Why is PVA a different kind of support?

[Dragon2781 0]

Hi,

I'm new to the 3D printing topic and I have some questions.

My first question is:

 

Why is the PVA support incasing the object and is it possible to deactivate this?

 

When I use the normal PLA as a support material, it is supporting only the overhangs.

But when I use PVA as support, the whole object gets wrapped in it, even on spots where no overhands exist.

The PVA is very expensive. Is Cura intentionally wasting PVA so Ultimaker is selling more of it?

 

I have the problem that little flakes of the PVA gets between the PLA layers.

After all PVA dissolved in water I end up with little gaps/holes in the object.

If the PVA would be used like normal support this would not happen.

[nallath 1,118]

Because of reliability. If you print very small bits of PVA, the PVA will remain above the degradation temperature longer, causing clogs. Combined with pva being less "study" as PLA, you need to ensure a min support size to ensure they don't get knocked over.

 

The default settings are a bit on the safe side, so in quite a few cases it should be safe to change it. You're looking for the  support horizontal expansion setting.

As for the problem that you describe, that sounds more like a wipe problem (the nozzle oozing a bit when it shouldn't be printing)

[gr5 2,097]

There's a second good reason for this.  The feature is called "horizontal expansion" and for pva support it defaults to 3mm.

 

While PLA sticks on top of PVA nicely it's not so good the other way around.  So if you are printing something shaped like a 4 legged table where legs are touching the build plate (a stupid orientation but whatever) you don't need horizontal expansion - there's lots of pva to hold up the flat part of the table.  But if you are printing a short tube with the hole being horizontal - the PVA doesn't stick well on top of the PLA inside this tube - you really need the PVA to be all connected together and to the glass plate to hold it all in place - so horizontal expansion has the PVA sticking out the sides of the tube by 3mm and then going all the way down to the glass.

 

This is a very crude feature that could be much smarter but it works for "most" designs.

[gr5 2,097]

And about the tiny bits of pva ending up in your print and causing holes in the walls - yeah I get that also.  A prime tower can help but if you try a prime tower make it at least 1/5 as wide as it is tall (skinny ones fall over) and put it in the back center of the glass - not in the corner (corner towers tend to fall over because the glass is almost always lower in the rear two corners.

[Dragon2781 0]

Thank you @nallath and @gr5 for the explaination. I have bad experiences with the tower. The PVA dont stick to it and carries over onto the printed object.

Edited February 16, 2018 by Dragon2781