New Raft design Maybe

[ronni-jensen 0]

 

 

 

So over the time of printing for the better part of this Year, I have had a lot of troubleshooting Regarding warping in the material while doing larger prints (8*8*8 cm and above, more or less). I have looked at a lot of things…. temperature of the build plate, nozzle heat and wall thickness. While they all have a saying in the outcome, warping of the raft is still common when I print lager objects.

So the raft design that is currently in place is way better then what we had before, but I am a construction engineer and I make prefab concrete structures for a living and there might be a better way of avoiding the raft warp as it does now when making big prints.

Okay so to those of you that is printing with rafts we know that warping comes from shrinking materials when it goes from hot to cold…. When a material is heated up it expands and when it cools it contracts. So let’s have a look at our current method.

Pic. from 1-4 shows how the raft is made to day. The lines is almost making the raft solid, and all the lines is facing in one direction. This model of the raft has greater cohesion then the old version yes, but it still lacking in dealing with the problem of all the shrinking forces in the materiel itself. This is shown I pic 5 where all the layers is shown in a cross section.

The first layer is closest to the heat bed and doesn’t shrink that bad because it does not cool as much, but keep in mind that it still shrinks, one of the options to reduces the shrinking is to crank up the heat, but in my experience it doesn’t always do the trick.

So in picture 6 I have tried to illustrate what happens to a material that have a difference in temperature from the bottom to the top, and the longer the individual string is the more it will bend because the differences in tension is bigger the longer a single string is. So when we look at layer 2-3-4 we see long continues lines that builds up the tension between the layers and make them warp when you are doing big prints.

So the smaller a line, the smaller the tension is in the material and I may have a way of dealing with that problem…. Now I am no programmer so I don’t know if this is even possible or not, but my solution can be seen in pic. 7

So this is building a 4 layer raft as is done now but instead of doing it in strait lines you do in a zigzag order, Should you need the first lines like in pic 1 that is no problem because it is in the bottom layer were shrinking is a minor problem. In this way you reduce tension in one strait direction and this may reduce warping In bigger builds.

So the most important thing is that we don’t want all of the tension forces to be going in one direction, but spread them out so that warping becomes more difficult for the raft.

PS I have found that when doing big prints a tall raft has helped me many times, making it about 7 mm with the heat of the build plate at 75 C, the warping tops at about 5 mm. so that the rest of the build can be done without warping, but this is only possible when the rafts stays in place and doesn’t warp to bad. The heat signature will look at bit like pic 8.

 

 

 

Let me know what you think

 

[woofy 12]

It would be helpful to say which machine and material you are using.

I can print 200mm x 200mm ABS prints on the UM2 without a raft at all, just brim.

I don't think a new raft design is really needed.

 

[ronni-jensen 0]

Using PLA on a UM 2

what's you're settings ?

 

[woofy 12]

For ABS I use 110 bed and 250 nozzle, for PLA I use 70 bed and 220 nozzle. I think 75 is too high for PLA, even my 70 is said to be too high. Readhttp://umforum.ultimaker.com/index.php?/topic/3404-printing-on-glass/ by gr5.

 

[svanbennekom 0]

You're spot on that the heat gradient is the killer. I've predominantly seen PVA glue used to combat warping.

Other options are building a wall around your object that keeps in hot air, or modify your printer to be enclosed and heating it up to just under the glass temperature of the material.

I'm not so sure whether your idea would fully work, since shrinkage is proportional to the size of the object, and changing the direction of your lines just changes the direction of shrinkage. Have you tried to write any gcode for it to do some experiments?

 

[ronni-jensen 0]

@ woofy: I have tried that but talking to the community here in Denmark it seems to me that most people is in the need of keeping their printer in a heated room so they do not need to apply more heat to the build and I think that is way I need the bigger temps.…. I understand that now. And thank you 4 the feedback.

I may have come to the conclusion that the room that I am printing in is too cold in the morning where I begin my prints, and when the printer has heated the room, the print stabilizes. And that I need to use glue to keep the print in place.

@svanbennekom: thank you for confirming that … so no I haven’t ever written any gcode before, and that is kind of my hope that someone will find this idea inspiring enough to make a gcode and test it.

So the theory of expansion and contraction in materials is the same in every material, according to elasticity theory. And in the pic below I have illustrated what will happen to an ordinary strait line contra a line that is zigzag shaped. Pic 1-2 and 4

So when we look at the first strait line we see that the longer the line is the more it will expand when heated, the small line will only contract a bit and the long one will contract more, and the longer it becomes the force in the direction it faces will become greater. When applying Newtonian law every action has and equal reaction there by the 2 arrows.

The same thing is happening in the steal in concrete elements, and in steel structures when heated and cooled.

Now bear in mind that cohesion to the build plate is to stop the object from moving upwards, so if we look at the bending line the forces will move the object ass shown pic 3. The upward motion is a side effect of the material cooling down.

So to minimize the forces in a single direction the zigzags will have to be small so the upward motion is less than the cohesion to the plate.

I have illustrated that as well, as you can see the small zigzag pattern could cut the force in one direction in half, and counter the upward motion that the long strait lines have, and making them zigzag will help counter line length reduction in one strait direction.

So when you make zigzag layers instead of strait lines you will help spared the contraction forces in the raft.

In the last illustration I have tried to illustrate how I think the forces will go in a zigzag line, and that making it zigzag maybe will help reduce contraction in the material a bit.

 

[ronni-jensen 0]

pic 4... the arrows that have an angel is pointet in the wrong direktion,,, you will have to imagene them pointing in the opposit direktion... sorry