Why does viscosity matter when formulating resins?

Why does viscosity matter when formulating resins?

In the dynamic landscape of Resin AM technologies, where performance, accuracy and productivity are paramount, every limitation when formulating materials emerges as a pivotal factor. Viscosity represents the inherent property defining a fluid’s resistance to flow, and it is the macroscopic manifestation of its microscopic composition. Therefore, viscosity assumes a crucial role in determining the outcomes of the final printed objects, shaping their structural integrity and functional performance.

For legacy resin AM technologies, viscosity poses a significant barrier that complicates the printing of their solutions. This difficulty arises from the crucial requirement to maintain viscosity within a specific range to ensure precise printing. Due to the limitations of existing technologies, only low-viscosity resins are viable, thereby becoming a prevalent constraint in legacy resin-based AM processes.

The Pitfalls of Low Viscosity Resins

Low viscosity resins are required in legacy AM photopolymerization methods due to process limitations. As a result, final parts have subpar properties, far from meeting rigorous industrial quality standards and making them unsuitable for end-use applications. There are several reasons why low viscosity resins lead to poor part performance.

Limited ingredient set

Due to the need to maintain viscosity within a certain processability range, the available ingredient set for formulating these resins is seriously restricted. Low-viscosity resin formulations primarily consist of monomers, with only a minimal amount of oligomers, additives, fillers, and modifiers included. Any attempt to incorporate additional components to enhance performance would significantly increase viscosity, rendering the resin unsuitable for processing.

The lack of diverse ingredients limits the resin’s ability to provide the required strength, durability, and resistance to environmental factors. Consequently, low-viscosity resins are unable to achieve the desired balance of mechanical, thermal, and chemical properties necessary for meeting industry standards.

Excessive crosslinking

When it comes to creating durable products through photopolymerization, the resin’s composition at a molecular level plays a crucial role. In this process, monomers (short polymeric chains) and oligomers (long polymeric chains) undergo through covalent bonding at their functional terminations to form a matrix that determines the material’s properties.  

However, the use of low viscosity resins, which are mainly composed of short-chain monomers, can introduce complications. This is because an excess of covalent bonds forms within the matrix, leading to a phenomenon known as over-crosslinking. This over-crosslinking has significant repercussions, affecting the macroscopic behavior of the material. 

In a rigid material, an over-crosslinked matrix leads to the obtention of brittle parts. These parts are highly susceptible to crack propagation, particularly from weakened bonds at functional terminations. As a result, the final products exhibit diminished structural integrity and are more prone to damage or failure under pressure.  

On the other hand, over-crosslinking has a negative impact on elastomeric materials. Elastomers are materials capable of stretching and returning to their original shape after being deformed. However, in an excessively crosslinked matrix, the materials can not properly stretch and deform because of the covalent bonds created during the curing of the resin act as anchor points. As a result, properties such as tear, strength or elongation at break are limited. This means that the material lacks the flexibility and resilience required for applications where deformation and stretching are common, resulting in reduced performance and durability of the final products.  

In summary, the over-crosslinking phenomenon in low viscosity resins leads to the production of brittle parts prone to cracking and inferior elastomeric behavior, ultimately undermining the quality and reliability of the end products. 

Inconsistent curing and VOC Emissions

As a workaround to manage higher viscosities, certain resin-based processes employ heating to reduce resin thickness for easier processing. However, this approach presents several drawbacks. 

Firstly, there is an exponential relationship between temperature and reactivity, which means that the heated printing material experiences a higher ratio of spontaneous polymerization reactions.  As a result, curing kinematics and matrix morphology vary over time, leading to incomplete curing and inconsistent properties among layers of a print job. 

Since the last layers are affected by partial curing during a longer pre-heating period, they may have different mechanical, thermal, or chemical properties compared to the earlier layers. This inconsistency can compromise the structural integrity and overall quality of the final printed object. 

Secondly, heating the resin to lower the viscosity exponentially increases the emission of volatile organic compounds (VOCs). VOCs are a group of chemicals that easily evaporate into the air. When released, VOCs can react with other pollutants to form smog and can have adverse effects on human health and the environment. As a result, facilities using heated low viscosity resins must invest in expensive equipment to mitigate VOC emissions and ensure a safe work environment for employees.  

High viscosity resins with VLM Technology  

VLM’s architecture drastically change the way of processing resins; ending on a system that takes advantage of high viscosity, giving a real tool for empowering manufacturers to create end-use parts that not only function but excel. By leveraging the benefits of high viscosity resins, customers can achieve superior mechanical properties. From automotive components to medical devices, high viscosity resins can pave the way for innovation and reliability in manufacturing. 


Viscous Lithography Manufacturing is a lithography-based AM technique that processes resins from 20 Pa·s to beyond 200 Pa·s. This substantial increase in viscosity translates to superior mechanical properties and performance in the final printed parts. With VLM resins, formulators can extend the available ingredient set, strengthening the polymer matrix, and mitigate material thermal aggression.  

Extended available ingredient set

With high viscosity resins, material scientists gain the flexibility to incorporate a wide range of ingredients into their formulations, all owing to greater customization and optimization of the printed objects.  

By leveraging high viscosity resins, manufacturers can explore various additives, fillers, and modifiers to tailor the properties of the printed objects to their specific requirements. Whether it’s enhancing mechanical strength, improving thermal resistance, or achieving desired surface finishes, the extended ingredient set available with high viscosity resins opens new possibilities for achieving desired characteristics in the final products. 

It is also possible to obtain functional materials that, on top of their inherent physical properties, by adding specific additives or fillers to confer the material a unique high added-value property. Some examples of functional materials are flame retardants, Biocompatibles or ESD-safe. 

Strength polymer matrix

During the photopolymerization process, when monomers and oligomers create bonds to form the matrix, using high viscosity resins makes the structure stronger and more resilient. This happens because the polymer chains in high viscosity resins are longer, allowing for tighter intermolecular and intramolecular interactions, leading to a less crosslinked network with more chain entanglement. As a result, VLM elastomeric materials behave similarly to industrial-grade molded elastomers or vulcanized rubbers, and therefore surpassing any traditional AM photopolymer.​ 


This strengthened polymer matrix translates to tougher and less fragile final pieces, particularly in rigid materials, where increased durability is desired. Moreover, in elastomeric materials, high viscosity resins contribute to improved resistance against tearing, higher energy return values, better compression sets, among other properties. By providing better mechanical performance, high viscosity resins ensure that elastomeric parts maintain their integrity and performance even under stress.

To avoid material thermal aggression

VLM technology with high viscosity resins eliminates the need for heating steps. In fact, VLM technology enables resin processing at room temperature, bypassing the challenges associated with thermal aggression. This absence of thermal aggression ensures consistent properties among layers, resulting in uniformity and predictability in printed objects.

Additionally, the reduction in VOC emissions contributes to a safer work environment and eliminates the need for costly equipment to mitigate health risks. By avoiding the complexities and drawbacks of heating, the use of high viscosity resins with VLM technology offers manufacturers a confident pathway to producing parts with consistent properties and uncompromised performance, ensuring reliability throughout the manufacturing process and in the final products.

Understanding the significance of viscosity in these areas is essential for unlocking the complete potential of VLM technology and attaining superior outcomes in resin AM technologies.