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How to reduce casting defects

Casting workshop

During the production process, foundry enterprises will inevitably encounter casting defects such as shrinkage cavities, air bubbles, and segregation, resulting in a low yield of castings, and they will face a lot of manpower and power consumption when they return to the furnace for production. How reduce casting defects is a problem that casting people have always been concerned about.


  1. Good castings start from high-quality smelting


Once castings are poured, the melting process must first be prepared, checked, and handled. Acceptable minimum standards may be used if required. However, a better option is to prepare and implement a near-defective melting schedule.


  1. Avoid turbulent inclusions on the free liquid surface


This requires avoiding excessive velocity at the front free surface (meniscus). For most metals, the maximum flow rate is controlled at 0.5m/s. For closed gating systems or thin-walled parts, the maximum flow rate will increase appropriately. This requirement also means that the falling height of the molten metal cannot exceed the critical value of the “static drop” height.


  1. Avoid the laminar flow inclusion of the surface solidified shell in the molten metal


This requires that during the entire filling process, the front end of any metal liquid flow should not stop flowing in advance. The metal meniscus in the early stage of filling must remain movable and not be affected by the thickening of the surface crust, which will become part of the casting. To achieve this effect, the molten metal front can be designed to expand continuously. In practice, only the bottom bet “uphill” can achieve a continuous upward process. (For example, in gravity casting, flow upwards from the bottom of the sprue). This means:


Bottom injection gating system;
No “downhill” form of molten metal falling or sliding;
Do not have large areas of horizontal flow;
There should be no stoppage of metal front flow due to pouring or cascading flow.


  1. Avoid air wrapping (bubbles)


Avoid the air bubbles generated by the gating system from entering the cavity. It can be achieved by:
Reasonable design of stepped sprue cup;
Reasonable sprue design, fast filling;
Reasonable use of “dams”;
Avoid “well” or other open gating systems;
Use a small cross-section runner or use a ceramic filter near the junction of the sprue and the runner;
use of degassing devices;
The pouring process is uninterrupted.


  1. Avoid sand core pores


Avoid air bubbles generated by sand cores or sand molds from entering the molten metal in the cavity. The sand core must have a very low air content, or use proper exhaust to prevent the generation of sand core pores. Clay-based cores or mold repair compounds should not be used unless they are guaranteed to dry completely.


  1. Avoid shrinkage cavity


Due to convective effects and unstable pressure gradients, upward feeding cannot be achieved for castings with thick and large cross-sections. Therefore, all feeding laws must be followed to ensure a good feeding design, and computer simulation technology should be used for verification and the actual casting of samples. Control the flash level at the mold and core junction; control mold coating thickness (if any); control alloy and mold temperature.


  1. Avoid convection


Convective hazard is related to setting time. Both thin-walled and thick-walled castings are immune to convective hazards. And for medium wall thickness castings:


Reduce convection hazards through casting structures or processes;
Avoid upward packing;
Turn over when full.


  1. Reduce segregation


Prevent segregation and control it within the standard range, or the component overrun area allowed by the customer. Try to avoid channel segregation if possible.


  1. Reduce residual stress


Do not carry out water (cold or hot water) medium quenching after solution treatment of light alloys. If the casting stress does not appear to be high, a polymer quenching medium or forced air quenching can be used.


  1. Given the reference point


All castings must be given reference points for dimensional inspection and machining.


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