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So, here is my LEGO Space Shuttle. Originally built in 1991, never disassembled (unlike the rest of my sets), so all parts are original and all were exposed to the same amount of sunlight.

However, the amount of yellowing is highly variable. The hinge for the manipulator arm (just back of the windshield) is like-new white, and the alignment pin for the arm (just in front of the green brick) isn't much different.

Two of the wing panels are much whiter than the others, and although it's hard to see, the hinged parts towards the back are two distinct shades of yellow.

What's the reason for this?

  • 1
    This is a great question. The Transcon 2 shuttle is notorious for its yellowing. – fabian Dec 31 '17 at 10:24
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There are several reasons for this:

  1. There isn't just one formula for the ABS plastic that is used in LEGO. There are actually quite a few. Depending on the color, translucency, and function, the mix of materials will vary.

    Translucent pieces are quite a bit harder and more rigid than other Lego bricks. Mini-figure accessories tend to be quite a bit softer than other pieces (so they don't cause internal damage if/when swallowed).

    If you pull out several types of parts and pay close attention to the hardness, flexibility, and stiffness, you will notice that not all LEGO is made the same.

  2. Even for similar parts, there is variance in the chemical mixes. When you get a set, it is highly likely that the parts weren't made from the same batch of plastic.

    Mixing plastics with minor variations can cause changes in the rates of UV damage and discoloration. It is really hard to get perfectly matched colors across multiple batches of plastic.

    The fact that LEGO colors are so consistent is due to the fact that they have really high standards for quality control.

Oh, and if you want to restore your set to that new pristine white color, you can treat it with retrobrite or oxyclean and hydrogen peroxide. There are plenty of articles online if you do a quick search for either.

  • So, what did they do with that hinge to keep it from yellowing, and what disadvantage does it have that keeps them from using it for everything? – Mark Jan 1 '18 at 8:59
  • +1 for part 2 of your answer - most likely this is btch variation within the parts - different shapes will almost certainly be different batches. Translucent elements are not made from ABS (as it can't be clear) but from a polycarbonate. – Zhaph - Ben Duguid Jan 1 '18 at 9:46

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