creating a negative from an stl file for vacuum forming mold

[Anonymous]

What is the recommended boolean operation for creating a negative from an stl file? I am trying to create a negative from a 3D scan in order to make a vacuum forming mold.

I also have Fusion 360 and 3DS MaX so I can try some operations in there as well.

the video shows a way to do it in 3DS max but doesn't list detailed steps.

https://youtu.be/38uffHwJbEg

[Anonymous]

If the parting surface between two molds is planar you do not have to go the Boolean way but use PlaneCut instead:

- Make your scan watertight either using ANALYSIS/Inspector (keeps the original surface as it is) or EDIT/MakeSolid (remeshes your surface according to the set parameters)

- Go to SELECT and SelectAll

- All selected run Edit/FlipNormals (this makes it a "negative" surface)

- Construct a cuboid shape (= mold material) including your object totally as a separate object.

- Activate both objects (hold down SHIFT while activating the second object) and run EDIT/Combine.

- Use EDIT/PlaneCut on this result and move the cutting plane to the desired position. If you set CutType to Slice(KeepBoth) you can construct two halves in one go. Run EDIT/Separate in this case to get the two halves as separate objects.

-----

If the parting surface isn't planar you've to use BooleanDifference (of course this works also if it is planar):

- Construct an object describing one half of the mold.

- Activate this object first and (holding down Shift) your scan as the second object (order important for you want to subtract the shape from the holding material) and run EDIT/BooleanDifference.

Note: If there's a bigger difference of mesh density (e.g. the stock object owns only a few triangles while you scan is made of many tris) you should try to get a similar density at the mold object before doing the boolean (simple way is to run MakeSolid on it)

[Anonymous]

thanks, i will try this out. it is not two part mold, just a negative. So you would place plastic sheet over a mold (created from the negative) and get your part.

Here is an example

https://youtu.be/VFkVxurKeAs

[Anonymous]

Yep, I know vacuum forming. Just in case you 'd like to do form two halves and glue them together finally...

If you're interested in one half only use PlaneCut at its default settings. If it results in the half you don't want click on the transparent blue arrow in the widget to inverse cutting direction.

[Anonymous]

@Anonymous no such luck. I tried Boolean difference (I selected a giant cube first and the part second) and it didn't quite give me the results I was looking for.  The part is a very thin walled part so that might be causing an issue. The giant cube is just my tool. Would it be better to sketch a profile of the part and use the extrusion as the tool instead?

Is there a way to upload stl files that are 21mb? I keep trying to attach it and the forum will not let me.

[Anonymous]

@Anonymous I sent you a pm with the stl files. I am thinking that because the stl is so thin walled that might be causing the issue with Boolean difference.

[Anonymous]

Thanks, got your files.

The problem aren't the thin walls. Here BooleanDifference does the job as expected: Your source shape is a cavity inside the mold's block...

Knowing your source: Seems to be better to go a different way by extracting the mold's surface directly.

Easiest way to demonstrate would be a movie of the workflow.

May I use your source file to record a screencast movie to post it here?

[Anonymous]

@Anonymous yes, please do post a video.

[Anonymous]

Here's the screencast.

Sorry, without audio. 

Some comments instead:

As a first step you need to use GenerateFaceGroups. Basically you could run this and simply delete all but the lower group. This is risky for there might be tiny groups included by the lower group (some group islands) and ichi might result in holes. This is why I do combine all resulting groups but the lower one first and select the upper group to delete. If there were islands in the lower group they will not be deleted this way (for they aren't connected to the upper group)

As a second step I use Transform to extrude in Y. You may wonder why I use Extrude with a zero offset before? Extrude doesn't create new groups at the extrusion sides instead an extrusion via Transform does. So Extrude at offset zero creates a two sided sheet where i transform one side of the sheet.

As a third step I split the side groups built by Transform to different groups. To select the plane surfaces you need to be in MESHMIXER navigation mode (> Preferences). Here RMB-drag allows to expand a selection to a creased edge.

Now I have groups to extrude via Transform. If this doesn't work (as it happened on the right side in the movie) you need to Modify/OptimiseBoundary (adds/removes faces of to/off the selection) first.

Finally I cut the resulting object via PlaneCut.

To do this in precise size I create FaceGroups for each side of your tool object and drop a pivot to the centres of the groups to be a guide for the cuts.

It might happen that PlaneCut fails to fill the cut. If it doesn't fill the cut in preview cancel and do all working cuts first. Maybe this solves the issue. If not you need to accept the open boundary and repair it via Inspector before doing another cut hitting this hole.

(sorry for the collapses when I change a shader. This is due to Screencast.

For WIN users that strange "stair-like symbolOpt" (modifier key I use to rotate the scene) in the recorded key commands means ALT - I'm on Mac)

[Anonymous · Accepted solution]

And another option:

As a first step again GenerateFaceGroups,

This time we need the small groups at the sides to be joined to the lower group.

Select all upper groups and Transform in Y. Needs to be higher than the stock is.

You might scale it up as well.

Finally (after refining the tool object) run BooleanDifference.

The result might have self-intersections. To get rid of them select the new group and run Smooth on it

[Anonymous]

I just found this forum because I too am trying to create a negative mold from an STL file as well. I downloaded meshmixer free trial, and for some reason when I try to import anything (be it the meshmixer bunny or sphere or plane, or my own .STL part) I get nothing, just a blank black screen with the meshmixer drop down on the left side. When I try to import my own .STL it gives me a progress bar, and says it is generating mesh, etc, but still only get the blank black background. If I try to reload it asks if I want to amend or replace, yet there is nothing to amend or replace. Anybody know what I am doing wrong?

[Anonymous]

This seems to be a rendering issue caused by the graphic card and/or its driver.

1. thing you should check if latest drivers are installed.

2. sometimes such issues are due to a not set printer. Go to the printers dropdown in the upper right corner (if not visible: Go to View menu and enable ShowPrinterBed) and set some printer (doesn't matter which one).

3. If that doesn't help try to disable ShowViewCube and AntiAliasedRendering in Preferences menu on the General tab.

4. If that doesn't help install Meshmixer 3.3 instead of 3.5 (available at the download page)

5. If all stuff above does not work: Enable BasicRendering in Preferences/General tab (worst option for some things are not useful any more)

Which graphic card? On a dual graphics system (as in some notebooks): Make sure the better card is used.

[Anonymous]

Its just a basic Intel 32-bit graphics card in a desktop pc, nothing special. I'll try changing the settings in meshmixer.

[Anonymous]

Ok, changed those things one at a time and needed to switch to basic rendering before it worked. But now it will open the application, and after running for about 2 minutes, I get a windows error saying that the application encountered a problem and needs to close...?

[Anonymous]

Maybe the crash is due to little resources on your system.

Could you try this:

- Locate meshmixer.ini on your system:

Windows: C:\Users\User Name\AppData\Roaming\Autodesk\meshmixer.ini
(NOTE: AppData is hidden by default, so you might have to enter that path manually or choose to show hidden files.)
Mac: ~/.config/Autodesk/meshmixer.ini

(NOTE: ./config is a hidden directory too)

- Close Meshmixer (if running) and rename that file to something like "meshmixer_cor.ini". This should reset MM to defaults

- Launch, test and close MM again (if that fixed your issues: skip next steps)

- Open meshmixer.ini (meanwhile MM created a fresh one) in a text editor.

-  Search for the "WireframeLineWidth=" line. If its value is 0.0000 change it to 1.0000 and save the file

   (a value of zero results in such blackouts)

- To ease the load for your system:

• you might go to "AutoSaveIntervalSeconds=" and set a higher value (e.g. 120 instead of 60 will make mm to write a autosave file at an interval of two minutes instead of 1)
• you might go to "MaxUndoMemorySize=" and set a lower value (means you can go back less steps using Undo in MM)

Can you attach the original .ini (the one we renamed to "meshmixer_cor.ini")? Maybe you need to zip it to post it here.

[Anonymous]

I've changed the things one at a time that you suggested and it still is encountering an error and forcing closed. Seemed to run longer this time around though, so maybe I need to change the auto save interval to even longer...? I drastically cut down the undo amount, was like 2,000,000+ and I changed it to 4,000

[Anonymous]

I can't figure out how to link a zip file to the forum. I could email it.

[Anonymous]

Ok, it is definitely an issue with just my pc, being drastically outdated and underpowered haha. I changed the auto save to 180, which got me 3 minutes before it shut down, so I changed it to 1800 (should get me 30 minutes) now. Now I can get the rectangular blank loaded, and the item I wish to change from a positive to a negative. I will try to duplicate your method to get my vacuum form style negative mold now.

[Anonymous]

Don't forget to save your file manually from time to time if you'd set such a long autosave interval on a weak machine!!

[Anonymous]

Well, that didn't work... Still only made it about 3 minutes before crashing. I may just try using a different pc, I'm pretty sure my laptop has a better graphics card, as well as a better processor. I think there is just way too much information for this pc to handle at one time.