London: Researchers Use Drop-on-Demand Method to 3D Print Latex & Rubber

Researchers at Queen Mary University of London are exploring a new drop-on-demand (DoD) method with latex, and rubber. They explain their work in ‘Additive Manufacturing with Liquid Latex and Recycled End-of-Life Rubber,’ detailing how they have been able to create a new method to overcome some of the challenges in compatibility between materials and inkjet systems.

The authors, Miguel A. Quetzeri-Santiago, Clara L. Hedegaard, and J. Rafael Castrejón-Pita are aware that while many companies have been interesting in using elastomers and rubbers in additive manufacturing, the end product has often been inferior to those made through traditional processes. Liquid elastomers have also been considered as an option, but viscosity restrictions prohibited their use, or success in fabrication. Clogging and agglomeration have also been stumbling points.

With drop-on-demand inkjet printing, the researchers considered a way to bypass previous challenges and take advantage of printing with droplets that can be printed close to each other, subsequently coalescing and creating strong layers. The process can be driven by different types of pressure pulsing that cause the droplets to be ejected from the nozzle. Advantages of this process include:

• Lack of substrate contact, preventing nozzle contamination
• Higher printing speeds
• Greater control
• Variability in droplet volume and speed
• Flexibility in production and options for customization
• Potential for recycling rubber

Two printheads, one small and one large, were created in the lab, with pulse generators driving the actuator—and each pulse resulting in one drop:

“The small printhead uses a 20 mm diameter loudspeaker (8 Ohms, 0.1 W), has an inner liquid reservoir volume of 4 mL, and a conical nozzle with an outer diameter of 1.0 mm,” stated the researchers. “The larger printhead uses a Visaton Structure-Borne Driver loudspeaker (8 Ohms, 25 W) with a 9 mL volume reservoir and a 0.85 mm conical nozzle.”

The inks used in this research were as follows:

• Pure liquid latex – from Liquid Latex Direct (UK), containing 60 percent natural rubber, 40 percent water, and less than .3 percent ammonia.
• Liquid latex – also from Liquid Latex Direct, containing 60 percent natural rubber, 40 percent water, and less than three percent ammonia.

“While being able to inkjet undiluted liquid latex is an interesting prospect in itself, the ability to add solid particles to form a colloidal ink widens the market applications for this technique,” state the authors. “The addition of particles can be used to reinforce the positive mechanical properties or improve other properties such as thermal and electric conductivity, stiffness, or elasticity of a given construct. Moreover, this includes the possibility of reusing discarded rubber materials, in the form of micronized rubber powder (MRP), in the manufacturing of new products.”

For 3D printing, the research team loaded each printhead, controlling backpressure and monitoring pulse duration and intensity. Each droplet was meant to coalesce into a form, building with layers. They used two different methods for curing, via ambient air and hot air. Continuous printing of the pure liquid latex was noted as ‘consistent and reliable’ in timeframes of up to one hour.

Printing with liquid latex. (a) The experimental setup showing the Grbl controlled stage and the printhead mounting (the 9 mL reservoir printhead is shown); (b) an example of a single layer structure made from pure liquid latex, using a droplet interval of 2.5 s with two close-ups of the corner resolution; (c) varying the droplet interval keeping the pulse signal and nozzle diameter constant: from left to right increasing the interval length from 1.0 to 5.5 s (inserts show bird-eye perspective). All scale bars 1 mm.

In adding parlon powder or micronized rubber powder (MRP), the authors discovered that both materials had a tendency to clump together in the nozzle, and in printing with MRP, ‘solid tire rubber could be clearly visualized in most of the droplets.’ The researchers theorize that a better mixing procedure and more vibration of the printhead reservoir could improve these issues. They also noticed that MRP particles combined with latex decrease the amount of elasticity but do not have any impact on stiffness at all. Overall, their work in 3D printing high solid content latex was successful though, and the researchers stated that this study offers ‘new possibilities’ for recycling tire waste.

“The capability of printing with a high particle loading (high solid content latex with the addition of parlon powder or MRP) and a heterogeneous particle size distribution shows that the printhead design can operate in a wide range of solid particle loadings,” concluded the researchers. “This is a great advance, as most conventional inkjet-based 3D printers cannot operate with viscous liquids or liquids with solid particle loading.

“A reliable method of AM with liquid latex would bring great merits to the industry, by reducing cost of manufacturing (no molds needed) and adding an unprecedented degree of flexibility in the manufacturing process. Moreover, the study has highlighted a novel method of recycling end-of-life tires. With this work, it is foreseeable that in the future we can create 3D printed objects with rubber tire waste, expanding the current recycling and waste management methods.”

Liquid latex with rubber particle loading. (a) Time-lapse of the printhead mounted to the x/y stage, jetting pure liquid latex (1 drop/1.5 s) (scale bar 1 mm); microscopy images of liquid latex with (b) 3.5 wt. % and (c) 6.7 wt. % parlon loading; (d) a defined array made by jetting liquid latex containing 6.7 wt. % parlon powder; (e) cast rubber samples of pure liquid latex and with increasing MRP loading (5, 9, and 16 wt. %); (f) a graph of Young’s Modulus, determined using indentation and tensile testing, of cast samples with 5, 9, and 16 wt. % MRP (control; 0 wt. % MRP), and printed samples of one and four layers (1LP and 4LP, respectively) (control thin: a pure latex cast). Data reported as mean ± standard deviation; (g) example of elongation of a one layer printed sample (1LP) under a constant strain tensile test (insert: original sample) and (h) tensile stress and strain at breaking point, derived from the constant tensile strain experiments. Data reported as mean ± standard deviation. MRP, micronized rubber powder.

The science and study of materials plays a huge part in 3D printing, and there has been great interest in exploring rubber also to its flexibility, as well as using it in the tire industry, and creating other materials like TPU to simulate its qualities. Find out more about liquid latex in 3D printing here. What do you think of this news? Let us know your thoughts! Join the discussion of this and other 3D printing topics at 3DPrintBoard.com.