Why the Casting Process Decides Impeller Life
An impeller is only as good as the metal it is poured from and the soundness of the casting beneath the machined surfaces. Porosity, shrinkage and inclusions that sit inside a blade do not show up on a dimensional inspection sheet, but they become fatigue crack initiation points once the fan is running. SINDFAN operates its own aluminium foundry, so alloy selection, moulding, heat treatment, inspection, machining and balancing stay under one roof instead of being split across subcontractors.
Alloy Choice: A356 and LM6
A356 is an Al-Si-Mg casting alloy, typically around 7% silicon with a controlled magnesium addition. The magnesium is what makes it heat-treatable: during ageing it precipitates fine Mg2Si particles that raise strength considerably. A356 is specified where aerospace-grade ductility and fatigue performance are required, and it responds reliably to T6.
LM6 is a British Standard aluminium-silicon alloy with roughly 12% silicon, close to the eutectic point. That high silicon content gives it excellent fluidity, so it fills thin sections and reproduces fine blade detail that other alloys struggle to achieve. It also offers good corrosion resistance, which matters in wet or mildly aggressive air streams, and LM6 is used for fire-rated duty. It reaches its working properties without heat treatment.
Practical selection rule: use A356 where maximum strength-to-weight and fatigue resistance dominate and T6 processing is acceptable; use LM6 where castability, thin blade sections, corrosion resistance or fire-rated duty dominate.
Die Casting or Sand Casting
Volume and physical size decide the route.
- Die casting uses a reusable steel die. Tooling cost is higher but is amortised over production quantity, dimensional repeatability is good, and it suits smaller impellers made in volume.
- Sand casting uses a sacrificial mould, so there is no die investment. It suits larger diameters, lower volumes, and one-off or heavily customised geometries.
Impeller blade count and blade pitch are custom in both routes. Overall casting range is Ø150 mm to Ø3150 mm.
T6 Heat Treatment for A356
T6 is a two-stage process. Solution treatment holds the casting at elevated temperature long enough to dissolve the alloying elements into solid solution. A rapid quench then traps them in a supersaturated state. Artificial ageing follows at a lower temperature, precipitating fine particles that obstruct dislocation movement. The net effect on A356 is a clear gain in yield strength and hardness over the as-cast condition, with ductility retained. T6 is applied to A356; LM6 does not require it.
X-Ray Inspection to ASTM E155
Die-cast impellers are X-ray inspected in accordance with ASTM E155, the standard reference radiographs for aluminium castings. Radiography reveals internal discontinuities that no external check can find: gas porosity, shrinkage cavities, inclusions and cold shuts. Comparing the radiograph against the ASTM E155 reference set gives a consistent, documented judgement of internal soundness rather than an opinion. For the buyer, this means the impeller's integrity is verified before it leaves the foundry, not assumed.
Machining and Balancing
After casting and, where applicable, T6 treatment, the impeller is machined: bore, hub, mounting faces and blade tips are brought to final tolerance. It is then dynamically balanced to ISO 1940-1 G4.0. G4.0 is a balance quality grade defining permissible residual unbalance relative to rotational speed, which directly limits the vibration and bearing load the finished fan will transmit.
What to Send When You Enquire
To quote accurately we need the duty: airflow and pressure, operating speed, air or gas temperature, the medium and whether it is corrosive or a fire-rated application, plus the impeller diameter and expected production volume. Custom blade counts and pitch are normal for us.
Contact SINDFAN on WhatsApp at +852 9524 8434 or email info@sind-tech.com with your duty and we will confirm the appropriate alloy, casting route and inspection scope for your impeller.