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What is Silicone Rubber?

Is silicone rubber or plastic? Silicone, scientifically known as polysiloxane, lies in a gray area, but it's generally considered a rubber. It’s a human-made elastomer (a polymer with elastic properties) that has the properties of both plastic and rubber. Silicone can be used to form liquids, hard solids, and rubbery products, but since elastomers are generally referred to as rubbers, it’s also usually referred to as a rubber. Silicone is also easily confused with silicon, though they are different materials. Silicon is an element, while silicone is a polymer (a large molecule with repeating units).

Types of Silicone Rubber

Built on a base of silicon and oxygen (as opposed to plastic’s base of carbon and hydrogen, which is less stable), groups of organic molecules (including methyl) lend silicone rubber its flexibility. Silicone rubber has several variants with traits that differ depending on the application for which it’s used.

Methyl groups go under the name MQ. These are the basic silicone rubbers that were first developed.

Methyl vinyl groups, which feature good compression traits, are known as VMQ. These are the most commonly used types of silicone rubber since the vinyl helps with vulcanization.

Methyl phenyl (MPQ) and methyl phenyl vinyl (PVMQ) groups are used to increase the rubber’s capabilities in low temperatures.

Flouro, vinyl and methyl (FVMQ) groups are more chemically resistant to oil, fuel, and solvent.

Among the families of silicone rubber variations, there are three main forms of silicone rubber, depending on the processing method: liquid, room temperature vulcanized, and high temperature vulcanized.

Liquid silicone rubber has the smallest molecular chains out of the three types, making its polymers a low molecular weight. It can be extruded or injection molded with specialized machinery. This type of silicone rubber is more weather resistant and transparent.

RTV (room temperature vulcanized) silicone rubber can be from one or a mixture of two components, and hardens at room temperature to soft or medium hardness. It’s generally used for potting, sealant, and encapsulations. Silicone rubber molds are also made from RTV silicone rubber.

Solid or HTV (high temperature vulcanized) silicone rubber has long-chained polymers with the heaviest molecular weights. This type usually comes in the unprocessed form and must be vulcanized at the same temperature as more traditional types of rubber.

Is Silicone Rubber Safe?

Silicone rubber has been considered safe in the United States for applications in consumer cookware and medical products since the FDA approved it in 1979. It is also approved in Canada. However, the European Union has labeled chemicals D4, D5, and D6, used in the production of silicone rubbers, as concerns, and some newer studies suggest that silicone rubber can leach these chemicals under certain circumstances.

How to Make Silicone Molds: A Practical Guide

Throughout history, artisans have used molds to make everything from Bronze Age weaponry to modern-day consumer products. While early molds were commonly shaped in stone, modern science has given rise to materials that are far easier to work with, such as silicone.

Today, a vast array of industries rely on silicone molding. Product developers, engineers, DIY makers, and even chefs all make silicone mold to create one-off or smaller runs of parts.

In this practical guide to silicone mold-making, we’ll break down everything you need to know before getting started, provide a list of items you’ll need, and give you a step-by-step guide for incorporating silicone molds into any type of project.

Why Make Silicone Molds?

Silicone is a strong choice for mold-making because it offers such a diverse array of benefits. You can easily create custom designs using silicone molding. The molds themselves are also quite durable, so you can use them repeatedly without fear of breakage. Silicone’s inorganic makeup—compared to rubber, its organic counterpart—makes it highly resilient to heat and cold, chemical exposure, and even fungus. Some of the benefits of silicone molds include the following:

Flexibility

Silicone’s flexibility makes it easy to work with. Silicone molds are pliable and lightweight compared to stiffer substances like plastic and they’re also easier to remove once a part is fully formed. Thanks to silicone’s high level of flexibility, both the mold and the fabricated part are less likely to break or chip. You can use custom silicone molds to shape everything from complex engineering components to holiday-themed ice cubes or confections.

Stability

Silicone withstands temperatures from -65° to 400° degrees Celsius. Additionally, it can have an elongation of 700%, depending on the formulation. Highly stable under a broad range of conditions, you can put silicone molds in the oven, freeze them, and stretch them during removal.

Achieving Stiffness for Compression Molding

Predominantly, if the metal part used in compression mold is tension or compression loaded, a ratio of metal part thickness to its extensional modulus, multiplied by the Bulk Molding Compound (BMC) material modulus (tension, compression, or flexural), is a successful formula to follow to achieve stiffness.

If the part is loaded in bending, it could be possible to make the metal part thicker, however, this is not necessarily needed. Ribs are easily added to a compression-molded part to add additional stiffness with the negligible material weight increase. Many companies, like Toray, give easy to navigate graphs for a variety of metals compared to BMC.