• Overview

  The purpose of this technical brief is to give the new designer or engineer the basics in the limitations of designing an aluminum extrusion. Extrusions are 3 dimensional shapes that can be fabricated after manufacturing to produce near net shape parts. To help minimize cost and maximize efficiency an extruded aluminum heat sink needs to be low in weight, high in exposed surface area, and simple in design. Violation of these basic design criteria will result in less than optimized heat sinks for the specific application.

• Tolerances

  In general, an extruded shape has a wider tolerance than a machined part. In many machined aluminum parts, holding feature tolerances can be done at +/- 0.005 inches or better. In extrusion, due to the nature of the process these feature tolerances can be up to +/- 0.100 inches or more depending on the overall size of the profile and its technical complexity. Most extruders, including Simply Products Company, comply with the Aluminum Association standard tolerances listings which vary allowable tolerances based on the overall circle size of the extrusion. In most cases the following tolerances apply to give dimensions:

  +/- Tolerance in Inches
  	Up to 10" Circle	Greater than 10" Circle
  Less than 0.125"	0.006	0.014
  0.125 to 0.249	0.007	0.015
  0.250 to 0.499	0.008	0.016
  0.500 to 0.749	0.009	0.017
  0.750 to 0.999	0.010	0.018
  1.000 to 1.499	0.012	0.019
  1.500 to 1.999	0.014	0.024
  2.000 to 3.999	0.024	0.034
  4.000 to 5.999	0.034	0.044
  6.000 to 7.999	0.044	0.054
  8.000 to 9.999	0.054	0.064
  10.00 to 11.99	---	0.074
  12.00 to 13.99	---	0.084
  14.00 to 15.99	---	0.094
  16.00 to 17.99	---	0.104

  
  
• Circle Sizes

  Most extrusions are made in large hydraulic presses that use a specifically shaped tool to form parts. The size of the press is designated by the number of tons of force it can apply and the maximum circle size of the tooling. The circle size of the tool limits the overall size of the extrusion profile that can be made by a given press. The largest allowable size is the combination of width and height that will fit within the given circumscribed diameter. Press circle sizes range from 1.0 inches to as large as 30.0 inches. In general the larger or smaller the tool size the fewer presses are available in the world.

• Extrusion Ratios

  One of the limiting factors in the extrusion process is the ratio of fin height to minimum air gap between fins. This air gap between fins represents the amount to tool steel material used in the production die that helps to hold back the flow of heated aluminum during the extrusion process. If the distance between fins is too narrow and tall the result will be die failure. Die failure can be either fin thicknesses that “shift” changing the width of the air gap setting fin thickness out of tolerance. The other failure mode is breaking of the steel resulting in redesign and remanufacture of the die.

  The limitation on extrusion ratios varies with the overall size of the extrusion profile. Extrusion ratios of 14 to 1 are commonplace for dies that are less than 4 inches in diameter. Ratios of only 10:1 or less are more typical for dies that are 10.0 inches and above. Some manufacturers can produce ratios of up to 28:1 depending on many factors. There is no set limitation on ratios that can be calculated. The limit of the ratio depends on many factors including the type of extrusion press, the skill of the operator, the alloy being pressed, and many other details.

• Physical Properties

  The most common aluminum alloy used for extrusion is 6063 which is 98.9% aluminum. The most common temper is T5. This alloy has excellent physical characteristics, can be made in various tempers, and is relatively low in cost. Following are typical properties:

  	Coef. Thermal Expansion			13X10-6 inch/inch
  	Thermal conductivity			209 W/MK
  	Electrical Resistivity			3.1 X 10-6 ohm – CM
  	Melting temp.			616 C
  	Density			2.7 g/cc>
  	Tensile strength, yield			145 Mpa

What characteristics make a heatsink a good one? There are a number of factors to consider:

• High Heatsink Surface

  It's at the surface of the heatsink where the thermal transfer takes place. Therefore, heatsinks should be designed to have a large surface; this goal can be reached by using a large amount of fine fins, or by increasing the size of the heatsink itself.

• Good aerodynamics

  Heatsinks must be designed in a way that air can easily and quickly float through the cooler, and reach all cooling fins. Especially heatsinks having a very large amount of fine fins, with small distances between the fins may not allow good air flow. A compromise between high surface (many fins with small gaps between them) and good aerodynamics must be found. This also depends on the fan the heatsink is used with: A powerful fan can force air even through a heatsink with lots of fine fins with only small gaps for air flow - whereas on a passive heatsink, there should be fewer cooling fins with more space between them. Therefore, simply adding a fan to a large heatsink designed for fanless usage doesn't necessarily result in a good cooler.

• Good thermal transfer within the heatsink

  Large cooling fins are pointless if the heat can't reach them, so the heatsink must be designed to allow good thermal transfer from the heat source to the fins. Thicker fins have better thermal conductivity; so again, a compromise between high surface (many thin fins) and good thermal transfer (thicker fins) must be found. Of course, the material used has a major influence on thermal transfer within the heatsink. Sometimes, heat pipes are used to lead the heat from the heat source to the parts of the fins that are further away from the heat source.

• Perfect flatness of the contact area

  The part of the heatsink that is in contact with the heat source must be perfectly flat. A flat contact area allows you to use a thinner layer of thermal compound, which will reduce the thermal resistance between heatsink and heat source.

• Good mounting method.

  For good thermal transfer, the pressure between heatsink and heat source must be high. Heatsink clips must be designed to provide a strong pressure, while still being reasonably easy to install. Heatsink mountings with screws/springs are often better than regular clips. Thermoconductive glue or sticky tape should only be used in situations where mounting with clips or screws isn't possible.

Good and bad example for contact area flatness