At Javelin Technologies, we have trained thousands of customers on how to use their brand-new Stratasys dual extruder 3D printers and the mechanics of tip calibration is always one of the trickiest parts to explain and demonstrate. In this article we are going to talk everything about tip calibration on Stratasys dual extruder printers such as Stratasys Fortus and F-series.
What is Stratasys tip calibration?
Tip calibration is nothing more than a process of telling the printer where the two tip nozzles (model & support) are relative to each other. The more accurate term for it would be relative tip offset distance calibration. Like everything else in the world, there are manufacturing tolerances in the components of the 3D printers. Although the printer knows the theoretical offset distances between the two tip nozzles, there are positional errors that typically occur in the range of +/- 0.010”, and sometimes as much as 0.020”. By performing a Stratasys tip calibration, we are looking to reduce the error down to +/- 0.002” or +/- 0.004” (depends on the printer model) in X-Y axes and +/- 0.0005” in Z axis.
OK, but why is this important?
The first reason behind the importance of tip offset calibration is to avoid model – support interference in X-Y axes. If you are printing parts that require support structures to run alongside of the model, you should notice that the air gap between the part and the support structure can be as little as 0.005”. If your tip is not calibrated correctly, the support structure could be too far, or too close. If they are too close, you will end up with a print that has witness marks of the support structure all over the surface. Worst case scenario, your print job will fail.
Secondly, correctly calibrated tip offset values promote good adhesion at the interfacing layer, where the top of the support structure and the model layer are touched. This keeps the build not only stable, but also affects the Z axis tolerance to some degree. When the Z axis tip offset is too close, the support base layer can be very difficult to peel off – which results in dissolving the part in the cleaning tank, which can take up to hours.
When do I have to run the Stratasys tip calibration?
The operator must calibrate the tip offset every time one of the tips (or print heads for F-series) have been unmounted. This of course includes changing the tips or print heads on an F-series printer.
How does one correctly calibrate the tips?
For the general procedures, refer to your user manual supplied with the printer. If you do not have a copy, please call or email our technical support to obtain a copy. In this article, we would like to focus on the information that is not discussed in the manual and something that could improve your day-to-day operation with Stratasys FDM printers.
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For Fortus 900 / 450 / 400 / 380 / 360 printers:
These printers go through 2 stages of tip calibration. The first is the automatic tip-to-tip calibration, then manually tweaking the values by reading the calibration box it prints.
During the automatic tip-to-tip calibration, it only measures Z axis offset. This is done by raising the platen up, letting the model and support tips to touch the ‘cone sensor’ – a simple on/off switch – while the printer monitors the Z axis encoder value and records it when each tip touches the cone. This way the printer can calculate the Z axis offset between the two nozzles. However, this measurement is only good for bringing the calibration value to a ball park figure and it can still be off by +/- 0.002” (sometimes more for certain materials such as Nylon) while the ideal tolerance is 0 to -0.0005”.
While this is all nice and easy if everything goes as described in the manual, some odd situations will rise time to time.
Case#1: the support material layer measurement does not respond to the changes made to “Material thickness” value from the calibration adjustment page.
This typically happens when the Z axis offset between the model and the support tips are way out of range. The very root cause of this behavior is the foreign material interfering with the cone sensor measurement during the automatic tip-to-tip calibration stage. This is somewhat common with materials that are hydrophilic (Nylon or Ultem based). High moisture content in the material filament causes excessive oozing of the material and it can lead to a false reading of the Z axis tip-to-tip offset.
To fight this, the printer will purge & clean the tips right before letting them touch the cone sensor but the material could still ooze again very rapidly or not get cleaned properly after the purging cycle. When this happens, it is important to check the cross section profile of the support layer that you peeled off from the calibration box. If the profile looks like the illustration below, it can be lead you to the completely wrong direction for making the offset adjustments.
Case #2: the X-Y calibration is completely out of bound.
This could happen sometimes after a tip change. The maximum range displayed on the calibration box is +/- 0.008” which isn’t a whole lot. If the tip physical offset is more than that, the calibration line will be out of bound and you may be lost in terms of which direction you need to change the values to. In cases like that, the diagram below will help you determine which direction (+X, -X, +Y or -Y) to move the values. Since you need to make adjustments in a large increment past 0.008” range, you can do so by unchecking ‘build calibration part’ when you enter the Stratasys tip calibration wizard and repeat adjusting the XY values until you reached the desired value.
For F-series 370 / 270 / 170 / 120 printers:
The automatic Stratasys tip calibration on F-series printer has gone through many improvements over the years and we’re at the point where it gets it right pretty close to the acceptable values about 90% of the time. However, it is always recommended to run at least one manual calibration box to confirm the automatic tip calibration has been done correctly.
Case #1: Automatic Stratasys tip calibration is far from being useful.
If your printer is running on firmware version older than 2.0, we highly recommend that you update the printer firmware to the latest. The automatic tip calibration process has been improved quite a lot since the very first release.
Once you have confirmed you have the latest firmware, we recommended to run the auto tip calibration while the materials are unloaded. The way these printers measure the tip offset values is by physically dragging the tip on the stainless steel Z axis platform around the ‘+’ crosshair cutout until the tip ‘drops’ into the cutout. This is sensed by a plunger sensor integrated to the print head design. Any interference (i.e. material oozing out of the nozzle) between the tip nozzle and the Z axis platform will lead to false measurement. By unloading the materials, you are eliminating the chance of getting oozed material from the nozzle. Additionally, it is always a good practice to visually inspect the nozzle area and make sure it is clean and free of any buildup.
Case #2: Stratasys Tip calibration values are confusing. Why is there a discrepancy between X1 and X2 (or Y1 and Y2) values?
These printers, like every piece of machinery, have some level of backlash in the gantry system. To mitigate the negative effect the backlash could have on the tip calibration, it moves the head from two different directions along the same axis (hence two readings per axis). So if your calibration reads +3 on X1 and -1 on X2, it means there is approximately 0.004” backlash and the true offset error is +0.001”, which is the average value between X1 and X2. As long as the true offset error is within +/- 0.002” range, the calibration is acceptable. The backlash error is automatically compensated by the printer’s motion controller – nothing for you to worry about.