Do you wanna build a snowman? (I do, but I have some questions...)

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The PhysX operators are only compatible with the PhysX solver. Mixing Particle Bind operators and PhysX operators won't work the way you're intending.

Ok, thats good to know. I was thinking that might be the case...

You do pose an interesting question though....how to allow particles to remain static until triggered by something, like a collider?

just to make sure: Physx sims DO work like this though, that you can simply switch them off in one event and switch them on once they get triggered by a collision or something else, right?

It's actually not an easy question to answer for a variety of reasons. For example, when the snowball hits the head of the snowman, you'd normally want the reverberations of the impact to affect the entire snowman, as he breaks and falls apart...but how does a particle at the base of the snowman know that a particle in his head was hit by something? Somehow they have to communicate.

Ok, generally I´d tackle a grain sim like a fluid sim then: Have it settle for a couple of frames (if there is gravity present) and then start my dynamic stuff frmo there on, that would get rid of that problem. In case of the snowman I´d want the inital frame to be rather stable though, so I´d probably crank up the stiffness much higher to have it "melt" less, when effected by gravity. (maybe you could still elaborate wether particle bind solver steps is enough or the other settings I mentioned have to be taken into consideration as well). or possibly do something with the clusters.

Currently, you could try adjusting the sleep settings in the Particle Bind Solver rollout. Make it sliiiightly greater than the strength of your gravity. This should cause particles to sleep until moved by a force greater than gravity itself - ie, the snowballs. The particle will fall asleep again if their velocity goes under that threshold, but you can keyframe that value in case you don't want sleeping to come on so strongly after the impact.

Hm, not sure if this is working for me...gravity is about 0.25 here, so I set "Velocity thresh" and "Wake thresh" to 0.26, but the snowman still starts melting...

I´m trying to make the snow stickier, so it breaks off in bigger chunks

Creating chunking effects like that can be done in a variety of ways. The most effective is probably using a particle clustering workflow. Basically, you create a Cluster operator and adjust the settings so that particles are grouped into smallish "clusters". Then, you have two Particle Bind operators....one that binds particles in the same cluster with strong and hard-to-break bindings. And one that binds particles in non-same clusters with weak and easy-to-break bindings. 

Gotcha. Still a bit fuzzy on the "hard-to-break" bindings part. is that basically just the stiffness value in the particle bind op (in combination with the steps)?
And how do I distinguish between the cluster binds and the in between cluster binds in two particle bind ops?
lets say I set the clusters to channel 1, do I then simply set the "strong" particle bind ops clustering channel to "1" and leave the "weak" particle bind op to "0"?
Or do I have to turn off the clustering in the "weak" one?

For a material like snow, you can also enable cohesion forces in your Particle Physics operator....keep the value verrrry small, but it will cause particles to slightly stract and cohere to each other...similar to wet snow or sand. You'll have to experiment a bit to get the right look.

How do I do that? I can see the "enable particle attractions" checkbox, but I don´t know how to change the cohesion force...

Eventually I'll add some example files showing how to do this.

Yep, Examples are the best.

You could either make your snowball a part of the Particle Physics sim (which is probably what you'd want...so it has the potential to break up as well), or simulate everything in a separate flow, set that flow to have a mesh and then import it as a collider in your snowman flow.

Yeah, I can think of both cases...Snowball seems to be the easier one.
I thought of a cannonball goin through it, so I tried to creata an extra tyFlwo for just the cannonball and picked that as a collision object in the snowman sim, but that didn´t work...