TPE vs. Silicone Elastomers in Additive Manufacturing

As we explained in our latest article “The different types of additive manufacturing”, Additive Manufacturing, is a disruptive series of processes that enable real physical objects to be built up in layers by depositing materials in three dimensions, challenging traditional manufacturing modes that eliminate materials from a block or an initial rough mold. Additive Manufacturing is therefore more efficient, less wasteful and more sustainable, since it only uses the materials that ultimately make up the final product. In this article, we provide information on two leading elastomers, TPE (Thermoplastic Elastomers) and silicone, which enable new and innovative parts to be made using Additive Manufacturing technologies.

What is an elastomer?

Let’s start with a simple definition: the term elastomer is a combination of “elastic” and “polymer”. A polymer, from the Greek poly (many) and mer (parts) is a molecule or material that binds very large molecules (macromolecules), through the polymerization of many small molecules, known as monomers. If these polymers have both viscous and elastic properties, they produce elastomers.

Elastomers are therefore part of the polymer family, which includes plastics or plastomers and other natural or synthetic macromolecular substances that integrate the characteristics of rubber.

Elastomeric materials differ from other types of polymers in that they offer, as their name implies, greater elasticity. This means that they can be engineered to have varying degrees of resistance before they return to their original shape. These variables include:

• Flexibility, with different degrees of stiffness or softness when subjected to pressure
• Tensile strength or the capacity to withstand significant deformation and elongation without tearing or breaking
• Prolonged elasticity and stability, with objects returning to their initial shape and retaining their elasticity over long periods of time when subjected to stress.

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What are TPE/Thermoplastic Elastomers?

Now that we have defined elastomers and examined their unique characteristics, let’s look more closely at the two materials we are focusing on in this article. Let’s begin with Thermoplastic Elastomers (TPEs). TPE is a polymer material that combines the characteristics of thermoset vulcanized rubber and thermoplastic. This means, from a processing standpoint, that TPE can withstand the high temperatures of thermoplastics, but does not rigidify when cooled or cured, providing the high elasticity of thermoset vulcanized rubber at room temperature. This is achieved by structurally varying their crosslinking bonds to alter the composition of the compounds and to generate differing degrees of viscosity and curing properties.

TPEs, along with their closely related alternative, Thermoplastic Polyurethane (TPU), are increasingly being used in Additive Manufacturing, for rapid prototypes, to replace traditional molding and milling techniques. However, there are still some technical hurdles to be overcome for their widespread use in all types of Additive Manufacturing printers, such as their melt index (which means they cannot withstand high temperatures once processed), a limited viscosity range and ability to replicate the end characteristics of injection molding for working parts. This research is ongoing, but there is yet no industry-wide consensus or standards for the use of TPEs in Additive Manufacturing.

What are Silicone Elastomers, and what are their main features?

Silicone elastomers are made by combining a silicone polymer with other molecules such as carbon, hydrogen and oxygen. Their versatile features are achieved thanks to their unique molecular structure that integrates both inorganic and organic components. This provides scientists and manufacturers with the possibilities of creating customized solutions that provide a wide range of characteristics, including:  

• Greater resistance to temperature variations in the final parts, ranging from -50°C to 250°C and beyond
• Enhanced chemical stability, even when exposed to harsh and aggressive substances
• Efficient electrical and thermal insulation
• High abrasion and vibration resistance
• Long-term weatherability when exposed to wind, precipitation, sunlight, UV rays and Ozone
• Low toxicity and higher biocompatibility than most other elastomers
• Strong mechanical properties, including maximum elongation and tensile strength, resulting in durable and efficient elasticity.

Why use an elastomer for Additive Manufacturing?

Elastomer materials, as we have seen, are available in various formats that can be adapted to different Additive Manufacturing processes. For example, TPEs are available in filaments or pellets, are meltable and offer reversible transformation. Silicone Elastomers, for their part, are available as liquids (in various viscosities) and pastes, are non-meltable (therefore withstanding high temperature variations) and are irreversible because of their covalent crosslinks.

Silicone elastomers are used in many types of processing – including injection molding, calendaring, cast molding, spraying, coating, sheet lamination, etc. – and are particularly well suited to Additive Manufacturing or 3D printing when properly formulated and when used with the right printers.

ISO (International Organization for Standardization) has categorized seven types of Additive Manufacturing processes, three of which are particularly well suited to the use of silicone polymers:

• UV-Vat (also known as stereolithography)
  • Very low viscosity silicone materials
  • Curing by photopolymerization
• Material Jetting
  • Higher viscosity silicone materials
  • Choice of curing methods, including photopolymerization, condensation or hydrosilytation (polyaddition)
• Material Extrusion
  • Highest viscosity silicone materials
  • Choice of curing methods, including photopolymerization, condensation or hydrosilytation (polyaddition)

The variable viscosity of silicone elastomers allows the material to flow at the required speed for creating and shaping a layer. Once cured, the resulting material, appropriate for both prototyping or end-use, remains elastic and rubbery, i.e. it can be stretched, compressed or otherwise deformed and has the capacity to return to its original shape.

In short, as Additive Manufacturing techniques evolve and as applications in an increasing number of industries are developed, Elastomers (and Silicones in particular) are becoming materials of choice for end-to-end solutions, from rapid prototyping to serial manufacturing, via customized batches to spare parts replacement.