Casting molds from liquid silicone at home

Materials and equipment

Two-component liquid silicone

What exactly and where I bought liquid silicone I described last time.

Silicone casting mold

The mold was custom-made for me from aluminum, but for simple and home tasks you can make it yourself from plaster.
I haven’t described this process yet, so that will be a topic for another post. Here is an example of a plaster mold that we made ourselves at the very beginning:

And this is the industrial form:

Mixing container

It is better to use transparent containers with measuring marks. But in order not to spoil the container, I took a shampoo bottle and cut off the top. And since the bottle is not transparent, additional steps were needed to determine the filling level. But these are minor things.

Stirring stick

Silicone consists of two components that need to be mixed together in equal proportions. Chinese chopsticks are great for mixing , which you can grab on the way home from any fast food with rolls.

Funnel

As a rule, casting molds have a small hole for pouring silicone into them. Therefore, a funnel will come in handy, which can be made from glossy paper and secured on top with tape.

Gloves

When working I used regular latex gloves. But any will do, as long as they are comfortable.

Degreaser

The mold into which the silicone will be poured must be cleaned and degreased.
The first time I didn’t do this, the mold ended up with small potholes from hard debris, and in a couple of places the silicone stuck quite strongly to the mold. To clean, I used a cleaner purchased at the construction market. Unfortunately, I forgot to take a photo of him. But you can use alcohol or WB.

What else might be useful?

Wax lubricant

The silicone supplier sells a special wax lubricant in spray form. It is applied to the mold before pouring and this guarantees easy removal of the finished silicone part from the mold. But I've managed without it for now.

Vacuum chamber

You can also use a centrifugal or vacuum chamber to remove air bubbles from the silicone. But for my purpose this was not so critical, and there were almost no bubbles.

Main properties and advantages of silicone for molds

High-quality silicone for casting molds has a number of characteristic advantages and properties:

• Excellent fluidity;
• Thermal stability;
• Resistance to temperature changes and aggressive chemical environments;
• Ease of application and unmolding. There is no need to use additional release agents;
• Insignificant degree of shrinkage (good elasticity, stretchability).

The finished forms are stable in size and have a high precision of the workpiece (they convey the smallest lines and bends of the sample in detail, create ornaments and subtle designs). They withstand numerous removals from finished products.

Important! Liquid silicone for plaster molds is characterized by its lack of adhesion to any materials (besides other silicones) and does not cause corrosion of metals and alloys (including copper).

About storage

You can buy affordable liquid silicone with a reserve. The product is suitable for repeated filling. The main thing is to store it in a tightly closed container.

Please note: the hardener is sensitive to air humidity. It should not be stored in unsealed packaging.

The forms themselves are shock-resistant, frost- and heat-resistant. Therefore, there is no need to create special storage conditions. They are inert even in relation to UV rays.

Rules of application

After mixing the components, the silicone remains in liquid form for 20 minutes (at room temperature). The lower the temperature, the longer this period.

The hardening period is approximately 5 hours. The form finally polymerizes in 1-3 days. After this, it can be removed from the flask and cut in half.

Important! Geometric stability is achieved during the polymerization process. The composition does not emit any hazardous components.

• Silicone for molds Polymerika Mold 20 (1.025 kg)
  1280
• Silicone for molds Polymerika Mold 20 (5.125 kg)

  4680

• Silicone for molds Polymerika Mold Plus 30 (1.025 kg)

  1280

• Silicone for molds Polymerika Mold Plus 30 (5.125 kg)

  4680

Manufacturing process

Level installation

Since my casting molds have a bathtub form factor, they need to be installed horizontally using a level. After measuring, I placed some cardboard under the low edge and everything was ready.

Determining the volume of a form

First you need to understand how much silicone is needed to make a specific mold.
The easiest way is to use water. Fill the mold with water and pour it from the mold into a measuring cup. In my case, the volume was 130 ml. The second form is almost the same. The total was 260 ml.

Now about the container for mixing silicone. Basically, you need a measuring cup with two marks of 130 ml and 260 ml to pour the two silicone components in turn.

I got out of the situation like this: I took one of the two Chinese chopsticks, placed it in a container and poured 130 ml of water inside. In the place where there was a line between the dry stick and the wet one, I drew a line. Then I added another 130 ml to the container and drew a second line.

Preparation of silicone mixture

To prepare a silicone mixture, you need to mix two components of liquid silicone in equal proportions and mix thoroughly.

Component A

Component B:

It is very important to mix well, otherwise the reaction will not occur evenly and some of the silicone will remain liquid and the mold will be damaged.

You have 15 minutes to do everything, after which the silicone will begin to harden and it will be impossible to pour it into the mold.

Pouring silicone into a mold

The prepared silicone mixture was slowly poured into the first mold into one point. It is important to pour at one point so that the silicone itself spreads throughout the mold and expels the air. If you start moving the jet, a cavity with an air bubble may form, and because of this, there will be either a hole or an empty space inside the silicone mold.

When almost the entire mold was filled with silicone, I closed it with a specially prepared lid with holes. He inserted a funnel into one of them and began pouring silicone into it. The second hole is needed for air to escape.

It is important to note that the lids must be pressed tightly into the mold. I did it by hand, but it was not very convenient. For the future I will make clamps or latches.

After the molds were completely filled with silicone, I placed a weight on top. It will remain in this form for 24 hours for complete hardening.

Removing finished silicone molds

After a day, the silicone molds are ready and can be removed from the equipment. To do this, carefully pry flat objects (I used a metal ruler) from the edge and slowly lift them up.

If the two components of liquid silicone are well mixed and 24 hours have passed after pouring, the product will easily fall off the mold. But if you don’t mix the mixture well, it will remain liquid in places and the part will be damaged.

Defects and excess removal

Still, air cavities have formed and now the shape is a little defective. For my task this is not critical at all, but the presentation suffers. Next time I'll take this into account.

There is a small gap between the lid and the mold; the silicone penetrates into it and excess remains on the mold. Silicone also remains in the filling holes and builds up. They can be easily removed with scissors and a utility knife.

MOLDING SILICONE FOR CREATION OF FOOD MOLDS

Soft silicone for platinum-based molds is presented in the catalog under the brands Smooth-On and Sorta Clear. A liquid compound with varying degrees of Shore hardness is used to make food molds, but can also be used to fill other soft molds.

This casting silicone is also two-component. According to the degree of hardening, it is divided into quick-drying and slow-drying. The latter's working time allows you to spend 40-50 minutes working on small details of the form.

What else can silicone molds be useful for?

When choosing silicone for casting, the main limiter is temperature, so pouring anything with a temperature above 120°C will not work.
Gallium, a metal with a low melting point (about 30 degrees), will be poured into these silicone molds, so there will be no problems. What else can you use molds cast from liquid silicone for:

• for the production of silicone baits;
• for the production of decorative stone and tiles;
• for casting from plaster;
• for molding soap during production;
• use when molding chocolate and jelly;
• for casting parts from two-component liquid plastic;
• in the manufacture of decorative figurines and handicrafts;

If anyone has experience in casting silicone parts and molds, it would be great to supplement this story with useful life hacks and techniques.

Range and features of silicone types

Even if you know what silicone for molds is, don’t rush into buying it. The fact is that products are offered in different types (classification depending on the main substance):

• Tin based.
• Platinum based.

“Platinum” silicone for molds cannot be used with sulfur-containing materials. Otherwise, the form will not be hard enough or will remain sticky.

Important! Use liquid silicone correctly, and you will get a mold that can cope with significant temperature changes and maintain its properties at t -80 degrees. Celsius up to +300 gr. Celsius.

Silicone for molds. Which one should I buy? Let's figure it out.

We care about our customers.
We can say this with complete confidence, because we not only offer high-quality products, good prices, convenient store opening hours, and so on. We help you understand and decide, find “your” product on the market. This article will be of interest to those involved in small-scale production, decor, art, etc. The list can be endless, because we will talk about a material that is widely in demand in a wide variety of industries - silicone.

What is silicone? This is a synthetic polymer material based on a chain of high molecular weight silicon compounds with organic substances. We will not delve into the formulas, but simply point out that thanks to its chemical composition, silicone occupies a special position in the ranks of elastic materials. Silicone is the only commercially produced elastomer that does not contain carbon atoms. And, therefore, silicone has one of the main physical parameters - heat resistance, i.e. the ability to maintain elasticity for a long time at high temperatures.

So, the first mention of silicone dates back to the early 20s of the last century. The material was patented in 1958. Used in the military industry. And only at the end of the twentieth century, silicone actively gained its position in construction, design, everyday life and even medicine. Actually, I would like to talk about the areas of application of this material, because the scale of its use is surprising:

• Construction and decoration - decorative gypsum panels, gypsum rosettes, stucco molding, figurines, restoration of monuments, making reproductions of stone figures, making artificial stone.
• Art - sculptures, candles, masks, artificial jewelry, reproductions of coins, medals, etc.
• Metal casting - molding of decorative products from alloys of tin, lead, zinc.
• Furniture - molding of decorative fittings.
• Archeology and paleontology - for the preservation and modeling of mosaics, statues, fossils.
• Automotive/Aviation/Aerospace - molding of rigid polyurethane foam parts: instrument panels, steering wheels, armrests, trim elements and prototype development.
• Medicine - silicone implants.
• Cosmetology - soap making, shampoos, hair masks.

The list can be endless.

There are silicone liquids, silicone elastomers and silicone resins. In this case, we are interested in silicone elastomers, these are:

• hot-curing silicone rubbers (high molecular weight);
• hot curing liquid silicone rubber (LSR);
• cold-curing silicone compounds (low molecular weight);
• cold-curing silicone sealants;
• silicone rubbers.

We will focus on cold-curing silicone compounds.

There are a lot of offers on the elastomeric materials market. Let's figure out how to choose silicone.

Silicones:
One-component (ready to use)	Two-component (consisting of a base (component A) and a catalyst/hardener (component B)). They are used most often because you can regulate the curing speed, and therefore the lifetime of the silicone

Two-component silicones are divided according to the type of catalyst:

tin based Tin-based silicone compounds are a good material that meets the price-quality parameter. They are easy to use, but have a significant drawback - they have low tear properties; after several years they lose their physical shape - they become brittle, split and tear. Therefore, they are more often used for less critical castings made of polyurethanes, polyester resins, plaster, wax, paraffin, gypsum, concrete, and liquid plastics.

platinum based These are more expensive silicone compounds that have a long lifespan for use. They are heat resistant. Can be used in cosmetology and food industry*. After curing, platinum-based silicones have the greatest chemical and microbiological resistance. Considered safe. Disadvantage: Reacts with many naturally occurring compounds, particularly sulfur, tin, amines, and newly manufactured polyester, epoxy, or urethane rubber products. Even after coating the product with acrylic varnish, silicone for molds on platinum will not harden if it interacts with sulfur- and tin-containing surfaces. This makes them incompatible with many natural objects.

*IMPORTANT!

Silicone compounds for use in the food industry must have appropriate food grade certificates!

Advice:

Silicone compounds do not hold alkaline compounds well, so it is not recommended to pour concrete into them - the molds quickly fail.

Suppose you have decided on the choice of silicone compound - platinum or tin based. Next, your choice will be based on technical requirements:

Hardness

Hardness is the property of a material not to undergo plastic deformation due to local contact action, usually reduced to the penetration of a harder body into the material.

This parameter is determined by the indentation method according to the Albert Shore scale. In accordance with the ASTMD 2240 standard, 12 measurement scales are described. The most commonly used options are A (for soft materials) or D (for harder materials). Usually the hardness is indicated in the name of the silicone compound, for example:

Mold Star 15 A Slow

The number (15) indicates the hardness value determined by the Shore method on the A scale (for soft materials).

For silicone compounds, hardness ratings can vary from 10 to 70. As a rule, silicones with a hardness rating from 10 to 15 are used for casting small, relatively light products with fine patterns: soap, candles, stucco molding. Silicones with a hardness of 20 to 70 are used for larger products, for example, if you need to make a mold for a large and heavy sculpture.

Viscosity/fluidity

Viscosity is the property of a liquid to resist shear of its layers relative to each other.

Silicone with a lower viscosity value is more convenient to pour into a mold; it flows better around complex surfaces (small parts).

Lifetime

This is the time during which the resulting silicone compound should be used while it has a minimum viscosity. After this time, the irreversible process of polymerization (hardening) will begin.

The most optimal lifetime is considered to be from 30 to 50 minutes.

Curing time

This is the time during which silicone gains the stated hardness and acquires elasticity.

Linear shrinkage

This reduction in the volume and linear dimensions of the casting during its formation is usually expressed as a percentage. For example, when filling a mold with a height of 300 mm with a compound with a linear shrinkage of 1%, the height after polymerization of the silicone will decrease by 0.3 mm.

Elongation at break

This is a parameter that characterizes how much silicone stretches before it breaks. The higher this parameter, the better the quality of the silicone.

Tensile strength

In simple terms, this is the threshold amount of force that must be applied to the silicone in order for it to elongate the maximum amount before it breaks.

Silicone compound stands out from other elastomers with a unique set of properties. No wonder it is so widely used in a wide variety of applications. However, like any other material, silicone has its pros and cons.

Advantages of silicone compounds:

• Copy any shape down to the smallest detail.
• A quick way to remove the mold.
• High strength and durability.
• Withstands a wide temperature range – from -50°C to +250°C. There are silicone compounds that are especially frost-resistant and especially heat-resistant, withstanding temperatures from -110°C to +350°C.
• Tensile strength up to 800%.
• They have low residual deformation and good mechanical characteristics.
• Water-repellent surface.
• High ozone resistance and radiation resistance of products.
• Excellent electrical insulation.
• High non-flammability, absence of toxic combustion products.
• They are considered the most environmentally friendly and safe materials from the elastomer series.
• Complies with the requirements of the German Federal Institute for Risk Assessment (BfR) and the US Food and Drug Administration (FDA).
• Resistant to the main human habitat and atmospheric phenomena: air, drinking and sea water, rain, snow, frost.
• Resistant to ozone, oxidation, UV rays, corona discharge, electric fields, cosmic radiation.
• High resistance to alcohol, mild alkalis, non-oxidizing acids, most salts, mineral oils, dampness and directed sunlight.

Disadvantages of silicone compounds:

• Conditional stability and accelerated thermal aging in closed systems in the absence of air or in an environment of superheated water or steam.
• The average price for a silicone product is higher than for nitrile-based products.
• When painted black, the product loses some of its properties.
• They are poorly painted externally (only with alcohol or silicone-based paints).

To summarize all of the above, we realized that silicone is one of the most popular materials, which has more advantages than disadvantages. We have studied the offers on the silicone compounds market, and are ready to offer you the most popular ones:

• Silicones on a platinum catalyst from the American manufacturer Smooth-On Mold Star series
• Silicones on a tin catalyst from a Chinese manufacturer of the Super Mold series

You can get acquainted with a detailed description of the silicones offered to you by following the link.

Read about how to work with moldable silicones (general principles) in our next article. Follow the news.

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