Silicone 3D Printing Technology

Today, additive manufacturing can not only be used for prototyping but also has great potential in pre-series manufacturing and serial part manufacturing. For 3D printed components, material properties are critical.

In this field, 3D printed parts can only compete with traditionally manufactured parts if they have the same mechanical and chemical properties. Biocompatibility, heat resistance, high elasticity, and other requirements make silicone indispensable in many applications. Therefore, people look forward to combining the advantages of additive manufacturing with materials such as silicon. However, the additive manufacturing process of silicone is not simple. This article lists various extrusion-based methods and processes, showing in detail the different cross-linking mechanisms and their respective advantages and disadvantages.

Silicone 3D printing: an extrusion-based process

A decisive advantage of the extrusion process is the wide variety of liquids and pastes that can be processed, including almost all types of polysiloxanes: from low viscosity to high viscosity, from RTV (Room Temperature Vulcanization) to standard LSR ( Liquid Silicone Rubber) UV curable and particle-filled silicone. The extrusion manufacturing process is very similar to FLM (Fused Layer Modeling) printing, where the material is extruded through the print head and deposited layer by layer. The two-component LSRs and RTV silicone process will be developed below. LSR requires thermal energy for cross-linking, while RTV silicone can react at room temperature.

△The liquid additive manufacturing (LAM) process is a bit like FDM, which is a material extrusion technology. Each layer is extruded, but it is different from methods such as FFF and FDM because the raw materials will not melt in the process. On the contrary, the LAM material is a liquid when it is deposited, and the liquid is vulcanized by heat exposure so that the material is thermally cured and crosslinked. This process enables the material to maintain its mechanical properties and creates many new applications. LAM 3D printing can make components have almost the same characteristics as injection molded parts, and has high application value in medical, footwear, and flexible materials.

Cross-linked LSR

Silicone printing involves liquid materials. Therefore, the decisive process parameters are the dimensional stability and crosslinking of the silicone. It is also important that the strength of the finished silicone can only be achieved through the chemical reaction of the material, such as through addition polymerization or condensation polymerization. Basically, LSR silicone has a difference between different cross-linking times: cross-linking during production and cross-linking after processing or post-processing.

Cross-linking in the production process:

Heating build platform: The hotbed in the additive manufacturing machine can provide thermal crosslinking energy. Depending on the reaction time of the silicone, crosslinking can occur within a few seconds after being extruded on a heated hotbed.

However, this process has a decisive disadvantage: the temperature distribution changes with the height of the component. This is because the farther the layer is from the build platform, the less heat it provides. This means that it is difficult to achieve a reliable and constant process for multilayer components. Experience has shown that it is no longer feasible to use a hot bed system to manufacture components larger than 2 to 3 cm.

Layer-by-layer cross-linking: Heat input to each component from above can achieve uniform cross-linking of any number of layers. It is directly heated during printing, so the components can be used immediately after printing. This method requires an additional heat source, such as an infrared lamp. Moreover, the intensity and exposure time must be adapted to the corresponding layer, which makes the entire manufacturing process more expensive and complicated.

Cross-linking in post-treatment:

Another method is to use the rheological properties of the material and use silicone that can retain its shape after extrusion. This can be achieved by the high viscosity or high thixotropy of silicone or a combination of both. The finished parts made of LSR silicone are placed in an oven in the post-processing stage and cross-linked at an appropriate temperature. At the same time, the heat input must not cause any deformation of the parts. In this process, there is no need to use additional equipment to provide heat during the printing process.

Support material

For overhanging or bridging (closed profile) structures, supporting structures must be constructed for manufacturing. This support material should adhere to the silicone during the manufacturing process and withstand thermal conditions during the cross-linking process. After that, it must be easily removed from the finished part. However, finding a suitable support material for each type of silicone is a challenge, and the construction of the support structure requires the use of additional materials and time in the manufacturing process.

RTV: made in hydrogel

But there is a process that allows design freedom without the use of supporting structures: manufacturing in different materials. For this purpose, a long needle can be used to extrude the silicone resin into a container containing a "support material". This support material can be, for example, powder or hydrogel. The process is shown in the figure:

△Silicone 3D printing: the manufacturing process inside the hydrogel

In the first step, the silica gel (orange) is extruded into the support material (blue), and the extruded strands are fixed by the surrounding material. After the desired structure is formed and the silicone is cross-linked, the component can be removed from the material with tweezers. Finally, remove or simply wash away the residue of the support material. RTV silicones are particularly suitable for this process because they crosslink in a short period of time (30 to 60 minutes). After removing the components, the support material can be used again.

△DNA structure made of silica gel, manufactured as a 3D printed component in a hydrogel.

The biggest advantage of this method is the high degree of design freedom. It is neither restricted by steep overhangs nor restricted by bridges. For example, the DNA double helix structure in the picture is printed with a 0.3 mm needle. So far, it is difficult to complete the fine connection between the spirals using other processes. However, the disadvantage of this technology is that a large amount of support material is required to manufacture larger components, and it is usually impossible to manufacture LSR materials using this process.

Summary-Silicone 3D printing

Due to the wide variety of extrusion-based manufacturing processes, almost all components can be produced with silica gel. Depending on the required geometry and type of silicone, the appropriate process can be selected. The basis and criterion for a successful manufacturing process is always the precise and repeatable dispensing of silicone. However, the potential of silicone 3D printing is far from exhausted. Especially with the use of new technologies, such as new support materials (such as hydrogels), Antarctic Bear believes that more new designs are still on the way.