Archive — This feature was originally published in the October–November 2019 issue of 3D ADEPT MAG. Interviews, product ranges and development plans reflect that period.
Silicone parts have long been produced by injection moulding, casting and compression moulding. Tooling costs can be significant for small series or customised parts. In 2019, emerging additive manufacturing approaches offered another way to process suitable silicone formulations, prompting this examination of their principles and applications.
ACEO®, German RepRap and Spectroplast AG shared their expertise for this feature. They represent three approaches discussed here, rather than an exhaustive list of silicone-printing developers.
ACEO® is a Wacker Chemie brand. At the time of the interviews, it offered 3D printing with silicone elastomers that the company compared with liquid silicone rubbers used in injection moulding. Egbert Klaassen, then Global Marketing Director, describes the approach below.
German RepRap, a German manufacturer known for fused filament fabrication systems, also developed the Liquid Additive Manufacturing approach discussed below. Lena Wietfeld, then Head of Marketing, answered our questions.
Spectroplast AG is an ETH Zurich spin-off specialising in silicone additive manufacturing. In 2019, the company reported a €1.38 million seed round led by AM Ventures. Co-founder and CTO Petar Stefanov contributed to this feature. Its healthcare ambitions are discussed separately from the qualification of any particular medical device.
Why print with silicone?
Silicone’s elasticity and other useful properties make it attractive for customised components. Processing it additively also requires control of how a liquid or paste is deposited and cross-linked into a stable shape.
Silicones are polymers with a silicon–oxygen backbone and organic groups attached to it. The formulation and cross-linking system determine the properties of the resulting elastomer.
A cured silicone elastomer cannot simply be remelted like a thermoplastic filament. The approaches described here instead process a suitable uncured formulation and solidify it through controlled cross-linking.
Depending on the grade, silicones can offer elasticity, electrical insulation, transparency and resistance to particular temperatures or environments. These properties must be checked for the formulation and finished part. Neither a universal −80 to +300 °C operating range, harmless fire emissions nor medical suitability can be inferred from the word “silicone” alone.
Spectroplast AG’s Petar Stefanov confirms this assertion while laying emphasis on the reason why silicone is used as a 3D Printing material: “Silicone is a high-performance elastomer that exhibits many beneficial properties including biocompatibility, fatigue resistance, chemical resistance, high temperature stability etc. Conventionally, only rigid materials such as metals, plastics and ceramics are accessible by additive manufacturing while there is a growing need for soft and compliant elastomeric materials. Introducing biocompatible elastomers to the world of additive manufacturing opens doors to the next generation of customized, patient-specific healthcare products.”
Stefanov’s comments describe Spectroplast’s development goals at the time. Other additive processes already offered flexible materials; suitability for contact with the body requires assessment of the specific formulation, processing and intended use.
As you may know, 3D printing is a technology that can process different forms of materials: resin, powder and filament. Now that silicone has joined the ranks of 3D printable materials, it should be noted that “the material begins its journey in liquid form and is selectively solidified using light.” “Highly precise light exposure gives the printed objects exceptional spatial resolution and outstanding surface finish”, explains Stefanov.
The light-based route described here is Spectroplast’s approach; it does not describe all silicone additive manufacturing processes.
How does the 3D Printing process work?
The main differences concern material formulation, deposition and curing. A printer must be compatible with the specific silicone and its cross-linking mechanism.
Spectroplast described light-based processes such as SLA and DLP as suitable for its formulations and highlighted resolution, surface finish and mechanical performance. These were the company’s claims for its own approach.
German RepRap and ACEO described different methods for depositing and curing silicone.
The “LAM” technology from German RepRap
German RepRap’s L320 used Liquid Additive Manufacturing (LAM) to process liquid silicone rubber (LSR). In the 2019 interview, the manufacturer also outlined plans to develop further materials. Wietfeld explained the process as follows:
“[Our technology] works with a material that is not – as with the FFF printer – melted and solidified again, but is liquid, and vulcanized under heat exposure. One speaks also of a thermal full network. This means that the individual layers that the printer deposits firmly connect to each other. In this way, it is possible to produce components that have almost the same properties as injection-molded parts – (…) a clear advantage because insights from the 3D-printed prototype can be transferred directly to injection-molded serial parts.
A special halogen lamp releases activation energy to accelerate the complete cross-linking, on a molecular level, between the two components. A finely tuned action, for both small and large objects, is ensured by the travel speed of the lamp. Due to this thermal cross-linking, the printing time is considerably reduced, at the same time the printing result, especially in terms of time savings, sets new standards. In comparison to conventional production methods, such as injection molding, a time saving of 50% and more can be achieved here. In addition, Liquid Additive Manufacturing eliminates the high tooling costs, which results in great price savings.”
“[This] is also very interesting for the development of new customer groups, since the process, even without an investment in tools or moulds, makes lot sizes of n = 1 possible or economical. That makes it very interesting for the bigger industries and I’m sure that this will lead to new possibilities for the additive manufacturing world.”
As far as their LSR material is concerned, unlike other manufacturers that have developed materials that would simulate silicone although it was never truly the same, Wietfeld lays emphasis on the fact German RepRap’s material is a real silicone, which is, as per her words, “thermally cross-linking”.
“The material does not contain any acrylic hardener, does not become UV-crosslinked and is almost identical to injection molding in all properties. Especially for products that require a high degree of fineness, the material is a big advantage. Companies already using this silicone can now combine the unique benefits of already used silicone with faster prototyping and small batch production of highly complex parts. The variation of different infill options of the same material, results in a wide range of possibilities, for example, for different damping properties. Depending on the mixing ratio, flexible products or even rigid printing results can be produced. This brings a decisive new possibility – away from the pure prototype process, individual parts can now be produced, in unit number 1, but with the technical requirements of injection molding quality” explains GermanRepRap’s spokesperson.
The quoted savings and property comparisons depend on the application. Mixing ratios should follow the qualified formulation and supplier instructions; changing the ratio arbitrarily is not a general method for obtaining a reliable flexible or rigid part.
Wietfeld also saw opportunities in construction and electronics. These were development prospects discussed in 2019.
ACEO® Silicone 3D Printing technology
Processing viscous silicone into controlled droplets was one of the challenges addressed by Wacker’s ACEO development.
Wacker presented ACEO at the K trade fair in 2016. Its 2016 account describes drop-on-demand deposition and layerwise UV curing. The approach changes the silicone through cross-linking; it should not be described as a process without material transformation.
“The ACEO® technology is based on a drop-on-demand principle. The print head deposits single silicone voxels on a building platform. These voxels merge smoothly into a homogeneous surface. After each layer, the curing process is activated by UV light. (…)
Due to the high viscosity and low surface energy of silicones, the formation of droplets requires a dosing valve capable to shear and eject the material with high frequency. With this method, there is no contact between the nozzle of the valve and the 3D-printed object and consequently no string of material between nozzle and object. Equipped with several valves, the ACEO® printers can handle more than one silicone at the same time – a process we call multi-material 3D printing.
A three-dimensional object is printed layer by layer with the use of support material to create complex structures, overhangs or cavities”, declares Egbert Klaassen.
At the time of the interview, ACEO reported elastomers spanning 20 to 60 Shore A, with colour and translucent options. This is the historical range described for that service.
Applications and limitations
In 2019, the interviewees described both practical uses and further applications under development.
German RepRap pointed to established silicone uses in construction, such as sealants, adhesives and coatings, and in electronics protection. These examples illustrate potential material needs; they do not mean that every such product was already being manufactured by 3D printing.
Spectroplast saw opportunities for customised healthcare products and patient-specific devices, including tracheal stents and aortic heart valves. These were application ambitions, not evidence that the named devices had been clinically validated or authorised.
ACEO reported a resolution of about 0.4 mm and identified viscosity as a challenge for improvement. This figure refers to the company’s description in 2019; it is not a universal specification for silicone printing.
For any application, the material data, curing and post-processing requirements should be checked with the equipment or service supplier. Availability of a printable silicone does not establish the performance or qualification of the final component.
This feature was originally published in the October–November 2019 issue of 3D ADEPT MAG.
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