HOW TO CHOOSE YOUR SILICONE
Ok, so you have your project ready, you are off to go buy some silicone and dive right in…. Whoa! Slow down, tiger….. There is a whole bunch of stuff you need to know first.
What are you doing? Is it a big piece, a small piece? Are you making a mould or a cast? Do you need to make a box mould, a brush-up mould, a matrix mould, a pour mould? How much are you going to need to fill it? Do you want a solid coloured, or translucent silicone? Are you in a hurry or is time not an issue? Will you need a long or short Potlife? What sort of Viscosity is required? What Shore Hardness do you require in your finished piece? Do you need to worry about Elongation/Tear Strength/Tensile Strength?
Yeah, working with silicones is a bit like doing high school chemistry all over again, but trust me, if you understand the basics, it will save you a lot of time, money and frustration.
Lets start with the Product Description.
When you are searching a website for silicon moulding rubbers, you will usually find somewhere a link to a.pdf saying “Technical Data Sheet”, “Product Overview”, or the like. In a catalogue there may be aTable giving comparisons between the products on offer.
Note that the information I am discussing here is NOT on the MSDS… (the what? I hear you say? It stands for the ‘Material Safety Data Sheet’- a topic for another time…).
A Technical Data sheet will give you all of the relevant information you need about a product. Lets go through one step by step, just to give an example.
At the top there will normally be a Product Description. This will describe the general characteristics of the particular silicone, its category and type, and an idea of its texture, hardness, mix ratio and suggested applications.
Here are a couple of examples I randomly pulled out of my file (you should always keep this info for future reference):
1. “ProSkin is a two-component silicone elastomer that cures at room temperature by a polyaddition reaction that may be accelerated by heating. It is designed as a 12 Shore A high tear strength rubber providing excellent physical properties for prosthetic and orthopaedic applications. It also displays good translucency and high elongation. Proskin is easy to process with a 100:100 ratio by weight or volume and has an extremely low viscosity when mixed.
Typical Product Uses:
- Orthopaedic Applications
- Prosthetic Devices and appliances
- Anywhere a soft skin-like feel is required
- SFX skins
- Cushioning appliances”
2. “Elastosil M4503. Pourable, condensation curing, two component silicone rubber that cures at room temperature and features:
- Good flow
- Low Shore A hardness (approx 25)
- High Tear Great extensibility and elasticity
- Excellent long term stability of the mechanical properties of the cured rubber.
- High resistance to casting resins, particularly polyester
Applications
Due to the excellent mechanical properties of the cured rber as well as its high resistance to casting resin, Elastosil M4503 is especially suitable for reproducing models with pronounced undercuts in casting resinc. Other materials, such as wax and plaster, may be cast without any problems in moulds made from M4503 Skin Moulds. “
Ok, so reading through those, what do they tell us? Which one is the Platinum Silicone?
Further down the sheet we come to Product Data, both Uncured and Cured. This is where the specific scientific measurements are listed. We will go through them one at a time.
No matter the format, each information sheet should have the following on it somewhere, although perhaps in a different order:
Colour-
usually translucent, white, or for certain products, blue, pink, yellow etc… Frequently, the colouring agent will be in one part, for instance the catalyst, or one part of a 1:1 system. This assists with the even mixing of the product. Proskin is a translucent silicone, that can be intrinsically coloured for the final result. This tells you that it is likely designed for making products or appliances. M4503 is a white silicone, which tells you that it is primarily used for mould making, as are most solid coloured silicones. However, there are times when you need to have a mould that is translucent, so you can see the piece through the mould itself for various reasons.
(The one thing no-one has yet managed to do is make a water-clear silicone. Well, they did, but it turned out to be so brittle it was unusable. It has since found a perfect purpose, as a form of breakable glass, too fragile to be put in a window but great for simulating crushed or broken glass on set with no danger to actors or crew).
Density, or Specific Gravity-
usually specified as @23 or similar, meaning at ambient room temperature of 23C. Density is expressed as a measure of Grams per cubic centimetre. Specific Gravity is a ratio of the Density of a substance to a reference substance, usually water. Water can be assumed to have a Density of 1 at sea level and 20C, and if a substance has a Specific Gravity of less than 1, it will float on water.
In our examples, ProSkin is listed as having a Specific Gravity of 1.08, Cured; while M4503 is listed as having a Density of 1.16, Cured. In plain terms, this translates to both kinds of silicon being heavier than water. Silicon is heavy, and fluid, and like water, follows gravity. The practical upshot of this is that if you leave a tiny hole ANYWHERE in your mould wall, or any edge is not completely sealed, the silicone will find that hole, and depending on the cure time of your material, when you come back several hours later to check on it, a great deal will have oozed out of that pinprick, and probably all over the floor……. (A great prank to play in the workshop is to pour a thin trail of silicone leading from the base of someone else’s mould to a puddle somewhere…. don’t tell anyone I told you… 😉 )
Mix Ratio-
this is not always listed as a category on its own, but will be in the information somewhere. Our examples give ProSkin a Mix Ratio of A:B 100:100. M4503 merely includes the information in the table as ‘With 5% wt Catalyst T35’. Note that Proskin, a Platinum Silicone, can be measured by weight OR volume because parts A and B have been ‘Specific Gravity balanced’ (they are the same Density as each other).
Pot-life-
This is your active working time, and starts from the time you first put the components together in the container, including mixing time, then whatever is left is the time that the silicone is still fluid, before it becomes gel-like. It can vary substantially from what is on the sheet according to your ambient working temperature, most significantly in Platinum silicones. Potlife is generally shorter in Platinum than in Tin silicones anyway, though there is a wide range. To give you an example, I was working on a documentary in the NorthWest of Australia last year, the average temperature during the day was around 35-40 degrees Centigrade. In 24C temps, the silicone I was using, Platsil Gel10, has a potlife of around 6 minutes normally with a demould time of 30. In the tent I was working the heat was around 45 degrees Centigrade, and potlife was around 20 seconds, demould about 2 minutes……. Having a short work time is sometimes good, sometimes not!
Our Proskin has a very long potlife for a Platinum silicone, at 60 minutes, and the demould time is merely described as ‘variable’. The M4503 is a fairly standard 90 minutes for Tin, with a 15-20 hour Curing Time to be tack-free.
Demould Time-
This is often the single determining factor in the final choice of silicone for professionals. Have you heard of the Designers Holy Trinity? Good, Fast, Cheap. Pick Two.
If you don’t have much money, but plenty of time, use Tin silicones. If you have no time, but a great deal of money, use Platinum. If you have neither, you are in trouble.
Our Proskin comes in at ‘variable’, which can mean anything between 30mins to a couple of hours, or less if heated to accelerate cure. M4503 will take 15-20 hours before you can touch it without it feeling sticky.
*IMPORTANT: If your silicone still feels tacky or sticky, or ‘wet’ on any surface after the expected cure time has elapsed, it is NOT going to set if you leave it longer. Platinum silicone can be slower in temperatures below 20C, and faster in temperatures above, or can be accelerated by application of heat (waring the mould in the oven on 50C, or blasting with a hairdryer, or directing a fan heater on it- I hasten to say, AFTER it has reached the gel stage). Tin silicone is an inexorable process that will cure in the allotted time no matter what… UNLESS there is a problem. We will be looking at possible problems later in this series.
Viscosity-
this is most simply described as the relative thickness of the liquid. In actual fact it is a lot more complex, but for our purposes we don’t need to get into Newtonian Fluid Mechanics….. Suffice to say that Water is measured at 1 centipoise, which is one one-hundredth of a poise, and equal to one millipascal-second (mPa·s) in System Internationale Units. Honey has a viscosity of 2 000 centipoise (or mPa s) and Molasses has a viscosity of 5 000 centipoise. Lard is extremely viscous at 100 000 centipoise. Our examples come in at 2500 mPa s for the (mixed) Proskin, and 40,000 mPa s (mixed) for the M4503.
From this we can gauge that Proskin is a really runny consistency, while the M4503 is quite thick and pasty. Can you think of situations that would require a very viscous silicone, and one that would be better with a thinner, more fluid silicone? Also remember that there are Thixotropic agents that can be added to silicones in small proportions that will change the viscosity of the material, and are often used when you need the properties of a silicon that is naturally runny (maybe because it is clear and soft) but you need to make a brush-on mould….
Hardness- (Shore #)
Ok, this is the big one. Probably the single most important measurement on the whole sheet. Hardness may be defined as a material’s resistance to permanent indentation and is measured by a durometer, on a scale invented by Albert F Shore in 1920. There are several different scales within the system, but the most commonly used in silicone rubber technology is the ASTM D2240 type A scale, mainly used for softer plastics. They measure between 1 and 100, with the higher number representing the harder material. As an example, tyre rubber is between 50 and 70 on the Shore A scale, dependant on the application. This link has a great chart explaining the Shore scale:
http://www.quickparts.com/LearningCenter/ShoreScale.aspxThe reason it is so important to us is that we need to know how hard our finished mould or piece will be. If you are moulding an item or sculpt with a lot of deep undercuts, you need a bendy, flexible mould, so you couldn’t use any silicone with a Shore A greater than say, 30. If you dont need to worry about undercuts and require a self supporting, rigid mould, then you would choose a silicon with a Shore A of greater than 30, and maybe even as much as 60 or so. If you are using silicone to make an item, be it a sculpt or a prosthetic appliance that will be glued to the skin, it is crucial that you know the hardness. If you are making a body part, then think- is it a flexible bendy part, or a stiff, bony part? Does you piece need to be soft and floppy, or stiff and self supporting? Choose accordingly…. Often the suggested applications will tell you what this product is best suited to. Our Proskin has an extremely low Shore A hardness, at 12, which is eminently suitable for the ideal purposes of skin-like products. M4503 is a fairly soft 25, so it is a fabulous moulding material for very detailed models with large or deep undercuts.
(*Note: SFX Prosthetics is a particularly specialised area and there are certain silicones that have been specially formulated with a third component that adds even more ‘fleshiness’ and ‘stickiness’ without affecting the cure.)
There are several other measurements that are relevant for industrial mouldmaking applications, but probably of less interest to most of us using silicones in the film industry. They include Tensile Stength, Elongation, Tear Strength, and in Tin Silicones, Linear Shrinkage and Expansion factors. We wont be going into those but information is readily available for those who are interested.
In the next part of what is turning out to be a slightly longer series than I had anticipated, we will look at using the silicones.
Summing Up- Important factors in your choice:
1. Colour- Do you need to see through your mould for careful removal of the item, or accurate placement of a prosthetic?
2. Pot-life and demould time- are you in a tearing hurry or can you afford to take time?
3. Compatibility- remember from Part 1, you can cast Platinum into Platinum, and Tin into Tin or Platinum, but you cant cast Platinum into Tin! What are you moulding, and what will your final piece be made from?
4. Viscosity- if you are pouring a matrix mould or a box mould over an intricate piece, you want runny. If you are brushing silicone over a large or vertical surface you want thick. Sometimes you need a combination of both.
5. Hardness- what does your silicone have to do? This will limit your choice to hard (High Shore A) or soft (Low Shore A) materials.
IN PART THREE: USING YOUR SILICONES.
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naomidlynch 
September 24th, 2014 at 8:26 am
Hi, Thanks for such a well written post!
When pouring a silicone box mold I’ve heard you can reuse your cured silicone leftovers by adding them into the mix as a filler. Are there any negatives of doing this? and are there any other materials which would make a good filler for larger molds?