3D printed rubber: What's possible and what isn't

Is it possible to 3D print rubber? In a strict sense, no. But with a huge number of flexible filaments and flexible resins out there, it is possible to make pliable, rubber-like parts suitable for a range of applications.

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05 Apr, 2022

Rubber is a suitable material for parts like seals and gaskets

Rubber is a suitable material for parts like seals and gaskets

Natural rubber, harvested from the rubber tree, is a highly desirable elastomer used in everything from waterproof boots to electrical insulation. But rubber is difficult to manipulate and shape. Once hardened, the gummy material can’t be melted back into a liquid state suitable for extrusion.

Fortunately, the desirable properties of rubber — flexibility, abrasion resistance, vibration dampening — can be approximated with other types of 3D printing material, using various 3D printing technologies.

This article takes a closer look at the ins and outs of 3D printed “rubber”, examining options like 3D printable filaments and resins, as well as new possibilities like 3D printed silicone.

Rubber treesNatural rubber is harvested from trees and cannot be 3D printed

Is it possible to 3D print rubber or elastomers?

When people talk about rubbers, they might be referring strictly to natural rubber — sometimes called India rubber or latex — which is an organic material harvested from the rubber tree, or they might be using the term in a slightly looser sense to describe elastomers, which are a large and varied group of elastic polymers.

So if we want to ask whether it is possible to 3D print rubber, we need to differentiate between natural rubber (specific) and elastomers (broad). In short, it is not possible to 3D print natural rubber because the material cannot be melted, cured, or manipulated in a way that is useful for any currently recognized 3D printing process.[1] With the right knowledge and equipment, however, it is possible to 3D print elastomers.

3D printable elastomers include flexible filaments like thermoplastic polyurethane (TPU) for fused deposition modeling and flexible resins for vat photopolymerization. In many cases, material suppliers offer these materials in different levels of flexibility, as the hardness level can be modified chemically.[2]

In recent years, a handful of companies have even found ways to 3D print silicone rubber — a material more valuable than typical 3D printable elastomers due to its biocompatibility.

Flexible filaments

Fused deposition modeling (FDM) is the most common 3D printing process, as it is compatible with a range of affordable thermoplastic materials like PLA, ABS, and PETG.

Although these materials offer different levels of rigidity, none of them is particularly flexible. The exception within the catalog of FDM materials is a category of thermoplastics called thermoplastic elastomers (TPE), the most notable of which is thermoplastic polyurethane (TPU). The printing characteristics of TPU and other elastomers are vastly different to those of stiff materials like PLA: although they have very low tensile strength, these materials possess properties like flexibility, impact resistance, high elongation at break, and good shock absorption.

There is a catch though. Flexible materials can be much harder to print than rigid ones, often producing blobs and stringing while offering poor bridging capabilities. To mitigate these problems, a direct drive extruder is required, and the printer should be configured with a slow feed rate and minimal retraction.

Common rubber-like filaments include:

  • TPE filament

  • TPU filament

  • Soft PLA filament

Flexible filaments are suitable for 3D printed parts like flexible prototypes, seals, soft-grip handles, smartphone cases, flip-flops, shoe soles, and vibration dampeners.

Flexible filamentFlexible filaments like TPU have rubber-like properties

Flexible resins

FDM is not suitable for all 3D printing projects, as it produces a substandard surface finish and limited level of detail. Fortunately, it is also possible to use flexible materials for vat photopolymerization processes — stereolithography (SLA) and digital light processing (DLP), for example — which typically offer superior smoothness and part resolution. Although less common than FDM, these technologies can be found in many consumer-level desktop machines.

An advantage of using flexible resins is that they are much more printer-friendly than flexible filaments, as the light-curing process is not susceptible to issues like blobs and stringing. However, photopolymerization printers and resins are more expensive than FDM hardware and filaments. Flexible resins are not easily categorizable, as each formulation is unique to the specific material developer.

Flexible resins are suitable for 3D printed parts like flexible prototypes, gaskets, seals, soft-grip handles, wearables, and stretchable enclosures.

Professional flexible materials

Another plastic 3D printing process suitable for the production of high-quality, rubber-like parts is selective laser sintering (SLS). Although SLS is generally limited to professional and industrial applications, it provides good results using materials like TPE and TPU. Print quality is superior to FDM, and larger parts can be made when compared to vat photopolymerization.

Individuals and smaller companies can still obtain TPE/TPU parts made using the SLS additive manufacturing process by using a 3D printing service.

Other professional-grade additive manufacturing processes that use flexible materials are traditional material jetting and Multi Jet Fusion from HP.

Printable silicone

Widely available 3D printing technologies like FDM and SLA are not capable of 3D printing silicone rubber — a high-value elastomer with useful characteristics like biocompatibility, thermal stability, and UV resistance. However, some companies have developed novel technologies for printing silicone, with promising results.

Silicone rubber starts out in liquid form, and can only be turned solid by curing, vulcanization, or catalyzation. To print silicone, the liquid must be selectively deposited (in a drop-on-demand manner, for example) and then solidified using one of the three processes. A further stage of curing is then usually required to achieve full strength.

Silicone 3D printing is not particularly accessible, and some technologies are still in early development. However, it holds promise in areas like medical 3D printing.

3D printing rubber-like materials vs injection molding

The properties of rubber that make it unsuitable for FDM 3D printing also apply — to a limited extent — to injection molding, as both technologies depend on heating up plastics to melting point then allowing them to cool and re-solidify.

Like 3D printing, injection molding can make flexible parts using materials like TPU, which can be processed like a rigid thermoplastic (i.e. melted, formed, then cooled) while still offering flexibility and other rubber-adjacent characteristics.

That being said, liquid silicone injection molding, a specific variant of the injection molding process that does not apply high levels of heat, has the additional benefit of being able to process liquid silicone rubber (LSR) for items like medical devices.

FDM 3D printers cannot extrude LSR because the material has a viscosity similar to water. In liquid silicone injection molding, however, the two components of LSR can be mixed together then cured within the injection mold. Despite initially being in a fluid state, the silicone will hold its shape, because it is enclosed on all sides by the mold (unlike with 3D printing, where the part must support itself as it is being built).[3]

Liquid silicone injection molding even offers greater design freedom than plastic injection molding, as silicone parts are flexible enough to be removed from the mold without the use of draft angles. However, the high viscosity of the silicone leads to a high degree of flash — material seeping through the parting line.

3D printing and injection molding have their own advantages and disadvantages when it comes to making flexible parts, as shown below.


3D printing

Injection molding

Advantages

Disadvantages

Advantages

Disadvantages

Affordable even in low quantities

Limited range of materials

Highly scalable

High tooling costs

High level of geometrical freedom

Surface quality and detail limitations

Suitable for liquid silicone rubber

Geometrical limitations

Molding rubberInjection molding is a good alternative to 3D printing when making flexible parts

3D printed molds for flexible parts

An alternative to directly printing flexible parts or injection molding flexible materials is 3D printing rigid molds — using a heat-resistant material like ABS — that can then be filled with a material like liquid silicone. This process combines some of the advantages of 3D printing and molding, and is inexpensive.

Molds can be designed using CAD software and fabricated like any 3D printed part. The cavity is coated with mold release spray then filled with the two parts of the liquid silicone, with curing taking around 90 minutes. A vacuum chamber may be used to de-gas the silicone, preventing the buildup of air bubbles.

This technique is slower than directly 3D printing elastomers, but silicone is much more flexible than printable materials like TPU, in addition to having unique benefits like biocompatibility.[4] The printed molds can also be reused.

Conclusion

If your parts must be made from rubber at all costs, then 3D printing is probably not the way to go. Techniques like rubber compression molding are more suitable for manufacturing natural rubber parts, as the material does not need melting.

3D printing does, however, offer several viable options for printing high-performance rubber-like parts with desirable characteristics like flexibility, impact resistance, and shock absorption. Flexible FDM filaments like TPU are great for tough prototypes and simple items like soft-touch grips, while flexible resins are suitable for slightly more intricate components. Rigid 3D printed molds are also a great means of quickly casting silicone parts.

References

[1] Drossel WG, Ihlemann J, Landgraf R, Oelsch E, Schmidt M. Basic research for additive manufacturing of rubber. Polymers. 2020 Oct;12(10):2266.

[2] What is TPU? [Internet]. Lubrizol. [cited 2021Oct19]. Available from: https://www.lubrizol.com/Engineered-Polymers/About/What-is-TPU

[3] Bont M, Barry C, Johnston S. A review of liquid silicone rubber injection molding: Process variables and process modeling. Polymer Engineering & Science. 2021 Feb;61(2):331-47.

[4] Fallahi D, Mirzadeh H, Khorasani MT. Physical, mechanical, and biocompatibility evaluation of three different types of silicone rubber. Journal of applied polymer science. 2003 Jun 6;88(10):2522-9.

05 Apr, 2022

Educated at King's College London and the University of Amsterdam, Benedict has been a freelance writer in the 3D printing industry since 2015. He is a contributing editor at Aniwaa and a senior writer at 3dpbm.

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