noob question: sprues and bubbles

[burner010101]

When making a mold, is there any basic theory I should know about choosing either (a) number of sprues, (b) location of sprues, or (c) thickness/shape of sprues?

Does increasing the number or thickness of sprues necessarily mean less bubbles in the finished cast? Should all of my sprues be the same thickness?

Background: I gravity cast with a two-part RTV silicone mold. I use SC 321 mostly. Sometimes SC 327.

[mangozac]

Just to clarify, the part you are referring to is the gate (and vents), not sprue. The sprue is the collection of gates, vents and excess material

It's pretty basic theory: the bigger the gate cross sectional area the more they will release bubbles. That only works to a point though - the viscosity of the resin also has a very large effect on it!

You will generally have just one gate but will need a vent for each of the high points of the part. The vents can be a smaller cross sectional area than the gate though.

Hope that helps!