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.