How to stop stress cracks (for free)

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Basically, when the shell is made, the whole process of injection moulding puts lots of stress on the shell, which is "set" in the frame due to the rapid production process. The molten plastic is squirted into the shell mould, cooled rapidly, then churned out of the mould. This leaves the plastic molecules stressed, and liable to splinter or crack during collisions or excessive twisting etc.

The below process is basically reassembling the plastic molecules without harming the frame shape, in order to stress-relieve the plastic.
I have successfully done the below process on my P4A shell, with no adverse effects and no more cracks. I used to do this process years ago on RC car parts, and it used to make a huge difference to the failure rate.

1. Strip everything out of your bird, ie anything electronic, that is located within the shell. Be prepared that you will almost certainly lose any stickers on the shell, and void your warranty, so it really is your choice to do this or not. Take your time, and take photos as you go to ensure you reassemble correctly. Only attempt if you are 100% sure what you are doing and the consequences of getting it wrong.

2. Put the shell in a large metal watertight cooking container that allows the entire frame to fit and has the depth to allow water to cover the shell. Only do one half of the shell at a time.

3. Pour in boiling water into the container to completely submerge the shell. This is easier said than done, as chances are you will need a couple of kettles in order to submerge the shell. I partially filled the container with boiling water, then topped up with hot tap water, and put on top of the cooker on a low flame to bring the whole container up to boiling point. Do not use a large flame as the direct heat in the area where the flame is may warp or melt the shell if it is in contact with the metal container.

4. Leave to cool naturally to room temperature (a good 90 minutes). This is the most important part!!! Do not pour in cold water etc or anything to speed up the process, the cooling down process and the time it takes naturally is what 'fixes' the stressed plastic.

5. Reassemble, and be stress free!
 
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I'm aware of this "heat treatment" for alloy structures, mostly welded aluminum structures (such as bicycle frames) to which the weld heat affects the surrounding area of the tubes and thus need HT and "aging" to bring back original or desired mechanical properties. It is also done to glass to relieve internal stress, but I never knew it could be done to injected or even molded plastic too, and also not to monocoque structures such as the P3/P4 shell.
 
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I never knew it could be done to injected or even molded plastic too, and also not to monocoque structures such as the P3/P4 shell.
It can be done. Plastic is plastic, no matter how it is made or into what structure, it is always heated which is what causes brittleness.
I was told about it years ago as wishbones etc kept on breaking on RC cars just through hardened use, and the failure rate dropped by literally 80% when i tried the method.
I did it to my P4A after the original shell showed cracks near the motor mount after my bird tilted over during landing a while back, so i did a test 1st on the old shell to make sure it kept its shape, and then on the new replacement shell as a preventative measure. I have not had any hard landings or crashes to fully 'test' the process but i have had a couple of tilt overs on landing with no damage.
 
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Not all plastic is the same. Actually, plastics (polymers) are categorized not only according to the arrangement of the chains of molecules, but also as to how they're affected to heat - thermoplastics, thermosets and elastomers.

Please don't take it as I'm disputing your claims of previous success using this technique. I'm just not aware of the use of heat treatment or aging for injection-molded polystyrenes (the P3 shell type), or how it could affect the mechanical properties of a pre-set thermoplastic in that way. In other words, I'm ignorant as to this even though I've dealt a long time with this matter - I'll do some research anyway, it's always good to learn something new ;-)

I've had problems with the RC of one of my P3P, but never with the shell. So my guess is the cracking issue is not caused by stress or repetitive movements, either extraneous or coming from the function of the object, but rather from the plastic mix or the molding/curing process.
 
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What is the melting point of the plastic used for the Phantoms?
 
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The glass-transition temperature for polystyrene is 100℃, relatively low. That's when it becomes fluid and can be molded. But I'm not sure what kind of plastic they used, from the level of fine detail and other characteristics (cheap, light, easy to work with, etc.) it is a thermoplastic polystyrene. It's very brittle too, maybe they got the mix wrong for some batches and that affected the mechanical properties and thus some P3 shells and Tx crack, but only a few.
 
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...maybe they got the mix wrong for some batches and that affected the mechanical properties and thus some P3 shells and Tx crack, but only a few.
That's what I think and posted about a year ago. Or they miss-timed their process.
And I think more than a few.
 
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That's what I think and posted about a year ago. Or they miss-timed their process.
And I think more than a few.
Who would have thought you could temper plastic! If this process will prevent cracking, why wouldn't DJI use the process. You would think it would prevent a lot of headaches related to returns etc...
 
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They probably outsource the process so their issue would be with their vendor.
 

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Who would have thought you could temper plastic! If this process will prevent cracking, why wouldn't DJI use the process. You would think it would prevent a lot of headaches related to returns etc...

Possibly because the time it takes to let everything cool naturally (remember we're talking many thousands of units) isn't worth it. It's possible the replacement rate is less expensive than the alternative or DJI hasn't made a definitive ruling as to what causes the stress cracks.

If this works it would be worth while especially if you were replacing the body any way.
 
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In my experience, reheating the plastic in this way is indeed likely to increase the perceived strength, but, as the OP notes, it will probably induce more brittleness to the plastic. So, on a high impact, it's more likely to shatter rather than deform and it could cause a complete failure if over-stressed when in flight. As I've said before, the most likely cause of the cracking on the shells relates to the injection moulding material, the spread and density of that plastic around the mold when liquefied, the speed of injection, and bubbles that can form inside the plastic itself when in that liquified state.

But if it worked for you, great, but I will not be rushing to strip the innards from my P3 and dunking the shells in boiling water any time soon....
 
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I would say that anyone who wants to give it a go tries it on an old frame 1st, if you are unsure of it, for your own peace of mind. Plus I am unable to tell if the 2x shells that I used were the new or old type, but I can assure you it did not affect the new shell at all.
If you are unsure of the validity of this (which I probably would be!!) then try on an old shell 1st.
 
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I would say that anyone who wants to give it a go tries it on an old frame 1st, if you are unsure of it, for your own peace of mind. Plus I am unable to tell if the 2x shells that I used were the new or old type, but I can assure you it did not affect the new shell at all.
If you are unsure of the validity of this (which I probably would be!!) then try on an old shell 1st.
Easy to check the type; DJI added more "strengthening" around the motor mounts IIRC on the MKII. Visually, you wouldn't know what chemical changes have occurred.
 
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Many years ago I was tasked with testing nylon tubing used in an air shock system for motorcycles. I tested dozens of different types with varying materials, wall thicknesses and diameters. Test samples were installed in fixtures and pressurized to the point of bursting. This data was recorded as a means of evaluation and selection for the final specification. At one point out of curiosity I wrapped a couple layers of common "Scotch Tape" around the tubing covering the small hole in the burst area and repeated the test. In every instance, the tubing would burst at a different area of the tubing rather than at the "repaired" site. We're talking about several thousand psi pressure in the tubing but with an area very small (burst hole) the actual pressure at that point wasn't enough to cause the Scotch Tape to fail. I was impressed by the integrity and strength of this tape. My point here is that if all the stress cracks are occurring at the same location on the shell, I would try a single tightly wrapped piece of this tape around that location. I would do it uniformly on all 4 arms. The weight of this tape is insignificant. This would be quite a bit easier than the tempering process described above.
 

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