There are basically two different types of alginate molds. One is a "bucket mold" and the other is a "layup mold".
In a bucket mold, the alginate is mixed and poured into some sort of a rigid container (bucket). The object to be molded (hand, foot, etc.) is placed into the alginate and when the alginate is set, the object is removed. The bucket is the "mother mold" or structural support for the alginate. The hole in the alginate is the mold.
In a layup mold, the alginate is applied to a surface (face, torso, etc.). It is applied fairly thinly (typically 1/4 - 1/2 inch). Since alginate is very flexible, a "mother mold" must be applied over the surface of the alginate so it doesn't flop around like a deflated balloon. Typically, a several layers of extra-fast set plaster bandages are applied to the alginate and when they harden, they maintain the shape of the mold. In this case, the exposed surface of the alginate is the mold.
In a bucket mold, you want the alginate to flow easily so it goes in and around all the nooks and crannies of the hand, foot, etc. A flowable alginate will get better detail and result in fewer air voids. For baby and child hand casts, we recommend 380-CC. For adult hand casts, we recommend our 380-CC or 390-S2 Formulas. For larger hand casting jobs, we recommend our 570-PGV or 580-CC formulas.
In a layup mold, the last thing you want is a high flow alginate. Most alginates flow too much for this type of work keeping the operators busy "catching" the alginate as it flows down off the model. Accu-Cast 590-IBG, 680-LS and 880 formulas have our unique "LowFlow" property so where you put the alginate is where it stays. It still is easy to apply and push around with your fingers, but once you stop pushing, it stops flowing.
A. For a hand cast, the best way to determine your alginate requirement is the "water volume" method.
First you must choose the "bucket" you plan to use. Half-gallon milk containers and 2-liter soda bottles can work fine for a single hand. For larger projects Tupperware makes a large range of buckets that work well.
Second, fill the bucket with water. Put your hand into the water and let it overflow into the sink.
Third, measure the remaining water. Since a pint of water weighs about 1 pound (1.043# to be exact), we would recommend 1/4 pound of alginate per pint of water for a 4:1 water/powder ratio. That'd be: 1/2 pound per quart, 1 pound per half gallon and 2 pounds per gallon. See Mixing Guidelines Document Here.
For other types of molds, we recommend:
Face Casting Job
4 ounces (590-IBG or 680-LS)- mixed with 16 ounces of water
Head Casting Job
1.25 pounds (590-IBG, 680-LS or 880)- mixed with about 2 1/8 quarts of water
Head Casting Job with shoulders
2 pounds (590-IBG, 680-LS or 880)- mixed with about 3 1/2 quarts of water
Pregnant Belly Job- no breasts
12 ounces (590-IBG or 680-LS)- mixed with about 42 ounces of water
Partial Torso- shoulders to hips- front only
2.5 pounds (680-LS or 880)- mixed with about 1 gallon + 1 pint of water
Q. When should I use Skin-Safe Silicone instead of Alginate?
A. Skin-Safe Silicone is a wonderful product. It makes extremely high quality molds. The great thing about silicone is that it doesn't dry out like alginate. The bad thing about silicone is that it is several times more expensive than alginate. We currently do not sell a silicone product.
Our recommendation is- "If you're making a production mold (one that can product many castings), you are much better off going with silicone." The extra cost for the material is paid for by the elimination of making multiple molds. We figure if you need to have 3 castings of a single object, silicone is a better deal.
Q. What do the numbers in the alginate names mean?
A. The numerical part of the Accu-Cast product names refers to a setting time/water temperature ratio. For example, Accu-Cast 570-PGV sets in 5 minutes when mixed with 70°F water, 880 sets in 8 minutes with 80°F water.
A. Well, that really depends on how much you're mixing. See "Alginate Mixing"
For small quantities (up to 3 ounces), we recommend mixing with a small spatula in a bowl. The type of spatula we're talking about is a wide, flat bladed knife sort of like you'd use to put frosting on a cake. If you don't have one handy, a tongue depressor or even a popsicle stick will do. We have the spatulas for sale Here.
For quantities larger than 2 ounces up to about a pound, a standard metal kitchen whisk is really great. A whisk is easier if you mix in a wide, round bottomed bowl. Put in your powder, then your water, stir for about 20 seconds, then whisk away. You'll see it smooth out pretty quickly. We have Whisks for sale Here.
For quantities of about 3 ounces to about 1 1/2 pounds, "mixing in a bag" is a great technique. The alginate powder is weighed out into a sturdy plastic bag (at least 3-mil). The water is poured in and the bag is rubbed, squeezed and mashed against a flat tabletop for about a minute. Done correctly this will mix the alginate very efficiently and cleanup is very easy. We also sell "Mix-In-The-Bag" Alginates and Casting Stone on our website.
For over a pound, consider using a power mixer. This is an electic drill with a "paint-mixer" attachment on it. The paint mixer attachments are available at your local home improvement or hardware store. The one that looks like a little propellor with a ring around it works much better than the one that looks like a black plastic hamster wheel. Put your powder and water into a plastic bucket (1 or 5 gallon, depending on how much you're mixing), start off slowly and increase the drill speed when all the powder is wet. We have Jiffy Mixers for sale Here.
To avoid mixing in lots of bubbles, look at how the flow of the alginate mix is moving in the bucket. You DON"T want it being sucked down the shaft of the paint mixer attachment. If it is, then reverse your drill. This is a very fast and efficient way to mix alginate and can be done with very large quantities.
Q. What are the factors that affect the setting time of alginate?
A. When we make our alginates, we test them for setting time under very specific conditions. If your environment is different than ours, it could affect the setting time.
Water temperature has the largest effect on setting time. Warmer water will speed up the set and cooler water will delay it. This is because some of the active compounds in the alginate dissolve faster with warm water and start "doing their thing" faster.
The next biggest factor in setting time is water/powder ratio. We test all our alginates at a 4:1 water/powder ratio (by weight). Tninner mixtures will set slower and thicker ones will set a little faster. This is because you are diluting the compound that causes the setting reaction by adding more water. Its not quite as strong so the reaction takes longer.
Other factors include air temperature (warmer will speed the set), humidity (higher will slow evaporation from the surface which will not cool as fast, which will speed the setting time), and type of surface being molded. Metal is a very good conductor of heat so it will cool the alginate mixture quickly and delay the setting time. Skin is warm (usually about 92°F) so this will heat the alginate causing it to set faster (especially up against the skin).
One final thing to mention is the pH (acidity/alkalinity) of the water. Since the setting of alginate is pH driven, using a slightly acidic water (pH <7.0) will make alginate set faster than using slightly alkaline water (pH >7.0). If you are getting setting times very different from ours, the water may well be to blame. Try a test with bottled water to determine this.
Q. I'm getting bubbles in my castings- what causes that?
A. Faults in castings fall into three categories: bubbles, runnels and voids. I'll talk about the other two later.
Bubbles in a casting are caused by bubbles either in the alginate or in the gypsum cement. (Positive) bubbles (sticking out of the casting are caused by bubbles in the alginate and "Negative" bubbles (actual holes in the casting) are caused by bubbles in the gypsum.
The only "real" way to eliminate bubbles is by putting both the alginate and the stone in a vacuum chamber, after mixing but before use. Some people do this, but it requires time and specialized equipment.
The best way to minimize bubbles in the alginate is to avoid putting them there in the first place. Overly vigorous mixing will whip air into the alginate mix. If you're mixing with a Jiffy Mixer on a drill, make sure to run the drill in reverse (counter-clockwise). This will not suck air down the shaft of the mixer and minimize air incorporation.
There is an effective technique for minimizing air bubbles In a hand casting. Have the subject moisten their hand with water before putting it into the alginate. Have them put their hand completely into the alginate then removing it immediately. With YOUR hands, rub the alginate onto the subject's skin- especially in the palm, between the fingers and around the fingernails. Do this for about 10 seconds or so and then have them plunge their hand back into the bucket and hold until set.
To minimize air bubbles in the stone, mix as usual but before pouring it into the mold, bang on the side of the mixing container with your fist for 30-40 seconds. You could also pick up the container and tap it against the table for the same amount of time. If you're mixing your gypsum "in a bag", then bounce the bag up and down on the table for about 30-40 seconds. This agitation will cause a good number of the bubbles in the mix to the surface.
Its also a good idea to pour the gypsum into the mold slowly, banging on the side of the "bucket" as you do so. This will also help. After you've finished pouring, bang on the side of the bucket for another 30-60 seconds to allow the bubbles to rise.
There is a device called a dental vibrator which is used by dental laboratories. A dental vibrator is a small box that vibrates quite a lot when turned on. You place the mixed stone bucket on the vibrator and the bubbles just start to rise to the surface. You can also put the mold on the vibrator when pouring. Small ones are not very expensive, but they're also not that effective. Large ones are several hundred dollars, but are really good at getting the bubbles out. A medium sized one will be pretty effective for most jobs. You can find them on E-Bay for under $100.
Q. I'm getting missing fingers and voids in my hand castings- what can I do?
A. If you're getting missing fingers its because the one or more of the fingers was pointing upward in the mold. If a finger is bent more than 90° AND NOT TOUCHING THE PALM, it can be very difficult or impossible to get the stone into that finger. There is really nowhere for the air to go.
The first step in avoiding this is to make sure your model is doing a pose that avoids this. If they want to do the American Sign Language "I love you" pose where the middle and ring finger are bent, then the tips of these two fingers MUST be touching the palm or you're going to have problems.
The second thing you can do (if the finger is bent just a little) is fill the mold about halfway with stone and tip the entire mold so the air can flow out of that finger allowing the stone to flow in.
This happens a lot in baby hand casts where you cannot control the position of the fingers. Our best solution is to fill the mold with stone about 3/4 of the way, cover the wrist hole completely with a piece of thin rigid plastic, and roll the mold around and around- tapping on it as you go. This usually works and is a good habit to get into in all such cases.
If you're getting voids in hand casts these will show up as ugly areas in and around fingernails, in the palm and between the fingers. The best solution for this is to have the subject put their hand into the alginate- then immediately remove it. YOU rub their hand(s) all over, especially in the abovementioned areas. This will rub alginate down against the surfaces most likely to trap air. Have them put their hand back in and hold until the alginate sets.
Q. How long do I have before an alginate mold must be poured?
A. Alginate very much resembles a wet sponge. The water is held but not bound in the rubbery solid. No one has yet found a solution to this shortcoming- but we're working on it.
The best answer we can give is: "Get it poured". If you absolutely cannot pour your alginate mold right away, there are a couple of things you can do to minimize the trouble.
First, keep the alginate moist. The thinner the cross-section, the faster it will dry- so layup molds will dry and distort the fastest. Cover the entire surface with wet paper towels and keep them wet until you're ready to pour. For hand molds "in a bucket", cover the top surface of the alginate with a wet paper towel.
The real problem is that alginate will lose its water eventually even if its submerged in water. This is called syneresis and we have no idea what do do about it. If you've had to wait a while before pouring a hand cast, make sure to pour off any water that has accumulated down in the mold before you start.
