Did you know that you can mix certain resins to not only get a specific color but even get the specific mechanical properties of the 3D-printed object? We’re talking about mixing the standard Prusament Model resins with flexible Flex80 additions. Here’s how it works:

The model resin allows printing of extremely detailed parts, it has very high shore hardness and may break upon stronger impact. On the other hand, the Flex80 resin is soft, flexible, and resists impact, but has a slightly limited ability to print tiny details. Sometimes, you may want to print a model with physical properties somewhere between these two materials. Luckily, the Prusament Flex80 and Model resins can be mixed in any ratio and help you achieve the desired properties. And, in some cases, you can reach the physical properties similar to the thermoplastic materials while maintaining the fine details of MSLA technology.

It’s a bit more complicated than just mixing 50% of each resin to achieve properties right in the middle of those two. And since the change in mechanical properties is not linear, we did some basic testing for you. With our results, you can easily tune the hardness, toughness, and flexibility of your prints to fit the properties of a desired application.

Mechanical properties

First, let’s talk about mechanical properties. The table below shows the results of our testing, but we’d like to explain the whole process of how we got these graphs and numbers. This way you can do the testing yourselves, if you wish.

The testing specimens were printed using MSLA 3D printer Original Prusa SL1S SPEED, dried with a paper towel and post-cured in CW1S for 0, 3, and 60 min. The specimens were then tested using the same methodology as described in TDS available on Prusa3D e-shop product pages. If you don’t know anything at all about the testing methods (tensile strength, Charpy impact resistance…), we’ve covered it briefly in one of our older articles, although it’s not related to the MSLA prints, rather standard filament-based prints.

Tensile curves of mixtures of Model and Flex80 white resins post-cured for 0, 3 and 60 minutes. The dotted lines present mixtures of model resin (M) and flex resin (F) tested as a greenbody (G). The striped lines represent mixtures of model resin (M) and flex resin (F) post-cured for 3 minutes (C3). Finally, the full lines represent the mixtures printed and post-cured for 60 minutes (C60).

Averaged values of mechanical properties of Model and Flex80 resin mixtures post-cured for 0, 3 and 60 minutes

The quite high elastic modulus and tensile strength (max. stress) of uncured Model resin prints are sufficient to withstand high peeling forces during printing without layer delamination or deformation. With post-curing, the material hardens, and the values of elastic modulus, max. stress, and surface hardness (Shore D) increase. On the other hand, flexibility (elongation) and impact resistance (Charpy) are reduced.

The Flex80 resin also hardens via post-curing. Its uncured green body is very soft, with low elastic modulus, strength (max. stress), and surface hardness (Shore D). This makes it vulnerable to the tearing of thin parts even during printing. Post-curing increases the values of all mechanical properties, leading to a relatively soft, flexible material with decent elongation and high impact resistance (above the measuring range of the used testing device).

Mechanical properties of Model and Flex80 resin mixtures post-cured for 0, 3 and 60 minutes.

The mechanical properties of the mixtures correlate non-linearly with the proportions of Model and Flex80 resin – as stated in the beginning, the mechanical behavior of these mixtures doesn’t change in a simple, straight-line way as you adjust the amounts of Model and Flex80 resins. The main transition happens when the mixture contains 25 – 50 % (weight) of the Model resin. The mixture containing 35 % of Model resin has a tensile curve with a yield point at 25 Mpa, elastic modulus 1.1 GPa, max. stress 27 MPa, and ~20 % elongation. These values, with relatively high impact resistance, resemble the properties of common thermoplastic polymers (used in FFF technology).

Now, what does it mean? When you have the basic data, you can tune the mixing ratio to get as close as possible to the required mechanical properties. All the mechanical properties correlate with the mixing ratio, so the resulting mechanical properties lie only somewhere between the properties of pure Model and Flex80 resins. In other words, the increase of elastic modulus and tensile strength (max. stress) is also connected with an increase in surface hardness (Shore D) and a decrease in flexibility (elongation) and impact resistance (Charpy).

Resolution

The Flex80 resin has a slightly lower resolution than the Model resin. To visualize this, we have printed testing objects (the Siemens star) in vertical orientation using Flex80 and Model resins and their mixtures. The center part of the object merges into an ellipse whose dimensions change with the resolution. For example, a wider than taller ellipse means lower resolution in the z-axis direction because the more horizontal the surface is, the more material is added by overcuring in the z-axis direction.

Siemens stars were printed vertically with different mixtures of Model and Flex80 resins, illustrating differences in printing resolution and the lower printing resolution of Flex80 resin in the z-axis direction caused by z-overcuring.

The size of the merged central part of the vertically printed Siemens star decreases with the increasing amount of the Model resin with higher printing resolution. The wider than the taller shape of the merged central part of prints containing a high portion of Flex80 resin is the result of more significant overcuring in the z-axis direction.

Visual appearance

Finally, we printed this Prusament Wardragon using mixtures of Model and Flex80 resins to compare the change in resolution of a complex shape. While the visibility of the details is mainly dependent on resin opacity, which decreases with a decreasing portion of Model resin, the presence of the details on the printed object seems unaffected by the naked eye despite the more significant overcuring of the Flex80 resin in the z-axis direction.

The visual appearance of the printed complex objects using white Model and Flex80 resins and their mixtures.

The visual appearance of the printed details using white Model and Flex80 resins and their mixtures.

Conclusions

When you find Model resin too brittle and insufficiently impact-resistant for your application, you can easily add some Flex80 resin to make it tougher. When you find Flex80 resin too soft, especially during printing as a green body, you may raise the strength a little bit with some Model resin addition. The change in the properties from flexible to rigid (tough) is a non-linear function of the composition. The most noticeable change in mechanical properties occurs in the 25 – 50 % portion of Model resin (and 50-75% of Flex80). We found that a mixture of 35 % Model resin and 65 % Flex80 resin offers a good compromise of mechanical properties (stiff, yet flexible and impact-resistant). This mixture has the mechanical properties of tough printing resin with a yield point at a tensile curve after 60 min of post-curing in CW1S.

Do you like the idea of mixing your resin compositions with specific mechanical properties? And do you already know about any applications? Let us know in the comments.

Happy printing!