Category Archives: Stereolithography

Episode 2 : Form1 calibration strikes back

[March 11th edit : these calibrations issues were enough to convince me to sell my Form1 printer, for details see this post]

In a previous post I was showing my Form1 was not really well calibrated and circles were turned into ellipses. After filling a bug report to Formlabs, their answer was on the line of “thanks for your concern we are working on it…”

I’m confident they will find a solution, but I started to wonder how a user could calibrate his printer himself. This is important as the warranty on the printer is only valid for 3 months and as any mechanical machine, I’m expecting the printer will require some calibration from time to time. Last but not least, this will be only possible if Formalbs is actually giving us access to more parameters than the currently closed/dumb down version of PreForm we have (Hint! Hint!).

Laser+Galvo based printer calibration challenge

A normal user has limited resources and not access to complex measure instrument, so the procedure should match these requirements:

  • Simple steps: if possible it should be as simple as print test objects, measure them, compute the corrective factors (Excel Spreadsheet/integrated to PreForm), enter the new parameters and re-print the test objects to validate the calibration.
  • Not too long and costly: printed test object should be small, we don’t want to waste time and resin
  • No expensive measure instruments: I’ll be only using a digital caliper available for less than $50 on Ebay/Amazon

My main assumption for this exercise is that the calibration required is only affecting linear parameters. I will not go into galvanometer fine tuning. We will see at the end of the article that unfortunately non-linearity are present and will limit your end result quality.

The main challenge with laser base printer is that it’s difficult to measure precisely the position of a moving dot at the bottom of the vat (absolute measurements). So one solution is to do relative measurements of printed object features. We will use a digital caliper to get a good number of data points to build a robust average estimation of the calibration.

In the graph under, you can see that the laser spot is covering each each slices of the object using two kind of paths. The perimeter/skin-paths are tracing the outline of the object, they will be responsible for the smooth finish. The In-fill paths are straight and parallel paths to cure the inside of each slice. Using simple shapes like circles and square will let us measure independently the performances of both X and Y axis galvanometers.

calibrationExplained1

The final dimension of the Slice is affected by both the axis amplification (deflection angle range) and the size/shape of the laser spot. When tracing the measures VS the theory graphs, the amplification correction is measured by the slope of the curve, while the shape of the laser spot is creating a constant offset.

calibrationExplained2

As PreForm software is not open source, I cannot be sure how to use these correction factors. For the amplification, it will probably mean the angles transmitted to the printer will need to be multiplied by the correction alpha found (different on each axis) . The offset is a consequence of the spot size mismatch. PreForm must use a boundary offset to draw the perimeter/skin paths. A positive offset means that the diameter of the spot need to be reduced by half of the value. Also if the laser spot is not exactly round it might be trickier to program…

Calibration object and Procedure proposal

The shape I used for my calibration is a simple series of square and circle extrusions of various small size and one large for the base. Each instance of the object provides 20 points of measures:
– Circles: [1, 2, 3, 4, 23] mm diameters, X and Y axis
– Square: [3, 4, 5, 6, 25] mm side length, X and Y axis

Cylinder_dimensions

To know where each object was printed on the platform, I’ve added a number engraved. That could be useful later if we are tracking non linearity issue depending on the location of the objects.

[STL & Inventor files for all squareCylinders Test] [5x .form file] [9x .form file]

I’ve used the 5 duplications file as it’s already providing 100 measure points. The base of the object is 3mm thick as I was hoping I could print it without supports to save time and resin. It turn out the Form1 is curing multiple times the slices in the first 2mm for the base. It creates an adhesion issue for the next layers, and I ended up with unusable objects with holes so I’m now generating support structures.

x5Cylinder_platformPosition

Here are my measures, with the 5x file, transparent 100micron material profile and default supports configuration. My first prints were done with the 0.8.1 software but reading that the 0.8.2 is supposed to have an improved laser tracking I decided to reprint everything. (Un?)fortunately the measures are consistent and the 0.8.2 didn’t improve the calibration…

Measure_x5Cylinder_0-8-2

The full spreadsheets with regression formulas are here [0.8.1] and [0.8.2].

x5Cylinder_calibration_results

From the regressions results we can see that the calibration required is consistent in both PreForm version. My printer has a 1.4% error in X axis and 0.35% error in Y. The laser spot diameter is also off by 0.2 mm.

For some reason the PreForm 0.8.2 is less consistent than the 0.8.1, the calibration is only reducing the overall error by a factor of 4 while it was reduced by 5 in the previous version…

Ain’t you doing anything about outliers?

When doing a root cause analysis on anything, you have to solve the first order issues that are dominating your calibration errors before looking at the rest. My Form1 is displaying non linearity on the X axis on geometries close to the back of the platform (2 = back-right and 5 = back-left).

cylinderX5_nonLinearity

When you look at the measures table, both objects are very different from the rest. I have no explanation yet for that phenomenon, but I hope that once I will be able to correct the offset and scale in PreForm I can build a new test to tackle this issue.

Until then, if you have a Form1 I would be glad to ear if your printer has the same calibration issue as mine. More reports will help Formlabs building a great printer!

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First steps with the Form 1 Printer

So here it is after many months of waiting the Form1 Stereo printer is sent to backers! and I received a few days ago my puppy, and now the 3D touch has some serious competition.

More on that later, but I’m thinking on selling my 3D touch printer with ~15 filament spools of various colors/materials. So If you are interested mail me, free training included if you are in the bay area 🙂

I will not do a full tear down of the printer, but if you are interested to see the guts of the form1 Bunnie’s blog has an interesting post.

cartboardWith3dTouch

The cardboard boxes are massive and Formlabs decided to avoid foam or bubble wrap to lock the printer in place. The printer was held by 2 plastic films, but you can see in the picture under that during transport it shifted out of the slot. Nothing serious in my case but I have already read some reports of people complaining they received their printer damaged. I’m not exactly sure if this packing method is completely fool proof, especially if the box are rotated during transport/handling.

InsideCartboardBox

The unpacking was really easy, just a few tape pieces to remove… But dang, that’s a sharp looking printer! I’m especially impressed by the build quality. As you can see in the following pictures, they didn’t try to cut on the details. The only small remark I’ve is that the printer detect when the orange hood is closed using a hall sensor and from time to time the magnet is not completely aligned so you need to fiddle/move a bit the hood to get it detected.

Form1ReadyToOperate form1BuildQuality

Now let’s move onto the printing. The samples I’ve seen during the Maker Fair were great but I have to say I feel the Preform software (V0.8.1 at the moment of writing) is still somehow limited and you need to practice a bit to understand how to get the best out of your printer.

My first test was a 25 microns print of one of the Cyvasse game piece but for some reason the support (surface of ~ 2 * 1cm) was not sticking to the platform and the cured resin ended up floating in the tank… So after 2 tries and a fishing party to retrieve the layers, I decided to start with something easier and stick with the default 50 microns layers for the moment.

My first success was with a pocket monster figurine by Andreas. I’ve scaled it down to 50% (.form file) and the result is quite stunning when you are used to the FDM quality. Once cleaned from the resin with alcohol and dry the pieces are a bit cloudy and not very shinny (left picture). I used a Varathane Gloss polyurethane water based spray (interior / Heavy Use Formula) to get a great shine (right picture). With one layer, the details are not lost and you really have the impression that it’s made of crystal.PocketMonster

After that, I was ready to test the limits of the printer. So I tried the simplest structure I could find on George Hart math page. When processed by Preform, the Goldberg polyhedron (.form file) had a large red/unsupported ring area but I decided to print it anyway…goldbergSphereError

It didn’t worked out and I could just  clean up the mess after. golbergFailOnTheTray

As you can see the resin is rather flexible and not fully cured after print. I couldn’t remove the remaining of the sphere from the support without tearing the sphere in half. So lesson 1 is: don’t pull 1mm thick walls… Maybe I should have tried to wash the result and wait until dry?golbergFailFlexibleResin

I was surprised to see how flexible the resin is after print, so I’ve put some of my failed pieces outside in the sun to see how the resin age. As you can see under, after the prints, the parts are white, but just 2 days outside (gyroid – top right) and the resin is much harder/cured but with a yellowish tint. After 4 days (sphere – top left) the tint is even stronger and  all the flex is gone, the cured resin is actually very stiff.form1_aging_resin

Now let’s have a look at print defects that you can get with the form1…

I’d like like to stress that Formlabs team is making progress and the new Preform software revision (0.8.1) is actually solving one of the issues, each layers had a visible seam but the new cleared that point.

seam_form1_defects

Visible seams in Preform 0.8 (Releaux spheres print in 50 microns)

These seams were visible but you couldn’t feel them on the surface so it was more a cosmetic defects. The real issue comes when some parts of the print are not cured properly. In this case the end result is rather ugly on some sides…

reuleaux_form1_defects_0-8-1

Reuleaux Spheres printed with 0-8-1 (50 microns, default generated supports)

To conclude this post, I have to say the form1 is an impressive printer, the details of the parts are great and the build quality is stellar. That said, I know being an early adopters has its disadvantage and even if the printing process is not yet “plug-and-print”, I have high hopes for the future!

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