BFFT Techblog: 3D printing combination of flexible and rigid filaments
Subject: Combining different materials (filaments) to print an object with new characteristics. – How can 3D printing materials with different degrees of flexibility be used to present new application scenarios for fully printed parts?
Initial situation: This trial is based on a 3D printer which produced parts made of thermoplastic materials using the FDM1 method. The printer which was used has two printing heads, allowing it to be fitted with two different starting materials.
This configuration is often used either to print parts in more than one color or to use soluble support materials which can be removed with a solvent from the actual printed object after printing.
The figure on the left shows 3D printed parts manufactured in two colors. The figure on the right shows the use of a support material. Support material makes it possible to print overhanging features.
The filaments commonly used in 3D printing are ABS4 and PLA5. These thermoplastic plastics are used for dimensionally stable objects. They allow for neat, accurate printing and are simple to handle. PLA is a relatively brittle plastic. Under mechanical load, it warps little before breaking. ABS is somewhat more ductile in this respect. It is more elastic and warps more than PLA. However, it also breaks under excess strain.
Flexible 3D printing filaments are rubber-like thermoplastic materials with significantly higher elasticity and much higher strain rates than “standard” filaments. This type of filament makes the connection between these individual layers of a printed part (layer adhesion) far more stable, and the objects also have a much higher mechanical load bearing capacity. For example, upon impact, the object would initially warp without breaking.
The combination of rigid and flexible materials allows for the targeted application of the positive characteristics of the original filaments in critical locations on the printed object.
One example of this is the design of a housing in which the surface consists of flexible filaments, and the core consists of rigid ones.
A housing printed entirely of flexible material would have a higher mechanical load bearing capacity and would be practically impossible to destroy without using a tool. But it is precisely due to this elasticity that such a part would not be suitable for use as a housing. It is not dimensionally stable and will warp under the slightest pressure.
A housing printed entirely of a rigid material is dimensionally stable but can break under mechanical forces, e.g. if it falls on the floor.
If the surface consists of the flexible material and a rigid material is used on the interior, we achieve a dimensional stability which is held together and protected from blows by the flexible material from the outside.
Figure 2: 3D- prinitng model of the test object
In this trial, ABS was selected as the rigid material, since it has a melting point close to that of the flexible material. This should ensure the best possible joining of the two materials. The trial uses a housing shell with a longitudinal length of 40 mm, a height of 15 mm and a wall thickness of 4 mm.
Figure 3: Flexible and rigid housing shell
To provide a comparison, the test object was printed in both starting materials. The print settings are identical except for the temperatures. The ABS part is printed at a nozzle temperature of 245° C and a bed temperature of 105° C. The flexible part requires a nozzle temperature of 220° C and a bed temperature of 70° C. There is no significant difference in the outward appearance of the two parts.
Figure 4: Flexible and rigid housing shell 2
The flexible housing shell can be temporarily warped with little effort without causing any lasting damage to the structure. The housing shell made of the ABS material cannot be visibly warped at all by hand. In addition, the outer walls of the flexible housing shell can be pressed in. This is also not possible with the ABS part.
Figure 5: Warping test with teh fexible and rigig housing shells
Depending on the Slicer6-software used, combining the two materials may require adjustments to the basic design in order to create one 3D model for the casing and one for the core. In this trial, we used the Simplify3D-Slicer7 .
This software supports the selection of different extruders for different features of the 3D model. This way, the perimeters (shell of the model) can be done using one extruder, while the infill (core of the model) is done using the other. The exact settings are shown in the figure below.
Figure 6: Configurations Simplify3D-Slicer
When setting the printbed temperature, a compromise has to be made. Since the trial uses ABS as a second material, the bed is kept at 105° in order to minimize the risk of warping during the printing process.
Figure 7: Created code (Blue coloration: flexible material, green coloration: rigid material)
Again, there is no major difference from the other housing shells in the outward appearance. Under load, the part warps without breaking. When the object is severely warped, we feel the material on the inside breaking. After being subjected to stress, the part again takes on its original form, and the structure does not appear visibly damaged. There are no subjectively noticeable effects on the stability or flexibility of the wall.
Figure 8: Overview all types of materials & warping-test with the combined housing shell
In the next test, pliers are used to inflict damage. The wall of the flexible part can be pressed in all the way. After removing the pliers, the wall resumes its original form, and only the indentations of the pliers remain. The ABS part is noticeably warped and stays that way permanently. The combination of ABS and flexible material retains only the indentation of the pliers and a slight bulge on the top surface.
Figure 9: Warping-test with tools
In the next test, a blunt force impact is used to test the limits of the parts. Again, the flexible printed part fully resumes its original form, the ABS part breaks and warps permanently.
The combination of ABS and flexible material again shows significant advantages. The blow is cushioned, and the housing shell does not break. The part is left with only minimal warping. Here, too, no subjective change in stability or flexibility is detected as compared to the undamaged part.
Figure 10: Blunt force impact is used to test the limits of the parts
The trial has shown that it is possible to use flexible and rigid material in a 3D printed part. If the parameters and part zones of the printed object are properly selected according to the materials, it is possible to print structures which combine the characteristics of both starting materials. This enables us to print objects which are more stable and more shock-resistant than was possible with the original materials.
The combination of rigidity and flexibility is also of interest for other application scenarios. For example, hinges or seals can be embedded directly in the printed parts
Author: Alexander K. (Entwicklung Konzepte & Tooling)
Picture sources: Alexander K.