes are faults in the casting piston movement or unsuitable selection of the changeover point from slow shot to fast mold filling phase as well as insufficient casting flow and gating design.
Moreover, cold laps (pressure die casting) originate from situations in which individual areas of the melt solidify partially or completely during mold filling, e.g. in case of retarded mold filling, too long flow paths or insufficient diameter of gates (that partly freeze during mold filling and thus form a necking in the available flow cross section). In addition, overlaying may also take place when
the solidified surface shell comes loose and molten metal flows into the gap between surface shell and mold wall.
The following remedial measures have proved successful:
1. Increasing the casting temperature to prevent premature solidification of the casting flow.
2. Ensuring short flow paths through optimization of the gating system.
3. Predominantly with thin cast metal parts high casting rates and highcasting pressure must be ensured.
4. Appropriate venting of air must be ensured; cold shruts have repeatedly been experienced mainly in moist and too warm mold materials due to most casting gasses that quench the metal.
5. A generally applicable measure is to improve the permeability to gas of the mold (e.g. through coarser grain size, reduction of fines, etc.).
6. In gravity die casting it may be recommendable to increase the ingot temperature, to check the runner and gating system and to widen the cross sections, to implement additional venting channels and to check the efficiency of grain refinement and refinement of the casting metal with regard to casting defects (non-metal contaminations in the melt cause viscosity).
7. In pressure die casting, optimum casting conditions must also be ensured (increase in casting metal and mold temperature), increasing the gating, runner and overflow volume, rounding edges and corhers, increasing wall thicknesses, if required, increasing the silicon content in the casting metal, increasing the casting pressure and modifying the chronological sequence of the hydrualic phases during mold filling, checking the injection piston and casting chamber for wear to prevent the piston from getting stuck during mold filling, prevention of pressure drop during mold filling (control of the gas cushion in the hydraulic cylinder) and control of the casting device (see Casting drive).