Air bubbles of one kind or another are inevitable if you deal with materials which start out life as a liquid and then later solidify such as plaster, latex, silicone and resin. Let’s take a look at what can happen, why, and what to do about it.
Air bubbles can be small, almost insignificant cosmetic issues or more substantial structural weaknesses if large or inconveniently placed.
Most liquid materials usually require some kind of preparation involving stirring or agitation to blend catalysts, pigments or to simply shaken to recombine components in a container which may have separated over time.
This is usually where air is added into a mixture, and is difficult to avoid. If the material is low viscosity (runny), then usually the air introduced in mixing can rise and leave the material quicker than if the material is higher viscosity (thicker).
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The other consideration is ‘pot life’, as many materials may set quickly, and this means a more fluid material which could allow added air bubbles to rise and escape will not do so for long before it starts to thicken and set – essentially trapping the air. To avoid lots of air bubbles therefore, the ideal combination is a runny material which doesn’t set too quickly.
This may mean modifying it…most silicones can be slowed down by adding a chemical retarder, or by refrigerating them beforehand. Perhaps in worse case scenarios when it is really toasty, you need to do both!
Materials will have an accompanying datasheet which will list its properties, and for liquid materials, one of them is to quantify the viscosity as a number, usually measured in ‘centipoise’ (cP).
The lower the number, the more fluid it is. For example, water is 1 to 5cP, honey is approx. 3000cP, Ketchup is about 60,000cP and peanut butter is about 200,000cP.
Degassing the mixture in a vacuum chamber (assuming there is time to do this process) is great if you have one, as this will remove the most amount of air by literally sucking it out.
The more fluid the mixture, the quicker and more easily it will degas. A good degasser will usually do its job in a few minutes, and the more powerful the pump, the better.
Taking the air out before pouring into the mould is the best thing you can do to prevent air bubbles in the first instance, but you’re not out of the jungle yet. You can still trap air in the mould with completely air-free silicone!
The other kind of air bubble is the ‘void’ usually created by air trapped in a high point in the mould, as the material gradually fills the space inside. Most of the air will usually get out, especially if the mould can be rotated carefully during the fill.
However sometimes in high points, like finer or ear tips, there will be a bubble of air which cannot escape or compress any further, preventing the casting material from making it in there.
You won’t know until the mould is opened, and then you can either repair the bubble or decide to pour another one. If you do, then you need to make a few changes to ensure it doesn’t happen again such as drilling/cutting small bleed holes or risers to allow the air to escape out from the high points.
This is best done during the moulding process by fitting thin wire into the points of the sculpt which will be the ‘high points’ for trapping air. Moulding with these in place, and then removing them afterwards will ensure a clean hole in the right place which is preferable to drilling than in afterwards.
Materials like silicone are hard to drill clean, and rigid materials can break drill bits (assuming there is a convenient place to position the drill which often there is not – in this instance length of piano wire may be a handy alternative.)
There are two categories of moulds to consider – open and closed moulds. Open moulds include simple flat moulds or a simple bowl-like mould of a flared out neck for example (see below) which allows bubbles to easily escape.
Closed moulds are those consisting of an inner core and outer mould, which is assembled before the mould is filled. These kinds of moulds offer the greatest likelihood of air bubbles, so some thought should go into mould design to minimise the chances of doing so.
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