I was doing a case study on the plastic used in cars for controlling the AC in a manual type system which actually snapped in mine it just rotates according to the knob position in the dashboard, pushing and pulling with a stiff cable/rod. It finally separated as you can see in the picture. So rotating the knob effectively does nothing. I am not considering electronically actuated (Automatic type) AC in cars enter image description here

Please note, the notch is indeed a design flaw as stresses are more concentrated there. I did ANSYS on a model of this however this is only a Material Selection Course that I am studying. So I need to limit myself to just that.

I went over the Ashby textbook and saw a (similar?) example in the 4th Edition.


$\frac{K1c}{E}$ was derived here by substituting the hooke's law and ignoring all material independent variables.

Everything else except Young's modulus and Fracture Toughness is independent of material so we exclude that according to Ashby. And arrived at ABS, NYLON PLA as the best materials like me in the textbook too. I reckoned it could apply here too.

There are three limits for the same objective, displacement limited, load limited and energy limited. I went with deflection limited since I imagined the part would require some deflection to snugly fit during assembly.

I also put an arbitrary limit of 10% strain since any significant cant change in geometry will result in obsolete part, prices not more expensive than ABS, not weaker than ABS.

I got cardboard and paper, ABS, PLA in level 2 on CES Edupack.

The part I don't understand

  1. Clearly, steel for example is cheaper and stronger than ABS OR any polymer, it wouldn't fail. But my problem is why do automakers around the world use plastic in all road cars when metal does the job better? Is it for lower production costs? My professor doesn't buy into the argument of "Planned Obsolesence" here.

  2. What is a better material index here? The Young's modulus in the denominator looks suspicious, so I should just be maximizing K1c?

  3. I should consider "Toughness" and not "Fracture Toughness" since clearly there are notches/imperfections and a better Fracture Toughness only makes things worse?

Yes, I know top trim cars have electronic actuators but I meant only on manual type AC controls

Here is the video of this mechanism

It is a mirror image to my picture because it's in an Australian car where they drive on the left.

  • $\begingroup$ It controls an Air Mix door (aka Blend Door) in case you are wondering - Temperature control knob. Please ignore the other mechanism which is for the vent selector or air flow selector knob. Which has also failed. I have observed that this fails often in hot places like the South of US (but not often as manual-type AC is rare), India, UAE (Middle East), Indonesia, Australia etc. $\endgroup$ – Ahmad Shumayal Feb 9 '19 at 19:36

You forgot one one parameter : mass... if all the plastic in a car was replaced with metal, then the total mass of a car would go up by 30% (wag) and the subsequent fuel consumption down....

That is why lots of bits are plastic... also cost of course...

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  • $\begingroup$ Yes, that is indeed true. Over here, I am trying to derive mathematically the best material index for this application, applying the cost or weight limits as a secondary criterion. The only way possible for me to get the plastic as the answer was with K1c/E but my professor considered "Fracture Toughness" as irrelevant here. And also objected to Young's modulus in the denominator. $\endgroup$ – Ahmad Shumayal Feb 9 '19 at 20:12
  • $\begingroup$ And also design to be “just strong enough”... Why does so much Victorian engineering survive - will car engines built today do as well? $\endgroup$ – Solar Mike Feb 9 '19 at 20:15

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