Thermoforming bend angle error PA11 2

[berg380]

Hi people,

I am trying to bend some PA11 tube material OD=1mm ID=0.3mm
Weird thing is that the bends are showing a forward bending relaxation behavior which continouse for 3 days. The mold angle is 90deg (inside angle). After thermoforimg for 10 mins at 160degC and rapid cooling (under 5 mins), 3 days later the piece has a 60deg inside bend angel.

Anyone have a explination for this behavior?

Best regards,

Mr. Berg

 

[Compositepro]

Obviously,you have residual stresses in the tube that are relaxing through creep over time. These stresses are due to the specific details of your bending process which you have not revealed. Is the wall thickness the same on the inside and outside of the bend? How is the part heated and cooled? How is it fixtured?

One scenario is that the inside of the bend became thicker in wall thickness and the center of the thickness takes longer to cool, but is restrained from shrinking by the solidified surfaces. So the inside of the bend has more residual stress than the outside.

 

[berg380]

I agree on the residual stress and relaxation through creep.
If the wall thickness stays the same on the inside and outside i dont realy know, havent measured this. Theoreticaly i would expect the outside to be thinner due tensile stress and inside to be thicker due to compressive strees.

Process
1. Convextion heated at 160degC for 10 mins.
2. Then rapid cooled to air. I calculated the that the cooling of the tube is in the order of seconds to room temperature. Lets say 10 seconds (tube geometery is small wall thickness 0.35mm). (OD=1mm ID=0.3)

Fixture
Both legs of the bend are held in two metal tubes, and the bend hangs in free air.


What i would like to know is what kind of polymer process is causing the residual compressive stress (in the inside of the bend) after rapid cooling, and ofcourse how it works?

I have some ideas but am not polymer expert.

 

[patprimmer]

As semi crystalline polymers cool from a soft to a hard state they form crystals. The slower the cooling rate the greater the level of crystallisation and the bigger and more perfect the crystals. This means more shrinkage.

Regards
Pat
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[berg380]

@patprimmer

The bend hangs in free air so cooling should be the same for inside and outside of the bend, which means that the crystal density (and thus shrinkage) should also be the same for the in- and outside of the bend.

The crystal density is expected to be different for the ouside and inside of the tube but i dont see how this can effect the bend angle?

 

[patprimmer]

Plastic is a very poor insulator and the thicker section cools slower and shrinks more.

Also if using a bending jig, the inside rad part of the tool might build up more heat.

Regards
Pat
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[Compositepro]

If the bend is hanging free, in air, The inside of the bend is on top and will cool more slowly than the bottom. In natural convection cold air contacts the hot plastic and rises so the air at the top of the tube is significantly warmer than at the bottom. This means that after the bottom wall of the tube has cooled to the point where it is fairly stiff the top of the tube is still cooling and shrinking but the shrinkage is restrained by the stiff bottom wall (and also cool skin on the outside of the tube). This restrained shrinkage results in stored stress in the plastic which then cause warping due to slow creep/stress relaxation.

You will probably see a big difference in spring-in if you blow cooling air down onto the bend.

 

[berg380]

@patprimmer

I guess you mean plastic is a good thermal insulator.

Are you realy sure that at the tube dimensions (OD=1mm ID=0.3) I mensioned, the diffence in wall thickness can realy cause the substantial bend angle error of 30deg?

My second post states the bend is suspended in free air. Only the two legs on each side of the bend are constrained by metal tubes. So there is no differentail build up of heat between inside and outside of the bend.

According to the reactions, I presume that the net residual stress which seems to be compressive in the inside of the bend is a result of slower cooling due to the increased wall thickness on the inside of the bend. Also the rapid coolign of the thinner outside wall would help the effect.

Although I have not measured the differential thickness between the inside and outside wall, I find it somewhat unlikely that it would cause such a big bend angle error, especialy at these dimensions.

 

[berg380]

@Compositepro

The bend is formed in the horizontal plane so the inside is not on top. Inside and outside of the bend are at the same level, so cooling due free convection is the same for in- and outside of the bend.

I calculated the cooling time to less then 10 sec. This means that when i remove the sample from the oven it is cooled alomst instantly. So I dont realy see a way how to sensibly apply force convetion cooling.

It is clear that the forward bending error of 30deg is cause by residual compressive stress which relax due creep over time. And that the stress is most likely cause by differential crystalization due to uneven cooling of the bend.

However, I dont see how in this free setup there would be uneven cooling of the part. Ofcourse the outside of the tube will cool first and will show less crystalization then the inside of the tube. But this should not result in a forward bending behavior.

Difference in wall thickness could explain the diffence in residual compresive stress but I find it somewhat unlikely.
Maybe someone have a other idea?

 

[patprimmer]

It is late. I did mean plastic is a poor conductor of heat.

In an injection moulding using semi crystalline materials the inside of the bend has a strong tendency to shrink more due to difference in the mould temperature.

It also has a strong tendency to shrink more in thicker sections which is exactly consistent with what you are saying is happening.

The reason for thicker sections shrinking more is due to slower crystallisation. This is an extremely well documented fact.

Regards
Pat
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[btrueblood]

"The reason for thicker sections shrinking more is due to slower crystallisation. This is an extremely well documented fact. "

Pat, restating: thicker sections cool more slowly, allowing larger/more crystallization to occur, and causing increased shrinkage after cooling.

Correct?

Sorry to nitpick, but I think you are hitting the problem on its head, at least it's the best explanation I've seen.

 

[patprimmer]

btrue

No problem. I actually explained crystallisation properly in my first post. I thought it was pretty obvious so I left it concise. My later posts where further explanation of what I already thought was obvious. I guess I did them in haste. It seems the nature of semi crystalline polymers is not obvious to all.

Regards
Pat
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[btrueblood]

"It seems the nature of semi crystalline polymers is not obvious to all. "

Not 'til you explained it.

 

[berg380]

Hi all,

First of all thanks for all the response.

It seems alot of poeple are cofident that the differential crystalization (shrinkage) is the cause for the forward bending effect.

Some other questions.
What time do the polymers need to crystalize? (ns,ms,s?)
And do you expect the differential crystalization to be substantial in cooling times of under 10 sec?

best regards

 

[patprimmer]

It varies greatly with circumstances, but the critical time is the transition from fully amorphous to semi crystalline.

The slower the transition, the more perfect and bigger the crystals. The more perfect the crystals the higher the shrinkage.

Things that influence it are initial melt temperature or just how much kinetic energy is stored in the molecules to break the forces of attraction between molecules.

The strength of the forces of attraction is directly related by the distance between the charged points. Force=1/distance squared.

In a semi crystalline polymer the crystalline regions have the molecules pretty well lined up at fairly uniform separation so the bonds are close and strong and uniform so the crystalline melting point is tightly defined.

The amorphous areas are randomly aligned and the bond length and alignment vary greatly so each bond breaks at a different temperature.

Nylon 11 has both crystalline and amorphous areas. As you heat it up the weakest bonds break first and it starts to soften. As the heat increases more and more bonds break and it gets softer until it gets to the crystalline melting point, where most of the remaining bonds break in rapid succession and the polymer is liquid.

On cooling, it depends on how many of the original bonds where broken and if any crystals remain. At thermoforming temperatures, the temperatures are often just below the crystalline melting point where the weaker bonds are broken, but the strongest remain in tact.

the slower the cooling the longer it takes for the crystals to reform and become more perfect. It is a dynamic process where everything is rearranging itself into a more relaxed position.

10 seconds cooling time really means nothing. rate of temperature change is the crucial measurement and that is also dynamic.

The presence of nucleating agents also has a strong influence as they increase speed of crystallisation and increase the number of crystals, but also tend to form less perfect crystals as there is less time for repeated rearrangements. Remember the kinetic energy (temperature) in the molecule is directly related to the strength of the bonds that will be broken or formed or where they can be formed broken reformed a few times to reach a more stable (stronger bond) state.

Also cooling air temperature and motion are not defined.

The official crystalline melting point for nylon 11 is 185 to 190 deg C and typical mould shrinkage is 1.9%.

In the absence of data specifically for nylon 11 I would guess that the extremes are 1.0% to 2.5%

what would 10% everywhere as formed and 1.0% on the outside rad and 2.5% at the inside rad do to the angle.

As the parameters are so poorly defined I can only take a wild guess, but

Regards
Pat
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