Exploring Print Adhesion Problems

There are many ways people get their ABS to stick to a heated bed, the most common being covering the bed in kapton, PET or a coating of ABS juice (ABS dissolved in acetone). I haven't used any of them (too lazy to buy any) so I instead use a slightly less conventional choice: water with a tiny bit of sugar dissolved in it which is applied to the bed's glass surface while hot. This has worked perfectly in the past, however I've been having quite a lot of lifting from the bed lately. The solution I currently use is about a 1:50 dilution. I apply it using a paintbrush, painting the mixture on using swirly movements. However I don't think I'm getting the best out of this sugar mixture given how well parts used to stick.

Lots of warping on a BeagleBone case - a recent pritn. Click on the image to see a
larger version and zoom in to see what I talk about below with layer deformation.

This problem seemed to have manifested itself as the ambient temperature fell due to the onset of winter. This led me to blame the lower air temperatures causing increased warping forces on parts(which does indeed happen), but on closer inspection, this didn't make sense. Why? Take a look at the original X motor mount which I printed for my machine (at a time when I had the best adhesion) as an example. Though deformation could be seen in the layers from large warping forces, the plastic never lifted from the bed; it probably would have de-laminated before becoming detached. Compare that to more recent prints - a BeagleBone case - large warping forces too, but not a sign of any such strong bed adhesion. The main reason corners were lifting wasn't to do with a lower ambient temperature creating more warping forces, the main problem was with bed adhesion - something that really doesn't have much to do with ambient temperature since the bed is heated up to the same temperature for every print.

Zoom in on this and see how on that edge all the layers near the bottom curl
upwards but the very bottom of the part still stays flat due to the layers
stretching to make up for this curl and extreme bed adhesion at the
same time.

So if it wasn't ambient temperature, what else had changed since then to produce such drastically different results? A lot, I realised. To list them: my bed wasn't heated to as high a temperature as it used to be, I didn't squash my layers down as much as I used to and the concentration and way I coated my bed in sugar water had changed. All these changes I had made for justifiable reasons at the time.

On my old machine I used to heat my bed to 120 degrees C when measured at the very edge of the bed on the surface. This is the very coldest point on the bed, meaning that I must have reached temperatures of at least 130 on the surface at the centre and even higher temperatures underneath the insulated bed). When I built my second machine I put the thermistor under the bed, right in the centre, like most people do. I also began using a bed temperature of about 115 degrees C (measured from the centre on the underside), this is about the bed temperature most people use so I thought there was no need to go any higher. This translates to perhaps 105 degrees on the top at the centre meaning that the temperature I now use is actually about 25 degrees colder than I used to use. Now I'd just like to get an IR thermometer to check all this rather than estimate.

As for squashing down the layers, on my old printer I had trouble maintaining a level bed (set it, do a print, and it's changed kind of stuff), so what I would do is set the layer height lower than what the slicing software thought(so I at least didn't have too much height if my badly-leveled bed dipped down). At times the hot end even dragged along the bed for parts of the first layer if the bed was higher-than-expected in some places. This wouldn't cause damage on that particular machine because the X carriage was able to pivot easily due to a bodged "floating bearing" on one side which could rotate upwards off the bearing and thus gain some height (though it was normally held down against the bearing due to gravity, quite hard to explain but I don't have any pictures). The end result was absolutely tiny and squashed first layers (0.1 mm).

I had also changed my sugar solution since then to be more dilute even though I hadn't had problems at the old concentration this was because I really wanted to stay far away from having too much which can cause the bed to become all slippery to the plastic and end up creating a big blob on the first layer. I used to use 4:100 but now use 2:100. My logic behind decreasing the concentration being concern about a particularly over-coated patch of the bed becoming slippery (because the solution doesn't tend to come off the brush perfectly). It also seemed that I was getting away with this lowering of the concentration until I really started to think about now vs. back then.

To test these three possible factors I will do some experiments, so far I have completed an experiment on getting the concentration right which is what the rest of this blog post will be about. When I complete the experiments on squashing down the first layer and on finding the best bed temperature I will post some more results.

SUGAR WATER COATING EXPERIMENT

For the sugar solution concentration experiment which this post talks about, I printed three copies of the same object at once, each placed on the bed such that they would be on areas of the same approximate temperature and each receive about the same airflow. The areas on the bed where these parts were printed were coated in their respective number of coatings of 1:100 sugar water to achieve the desired equivalent coatings of 1:100, 2:100 and 3:100, with multiple coats also helping to ensure even coverage on the bed. Printing the parts at the same time had the advantage of saving time and also making sure that all parts were subject to the same conditions (not including sugar water concentration of course).

The spacing and positioning of the parts.
The prints shown here are three Mendel90
vertexes which are small in size for a quick print,
but also high-warp due to high infill percentage.

COATING EXPERIMENT CONDITIONS:
Ambient temperature:                       12.5 degrees Celsius
Print Material:                                   ABS
Sugar Solution Concentration:          1:100
Bed Temperature:                             105 degrees Celsius
Object 1:                                            1 coat of sugar water
Object 2:                                            2 coats of sugar water
Object 3:                                            3 coats of sugar water


RESULTS:

Here are the undersides of the prints, the blue lines were drawn on
with a permanent marker to indicate the boundary between where the part
has stuck to the bed or come off the bed.

Interestingly, print 2 did the worst, with print 1 coming in at a marginally better second-worst and print 3 with the 3:100 coating being the best. Perhaps print 1 beating print 2 can be explained by a difference in airflow, being that that print was off to the far left - maybe I didn't get the airflow quite equal for all the parts. Though I doubt the airflow explanation since the ring of separation from the bed is so uniform and an airflow in a particular direction would show as a couple corners lifting significantly more than all the others. Also the low profile of the parts and the spacing really should mean that they cannot shield each other. Then again it could have been really minute differences in bed height that caused this(I did take a lot of care leveling the bed before this experiment). Or maybe it is just that a 2:100 coating is really bad, either way it looks like I will use the 3:100 coating from now on, being visibly better.

RepRap Z-Wobble Woes

While I am happy with almost all aspects of my Mendel90 that I built, I have never really been happy with one aspect of the print quality. That aspect of print quality is z-wobble. Or so I thought it was, as I have so far found out, it must be a much more difficult to remedy Z-artifact problem.

This is how prints looked when I first made my printer:

Very typical Z-Wobble - it has the same pitch as the leadscrews of the Z axis and is easily fixed in theory by removing a bit of constraint. However, even at this point in time, the Z-artifacts were inconsistent - they could be horrible one print and then hard to spot on the next.

The first thing I tried was to install a new type of nut trap (pictured below) which transferred virtually no movement except for movement in the Z direction - a desirable thing. Along with this initial modification I then drilled out the holes that the Z rods pass through to give them a bit more room to turn without knocking about, pressing against the X ends and to make up for inevitable misalignments. But if you looked closely there were still artifacts, and now they were looking different and more irregular, I guess the Z-wobble, which was now reduced by a great deal, had been concealing these other artifacts.

I then tried re-seating the rods in the old couplings and not over-tightening them in an attempt to align them more accurately and make sure they couldn't exert too much force in the X and Y directions, but this still didn't grant results I was happy with. I eventually moved to another coupling method - using my lathe to turn down the ends of the rods to 5mm so a flexible section of tubing could be used as a coupling. Alternatively I could have just used 5mm threaded rod.

The coupling system installed. I may have to replace
the cable-ties with hose clamps because the tube
softens up a bit due to heat - no problems so
far though.

The turned down end, I was very impressed given that
it was done using my first home-made/ground HSS
lathe tool. Cutting stainless-steel too!
In the background is half of an old coupling, a nut trap
and also a badly heat-deformed (from the motors) piece of
tubing that went inside the old coupling.

After all these mods, done over much time, I got this print quality on an X Motor End for a Mendel90 - unsatisfactory to say the least:

However, I was now certain that the Z-wobble was gone and that this was something else. This is due to the fact that the misalignment does not happen at regular intervals and does not look like a sinusoidal wave of  X-Y layer displacement. Instead it happens very randomly and can go from the displacement varying every layer to happening every 10. The displacement is also very slight - about 0.1mm most of the time, however under certain lighting and angles, and with my highly-opaque white plastic, I can either hide it completely or make it look ugly. I suspect that if I put my plastic into an UP! or a Makerbot Replicator, the prints would come out with a bunch of highly-visible ugly artifacts like this too, but the photos of their parts sure do look nice. Having actually seen UP! prints I can say that the slight transparency that their white plastic has hides a lot, for it is very, very hard to distinguish layers by eye even at large layer heights. Holding hard, 90 degree edges up to light gives away a lot of small, but visible layer alignment issues on the UP! too though.

So, still looking for answers, I thought it could be backlash or skipped micro-steps as per Nophead's excellent article on micro-stepping. So I tightened up my belts and changed a few things with how my StepSticks were set up. I didn't read too in-depth into his article to know how it all related to my motors as they are low-current and high-resistance and I didn't really bother with the maths. So I didn't calculate the off times to get the PWM right for small-current portions of the micro-step cycle given current decay in the windings, let alone how to set this on the driver. But reading through the data sheets and his post, I realised that if I set my current too high with the potentiometer, say 0.4A (hey, I thought having a bit of juice available to them was a good thing) when my motors only draw 0.3A per winding at the drive voltage, the motors would have very bad and uneven micro-stepping. This is because the potentiometer does not directly limit the current to the motor by being in the path of current travelling into the winding, it is instead a user-set way of telling the driver at what current the 100% current level should be. This 100% current level is when all the current is in one winding and the other is completely off - a full step. If you set it above the current a motor can actually draw due to its resistance, the motor may actually reach full current in one winding while the driver is on the 13th out of 16 micro-steps or whatever, causing some really dodgy stepping.

I was in fact able to see and hear this, the motor would, at very slow speeds, change its sound and start and stop when motion should have been continuous. After setting the current properly, movement was smooth and the tone of the motor didn't change nearly as much. I might still only be getting effectively 1/8 or 1/4 micro-stepping for all that I know though, but at least it's smooth. That is something to look into another time. Here's the resulting print, it's still got all the same problems so it looks like the micro-stepping or belt backlash wasn't the big culprit:

Ah well, after all that effort investigating the motors I at least have gained some knowledge on stepper motors and hopefully eliminated that visible artifact on shallow curves and low-angle X-Y lines where it looks like the line is uneven with little bumps due to jerky movement.

There are a few more things to test including an unevenly hobbed bolt and badly-meshing gears on the extruder, my bed changing height as it expands and contracts with temperature swings and also the filament pulling the carriage around a tiny bit - for which I have ordered a PTFE guide tube which will arrive soon for me to test, assuming free shipping from China on eBay is quick.

The RepRap magazine has an article on Z-artifacts if you are having trouble with them like I am, and I hope that you have gained something from this post.