Material Notes: SILICONE
works of Carl Pisaturo
2 part silicone rubber starts as a thick goo. It will take the shape of whatever you pour it into, down to the finest detail, and cure into a rubbery solid. This rubber has the outstanding property of not sticking to anything. And that makes it great for molds.
The less desirable traits include poor tear resistance, bubble entrapment, high cost and short shelf life.
In short, silicone can do great things, provided you are careful and patient.
The following sequence illustrates how silicone fits into the making of a composite cast part. The process involves making and using an open silicone mold.
The less desirable traits include poor tear resistance, bubble entrapment, high cost and short shelf life.
In short, silicone can do great things, provided you are careful and patient.
The following sequence illustrates how silicone fits into the making of a composite cast part. The process involves making and using an open silicone mold.
MOLD HOUSING
MASTER
SILICONE MOLD
Working with molding and casting gets you to think in terms of positive and negative space. A mold is a negative of something. That something is the MASTER (sometimes called a pattern).
The master is the shape you want your final thing to have. Sometimes a master is an existing thing you want to copy, and sometimes it is something new, as in this case. Here, the master has been machined out of a block of delrin to a precise shape.
There must be some kind of container to pool the liquid silicone around the master, and this is the mold housing. Mold housings can be as simple as a paper cup, or more complex like this one which has the additional function of guiding "placeholder rods".
We will pour almost to the top of the master, not fully cover it. That's why its an OPEN mold.
The master is the shape you want your final thing to have. Sometimes a master is an existing thing you want to copy, and sometimes it is something new, as in this case. Here, the master has been machined out of a block of delrin to a precise shape.
There must be some kind of container to pool the liquid silicone around the master, and this is the mold housing. Mold housings can be as simple as a paper cup, or more complex like this one which has the additional function of guiding "placeholder rods".
We will pour almost to the top of the master, not fully cover it. That's why its an OPEN mold.
POURING THE MOLD: The silicone shown here is "TAP silicone RTV", a typical mold making formula, but they all differ so read instructions carefully. This isn't an endorsement. I've had trouble with this stuff as well as others.
MIXING IT UP. This is the tricky part. Do it right or you may end up with an expensive mess! 1) Don't use opened containers more than a few weeks old. Even sealed containers go bad in under a year. If the mixed consistency is more "snotty" than liquid, it is not usable. 2) Everything the silicone will touch must be squeaky clean - some contaminants can inhibit the cure of some silicones. 3) Mix accurate ratios - this may require a .1g resolution scale, especially for small volumes 4) Mix very thoroughly, this stuff is thick so it takes effort. Any unmixed areas will ruin the mold.
Now it is mixed and full of bubbles. If you want quality, you need to de-air with a high vacuum pump & chamber. These pumps cost $300+ and get most bubbles out. Keep in mind that this de-airing process expands the silicone 3-4 times, so use a big enough cup. If not de-airing, let the stuff sit for a few minutes to stratify and then pour from the bottom (puncture cup) where the silicone has the least bubbles. The problem with bubbles is that they get stuck on the bottom of the master, and the negative of that bubble in the mold will be a pimple on the final product.
Finallly, the pour. Tipping the housing / master helps prevent bubble entrapment in this case. Pour slowly starting at the low end.
MIXING IT UP. This is the tricky part. Do it right or you may end up with an expensive mess! 1) Don't use opened containers more than a few weeks old. Even sealed containers go bad in under a year. If the mixed consistency is more "snotty" than liquid, it is not usable. 2) Everything the silicone will touch must be squeaky clean - some contaminants can inhibit the cure of some silicones. 3) Mix accurate ratios - this may require a .1g resolution scale, especially for small volumes 4) Mix very thoroughly, this stuff is thick so it takes effort. Any unmixed areas will ruin the mold.
Now it is mixed and full of bubbles. If you want quality, you need to de-air with a high vacuum pump & chamber. These pumps cost $300+ and get most bubbles out. Keep in mind that this de-airing process expands the silicone 3-4 times, so use a big enough cup. If not de-airing, let the stuff sit for a few minutes to stratify and then pour from the bottom (puncture cup) where the silicone has the least bubbles. The problem with bubbles is that they get stuck on the bottom of the master, and the negative of that bubble in the mold will be a pimple on the final product.
Finallly, the pour. Tipping the housing / master helps prevent bubble entrapment in this case. Pour slowly starting at the low end.
After the pour, the mold cures overnight at room temp, it should be "post cured" with heat and humidity. Placing the mold / housing in an inverted aquarium with wet rags and lamps work well.
EXTRACT THE MOLD: Although silicone doesn't really stick, it is rubber and has "gription", so extracting can take a bit of work. Refrigerating may help by shrinking the silicone more than the aluminum housing. Finally you get the mold free (left)
More post-curing will make the mold stronger. 6 hours at a humid 150F would do it.
EXTRACT THE MOLD: Although silicone doesn't really stick, it is rubber and has "gription", so extracting can take a bit of work. Refrigerating may help by shrinking the silicone more than the aluminum housing. Finally you get the mold free (left)
More post-curing will make the mold stronger. 6 hours at a humid 150F would do it.
Now the mold is ready to do its job in life: make a casting. Silicone molds last well and should be able to make many such castings. Mold release spray is not necessary, but may improve the lifespan of the mold. Silicone can cast almost anything, including epoxy, the red stuff being poured from the cup. This is a prototype finger bender being made. A urethane part has been loaded into the mold. (clamping is not typical, in this case it helps the mold seal up against the urethane component)
This is an open mold - the easiest kind. The limitation of an open mold is that the top surface of the casting must be planar and it will be imperfect due to the meniscus (the little curve a liquid makes where it contacts its vessel's walls). Sometimes a subsequent filing or sanding step is required to clean or this side of the casting. Most often in arts and crafts, this side would be the hidden back side of the casting.
This is an open mold - the easiest kind. The limitation of an open mold is that the top surface of the casting must be planar and it will be imperfect due to the meniscus (the little curve a liquid makes where it contacts its vessel's walls). Sometimes a subsequent filing or sanding step is required to clean or this side of the casting. Most often in arts and crafts, this side would be the hidden back side of the casting.
The casting has cured and now the fun part: CASTING EXRACTION. In this case, there are tunnel placeholder rods, so they get pulled out first. These are smooth teflon tubes (with music wire cores), so they wont get stuck in the epoxy.
The casting pops out easily because the mold can be "peeled back".
The casting pops out easily because the mold can be "peeled back".
The finished product, complete with cast-in tunnels.
This was just one of many ways silicone can be used for molding.
This was just one of many ways silicone can be used for molding.
SILICONE AS A CASTING (as opposed to molding) MATERIAL
Silicone isn't always used to make molds, sometimes it is the final product. It is a desirable material for some applications because of chemical inertness and high temperature tolerance.
Chemical inertness means nothing sticks to it or grows on it and it keeps its suface qualities. This makes it "biocompatible" and thus silicone castings are used extensivly in medical devices and in food-contact situations.
Heat tolerance of 480F (250C) puts silicone in a class by itself among elastomers. Silicone can be used for high temperature gaskets and seals, and wherever you need a rubbery thing that will get hot. The electrically shielded heating pad being cast at right is an example.
Casting silicone is easy: since it sticks to nothing (except other silicone) you can use just about anything as the mold.
Chemical inertness means nothing sticks to it or grows on it and it keeps its suface qualities. This makes it "biocompatible" and thus silicone castings are used extensivly in medical devices and in food-contact situations.
Heat tolerance of 480F (250C) puts silicone in a class by itself among elastomers. Silicone can be used for high temperature gaskets and seals, and wherever you need a rubbery thing that will get hot. The electrically shielded heating pad being cast at right is an example.
Casting silicone is easy: since it sticks to nothing (except other silicone) you can use just about anything as the mold.
