More of the same ‘out loud’ thinking on the issue of warping in this post to follow on from the last post on warping fundamentals. In particular, thinking about how the speed of a print, the size of an object and they type of plastic all effect warping. Also some other obvious points about why a heated print bed helps reduce warping.
The effects of printing speed, object size and plastic types on warping.
By printing small objects quickly it seems you can limit the amount of warping that takes place. This may be due to the limited time that the outside of the object has to cool and so results in the whole object being of a more even temperature over the short duration of the print.
Again, the image above also applies if the top object was larger than the bottom and both were printed at the same speed. The larger object will take longer to print and so will have more time for the sides of the object to cool and possibly resulting in less warping.
The image above also applies if different plastics are used, with the top object being a high temperature Tg plastic (Eg: HDPE) while the lower image being a low temperature Tg plastic (Eg:PLA). The higher temperatures relative to ambient (often 25ºC) required to print HDPE will result in the outside edges falling in temperature considerably faster with respect to the centre than compared to PLA.
Although the sides of the HDPE object are hotter than the core of a PLA object, they are still relatively a lot cooler than the core of the HDPE object. It is this steeper temperature gradient that leads to warping. Differences in the expansion coefficient between the different types of plastic may also play a part.
Heated beds and warping
Heated beds are the obvious solution to the problem of cool down on big objects or high Tg plastics. The wide spread adoption of heated beds is a tribute to their effectiveness.
Unfortunately it seems even a heated print bed has its limits. As an object is printed the warming effects of the heated bed will diminish with height. I imagine this then leads to the same warping effect in the top section of the object that is present in objects printed with out a heated bed. These internal stresses that build in the top layer would weaken the object even if it is not enough to cause the lower, stress free layers, to curl up at the edges.
This problem could be solved by streams of hot air blowing from above, a heated build chamber or even an infrared globe above the print bed. However, with out the dissolvable support material used in commercial printers (work in progress for reprap’s) I imagine this will also have its limits as slight overhangs or teardrop through holes begin to slump on larger objects. A simple solution to slumping is to strap on a lot of fans, but then you would be back at square one with the warping problem*…
It may also be possible to greatly reduce warping by having a slow controlled cool down such as used when casting large objects. So instead of the heated bed switching from ‘full on’ to ‘full off’ there could be a gradual decline of 2 or 3 degrees per minutes until room temperature is reached. This might aid in reducing warping but has not been tested as far as I’m aware.
It should also be noted that the heated print bed seems to also allow for greater adhesion between the print bed surface and the first layer of plastic. From what I can gather this is due to an increase in intermolecular contact brought about by the higher temperatures (lower molten plastic surface tension) and the longer time frame were the plastic is molten at the surface and so can spread (wet) more. More info.
Anyway, I hope those new to the reprap community find this helpful. If I missed a few things or something doesnt seem quite right please, by all means, let me know.
* Just as a side note: What would be really nice is if there was a way to analyse a 3d object and determine where slumping is most likely to occur. Then throughout the print a fine jet of room temperature air aimed at the print nozzle could be turned on or off as the print head prints the layers above these ‘high slump risk zones’. You might be able to get away with having an elevated build chamber temperature and reduce slump at the same time. Just a thought.