Z-Compression Correction question

Hi there, has anyone tested the new PreForm 2.20 Z-Compression feature ?

I have just printed a part in Rigid directly on the BP, it has a relatively big surface area in contact with the platform ( ~10’000mm^2 or about 100 by 100mm). I have left the default Z-Compression Correction value of .75mm and my part ended up .73mm taller.

Have any of you seen anything similar ? I will try to print the next iteration with a .05 or 0mm value for Z-Correction.

I suspect that the force exerted by the BP is creating a comparatively low pressure on the tank compared to smaller prints, and that this .75mm is there for very low surface area situations.

Here are some notes from formlabs when it was in beta.

PreForm 2.20.0

Compression Correction for printing on the Build Platform: Under Supports > Advanced there is a new feature called Compression Correction.
This adds extra height to the bottom of your part to compensate for the compression that takes place in the first printed layers.
We recommend you print a small cube, measure its height, and enter the difference from nominal into this Compression Correction value.
You will see this visually added to the bottom of the part.

Base Merge Height:

In the previous PreForm beta (2.18) we added “early layer unioning”. This improves the print success rate of parts with bumpy or slightly curved bases by adding extra material to make them flat and smooth for better adherence to the build platform.

This feature is called Base Merge Height and can be found under Support > Advanced. It is also visually shown in PreForm.

Small tips for small minima:

PreForm will now automatically put a smaller support touchpoint when it detects that the minima is also small. This should help maintain small details.

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Yes I understand how the feature works, it’s just that I’m surprised that for this parts I don’t need any. I have a standard print with 40 parts printed in Tough directly on the BP, each part has a surface area of about 70mm^2 and with the previous version of Preform I was indeed loosing .7mm (had to add these manually).

I’m surprised that with this rigid, 10’000mm^2 part I have 0mm to correct and with a tough, 2800mm^2 print I have .7.

Took me a while to figure it out, sort of anyway. It appears that the original built in compensation for enhancing adhesion to build platform may be gone. Instead you have the default .75. I tried it at zero and while it adhered to the BP the part was much thinner than before. I think I ended up at .7 being close to the original. Plus I have my Z axis at .4. Once you find a good starting point it is great. Nothing replaces a good micrometer.

I’m in a hurry iterating on a design so I can’t make tests for the sake of it or take too many risks. I did however lower the Z compression value from .75 to .4 and had almost exactly .4 mm added to my final print. This was the same kind of print as the one that triggered this post.

I definitely think you’re right about the original offset being now user-modifiable as the Z-compression correction but I was surprised that the amount of correction ended up not being used at all.
It will take me a little while before I can do a new test in Rigid (for comparable results) with a print directly on the platform that has a much lower first layer surface area, but when I can do that I expect that the Z Compression Correction will behave differently and that I may have to raise the number.

I think we might have to end up creating a table with featuring the relation between Z Compression Correction and first layer surface area… but if this table is different for each printer+BP+tank+resin+Z-offset combination it will be too much work I think. Maybe Formlabs could provide some calibration guidance or at least allow us to create presets to facilitate this kind of work.

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My 5 cents, I have made a test cube print with 0 value, and I measured 0.75mm less height than expected, so I use the default 0.75mm value which gave me 0.01mm difference of height when I printed the same cube over again. Regarding adherence I see no difference between the 2 (0 value and 0.75).

What was the surface area of the first layer ?

These are the settings I use: (I have however tested it first printing a cube at 0 value and measured how much I was missing and found it to be around 0.75-0.8mm)

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If you print a test cube that you know its dimensions and measure it at 0mm Z Compression Correction, you might be missing less- or more, I guess it varies from printer to printer.

Thanks for keeping the thread updated ! I have printed my ~10’000mm^2 a couple more times and ended up with the right dimensions with a .1mm Z-Compression Correction value.

The reason why I asked about the surface area of your part is because is has en influence on the compression of the first layers, this has been confirmed by Formlabs. The printer always pushes the BP down with the same amount of force (driven by the springs under the tank receiver), so the pressure is changing based on first layer surface area, pressure which then influences the amount of compression.

@JohnHue Is this on a Form 2 or 3?
Does the confirmation of Formlabs apply for both?

Edit: oh, your Bio says you have a Form 2