Die casting for LED fixtures;
The thermal aspects

 

  As we all know the thermal design of a LED application is paramount for the lifespan of the light fixture.
  More and more designs of light fixtures for LED are made through aluminium die casting.
  The flexibility in graphic design and the relative low unit price in mass production are the main trigger points to use this process.
 
 

 But are you aware of the real thermal result you will get?  

 Ever wondered why after you have made your prototypes by CNC you measure a higher die temperature on the final die casted

product of up to 10°C ?  

 The explanation is relatively easy when you understand the process and doesn’t have to be a breaking point for not using die casting, as long as you take the necessary measures in account during your design.

 In the die casting process the hot aluminium is forced with high pressure into the cavity halves of the mould.

 The mould is foreseen from air vents to let the entrapped air out at the moment the mould is filled.
 
 

 Despite this air vent technology there will always remain some air in the product known as micro blow holes.

 A second reason is the formation of gas in the aluminìum formed during the heating process.

 The entrapped air and gas compresses randomly in air bubbles of around 5-10µm during the injection cycle and slightly expands

during the cool down cycle.

 The amount of air and gas trapped varies according to the size and the complexity of the design and can vary from 0.1 to 0.5%.

 This numbers on themselves don’t seem to be so important, but can influence the thermal conductivity with as much as 5% and can
lead to a major offset in the expected LED die temperature and the lifetime result.
 

 

  A practical case to show the impact:

 

 This heat sink is made in 2 different production methods – die casting and warm forging. By warm forging the raw aluminium is

preheated but stays below the melting point of the aluminium and is for this design pressed vertical in an impression-die mould.

 The die casted pin heat sink gives an Rth of 7.6°C/W The forged pin heat sink gives an Rth of 6.3°C/W or a difference of 17%.

 When tested with 6x the Philips Rebel 1.2W LED the result led to a deltaT of 15°C on the dies which will bring the life time down as

much as 25%.

 As long as you keep in mind during your design made for die casting to calculate with these numbers it still is one of the best and

most preferred processes for higher quantity and complex products.

 We start die casting from volumes of 300kg for small parts and 500kg for bigger parts with a maximum capacity of 600 Tons injection (around 4Kg per injection cycle).

 Of course forging as a process has despite the perfect thermal results also its limitations to the shapes which can be obtained.

 

End of document

 

 

 
 

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