How strong are 3d printed objects

Stefan constantly tests different materials with this method and the results are shown in the graph above. In a video where he tests Siraya Blu resin, Stefan found that Siraya's rigid resin was stronger than some FDM materials such as ABS and ASA, although not as strong as PLA and PETG.

SLA print (left) has the same strength regardless of orientation, but FDM print does not (right) (Source: Formlabs)

Although material properties largely determine the strength of a given print, there are other important factors that can affect mechanical performance.

How a design is printed greatly affects its durability. For example, a certain component can be strengthened by distributing more material in places where stress will be concentrated. The freedom of design afforded by 3D printing means that there are often computational tools such as topology optimization that automate the process.

How a part is made also depends on how well it can withstand external influences. In the context of 3D printing, this is determined by the print settings determined during slicing. It is well known that the height of higher layers greatly affects the strength of FDM parts and this is also true for SLA prints. Most FDM slicers have multiple infill densities and patterns to reduce media and print time, however they are rare in SLA. However, the third-party ChiTuBox slicer includes these features, which provide the ability to increase toughness while optimizing material utilization.

Anisotropy is when a physical property of an object or material has a different value when measured in different directions. This happens in 3D printing when FDM printed objects display different material properties depending on the direction in which the part is assembled. More specifically, this happens when a lower adhesion is created between layers on the Z axis, which causes lower tensile strength compared to the X or Y axis. As you can see in the image above, when a weight is applied to an FDM print that is not oriented or not modeled for anisotropy, it may break. However, when it comes to SLA printing, the opposite is true because the resin is isotropic. During his testing with Siraya's Blu, Stefan confirmed this by experimentally showing that the parts are equally strong regardless of the assembly direction. Formlabs also came to the same conclusion, which is discussed in their article on the isotropic properties of SLA printing.

Orthesa on an ankle joint from a durable resin (Source: Animake)

Given everything that we learned, it becomes clear that when choosing the correct resin, 3D firsts of SLA can make imprints of stubborn like FDM.

To illustrate this point, here is an inspiring example of how resin prints strong, functional, and complex parts in the world of orthopedics.

Matej Vlašić's son Nick from Slovenia was born with cerebral palsy and could not stand or walk without assistance. Mr. Vlašić created special orthopedic ankle orthoses for Nick using SLA printing. About a year later, the final product design allowed Nick to walk for several days.

Like many other orthotic devices, AFOs must be strong enough to provide support while supporting the entire body weight during movement. Mr. Vlašić used Formlabs resin, which proved to be durable and comfortable even over long periods of use. Thanks to this success, Mr. Vlašić founded Animake. He and his team provide diagnostic treatments and 3D printed orthotics to help other children in need of special AFOs.

Source

Tags:

SLA 3D-printing, 3D-printed orthopedic devices, FDM resin, CHITOOX Skoton SLA, ANYCUBIS resin, FormLabs TUUGH. actual product size after 3D printing?

05/10/2018 in 3D Modeling, Expert Blog, Instructions

In this article, we will consider the most common reasons for changing the final dimensions of manufactured products printed on a 3D printer. This article will be helpful for designers and inventors to understand how to design 3D models for 3D printing.

First, let's introduce some terminology. All deviations that occur in the technological process of 3D printing and related technologies, we will call “error”. Consider what types of errors are and delve into each of them.

1. Extrusion width error.
2. Approximation of radii depending on layer height.
3. 3D printer extruder positioning accuracy.
4. Inertial beats.
5. Uncontrolled material shrinkage.
6. First layer error.
7. Porosity.
8. Matching accuracy after printing details, splitting into components.
9. Support material tearing points.
10. Thin wall roughness.

Extrusion width error.

Preparing a 3D model for 3D printing comes down to creating a so-called G-code. G-code is a computer code that sets the movement for all the electric motors of a 3D printer. These motors include those responsible for moving the extruder (the 3D printer's 3D printed head). The movements and movements of the extruder are given by the trajectory, which is determined by the line. In preparation for 3D printing, the model is split along the Z axis into layers, determined by the selected layer height. The trajectory of the extruder is built based on the dimensions of these layers, and in the case of the layer perimeter, it describes the outer surface of the model, averaged along the Z axis. In turn, the extruder repeats the trajectory laid down in the G-code, namely the center of the nozzle. Ultimately, when designing models, it is necessary to take into account the fact that the nozzle itself has a hole diameter through which plastic is extruded. In simple terms, the nozzle radius is added to the size of the 3D model. When choosing a 3D printer, in terms of extrusion width error, the accuracy of the part will be higher on the one with a smaller diameter nozzle. And when designing three-dimensional models for 3D printing, it is necessary to lay a margin for model widening. I want to note that in other 3D printing technologies that use a laser or an illumination device, the width of the outer perimeter line depends on the focus, that is, on the thickness of the beam. As a rule, these parameters can be clarified with the manufacturer of the 3D printer or with Studia3D specialists.

Let's see how it works on the example of a 3D model that has a hole.

The figure shows the 3D model, rendering of the prepared G-code and the trajectory of the center of the extruder nozzle for the 7th layer of 3D printing. Select the perimeters of the holes and overlay the actual extrusion width on the trajectory (Highlighted in light green).

As you can see, the hole diameter is smaller. How much? On the nozzle radius on one side + the nozzle radius on the other. That is, when designing a hole, I need to lay a gap equal to the width of the nozzle. The same goes for all other cars. The smaller the nozzle size, the closer to the nominal dimensions the part will be. At the same time, printing with a nozzle of a smaller diameter will cost more. This is due to performance. Through a larger nozzle, more plastic comes out per unit time, which affects the speed of printing. The operating time of the printer, as well as the amount of material, affects the cost of obtaining the product.

Approximation of radii depending on layer height.

The specified layer height directly affects the accuracy of 3D printing. This is clearly expressed on the radii in the sections of the model along the vertical. Consider the part from the previous paragraph, positioning it on the edge in the 3D printer chamber.

As you can see in the figure, the quality of the hole depends on the selected layer height. The lower the layer height, the better the detail. In this case, it is worth considering the time of 3D printing. As the layer height increases, the print time decreases by reducing the total length of the trajectory described by the extruder. Accordingly, the price of the part is reduced, because. The operating time of a 3D printer directly affects the cost of 3D printing.

3D printer extruder positioning accuracy.

This parameter determines the accuracy of repetition by the center of the extruder nozzle of the trajectory specified in the G-code. In other words, this parameter characterizes the maximum possible deviation of the center of the extruder nozzle from the trajectory during printing. This parameter is specified by the 3D printer manufacturer for a specific printer model. But it should be noted that this parameter corresponds to the manufacturer's recommended 3D printing speed. In turn, Studia3D specialists, when preparing the G-code, reduce the influence of this parameter to a minimum, but do not exclude it at all. In addition to speed, this parameter is affected by the rigidity of the 3D printer and the extruder drive system.

Inertial beats.

As a special case of positioning accuracy, we single out inertial beats that occur during 3D printing as a separate point. This parameter is affected by the same positions that were described in the previous paragraph, however, this error is reduced in a different way. This can be understood by studying the moment at which the influence of this error on the print quality arises - a sharp change in the direction of the extruder's motion vector. When the electric motors, together with the drive system, abruptly change the direction of the trajectory, the print head, which has some characteristic weight, continues to move in the previous direction by inertia. As a result, dynamic damped oscillations occur, which negatively reflects the surface. The influence of this error is reduced in two ways. Decreasing the speed of printing the outer perimeter, which we use when printing all models without exception and is laid at the stage of preparing the G-code. The second method is laid down when designing the model: if possible, it is necessary to add mates to acute-angled faces in the model, which, when the part is positioned on the 3D printer platform, in sections parallel to the XY plane, will give sharp corners in the perimeters. As practice shows, a pairing of 2 mm is considered sufficient. We show with an example.

As you can see in the figure, by rounding the corner (by entering a fillet on the face), we minimized the influence of this error.

Uncontrolled material shrinkage.

This parameter has its effect, as a rule, in the case of 3D printing of large objects. Large - this means more than 30% of the maximum size of the 3D printer camera along any of the coordinate axes. Most often, this error manifests itself in interlayer adhesion and a decrease in dimensions in planes parallel to XY.

Our observations have shown an approximate shrinkage for some of the most common materials in relation to cross-sectional dimensions parallel to the coordinate axes.

ABS: XY ≈ 5%; Z ≈ 1%

PLA: XY ≈ 2%; Z ≈ 0.5%

Polypropylene: XY ≈ 7%; by Z ≈ 10%

First layer error.

The G-code preparation program considers the 3D printer platform to be absolutely parallel to the XY plane, however, in practice, platform calibration also has its own error. In Studia3D, the norm for the difference in thickness of the first layer is:

Δ=0.1 mm from a layer height of 0.2 mm

Δ=0.05 mm from a layer height of up to 0.2 mm

Porosity.

Porosity in the case of 3D printing is usually discussed only in the case of the percentage of filling of internal floors up to 100%. However, when printing with 100% infill, the porosity does not go anywhere.

This can be clearly seen by examining the gap in the workpiece through a microscope to determine the strength characteristics.

If the model is not included in the printable area of ​​the 3D printer, it is split into components. In the best case, grooves are provided for the most accurate assembly. You need to understand that the assembly of the model from the components also affects the accuracy of the overall design. This accuracy is very difficult to assess. We accept deviations from the given model of the order of + - 2%. To fix the joint, we almost never use glue, but use a special chemical solution. Ultimately, the parts are not glued, but soldered, only not by thermal melting of the material, but by chemical. As for the adhesion strength: it is higher than the adhesion between layers. In simple terms, if you throw such a part against a wall, it will break, but first of all not at the joints.

Locations where support material has been torn off.

This item applies to mechanically removed supports. Since the material of the support and the material of the main part consist of the same material, they are simply soldered together under the influence of temperature. When such supports are torn off, traces, threads, chips, etc. remain.

To minimize this error, it is necessary to order 3D printing with soluble supports. Due to the expensive soluble support material, the cost of 3D printing also increases significantly. Accuracy, like beauty, requires sacrifice.

Thin wall roughness.

When preparing a G-code in standard form, the program strives to make your part as strong as possible. This is bad only in one case, when the product has thin-walled elements. When constructing a trajectory, we set the minimum wall thickness inside which the infill will be built. As a rule, this thickness is equal to the 3rd nozzle diameter. But in cases where there are places where the thickness is less than the thickness of the outer wall, when constructing the trajectory, the program will draw one perimeter on each side, and fill the void between them. Due to the fact that filling will occur at a distance of 1 to 2 nozzle diameters, the extruder will begin to vibrate, inertial beat will begin (see point inertial runout ). The more this distance is reduced to 1 nozzle diameter, the faster the near-resonant frequency occurs. All this negatively affects the appearance of this wall, since the vibration is transmitted to the nearby material. Let's look clearly.

To minimize this error, it is necessary to design the part in such a way that the wall thickness is a multiple of the nozzle diameter. Face difference!

Conclusion.

In conclusion, I want to note that when designing three-dimensional models for 3D printing, it is necessary to take into account the totality of all factors affecting the accuracy of products, and not consider any particular one. The Studia3D team works every day to ensure that the 3D printing process is completely predictable and controllable, however, like any kind of production, 3D printing has its own precision. There are no ideal sizes. Therefore, we are very pleased if, in addition to three-dimensional models, a working drawing of the product is provided, where all fits, tolerances, roughness, etc. are indicated. Only in this case we can choose the technology, properly prepare the G-code and guarantee the quality laid down by the designer.

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