The casting process: making your part
Once you have created your core, mould and selected your alloy, you can start to cast your part.
Controlling the rate of solidification is vital to achieve a quality casting.
This is the point that all of the previous process steps converge and you finally get to make your part. But it's not as straightforward as just pouring the metal into the mould. There's quite a bit to think about- including controlling the rate of solidification.
What does a good sand casting foundry do?
A good foundry will ensure that the furnace is kept clean to stop any impurities contaminating the casting. The understanding of the technical aspects of sand casting now allows us to fine tune factors such as hydrogen content.
The furnace needs to be degassed to control the quality of aluminium as it will react with hydrogen present. Degassing can be combined with flux treatment.
What is the impact of solidification rates?
Solidification rates are important in aluminium castings, with faster solidification improving the quality and material properties in specific areas of the part.
Accelerating the solidification rate will produce desired higher mechanical properties by altering grain size.
How are solidification rates controlled?
The sand casting process is designed and modified for each project to control solidification rates. Solidification rates can be influenced by the addition and placement of chills in the mould and feeder positioning.
The thermal conductivity properties of the sand type chosen also has an impact. For example, Zircon sand has higher thermal conductivity than other sand types- cooling the entire casting down faster.
What is the impact of temperature on the sand casting process?
Temperature is important. It's vital that the temperature isn’t too high to boil off some of the alloying metals in the aluminium. Maintaining temperature also confers repeatability benefits.
Temperature is also critical because we have modelled a very precise solidification behaviour to get the right product, and this will have assumed a particular starting temperature. If this isn't met then the final
part won't reflect what the designer intended, potentially causing scrap or out of tolerance material properties.
How does good sand casting process design influence the final part?
Good casting method design reduces potential defects such as oxides in the metal, inclusions, porosity and cavities. Optimising metal flow with good mould pack design reduces potential resistance in the mould. This includes radii on internal features to avoid turbulence.
What problems are caused by turbulence in the casting process?
Low turbulence runner feeder systems are vital to achieving a quality casting, helping to control the rate at which metal is poured
into the mould.
This decreases porosity by reducing gas entrainment. Porosity is linked to lower strength, and is often only found if the part is X-rayed, CT scanned or when machining surfaces of the part.
Read more: What is sand casting used for?