Warp
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What is Warp
A picture is worth milion words so here are some examples showing warp problem:
 HDPE Warp |
 ABS Warp |
There are "few" discussions online about this problem, some of them can be found here, here and here.
Why does it occur - theory1
(following text taken from here)
The plastic is a very viscous material when we print it, I am told it is a non Newtonian fluid but never the less, it is a fluid, and at a fixed temperature, I think it fair to say it might approximate to a standard fluid.
If we run with this idea we can say that the pressure drop across the nozzle is = flv^2/2gd
f being the friction factor l the length of the nozzle v the fluid velocity g gravitational constant d the novel diameter
To evaluate the friction factor we must know the Reynolds number Re, this is equal to vd/Kinematic viscosity
v fluid velocity d pipe diameter
We can see that for increasing viscosity the Reynolds number (Re) tends to zero. Our viscosity for the molten plastic is very high so Re will be very low.
Re < 2000 confirms that the flow of plastic through the nozzle is laminar therefore the friction factor in the first equation can be evaluated by f = 64/Re.
As Re tends to zero the friction factor gets very large exactly as you might expect.
In the pressure drop equation, the length of the nozzle is in the order of the diameter of the nozzle so for arguments sake lets say they cancel each other out.
This leaves us with the pressure drop approximating to fv^2/20
So the stress in the fluid is related to the friction factor and the fluid velocity squared In practice we can make a small change to the friction factor by increasing fluid temperature and thus decreasing the fluid viscosity, but this is a small change. The greater benefit is to reduce the velocity, any small change here will greatly reduce the stress at source.
The above describes the Pressure stress within the fluid, I think this would be proportional to the residual stress remaining in the plastic at the exit of the nozzel.
In the case where the fluid remains liquid, the internal stress can come out as the liquid will allow movement of the molecules, in our case the molten plastic is discharged into air and is frozen almost instantly. How much of the residual stress remains in the filament must be related to the speed at which you freeze the fluid. The rate of cooling is a function of the difference in temperature. Reduce the rate of cooling and this will give chance for some of the stress to relieve.
Following on from this we might say “Warp Factor” is related to (T2-T1). f.v^2/20 T2 Extrusion Temperature T1 Local ambient.
There are almost certainly other factors that we should account for, elasticity of the plastic may be one.
I am working on this to see if any of this idea pans out, certainly the fluid velocity has a significant effect.
Raising the extrusion temperature has very little effect as it might reduce the viscosity but at the other end it would raise the Delta T. more or less cancelling out(?)
Raising the local ambient has a good effect as in the limit if the temperature of the ambient matched the filament then all stress would have time to relieve but the plastic would remain fluid.
If this idea is on the right track then it gives a little insight as to what might work for reducing the warping. The hot table would work certainly in the lower layers of the print, perhaps less as the hieght increased. Printing in a bag, this has been tried, elevating the local temperature round the print reducing the Delta T Printing with a hair drier, hot air again reducing the delta T. My idea, print slow and do not put as much stress in the fluid in the first place.
As far as I know this is all newish territory so feel free to experiment and chuck in any ideas you have. More academic types out there may have issues with the content, please jump in and see if we can knock a few ideas about to understand this problem more fully.
Why does it occur - theory2
(following text taken from here)
The warping occurs because the layers have time to cool (and shrink) between laying down each layer - for instance (thermal expansion is ~1% of ABS at nozzle temp):
- Layer 1 laid down as 100mm line.
- Layer 1 cools to 99mm.
- Layer 2 is laid down on top of layer 1 - 100mm of melt on top of 99mm layer - overlapping by 0.5mm each end (not quite true, but approx)
- Layer 2 cools to 99mm, compressing layer 1 - layer 1 is now in compression, layer 2 in tension, causing a bending moment.
This happens on every layer pulling the ends away from the bed, and bending the middle down. That's why eventually one end or the other seperates from the bed - ruining the print, or producing a warped item.
Running inside a hot box, or a hot table, reduces the cooling and therefore shrinking between each layer, allowing the whole object to cool consistently after finishing.
Coloured filament
HDPE and PP tests I made show that coloured filament warp slightly more then natural / uncoloured one.
How to prevent it
Print Slower
I managed to reduce warp to minimum printing with PP (worse the HDPE and ABS by factor of 2) by reducing speed of the print to 6mm/s. Note that reducing print speed requires also the extruding speed to go down in order to output just the right amount of filament.
Find right temperature
Finding the right temperature is crucial. If you heat filament too much it will stick better to underlying layer but the effect of the warp will be increased as difference in temperature of upper layers to lower layers is too big. Find the lowest temperature that gives you good interlayer adhesion and use it.
Let it stick
There is 2 levels of warping. Warp can start while printing; the corner of the object will detach from the raft/print bed. This is "unacceptable" warping and must be prevented. In order to prevent this
- Use bed that your filament stick good to (PP stick good to PP, HDPE to HDPE and PP, ABS stick good to acrylic and MDF)
- Make good raft - double width base layer, single width 70% fill double interface layer is good start. Let your raft cool a bit before you start printing the object
Cooling / Ambient temperature
The second level of warping happens after the object is removed from the print bed. Due to stress accumulated in object after removing from print bed the forces trapped in object will try to warp the object. To prevent it you can try some of the suggestions
- Let the object cool slowly (do not cool it artificially), and then, before removing from bed, heat the "whole object" uniformly and let cool again - only then remove
- Print object in higher ambient temperature - experience show that object printed in higher ambient temperature accumulate less stress
- Tony experimented with adding additional heater. You can see picture here. This way you can locally increase ambient temperature reducing speed of object cooling hence reducing accumulated stress
Warm Bed
Few people tried printing on warm bed (copper plate) with good success.
