Thread Rating:
  • 0 Vote(s) - 0 Average
  • 1
  • 2
  • 3
  • 4
  • 5
Model Stress
I've just printed part of a control box (for me Acetone vaporiser) that is 100 x 100 x 50mm tall. The design has cut out's in the front to fit the temperature controller and power switch. Quite a simple design really. I printed this at 0.3 hieght and max quality. It printed flat base down and with supports for the two cut outs. Overal I am very pleased with the quality of the print, using just the bog standard software, firmware version J.

The supports broke out easily and cleanly, but as I did this a crack appeared on the right hand side, about half way up the model, streaching about 40% down the side of the box. The crack is at the front corner about 1mm open. The box will clamp toghether and glue without problems, I hope, but the nature of the crack and the fact that it is open seems to sugest it is due to stress in the model.

From those who are likely to know more about this than myself, does this suggest that the bed temperature could have done with being higher or the nozzel temperature being higher. In either case I guess this would mean using other than the standard software. Any thoughts would be welcome.


To me whom has welding knowledge. It sounds as if it was not allowed to normalize. IE, when you let the bed cool, did you allow the cabinet to remain closed and let the part cool as well? Just a thought of something to try. Otherwise I thought it was common practice to print openings (if possible) rotated so that the large opening was at the top???

I am eager to hear wether my thought process is correct here as well.
Cooling could very well be a factor. Abs is designed to shrink when it cools. Unfortunately this sometimes causes exactly what you are describing.

A hotter bed and higher ambient (room) temperature would probably help. Make sure you aren't opening the door during the print. You could also try placing a towel or something else to block air from the top of the printer. Sometimes I see a print problem occur within minutes of the air conditioning turning on.

You could try to save the print; I would paint the crack with a few coats of acetone and then clamp it. Once it dries try dipping the entire model in acetone for 2 or 3 seconds. Then hang it up to dry. This should help melt the layers together and allow the model to be quite strong. You risk deforming the model if you leave it in acetone too long, if the walls are to thin, or if there is not enough infill.

Thanks for the replies.
In fact the model print was started, door closed and then I went and had a few hours kip, so by the time I woke up the model was long finished and cooled.
Ironically this unit is for the controls of me Acetone vapor box so it will remain un-soaked, but I like the idea of using Acetone to glue it, probably much better than super glue!

Question, is ths problem the same when using PLA?
I'm thinking of going down the play with the chip route and then using some Voltivo PLA.
ABS "crackage" is one of the downsides of using ABS over PLA.

This cracking mainly happens because of the temperature differential between the older (bottom) layers, and the newer (upper) layers.
ABS expand/contracts more than PLA.

The main things to monitor/tune to reduce cracking are:

- Correct filament temperature. - To print with the correct temperature, you need to find the ideal temp. for your filament. Take note that all filament comes with a recommended setting, but this is only a recommendation. There are variations between the temperatures of extruders, even between similar printers.
Setting my DaVinci printer to print at 220DegC will give me a different temperature than yours, set at the same temperature.
In order to calibrate filament for your printer, you need to print a 25X25X100mm rectangle. Use single wall, no infill, no top, 0.2mm layer height. Once it is sliced, edit the GCode, and set a range of temperatures manually in the GCode.

For exmple, on my setup, my rectangle slices to 391 layers, so I search through the GCode, and insert these commands in the correct places:
1 - M104 S230 0-48
2 - M104 S225 49-97 - 9.8
3 - M104 S220 98-146 - 19.6
4 - M104 S215 147-195 - 29.4
5 - M104 S210 196-244 - 39.2
6 - M104 S205 245-293 - 49
7 - M104 S200 294-342 - 58.8
8 - M104 S195 343-391 - 68.6

Here you can see that I set the temperature range from 230DegC down to 195DegC, in 5 deg steps. Once it is printed, remove it, and check it visually, marking off the best layers. That should give you a good idea of what temperatures you should be using. Write these temperatures on your roll of filament, and create a new setting under filament in Slic3r, save it as the name of the roll you configured.
If you want to really get good results, do the same thing, but set the layer height to 0.1/0.08, and the temperature range to +- 1 steps in both directions, in 1 DEGC steps.
Example: in previous test, you find filament prints better at 215. set the range to 220-210. this should give you a better value.
Remember, this temperature is optimized for YOUR printer. No use sharing these temperatures with others, as each printer has their unique temperature fingerprint

- Correct bed temperature - Once the temperature is calibrated for your filament, you have to calibrate bed temperature. This is a much longer process, but if you are using your machine to make money, or want the best prints possible, you can't skip this step.
This time, we are going to print a 5mmX120mmX60mm rectangle. set first layer to 0.3, and other layers to 0.2/0.15.
Do this 3 times for 80, 100 and 110 deg set for bed temp.

Let the models cool completely, remove and compare. Look for layer quality, cracking and warping.

- Correct extrusion rate - If your extruder isn't laying down the correct amount of ABS, or that amount varies throughout the print, this will cause the stresses to focus on low adherence zones, increasing the cracking.
To calibrate this (also needs to be done with each roll of filament), print out a hollow 20mmX20mmX20mm cube. Set layer height to 0.2mm, 1 perimeter, no infill or tops.and in advanced settings in slic3r, define the wall thickness specifically as 0.4mm.
Print it out, let it cool, then measure the wall thickness.
Calculate the ratio needed between the measured value and the value you defined, and insert that ratio in to the flow rate settings in Filament section in Slic3r, under filament width.
Repeat. Adjust. Repeat, lock in value. If you find that you are getting inconsistent results, then you are either doing the calculations wrong, or there is some hardware problem causing this issue. There are some nice writeups in this forum about how to adjust/tighten/fix/tune various parts of the printer.

- It's a trap - For hi-res prints (0.08mm layers), I will use a very helpful setting in slicer. Skirts. I will create a 2 perimeter skirt, which goes up, to about 10mm above the model height. This creates an enclosure that maintains the print evenly heated (A heat trap). Let the print cool down. Remember, you have printed a large insulating cover over a heat source. It will still be hot in the middle for a long while. Resist temptation to start hacking away, trying to get the model off the bed. Be patient, go read a book, have some coffee, watch through the first 3 seasons of LOST, then you may attempt to remove the print. Another trick I'll use is to tell slicer to build a solid infill every 15mm. Two layers before the solid infill, I'll increase the bed temperature to 105Deg. Once the solid layer is done, I'll drop it to 95. This helps lock in the heat a little more.

- Levelled bed - Level your bed. Seriously, if you don't have the same clearance (1 paper thick) on the back two corners, center, and front two corners of your bed, forget it. You may be able to pull off some nice small prints, but start increasing the size, or decreasing the layer height, and you're gonna have a bad time.
Once you have your bed levelled, print out a 180mmX180mm square, no infill or top/bottom layers. Tell slic3r to build a 1 layer high skirt, 5mm from the object.

Once it has completed the first layer, reset the printer, and lower the bed. Analise the plastic laid down all around the print bed. It should be nicely squashed against the glass, and uniform around the whole print bed.

- Good quality filament - Get the good stuff. Dont buy cheap chinese/ebay stuff if you're serious about printing. Just dont. Cheap stuff will almost always crack with big & hirez models. It will also probably block up your extruder.

There are other tweaks you can use as well, but those are probably outside the scope of this forum,as they are very specific to the slicer/printing sofware.

Oh, and no, PLA doesnt suffer from this problem as much as ABS, as in most cases, it is slightly more flexible than ABS. It also uses lower temperatures, wich means temp. deltas are lower.

Hope that wasnt too long winded.

best of luck on your prints!

Forum Jump:

Users browsing this thread: 1 Guest(s)