Elastomeric parts: injection molding silicone vs 3D printing flexible materials

Elastomeric parts: injection molding silicone vs 3D printing flexible materials

From industrial gaskets to the soles of your shoes, elastomeric parts are both ubiquitous and unique. Made from elastomers, a group of rubber-like polymers, these flexible, stretchy, and shock-absorbing components are easy to make — and can be produced in several ways.

One of the ways to produce elastomeric parts is injection molding. Not the injection molding of thermoplastics, but the injection molding of thermosets like liquid silicone rubber: this material is injected in its liquid state then cured to make it solid. But today there’s a great alternative to liquid silicone injection molding, and that’s the 3D printing of flexible materials like TPE and TPU. No longer specialty materials, TPE and TPU can now be 3D printed on low-cost FDM printers.

This article looks at the basics of elastomeric parts: their properties, their applications, and whether you should opt for injection molding or 3D printing.

What is an elastomer?

An elastomer is defined as a polymer with a low Young’s modulus, a high failure strain, and viscoelasticity — a combination of viscosity and elasticity. Sometimes referred to simply as rubbers, elastomers are actually a broader category that includes non-vulcanized materials.

In layman’s terms, elastomers (elastic polymers) are materials that are, to various extents, elastic, stretchy, flexible, or deformable at room temperature. This is due to their weak intermolecular bonds. All of this makes elastomers incredibly useful. Since they can be bent and stretched without breaking, they are used in many applications where a more rigid material would fail in its purpose.

Elastomers can be either thermosets (do not melt when heated) or thermoplastics (melt when heated), and some of the most common elastomers include:

  • Natural rubber
  • Butadiene rubber
  • Styrene-butadiene rubber
  • Ethylene-propylene monomer

However, since this article focuses on prototyping and two manufacturing processes in particular, we will pay greater attention to silicone, thermoplastic elastomer (TPE), and thermoplastic polyurethane (TPU). Silicone is a thermoset, while TPE and TPU are both thermopolymers and can therefore be extruded by a 3D printer.

Examples of elastomeric parts

Elastomers are incredibly common and can be found in simple everyday objects and complex industrial assemblies. Here we look at various elastomeric parts categorized by industry, as well as a few material-specific applications.

1. Automotive

For tires and much more, elastomers are essential for the production of many automotive components.

  • Tires
  • Vibration dampening equipment
  • Windshield wipers
  • Suspension components
  • Seals and gaskets

2. Industrial

Elastomers are widely used in industry, especially for seals and gaskets for watertight and airtight assemblies.

  • Seals and gaskets
  • Electrical insulation
  • Conveyor belts
  • Drive belts
  • Hoses and tubes

3. Oil & gas

Elastomers, especially synthetic ones, are found in the oil and gas industry, where they often serve to prevent oil leaks.

  • Hydraulic seals
  • Pipe wipers
  • Packer cups
  • Hoses

4. Consumer goods

A huge number of consumer products are made from elastomers, from footballs to pencil erasers.

  • Footwear including outsoles
  • Raincoats
  • Sponges
  • Pencil erasers
  • Sporting goods including balls
  • Food containers

Material-specific examples

Since this article focuses on injection moldable liquid silicone rubber and 3D printable TPE and TPU filament, here are some common uses for these three materials:

  • Silicone: Biologically compatible medical equipment, prosthetics, food-safe devices like ice cube trays, seals and gaskets, overmolded components
  • TPE: Seals and gaskets, bottle cap liners, snowmobile tracks, medical components including catheters, tool handles, suspension bushings, overmolded components
  • TPU: Electronics cases, caster wheels, medical components, drive belts, footwear, inflatable items, wire jacketing and tubing

Injection molding elastomers

There are numerous ways to make elastomeric parts, but one of the most reliable — especially in large quantities — is injection molding.

Injection molding is the process of making plastic parts by injecting, at pressure, a liquid material into a metal mold, usually made from steel or aluminum. During the process, the injection molding machine feeds the raw material into the mold, the geometry of which is a negative impression of the final part and which usually consists of two sections with a cavity between them.

Injection molding is compatible with thermoset elastomers like liquid silicone, and this is an excellent way to manufacture pliable, impact-resistant, and temperature-resistant parts in large quantities. Before molding, a two-part silicone solution is mixed together, one half of which contains a platinum catalyst that enables the silicone to cure into a solid.

Injection molding liquid silicone can produce parts like gaskets, medical equipment, kitchen equipment, overmolded sections of multi-material assemblies, and more.

Pros

There are several advantages to injection molding silicone, including rapid production of large volumes, high detail level, good material properties, and a low cost per part at certain medium-to-high quantities.

Cons

Limitations of injection molding liquid silicone include high metal tooling costs and the geometrical restrictions inherent to the molding process, including thin walls and radiused corners.

3D printing elastomers

An alternative to liquid silicone injection molding is the 3D printing of flexible filaments like TPE and TPU. Although these materials can also be injection molded, 3D printing offers a more radical alternative that may be suitable for different kinds of projects, including one-off prototypes.

One of the biggest advantages of 3D printing elastomers like TPE and TPU is that these materials are thermoplastics and can therefore be made into FDM 3D printing filament suitable for melting and extrusion. FDM is the cheapest form of 3D printing — both in terms of machine and operational costs — and this means that high-quality elastomeric parts can be made by anyone.

TPE and TPU filaments both have their uses. TPU is a harder, denser, and more rigid material than TPE, with better abrasion resistance and less chance of shrinkage. However, it is less flexible than TPE, which can be used to make parts as flexible as rubber bands.

Although care must be taken when 3D printing elastomers, especially TPE (print speeds must be slow, and direct drive extruders work better than Bowdens) virtually any FDM printer can process these materials.

Common 3D printed elastomeric parts include TPU footwear midsoles and wearable electronics, and TPE handles, hoses, and smartphone skins.

Pros

Advantages of 3D printing flexible filaments include very low costs even for small quantities, no tooling costs, and almost unlimited geometries (internal and external). This makes the process highly suitable for elastomeric prototyping.

Cons

Limitations of 3D printing elastomeric parts from TPE and TPU include accuracy and detail level (compared with injection molding), production speed, and scalability, since there is no economy of scale either in terms of cost or time savings.

Conclusion

Liquid silicone injection molding and FDM 3D printing are both excellent ways of producing elastomeric parts. Injection molding is the frontrunner when it comes to part quality and scalability, but 3D printing is a convenient and affordable option for prototypes, especially one-offs.

When making elastomeric parts or prototypes, you should consider factors such as budget, number of units, required material properties, and future production needs before deciding on your next course of action — a decision that 3ERP can help with.

3ERP is a prototyping specialist offering both injection molding and 3D printing, as well as many other services. Get a free quote now for your elastomeric parts.