Cracking the P3 crack

[Chuck Young]

2 ideas for the experts out there: 1) A full replacement of the end of the arm for a better material. Cut the end of the arm and insert a new one. 2) Place a sheet of insulation between the motor and the plastic. Just a thought

I am simply going to lay down two layers carbon fiber inside the shell and two layers outside the shell of 1K plain weave, maybe add some graphene oxide (to add some strength), bag it up with resin, and add vacuum. I think it will work fine for my bird. After warranty expires, until then I send it in.

 

[Phil Tuggle]

One thing DJI can do very easily is to modify the material to something MUCH better in terms of toughness and flexibility over time; EVERYBODY agrees with that (except maybe DJI or their molding contractor).

Not quite as easy, but certainly do-able is to modify all the bottom shell injection cavity tools by removing material (in the tool, not the part itself) to create radiuses which will spread the stress more gently and over a wider area. This is an extremely common practice in injection molding, to remove inadvertent stress risers from a part. It involves machining and replating each cavity, but is FAR cheaper than creating new molds.

The photo below (borrowed from another poster for illustration purposes) shows the existing sharp intersections of the ribs and other features on the left; the image on the right is an illustration of what hopefully looks like gentler - and thus more durable - transitions.

To achieve this, each tool should have material removed. This is what the tool might look like before and after the change. Just a diagram of the TOOL, before and after...

Since I can't do this for the dozen tool cavities x 4 arms, I have cleaned and roughened all those areas of my bottom shell (to remove mold release agents and aid adhesion) and applied fillets of my own, using a strong epoxy. No cracks were present when that was done and I hope it stays that way.

Oh, yeah, I do confess to having detachable prop guards (leave brackets on), AND have balanced my propellers.

Any way you look at it, I wish DJI would think about some changes.

 

[Simonb3526]

Hi All. I read through 4 pages of this before commenting, didn't get time to read it all! I manufacture plastic parts for a living. Cosmetic parts for electronic companies as well as very function parts for machines such as parking meters all the way through to medical devices. Im more hands on than scientific as I have spend many a conference call talking about flow lines with Hitachi when what I needed to do was spend a few days at the machine to fix the issue. Sometimes its science and sometimes its a bit of a black art.

I haven't checked but if anyone has a spare case there should be a marking on it to identify the material used. I suspect it some form of ABS. ABS is used in automotive and most cars go way above the temps were talking about for far longer. However there is ABS and there is ABS, many grades from many suppliers, I have had a lot of issues with factories using regrind material (where they take the plastic parts, regrind then and put them with the virgin material). So product works well, you test the hell out of it then some moron regrinds to save a few bucks. Impossible to track other than through spot inspections and returned product.

This effects the quality of the part and normally makes it more brittle. Also ABS manufacture vary and so do grades. So without looking at their QC records and goods in receipts you would not know if this was a batch issue die to alternative material or something more fundamental. Also just because a tool is good and worked once doesn't mean your get the same result next time. Running temps - ambient temp all can have an effect. For most mouldings its not an issue but I have had a few parts in my time where it would work one week but not the next, the conditions were so on the line it was hard to control.

I had a part once where we were importing Bayer material from Germany, worked well, then the next delivery was the same grade and spec but processed in China. Didn't work, mould flow lines over this cosmetic part. Bayer didn't know why said there was no difference, same product number everything. Imported it direct again and worked...

Stress fractures at these key areas are always an issue. It can be the amount of material around the boss but also the mould flow. The injection point is vital, as the material flows into he part it starts to cool. If its cooling to much at the extremities it may not fill the part correctly or the material has cooled too much and effects the mechanical properties. As the material flows around say a screw boss the plastic meets and joins - if its too cool that joint will be the first stress part to go. You will find batch to batch the shot weight of the part will change unless they have rigorous QC. Again many time I check product and I always have a golden sample which I measure and weigh - the clever machine operator will reduce the shot weight to improve on issue while causing another. If anyone one has old and new cases it would be interesting to accurately weight them. Less material, less packing in key areas, less strength.

So what you do in real life is increase the flow to those areas by having larger channels to direct material to those areas as well as increase some wall thicknesses - you can do that to much as you get sink in thick areas. You use hot runners in larger parts to make sure the material stays as hot a possible before entering the part.

As Phil said radiuses edges are the norm and I add small ones on every part im doing, especially in the non cosmetic areas, they are simple and easy and this could be done in 1-2 days. You take metal off in a tool in two ways, either with a CNC or bench mill or for more complicated or cosmetic parts (bits you can see on the outside that need a texture) with spark erosion. This is the easy one to do. Im surprised they are not larger than they are in the pics. What we don't know is how many moulds or how many cavities DJI has. They may have multiple moulds where they have modified some but not others.

In parts like this its tricky - looks like a slight change in moulding condition or material means the difference between it working and not. I have had a few products like this and I always go belt and braces. Either use a far stronger material (cost more but returning product is not cheap either) of have an additional support in there - Such as a carbon fibre part to sit between them and go down into the arm to act as support if it does crack for example. Or just as your developing here an outer part to stop total destruction.

I don't think the heat is effecting it directly as the melt point of ABS is high, the vibration and twist will be a big factor. The problem is with this and the strong arm parts is testing it, I do medical products as well which have very stringent testing and trials and still we find in the field we have issues we cant replicate. This is why critical system such as aircraft have failsafes and backups.

Personally after 15 years of moulding experience I have taken the high tech approach of a bit of duck tape and wrapping it around the arm instead of cable ties. I noticed the fit to the top case to bottom was stressing the little stickers that go around it so bonded the two together. Thats not going to help the motors ripping off so try to remember to check the bird before flight and not fly it if I see anything. I also now try not to fly over buildings or roads in case it does drop out the sky. Tricky problem for DJI, but they are huge now with a lot of cash so the next generation of product needs to fix this issue and reading a number of threads they seem to be quietly fixing them and replacing parts.

If they do the easy part in the tool and you have an outer holding place and ideally something inside to fill the voice to sandwich it all together then your belt and braces. I have access to 3D printing and CNC equipment. The one thing I need is a scanner to get a 3D shell part into Solidworks. I would inject or CNC the parts out of a Nylon 6 or POM. Very strong parts but light.

im sure DJI could use us as testing monkeys!

 

[Nicoloks]

Hi guys,

Nothing constructive to add to this thread. Just thought I would add I am the latest victim to the stress fracture issue. Both rear screw holes closest to the battery compartment have been damaged. One has cracked right through, the other looks like a crease line in the plastic. I noticed last flight that the drone appeared to have a very slight rotation to the left which prompted me to check. The longest single session I've had is 2 x full batteries. P3P arrived late August, have had less than 20 flights in total, no crashes.

Timing wise this sucks for me as I have 5 days leave booked at the end of October which I was hoping to get some serious drone time (just purchased the 6 pack set of Polar Pro ND filters as well as their 3 pack graduated ND filter pack). I live in Australia, so warranty wise I imagine would be back to DJI China which I doubt will be done inside 4 weeks.

Think I am going to get a replacement shell (maybe even buy 2 while I am at it) from Helipal and just do it myself. How hard is the replacement process? Any guides I should be looking at?

 

[MikeON]

Don't want to take apart my P3A to measure it, so borrowed this picture from another post. Maybe someone could answer a question. When the shell halves are assembled together, do the corresponding bosses in the top and bottom shells contact each other, or not? If not, that would make the clamping force of the screws be reacted by the contacting surfaces around the shell edge. This in turn would cause shear stress where the bosses join the shell, right where the cracks are occurring. The lack of a fillet radius there would certainly aggravate this, especially if vibration is also a factor.
If the bosses did contact, there would be zero stress in the cracking area due to screw tension, albiet perhaps stress from other sources such as vibration.
Of course, I'm referring to the screws holding the shell together, not the motor mount screws.
I have no experience with plastics, but have designed highly-stressed steel components for the mining industry for 40 years. If the bosses don't contact, I would expect this design to crack.
Maybe a few thin washers surrounding the screws, that the bosses could bottom against when the screws are tightened, would help.
My second P3A is a week old, the first having been bricked by firmware upgrade and replaced under warranty. No cracks observed in either, so far.

 

[Simonb3526]

Check you tube. Found this but there are longer ones for p2 out there.

 

[Phil Tuggle]

Don't want to take apart my P3A to measure it, so borrowed this picture from another post. Maybe someone could answer a question. When the shell halves are assembled together, do the corresponding bosses in the top and bottom shells contact each other, or not?
View attachment 32148

That is an EXCELLENT question and observation - Thanks! I will certainly check this out as soon as time permits during our current poor flying weather in South Carolina. If the bosses don't touch, perhaps a really tiny and thin o-ring or something thin and flexible should be put there? I think you are right.

And thank you, Simon, for some great comments - especially regarding flow lines and resin variation and the rest. Maybe they are forcing cycle time to get production numbers, too. Who knows? So important! Even though DJI is a huge provider of some great flying machines, I really have to think they must contract out the shells. They need to buy the moulder and get things right, or at least have some engineers camp out at the molder(s); they need to get EXPERT control of everything that is critical to their product.

Oh, and the duct tape idea is GREAT! Will definitely do that...may not be pretty, but will probably help a lot to absorb vibration energy a little better than zip ties could, while also reducing any torsion.
The two greatest inventions: The wheel. Duct tape.

 

[fhagan02]

How does this help the guy whose motor and prop ripped themselves right out of the end of the arm? Nothing was left but a shattered plastic stump and some twisted wires.

Ditto this. This whole inevitable "crack problem" is astonishing. If I walked into a drone shop, hand the guy $1300 for a drone and then the guy hands me 50¢ worth of zip ties and some JB weld saying, "You'll need these to keep your motors from flying off"... Wouldn't I be an idiot if I didn't ask for my $1300 back?

 

[brianb87]

Ditto this. This whole inevitable "crack problem" is astonishing. If I walked into a drone shop, hand the guy $1300 for a drone and then the guy hands me 50¢ worth of zip ties and some JB weld saying, "You'll need these to keep your motors from flying off"... Wouldn't I be an idiot if I didn't ask for my $1300 back?

Completely agree.. If my bird falls out of the sky due to stress cracks and hurts someone DJI can expect a lawsuit.

 

[StumbleBee]

Ditto this. This whole inevitable "crack problem" is astonishing. If I walked into a drone shop, hand the guy $1300 for a drone and then the guy hands me 50¢ worth of zip ties and some JB weld saying, "You'll need these to keep your motors from flying off"... Wouldn't I be an idiot if I didn't ask for my $1300 back?

LOL, do you feel any better for having plunked down your $1300* entry fee (!) and coming to Phantom Pilots to be told the same thing?

*That's the cover charge, drinks will cost you a lot more.

 

[StumbleBee]

Completely agree.. If my bird falls out of the sky due to stress cracks and hurts someone DJI can expect a lawsuit.

Maybe eventually, but the injured party is immediately going to go after you. None of us would want that aggravation or expense.

 

[StumbleBee]

ne of us

[snip]

I'm sure DJI could use us as testing monkeys!

Don't want to quote your entire post, just the last part. I sometimes think DJI is using us as test monkeys.
I have 16 years in molding, and a Degree from Newark College of Engineering in Thermoplastic Injection Molding Technology. Everything you wrote is spot on.

 

[Zoti]

How about a former plastics expert with an engineering degree in both thermoset and non-thermoset resin molding technology. Back in the early 70's I did some pioneering work for American Standard Lab's in Piscataway, NJ on photoelastic stress analysis of melamines. I then ran a Reliability Test Lab for Mattel, Inc. (toys) for four years until 1977. Lots and lots of experience with polystyrenes, vinyls, polyethelenes and many other thermoplastics, both extruded and molded. I was involved with resolving many cracking issues including on one ill conceived wind up airplane toy (the ZipCord Fighter). Every kid who got one of those on Christmas morning had it shatter to pieces the first time it hit the ground at 20F or less.

Anyway...there are many factors which can cause stress related cracking in molded plastics, including of course residual stress from the molding process itself. Another very important factor is how much regrind (now called recycled) plastic was used along with virgin resin. Often regrind varies greatly (and badly) in composition. Bad regrind equals trouble. And it's intermittent and variable trouble, the worst kind.

To your point about temperature and that you believe the temp's you recorded would not be a problem. It would be. Or at least a strong could be. All plastics contain plasticizers, that's the ''new car" smell we all love, poisonous plasticizers. These naturally exude from plastic and heat greatly accelerates the process. The more these plasticizers are lost the less 'plastic' the plastic becomes, i.e. the more brittle it becomes. Ever see a cracked knob in a car, or a split dashboard? Lost plasticizer. So any heat at all is bad, the more the badder.

Yes but cracking due to lost plasticizer in 5 hours of total use?

I'm obviously not a plastics expert but a car dash takes 10 years of sitting in the sun heating and cooling up to 120-130 degrees to crack. Maybe the plastic is really of crappy quality which I doubt.

I think these are all vibration and stress cracks. If it was heat we'd see it on all arms or randomly but most cracks are on the rear arms.

 

[Zoti]

Hi All. I read through 4 pages of this before commenting, didn't get time to read it all! I manufacture plastic parts for a living. Cosmetic parts for electronic companies as well as very function parts for machines such as parking meters all the way through to medical devices. Im more hands on than scientific as I have spend many a conference call talking about flow lines with Hitachi when what I needed to do was spend a few days at the machine to fix the issue. Sometimes its science and sometimes its a bit of a black art.

I haven't checked but if anyone has a spare case there should be a marking on it to identify the material used. I suspect it some form of ABS. ABS is used in automotive and most cars go way above the temps were talking about for far longer. However there is ABS and there is ABS, many grades from many suppliers, I have had a lot of issues with factories using regrind material (where they take the plastic parts, regrind then and put them with the virgin material). So product works well, you test the hell out of it then some moron regrinds to save a few bucks. Impossible to track other than through spot inspections and returned product.

This effects the quality of the part and normally makes it more brittle. Also ABS manufacture vary and so do grades. So without looking at their QC records and goods in receipts you would not know if this was a batch issue die to alternative material or something more fundamental. Also just because a tool is good and worked once doesn't mean your get the same result next time. Running temps - ambient temp all can have an effect. For most mouldings its not an issue but I have had a few parts in my time where it would work one week but not the next, the conditions were so on the line it was hard to control.

I had a part once where we were importing Bayer material from Germany, worked well, then the next delivery was the same grade and spec but processed in China. Didn't work, mould flow lines over this cosmetic part. Bayer didn't know why said there was no difference, same product number everything. Imported it direct again and worked...

Stress fractures at these key areas are always an issue. It can be the amount of material around the boss but also the mould flow. The injection point is vital, as the material flows into he part it starts to cool. If its cooling to much at the extremities it may not fill the part correctly or the material has cooled too much and effects the mechanical properties. As the material flows around say a screw boss the plastic meets and joins - if its too cool that joint will be the first stress part to go. You will find batch to batch the shot weight of the part will change unless they have rigorous QC. Again many time I check product and I always have a golden sample which I measure and weigh - the clever machine operator will reduce the shot weight to improve on issue while causing another. If anyone one has old and new cases it would be interesting to accurately weight them. Less material, less packing in key areas, less strength.

So what you do in real life is increase the flow to those areas by having larger channels to direct material to those areas as well as increase some wall thicknesses - you can do that to much as you get sink in thick areas. You use hot runners in larger parts to make sure the material stays as hot a possible before entering the part.

As Phil said radiuses edges are the norm and I add small ones on every part im doing, especially in the non cosmetic areas, they are simple and easy and this could be done in 1-2 days. You take metal off in a tool in two ways, either with a CNC or bench mill or for more complicated or cosmetic parts (bits you can see on the outside that need a texture) with spark erosion. This is the easy one to do. Im surprised they are not larger than they are in the pics. What we don't know is how many moulds or how many cavities DJI has. They may have multiple moulds where they have modified some but not others.

In parts like this its tricky - looks like a slight change in moulding condition or material means the difference between it working and not. I have had a few products like this and I always go belt and braces. Either use a far stronger material (cost more but returning product is not cheap either) of have an additional support in there - Such as a carbon fibre part to sit between them and go down into the arm to act as support if it does crack for example. Or just as your developing here an outer part to stop total destruction.

I don't think the heat is effecting it directly as the melt point of ABS is high, the vibration and twist will be a big factor. The problem is with this and the strong arm parts is testing it, I do medical products as well which have very stringent testing and trials and still we find in the field we have issues we cant replicate. This is why critical system such as aircraft have failsafes and backups.

Personally after 15 years of moulding experience I have taken the high tech approach of a bit of duck tape and wrapping it around the arm instead of cable ties. I noticed the fit to the top case to bottom was stressing the little stickers that go around it so bonded the two together. Thats not going to help the motors ripping off so try to remember to check the bird before flight and not fly it if I see anything. I also now try not to fly over buildings or roads in case it does drop out the sky. Tricky problem for DJI, but they are huge now with a lot of cash so the next generation of product needs to fix this issue and reading a number of threads they seem to be quietly fixing them and replacing parts.

If they do the easy part in the tool and you have an outer holding place and ideally something inside to fill the voice to sandwich it all together then your belt and braces. I have access to 3D printing and CNC equipment. The one thing I need is a scanner to get a 3D shell part into Solidworks. I would inject or CNC the parts out of a Nylon 6 or POM. Very strong parts but light.

im sure DJI could use us as testing monkeys!

Make a replacement bottom shell that is strong and you can make some good money.

 

[Simonb3526]

Thinking about that last night - getting some tooling quotes. Not sure of the legalities of doing so and would have to change the look to get away for any design registrations. Thinking about it it only needs to be the bottom case - gout put some extra room in there for GPS slots or other things people strap to their bird.

 

[bobmyers]

Hi guys,

Nothing constructive to add to this thread. Just thought I would add I am the latest victim to the stress fracture issue. Both rear screw holes closest to the battery compartment have been damaged. One has cracked right through, the other looks like a crease line in the plastic. I noticed last flight that the drone appeared to have a very slight rotation to the left which prompted me to check. The longest single session I've had is 2 x full batteries. P3P arrived late August, have had less than 20 flights in total, no crashes.

Timing wise this sucks for me as I have 5 days leave booked at the end of October which I was hoping to get some serious drone time (just purchased the 6 pack set of Polar Pro ND filters as well as their 3 pack graduated ND filter pack). I live in Australia, so warranty wise I imagine would be back to DJI China which I doubt will be done inside 4 weeks.

Think I am going to get a replacement shell (maybe even buy 2 while I am at it) from Helipal and just do it myself. How hard is the replacement process? Any guides I should be looking at?

There are a couple of Youtube videos that walk you thru the entire process.

 

[FatTwoTonTony]

Sounds to like I should pick up a small sheet of aluminum and cut out a few pieces and mount them to the arms?? Do you think this will help?

 

[bobmyers]

Sounds to like I should pick up a small sheet of aluminum and cut out a few pieces and mount them to the arms?? Do you think this will help?

Are you taking about mounting the aluminum to the arms with screws like the strong arm's from UAV or doing something else.

 

[FatTwoTonTony]

Are you taking about mounting the aluminum to the arms with screws like the strong arm's from UAV or doing something else.

Yes I am wondering if that might work...what do you think?

 

[Gantech]

I just bought a new p3 shell a and I'm going to make a mold then lay up carbon fiber shells. I'll post here when I'm done