Above: In the art of mold making and casting, air is not your friend.
But it’s only air. You may wonder why an article is devoted to air bubbles and air traps is contained in a section titled Materials. Agreed – this topic is the antithesis of materials – it is the absence of material. But, as without understanding its effect you will be unable to achieve professional results in your mold making and casting activities while using your materials. You want your material to be fully present instead of partially absent as caused by unwanted air.
Air bubbles and air traps are the primary enemy of mold makers everywhere, as they weaken and disfigure materials such as concrete, polyurethanes, silicones, gypsums, and most other liquid materials used in mold making and casting. The absence of material caused by air bubbles disfigures the surfaces of molds and castings, creating pits and bumps, and if the air spaces are very large, such as those that are caused by air traps, they will weaken the internal structure of the mold or casting, rendering them useless.
Above: Gas bubbles collect around a finger inserted into seltzer water. This is analogous of what takes place in mold making and casting material that isn’t de-gassed. Left de-gassed the result will be a mold or casting with an imperfect, mottled surface.
Figure 2 provides an extremely visual example of what damage air bubbles can do. A finger is placed in a glass of soda water (analogous to any mold making or casting materials in this example). The CO2 in the water tends to gravitate around the finger, like what air in a material will do if not removed from the product. The resulting gas bubbles will cause the surface of the mold to be filled with unsightly holes surrounding the finger.
Above: This diagram shows the differences between air bubbles and air traps. The picture on the left shows how air bubbles collect on the surface of an inverted cone. A better way of casting this model is to flip it upside down, allowing air bubbles and easier escape. The image on the right shows how air is trapped in unvented spaces in a complex model.
Differences between air bubbles and air traps. Air bubbles are internal to mold making and casting materials. They appear from air failing to escape from the material during the curing process; either its unintentionally added during the mixing process, or it arises because of chemical reactions with contaminants as common as moisture, or more exotic, such as sulfur in clay. Whereas an air trap is formed external to mold making and casting materials as a result of air having been improperly vented when the materials are filling the matrix or mold. If the air within the mold has no place to escape, as the liquid fills the mold, the material is unable to fully fill it, prevented by the air in front of it. Experienced mold makers remedy this by constructing vent holes so that air can be released in front of the material that is filling the mold. Inexperienced mold makers are often left frustrated by this phenomenon.
Why air bubbles are created. There are many reasons why air bubbles are created. But air added during mixing is the primary cause. Due to the incidental introduction of air while mixing, by the time they are poured, mold making and casting materials will eventually contain a 5% to 20% volume of air. This small amount of added air seems like it wouldn’t be such a bad thing, after all it’s just air and air bubbles tend to rise to the surface and escape the material altogether. So that under ideal circumstances, with a little patience, the air bubbles will eventually release from the liquid. But, in reality, most applications do not allow for this as the material is far too viscous, or it lacks the proper conditions or time for the materials to self-de-air. So, the mold maker must utilize a mechanical means to hasten the release or air through either vibration, pressurizing or vacuuming.
Above: The alginate mold on the left exhibits air bubbles (negative spaces). This results in bumps in the casting, which will require repairs.
Here is a list of how air bubbles can contaminate your materials:
- Air incorporated during the combining and mixing of materials (see Figure 4)
- Humidity and damp materials, especially polyurethane materials
- Moisture contamination from a damp pattern or improper material storage (see Figure 5)
- Excessive solvent vapors from certain types of mold releases
- The air contained in the surface of a pattern.
- Air from improperly de-aired materials
Above: This is the result of moisture encountering this clear polyurethane casting material. The moisture creates the appearance of unsightly champagne-like bubbles.
- Air that is trapped in molds (air traps) through improper venting (see Figure 3)
- Air trapped due to improper pouring or application.
Common sense provides that the best method to eliminate air bubbles is being careful not to introduce air while handling the materials. But this is often unavoidable, as when you combine two component mold and casting materials they must be mixed out of necessity. But you can reduce the addition of air during mixing in several ways. For hand mixing, use a wire kitchen whisk, as it will introduce the least amount of air while mixing. For mechanical mixing, use a Jiffy Mixer attached to an electric drill, as the mixing blades are designed to minimize the introduction of air during mixing. Still, it is unavoidable not to introduce air during mixing. Therefore, it is highly desirable to remove the trapped gases under vacuum before using the mix. An alternate method is using a pressure pot, whereby gas bubbles trapped in a liquid are reduced to microscopic size.
Prior to mixing, which surely will introduce air into materials, there are a number of things you can do to reduce the potential for air bubbles, especially as it pertains to polyurethane materials that are moisture sensitive. You can control the humidity where you are working, you can make certain your model or pattern is dry and you can make certain to use the correct mixing containers and stirring sticks.
Because of the longer pot life of polyurethane materials, they can absorb more humidity in the air. So, it's important to keep the relative humidity no greater than 50% with 30% being the better target. Working in an air-conditioned environment should eliminate the humidity issue.
A wet or damp pattern will cause unsightly air bubbles (see Figure 5). They often result in molds made from plaster that has not yet cured or from condensation cure silicone. The remedy is to place them in an oven on a low temperature for several hours. A common complaint is bubbles in clear casting resin during encapsulation, or bubbles appearing during cold casting of simulated woods using resin and pecan shell powder. These are the result of damp material, which oven drying will quickly remedy.
A surprising source of air bubbles is due to dampness arising from improper mixing containers and mixing sticks. Paper containers and wooden sticks can and do hold moisture. So be certain to use a plastic, metal or glass mixing container, as well as plastic or metal stirring sticks. Also, mold boxes of cardboard also contain unwanted moisture.
Not All Air Bubbles are the Same.
Pin Hole Bubbles. These are usually caused by an excess application of mold release being applied to the mold surface before casting. This especially negatively affects plastic resins. The proper application of mold release is done in three steps. First, spray a light coat on the mold’s surface. Second, use a soft brush to smooth the mold release. Third, spray a very light second coat and wait at least ten to fifteen minutes to make certain the mold release is dry. If it isn’t dry, your casting will exhibit the same set of fizzy bubble patterns you find in a fine champagne.
Mold Bubbles. These bubbles appear onneeds surface of molds and ultimately result in bumps on the final casting that need to be removed. The cure for this phenomenon is to apply the first coat of your mold material thinly, using a soft brush. The brushing action pushes out any surface air contained on the pattern and will provide a better-quality mold. Just remember to refrain from a heavy application of mold rubber when applying the face coat. The ideal application is four thin layers, as opposed to three heavy layers of mold rubber.
To reduce air bubbles in plaster casts that accumulate in the upper cavities of a mold, a wetting agent applied to the mold is beneficial. A homemade wetting agent consists of a tincture of green soap with alcohol to the viscosity of water. It is applied by spray bottle and then allowed to dry before pouring your plaster.
There are commercial debubbizers available as well. You might check dental supply houses in your area. But be careful using such formulations if you plan to use de-airing systems, such as a vacuum chamber, as debubblizers will actually increase the propensity for bubbles under vacuuming.
If you wish to make your own debubblizer, this is a very effective recipe:
The formula works by having the acetone dissolve any grease or oil. The methyl alcohol allows for quick evaporation, and finally the detergent is what actually prevents the air bubbles by wetting them out and acting as a suffocant.
The Air Bubble Avoidance Process
Air bubble avoidance begins with the material selection. The objective is to use the lowest viscosity of liquid acceptable for your project. The higher the viscosity, the more difficult it is for air to escape prior to curing. Once you have selected the appropriate material, by following the steps outlined here, you will have a bubble free mold or casting.
The Mixing Step
- Use a gentle, consistent motion while mixing. A vigorous motion will introduce unwanted air.
- Make certain the material is fully mixed by scraping the sides of the container. Failure to do so will result in soft spots or uncured material.
- For best results, Use a vacuum chamber to de-gas your mixture, if available.
- When using fillers, de-gas each individual component before combining.
- If a vacuum chamber is unavailable, letting the material sit after mixing for a few minutes will give air bubbles time to reach the surface and dissipate.
Mixing larger quantities of material, especially those that are more viscous, may call for mechanical mixing using an electric drill.
- Use a mixer designed to reduce the introduction of air, such as a Jiffy Mixer.
- Make certain the material is fully mixed by scraping the sides of the container. Failure to do so will result in soft spots or uncured material.
The Pouring Step
- Pour slowly in a high narrow stream to let the air have time to escape.
- Pour into one corner of the mold and continue in that location to allow the mold material to envelope the pattern. This method pushes any surface air ahead of the material without trapping it inside.
The Curing Step
The best practice is to de-gas the material contained in your mold or on your pattern after mixing, then placing it in a vacuum chamber while curing. But if you cannot do this due to size, at least de-gas the poured material prior to the curing process. Complicated shapes are conducive to air entrapment and are best de-gassed prior to curing.
If the material cannot be de-gassed in a vacuum, then placing the mold or pattern on a vibrating table will help in eliminating air bubbles, but not as thoroughly as when a vacuum is used.
For all hard setting materials, especially transparent acrylics and urethanes, placing the mold in a pressure pot during the curing process with compressed any air to microscopic size rendering the bubble so tiny, they will be unseen.
Finally, the slower the setting time during the curing phase, the long air has to reach the surface and dissipate. Cooler temperatures tend to delay setting times in most materials, so you may wish to adjust the temperature to slow the gelation process.
Considerations affect the removal of trapped air:
Viscosity: This is the thickness of the material. The thicker a liquid is, the harder it is to remove trapped air. Think water versus hot bubbling tar.
Surface Tension: The surface of a liquid resists escaping air. Tension is reduced by heating the liquid or the addition of wetting agents, called surfactants. A surfactant is a material that can greatly reduce the surface tension of water when used in very low concentrations.
Temperature: Heating the material will reduce its viscosity. However, it will also increase setting times in many materials, so read the manufacturer’s label for a set time to allow yourself a sufficient pot life.
Exposure: The larger the surface area that is exposed while vacuuming the faster the air is removed. The shallower the amount of a liquid in a container means the rising trapped gases have less resistance to escape.
Mixing: Mixing or agitation during de-gassing when vacuuming will considerably speed the removal of air.
Two Alternate De-gassing Methods.
Pressure: Placing the mold in a pressure vessel during the curing process will compress air bubbles to microscopic sizes. The air is still present; it is just so small that it cannot be detected. Pressuring rubbers don’t work well, however, because as soon as the cured material leaves the pressure vessel, air expands again, due to the rubber’s flexibility. So, this method is best reserved for hard setting materials. To be effective, the amount of air pressure should be in the range of about 50 to 80 psi depending on the capacity of the pressure vessel.
Vibration: Placing the mixture on a vibrating table will greatly assist the bubbles in travelling to the surface by lowering the coefficient of friction in the material. This method is not a substitute for using the vacuum technique, as it can never fully de-gas a product – only improve on it.
More About Air Traps
Air traps prevent molding making or casting materials to fully fill a mold, resulting in missing sections of cured material. The result often means that the casting or mold cannot be used. Mold makers add vents in their mold designs to allow air to dissipate as the product is added to the mold. The chapter on Injection Molds more fully explains this.
Above: This shows the effect of a damaging air trap. A mold of a hand with its fingers curled is filled with casting plaster. Since the fingers are pointed upward, the air being pushed in front of the incoming plaster, as it is poured, has no place to go, resulting in voids in the fingertips, completely ruining the casting.
To better understand air traps and how to overcome them, I will use an example of a hand molded using alginate. If one simply placed their hand into a container of alginate with fingers pointed downward, the resulting mold would result without air traps. However, if that same hand curled its fingers so the fingers were pointed upwards, as in an ‘U’ shape, the upward pointed fingers would be missing their fingertips after being cast. That is because the air in the mold is driven up the fingertips as the mold material is added to the mold. The air has no place to go, so it prevents the casting material from fully filling up the fingers.
The solution, of course, is to provide venting holes at each fingertip. Technically, one could Super Glue a rigid wire to each figure tip and allow the wires to protrude from the surface of the alginate container. When the alginate cured, the wires would be pulled out along with the hand, leaving behind some vent holes.
Above: This is a diagram of the empty mold showing the fingers pointing upwards. This will result in air traps if the mold maker fails to remedy the problem. The first step in preventing an air trap is to mark the container with a Sharpie on the side where the fingers curl up.
Above: The next step is to fill the mold only one third of the way with casting material. Note that the fingers still show the trapped air in the fingertip region.
But this process is a bit ungainly. There is another solution. But you must know in advance which side of the mold the fingers are located. Mark the container with a Sharpie mark so you can tell this after the hand is removed from the mold. Then fill the mold about 20-30% full of your casting product. Stuff a paper towel down the arm hole to stop any casting material from escaping. With the finger side facing downward, flip the mold ninety degrees three or four times to get the casting material into the fingertips. The flipping action will allow the air in the fingertips to escape. You can the fill the mold completely being satisfied that there will be no air traps.
Above: Paper towels are placed in the wrist opening to seal it tightly to prevent any casting material from splashing out during the next step.
Above: The container is held on each side with the top of it facing the mold maker’s stomach and vigorously flipped upwards so that the bottom of the container is momentarily pointed toward the ceiling. This action is done three to four times. The resulting movements will break the air traps in the fingertips, allowing air to escape. When the container is placed back down the fingers will be filled with casting material.
Above: When the flipping action is completed, the pouring of the casting material can continue to completely fill the mold. The result will be perfect fingertips and a victory over air traps.
Eliminating air bubbles and air traps is a continual learning process. But by practicing the suggestions offered here you will be able to create professional quality molds and castings. Make certain you read the chapter on Mold Making Tools to learn more about the tools you need to prevent air bubbles and air traps from damaging your artistic work.
The best practice is to de-gas the material contained in your mold or on your pattern after mixing.