How does infill work around screw holes?

[mr ed]

Newby to 3D and learning Freecad. I have a plastic piece designed about 400mm long. It will need to accept 4x M3 bolts or screws (tbd) holding a small platform. I assume the printer slicer will generate an infill pattern of some sort, typically 30%?
Question is:: how much solid material will be put surrounding the screw hole?  I don't see a way to force a solid boundary around the hole. I am unsure about the strength of the attachment.

[sleemanj]

Depending on slicer there you can add a modifier object to set extra perimeters in that region, or the slicer may have an option for extra perimeters on holes.

If neither of those, a trick I have used is to surround the hole with a ring of some very small holes, smaller than your printer can resolve, this will force the slicer to try and print a load of perimeters in that area as it tries to print all those 0.1mm or whatever holes.

[MarkF]

In the PrusaSlicer (and I believe Cura also), the amount of solid material is dependent on the number of vertical walls set by the user.  For holes on the sides, it's the number of top and bottom solid layers set. 

The percent of infill has no effect on the amount of solid material around a hole.

Simple use case of a hole with 3 vertical walls on the left and 6 walls on the right:



The number of vertical, top solid layers and bottom solid layers will determine the amount of plastic around the hole.  The percentage of infill will determine how strong the hole is secured into the block.

Also as mentioned, there are ways to localize these parameters if you don't want them applied to the whole block.


I also use FreeCAD and have been pointing people to MangoJelly's Tutorials. 
He has now started a new series for version 1.0 of FreeCAD. 

You may also be interested in the very long "Resize a STL File For Me" discussion involving FreeCAD design.

[Whales]

Sidenote: if your M3 screw passes through the hole then you're good. 

If you're trying to thread it in then it might be fiddly.  Set the hole diam to about 2.5mm (but you will have to adjust depending on your printer & filament, the hole might shrink and make it too hard).  Have at least 5mm of threaded depth (ideally much more) as M3 machine screws do not bite into plastic very well.

I have found #6  ("No. 6") screws to be good drop-in replacements for M3 but with much more plastic compatible threads.  Easier to install, harder to pull out, far less likely to strip threads.

[mr ed]

What about metal inserts? I could glue them in (using a larger hole).

[Whales]

Metal inserts also work well.  You can install them with a conical soldering iron tip. 

Compared to self tappers:
+ can survive more cycles
+ smoother to turn screw (after the insert is in)
~ come in many slightly different types and sizes (with little standardisation), make sure you size your hole to match exactly what you have at hand.
- can be installed crooked
- extra part, extra step
- soldering iron tip sometimes gets jammed and then it's a pain to wiggle it out without disturbing the insert (maybe blunter tip shapes are better?)
- sometimes plastic melts somewhere you don't expect and requires drilling out

I did a test on PETG a while back and managed to get a No. 6 self tapper in and out of the same hole around 20 times before I started having issues.  Albeit I do a lot of screws so maybe I was gentle and avoided cross threading, not sure.

I prefer self tappers over inserts, much faster.  I'm yet to make something that a self tapper or metric + through-hole doesn't suit.  I do not recommend inserting nuts, it's even fiddlier

[Coordonnée_chromatique]

The metal insterts are designed for the plasic injection process, there is no correct ways to use it in a FDM part 

[nali]

The metal insterts are designed for the plasic injection process, there is no correct ways to use it in a FDM part 

Depends on the insert, no? 


For one-off or hobby I usually glue them in. FDM isn't suited for press fit as it can cause splits and using a soldering iron makes it hard to get them straight.

BTW, if the holes are clearance get yourself a reamer! Much easier and more accurate than using a drill to clear holes and they're pretty cheap now from eBay, Ali etc.

[Coordonnée_chromatique]

The metal insterts are designed for the plasic injection process, there is no correct ways to use it in a FDM part 

Depends on the insert, no? 

No, it don't depends on the inserts, the horrors that you've linked are garbage, i've never seen them in a clever design.
Here is a not too bad solution in order to have some dimentionnal pecision and robustness : HEX SPACERS

[nali]

That was just a random Google, the point was some inserts ARE designed for press fit; they are not all for injection moulding. I have some M3 inserts at home, but they're the split type.

Hex inserts isn't a bad idea though sounds like a bit of tinkering might be needed to get them to fit properly. Or are they heated & pressed in?

[Coordonnée_chromatique]

That was just a random Google, the point was some inserts ARE designed for press fit; they are not all for injection moulding. I have some M3 inserts at home, but they're the split type.

Hex inserts isn't a bad idea though sounds like a bit of tinkering might be needed to get them to fit properly. Or are they heated & pressed in?

A reliant car is designed to be driven on the road, will you buy it ... yes these kind of inserts very often rip on the surface when tightened and often have a small play and is not aceptable mechanically speaking you have zero dimentionnal precision.
For this reason I NEVER heat a printed part, hex spacers are not precise parts and i have to measure all of them each time to ensure a tight adjustement (no play at all) in order to have a good dimentional accuracy.

[mikerj]

The metal insterts are designed for the plasic injection process, there is no correct ways to use it in a FDM part 

Depends on the insert, no? 

No, it don't depends on the inserts,

YES it does depend on the inserts.  Some are designed for heat setting, some are designed for injection moulded parts.  Heat set inserts are very widely used on 3D printed parts with zero issues, you just need to size the hole correctly and push the insert in straight, whilst heated to an appropriate temperature.  If you are ripping these things out then you are either doing something very wrong or you have very unreasonable expectations.

[janoc]

Metal inserts also work well.  You can install them with a conical soldering iron tip. 

Uh don't! That's one reason why people find it difficult to press these in straight. And the other problem is that the conical tip will wedge itself into the insert when you are pressing it in - and you will pull the insert back out of the molten plastic.
Don't do this. Buy a proper insert setting tip. Those have a cylindrical end that fits into the insert, so there is no way they can wedge in the insert. They exist for common irons - or buy a cheap iron for a few bucks and dedicate it to inserts.

E.g.: https://cnckitchen.store/collections/soldering-tips-and-sets

And Stefan did also strength testing of various inserts:
https://www.youtube.com/embed/G-UF4tv3Hvc

[mr ed]

Useful video. I was surprised at how strong inserts can be.
The cheapest Inserts are definately to be avoided.

[Monkeh]

The metal insterts are designed for the plasic injection process, there is no correct ways to use it in a FDM part 

Depends on the insert, no? 

No, it don't depends on the inserts, the horrors that you've linked are garbage, i've never seen them in a clever design.

What counts as a clever design? These inserts are used all over industry, from the battery door on your multimeter to the various plastic parts mounted to the engine in your car (I make the assumption you have either or both of these things..). They work well. In both 3D printed and injection moulded applications, they are routinely heated and pressed in after the part is formed.

[sleemanj]

The metal insterts are designed for the plasic injection process, there is no correct ways to use it in a FDM part 

Complete and utter nonsense.  Inserts literally designed for heat or ultrasonic staking are readily available.

https://www.pemnet.com/products/product-finder/iutb-m3/

It is true that many inserts are made for injection molding (the ones with a simple crown at either end), but absolutely not all inserts.

These can be easily purchased online from many vendors.

[Coordonnée_chromatique]

The metal insterts are designed for the plasic injection process, there is no correct ways to use it in a FDM part 

Complete and utter nonsense.

It is my opinion thanks for you apreciation, so if you think that someting is good if it is commercialized it is ok i've got no problem with that.
Regards.

[mikerj]

The metal insterts are designed for the plasic injection process, there is no correct ways to use it in a FDM part 

Complete and utter nonsense.

It is my opinion thanks for you apreciation, so if you think that someting is good if it is commercialized it is ok i've got no problem with that.
Regards.

It's ok to admit you are wrong.  https://shop4fasteners.co.uk/heat-ultrasonic-threaded-inserts-brass-m2-x-4-0mm.html

• For creating a strong, re-usable thread in thermoplastic materials
• Can be installed by applying heat or through ultrasonic vibration
• Where heat is used the plastic should be softened and not totally melted to prevent flash forming around the hole
• Heated plastic flows into the knurls and undercuts to lock the insert in place

[Jeroen3]

Iirc Made With Layers (Thomas Sanladerer) recently posted video explaining how to do better self-tapping standard metric machine screws in 3D prints.
It came down to making the hole larger with small bumps. The insertion would move the bumps and it would create thread.
Due to not being too tight it didn't melt and was re-usabe a few times, just like regular plastic.

If you are using standard holes you can just use 3.2mm, if you want extra plastic around you can perhaps increase the perimeter count to 3 or 4, if that doesn't add lots of plastic/time.
Otherwise you can draw bodies overlapping your hole and export them inside the same STL/3MF, then you can use them a modifier objects for the perimeter/infill setting only applicable inside that object.

[Coordonnée_chromatique]

Iirc Made With Layers (Thomas Sanladerer) recently posted video explaining how to do better self-tapping standard metric machine screws in 3D prints.
It came down to making the hole larger with small bumps. The insertion would move the bumps and it would create thread.
Due to not being too tight it didn't melt and was re-usabe a few times, just like regular plastic.

If you are using standard holes you can just use 3.2mm, if you want extra plastic around you can perhaps increase the perimeter count to 3 or 4, if that doesn't add lots of plastic/time.
Otherwise you can draw bodies overlapping your hole and export them inside the same STL/3MF, then you can use them a modifier objects for the perimeter/infill setting only applicable inside that object.

Printed PLA parts are very porous and vulnerable to delamination forces, by far the the worst delamination force is traction, all the insterts are applying a lot of traction on the part.

[caulktel]

You might want to check this guys videos out.

Joel
N6ALT

[mr ed]

What if I decide to use other than pla, say peg, nylon or abs with hot press inserts;
Do I have to use different holes sizes depending on the plastic type? 
Is there a plastic type that works badly with hot inserts?

[Coordonnée_chromatique]

The delamination problem is still here even with a 100% infill and a local melting, the contact zone between the deposits have a geometrical constraints concentration and a solidification gradient.

[Monkeh]

What if I decide to use other than pla, say peg, nylon or abs with hot press inserts;
Do I have to use different holes sizes depending on the plastic type? 
Is there a plastic type that works badly with hot inserts?

I wouldn't like to try in, say, TPU.. Otherwise, they're known to work well (despite claims here..) in PLA, PET(G), various nylons, ABS, ASA, PBT.. not sure I've seen anyone use them in PS, PP, straight PC (I have with blends), or PEEK (Uh.. I don't think any of us are printing in PEEK..) but I imagine they'll work.

[janoc]

The delamination problem is still here even with a 100% infill and a local melting, the contact zone between the deposits have a geometrical constraints concentration and a solidification gradient.

Sorry dude - it is irrelevant.

When an M3 threaded insert can withstand 180 kilograms (1800 Newtons) of force before you manage to rip it out out of a sheet of 3D printed PLA then it literally doesn't matter. What are you trying to hang on your inserts that this wouldn't be sufficient for all use cases where these are used? Even the crappy cheap inserts meant for injection molding melted in using a soldering iron will still manage 30kg (300 Newtons) on an M3 bolt.  And if you try to torque the insert out, you are more likely to shear the screw head off than to break the insert out.

In practice you will break off the boss which holds the insert first before you manage to pull the insert out or to delaminate the print.

You have the measurements & testing methodology in the video I have posted - that guy is actually an aerospace engineer, he does know what he is doing. These things are used everywhere, both in injection molded and 3D printed parts without any issues for many years, whenever you need to fasten two parts together in a way that will withstand repeated mating and de-mating.

So what are you endlessly theorizing about? Have you actually used any of these inserts?