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DTU and TU Delft: Stress Adapted Orthotropic Infill for 3D Printing

A team of researchers at the Technical University of Denmark (DTU) and Delft University of Technology (TU Delft) teamed up recently to improve functionality with infill in orthotropic materials as well as studying how they could further optimize performance and overall quality in such objects. Authors Jeroen P.Groen, JunWu, and Ole Sigmund detail their findings further in ‘Homogenization-based stiffness optimization and projection of 2D coated structures with orthotropic infill.’

Overview of the proposed methodology to obtain high-resolution coated designs, with composite orthotropic infill.

Emphasizing better ways to produce coated structures with regular infill, improve quality, and still enjoy better affordability in production, the research team outlines their new method for generating stress adapted orthotropic infill for 3D printing. While pointing out that methods like FDM 3D printing, known for manufacturing solid structures, offer consistently stable structures, the researchers state that optimization of complex geometries is an ideal challenge that technology like 3D printing and additive manufacturing should be able to overcome when using infill too. They begin by discussing homogenization-based topology optimization for coated structures, and then the second half of their paper explains their method for creating high-resolution objects on fine mesh.

Coating is the first topic at hand, although the researchers point that the technique to discern between both infill and coating is almost identical.

“The procedure to distinguish between coating and infill makes use of two well-established filter methods in topology optimization,” state the researchers. “The first is a smoothing operation using the density filter. The second is a projection step to force the smoothed values on the interval towards either 0 or 1.”

The team noted that in using a single smoothing and projection (SSP) technique, the structures tended to have better compliance, but they found voids within the structure in areas not covered by coating. With double smoothing and projection (DSP), they found that there was almost a 90 percent reduction rate of vanishing coating—in comparison to SSP. And although further testing would be required, the researchers also theorize that ‘vanishing coatings’ could be prevented if image processing was applied after homogenization-based optimization and enforced coating. Overall, they found DSP to ensure ‘clear distinction’ between the coating, infill, and any voids.

As they began mapping coated designs in the second half of their study, they explained how their innovative method refining periodicity allows for regular infill spacing.

Example of the mapping procedure.

“Numerical experiments demonstrate that the projected designs, despite a lack of separation of scales, are very close (within 1%–2%) to the homogenization-based performance,” stated the researchers.

Such optimization of infill also produced designs with finer resolution and higher performance—all with a computational cost they project to be ten times lower, or more. This approach also results in 31% stiffness improvement (or similar weight reduction) when dropping conventional isotropic infill in favor of orthotropic stress adapted infill.

“This overall promising approach allows for extension of the method to 3D or to more complex loading situations. The main challenge here will lie in finding a parameterization that allows for smoothly varying microstructures through the domain,” concluded the research team. “We are confident that such a parameterization can and will be found.”

Infill can be a critical component in the stability and overall success of a 3D printed structure. Many users tend to overlook or struggle with optimizing this variable, but different customization techniques can lead to improved functionality in experimenting with new 3D printing materials, seeking properties like better tensile strength, and enhancing innovation overall.

Find out more about infill and orthotropic materials 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.

Visual explanation of adaptive periodicity and required transition zone.

[Source / Images: ‘Homogenization-based stiffness optimization and projection of 2D coated structures with orthotropic infill‘]

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3D Printing News CrAMmed, University of Maryland, SUTD, TU Delft, RWTH Aachen University

Welcome to CrAMmed, the first edition of our 3D printing digest based on the latest academic additive manufacturing research. Today, CrAMmed details the latest applications for 3D printed molds from the Ming Chi University of Technology; 3D printed microfluidic circuitry for medical devices and pharmaceuticals from the University of Maryland; as well as 3D printed […]
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3D Printing Industry review of the year October 2018

For the 3D printing industry, October 2018 was a month of progress and milestones. This month brought an extinct species back to life, and Apple’s 3D printer closer to birth. Flying higher with 3D printing October was a big month for GE Additive. The company announced a landmark event in the history of aviation and […]
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Researchers Decrease Support Structures for Models Through Multidirectional 3D Printing

An illustration for the idea of the algorithm: (a) a progressively determined planar clipping results for generating the optimized base planes, and (b) the inverse order of clipping planes results in a sequence of regions to be fabricated where the printing direction of each region is the normal of its base plane. The orientation of a printing head is fixed during the procedure of physical fabrication. The parts under fabrication are reoriented to realize the multidirectional 3D printing.

In most planar-layer based 3D printing systems, material collapse is prevented on large overhangs by adding support structures to the bottom. But support structures in single-material 3D printing methods have some major issues, like material waste and the possibility of surface damage. This can be helped by introducing rotation and turning the hardware into a multidirectional system, where models are subdivided into separate regions and each one is 3D printed along a different direction.

L-R: Snowman models fabricated by an FDM 3D printer and the team’s multidirectional 3D printing system by adding only one rotational axis on the same 3D printer.

A team of researchers from Tsinghua University, TU Delft, and the Chinese University of Hong Kong developed two types of multidirectional 3D printing hardware systems: one modified from an off-the-shelf FDM 3D printer with an added rotational degree-of-freedom (DOF), and the other implemented on an industrial robotic arm to simulate a tilting table for two rotational DOFs. They outlined their work in a paper titled “General Support-Effective Decomposition for Multi-Directional 3D Printing.”

The abstract reads, “We present a method to fabricate general models by multi-directional 3D printing systems, in which different regions of a model are printed along different directions. The core of our method is a support-effective volume decomposition algorithm that targets on minimizing the usage of support-structures for the regions with large overhang. Optimal volume decomposition represented by a sequence of clipping planes is determined by a beam-guided searching algorithm according to manufacturing constraints. Different from existing approaches that need manually assemble 3D printed components into a final model, regions decomposed by our algorithm can be automatically fabricated in a collision-free way on a multi-directional 3D printing system. Our approach is general and can be applied to models with loops and handles. For those models that cannot completely eliminate support for large overhang, an algorithm is developed to generate special supporting structures for multi-directional 3D printing. We developed two different hardware systems to physically verify the effectiveness of our method: a Cartesian-motion based system and an angular-motion based system. A variety of 3D models have been successfully fabricated on these systems.”

The researchers wanted to create a 3D printing system that would be able to “add rotational motion into the material accumulation process” to ensure fewer supports, if any. To do so, they created a general volume decomposition algorithm, which “can be generally applied to models with different shape and topology.”

“Moreover, a support generation algorithm has been developed for multidirectional 3D printing,” the researchers explained. “The techniques developed here can speedup the manufacturing of 3D printed freeform models by saving the time of producing and removing supports.”

Progressive results of fabricating models on 4DOF multidirectional 3D printing system and a 5DOF system realized on a robotic arm.

The research team’s paper made several technical contributions, including their support-effective algorithm, which is based on beam-guided search and can be applied to 3D models with handles and loops. In addition, they also summarized decomposition criteria through their multidirectional 3D printing process and created “a region-projection based method” for generating supports for multidirectional 3D printing.

There are, however, some drawbacks involved when changing from one 3D printing direction to another, such as slowing down the process, which is why the researchers “prefer a solution with less number of components, which can be achieve by considering the following criterion of clipping.”

A comparison of decomposition results obtained from three schemes introduced in this paper.

“After relaxing the hard-constraint of support-free into minimizing the area of risky faces as described in JG, the scheme of generating support is considerately vital while both feasibility and reliability should be guaranteed,” the researchers wrote. “To tackle this problem, we propose a new pattern called projected supports that ensures the fabrication of remained overhanging regions through a collision-free multi-directional 3D printing.”

The decomposed and 3D printed results fabricated by the system with 4DOF and 5DOF in motion.

The team applied their algorithm to several models, and were able to reduce, and even eliminate in some cases, the need for support structures. In addition, their method’s “computational efficiency” was on par with general 3D printing time.

“We present a volume decomposition framework for the support-effective fabrication of general models by multidirectional 3D printing,” the researchers concluded. “A beam-guided search is conducted in our approach to avoid local optimum when computing decomposition. Different from prior work relying on a skeletal tree structure, our approach is general and can handle models with multiple loops and handles. Moreover, a support generation scheme has been developed in our framework to enable the fabrication of all models. Manufacturing constrains such as the number of rotational axes can be incorporated during the orientation sampling process. As a result, our algorithm supports both the 4DOF and the 5DOF systems. A variety of models have been tested on our approach as examples. Hareware setups have been developed to take the physical experiments for verifying the effectiveness of our system.”

Co-authors of the paper are Chenming Wu, Chengkai Dai, Guoxin Fang, Yong-Jin Liu, and Charlie C.L. Wang.

Discuss this research and other 3D printing topics at 3DPrintBoard.com or share your thoughts below.

 

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Technical Possibilities for Making 3D Printed Engineering Components Based on Reused Polypropylene

Tested specimens for the three print directions.

From bacteria and metamaterials to shape-shifting and support-free, the innovative researchers at TU Delft have worked with a wide variety of 3D printing materials over the years. Now, their focus is shifting to polypropylene, a thermoplastic polymer used in a variety of applications, though engineering is not typically among these.

TU Delft researchers Fred Veer, Foteini Setaki, and Ton Riemslag, together with P. Sakkas from The New Raw, have published a new paper, titled “The strength and ductility of glass fibre reinforced 3D-printed polypropylene,” that discusses the technical possibilities for making 3D printed engineering components based on reused polypropylene.

The abstract reads, “The possibility of using a mix of recycled polypropylene (PP) with new glass fibre reinforced polypropylene as a materials source for 3D printed engineering components is investigated. The strength and elongation to fracture are determined for various grades of material and in relation to the print direction. The measured values are compared with literature values for these materials in an as new condition. It is shown that the use of recycled PP degrades the material properties. PP recycled from house hold waste has significantly worse properties than PP recycled from industrial waste.”

Test setup: Zwick z10 universal testing machine with Test Expert 4.12 software.

A lot of primary material is used to create disposable molds out of virgin plastic for construction purposes, which is not great news from an environmental standpoint. It’s far more practical to use recycled plastics as raw materials, and research has been conducted in the past regarding the use of recycled high density polyethylene. But PP has better mechanical properties, and has the correct thermal properties for 3D printing.

Unfortunately, recycled PP is far less strong than unused PP. In order to achieve the desired properties, recycled PP is often mixed with the virgin material and fibers.

“For this research different mixtures of recycled, re-recycled and virgin polypropylene with short glass fibres were tested to look at the various factors influencing the overall properties,” the team wrote. “This research focussed on the failure strength and strain of the material as these are good indicators for materials performance and are also suitable to compare the different mixtures.”

The researchers blended mixtures of recycled, re-recycled and virgin PP with short glass fibers, then inserted the material into a heated extruder with four chambers to be 3D printed into sheets. The sheets were then laser cut into dog bone-shaped specimens and tested using a Zwick z10 universal testing machine.

“For mixtures 1 and 2 the properties were determined in the print direction, 0°, at 45° to the print direction and at 90° to the print direction,” the researchers explained. “Mixtures 3, 4 and 5 were only tested in the 0° direction in order to allow comparison between the mixtures.”

The results show clearly that the predictability of the strength of a material mixture was degraded by the use of recycling, unfortunately. In addition, it’s implied that the print direction has to be taken into account in any design, and that the structure must be modeled using direction dependent properties. Because we’re dealing with composite materials, the researchers explained that “the engineering effort will be much greater than with conventional materials.”

Test specimen; dimensions are in mm.

Another important factor to take into account is the quality of the recycled material: the average of mixture 1’s strength was only about 85% of the average strength of the virgin 10% glass fiber-filled PP homo polymer. There’s also a major decrease in properties – a 35% loss of strength – when a print was recycled. Properties were also significantly degraded when household waste was used as a recycled PP source, as opposed to industrial waste.

“Adding more glass fibres and using less recycled polypropylene gives a mixture that more clearly approaches that of virgin material. An eco-friendly design using large amounts of recycled material will thus always have significantly decreased properties, leading to the use of more material,” the researchers concluded. “In itself this does not have to be a problem, using a larger amount of waste material also means less waste to burn. It is, however, also clear that reusing the material more than once leads to more significant loss of properties as is evident from the loss of properties of mixtures 3 and 4 compared with mixtures 1 and 2. Using recycled polypropylene for products with a short service life is thus counterproductive as it produces unusable waste which can only be burned, as it will not biologically degrade in a land fill. It is thus important to use recycled polypropylene in such a way that a sufficiently long life time is achieved with a clear route for final disposal at the end.”

The team also stated that there appeared to be no clear relation between strain at fracture and failure stress, and determined that properties at 45° or 90° to the print direction are much lower than in the print direction.

Discuss this research and other 3D printing topics at 3DPrintBoard.com or share your thoughts below. 

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Watch: TU Delft’s morphing chaise lounge proves applications for 4D printed furniture

Exploring the capabilities of 4D printing, members of the Robotic Building (RB) research group at Delft University of Technology (TU Delft) in the Netherlands, have created a 3D printed chaise lounge that can swiftly transform into a bed. Project: Adaptive Stiffness Last year, Henriette Bier, TU Delft Associate Professor, and RB Group Leader and Arwin […]
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TU Delft Researchers Develop Heat Accumulation Detection Procedure for SLM 3D Printing

Selective Laser Melting (SLM), a powder-based 3D printing technique also known as Laser Beam Melting or Laser Powder Bed Fusion, has been used to process metal in a variety of sectors, such as automotive, medical, and aerospace. Because this AM method offers excellent freedom of form, it’s a perfect enabling technology for designs that are topology optimized; this means they have a complex layout, but still offer a superior performance. But, SLM 3D printers don’t always realize the dimensional accuracies that are necessary for very precise components.

Because of laser-induced heat, SLM 3D printed layers go through stages of rapid heating-cooling, which can cause inaccuracies, such as unwanted mechanical properties and poor surface finish. If certain design features, like thin sections and overhangs, that can cause local heat accumulation could be detected earlier in the design stage, this issue could be avoided more easily. To do this, next generation topology optimization (TO) methods need to be developed.

A group of researchers from TU Delft recently published a paper, titled “Towards Design for Precision Additive Manufacturing: A Simplified Approach for Detecting Heat Accumulation,” focused on a simper heat accumulation detection procedure – very important for creating a TO scheme that can account for thermal 3D printing aspects.

“In order to address thermal aspects of AM into a TO framework, an appropriate AM process model is required. This becomes problematic because a high fidelity AM process model is computationally very expensive and integrating it within a gradient-based TO framework becomes even more cumbersome,” the researchers explained in the paper. “Therefore, in this research, a physics based yet highly simplified approach is proposed in order to identify zones of heat accumulation in a given design. The computational gain offered by the simplification, makes it feasible to integrate the heat accumulation detection scheme within a TO framework.”

Definition of overlapping cells for heat accumulation detection.

In addition to being used in a TO process, the team’s new procedure can also be used to independently analyze 3D printing designs, manual design improvements, and even determine the best build orientation.

Equivalence of a 3D body (A) to a simplified body (B) with equal thermal capacitance
and conductance.

Two simplifications made in this research can be used to help lower the computational cost that’s associated with the thermal analysis of 3D printable designs. The first, “motivated by the fact that the local geometry of only few previously molten layers” can significantly effect the new layer’s initial cooling rate, is to perform thermal analysis in the vicinity of the 3D printed layer being deposited.

The second is to use a steady, rather than transient, state thermal response to predict heat accumulation.

“For this purpose, a physics based conceptual understanding is developed which enables estimation of spatially averaged transient thermal behavior of a local geometry just from its steady state response,” the researchers wrote.

A structure’s topology can influence its internal heat flow; as such, different geometrical features in an AM design can obstruct or facilitate heat flow during the 3D printing process differently.

The researchers explained, “In this work we explore the possibility to approximately quantify, and hence compare, different geometries from the viewpoint of heat accumulation. For this purpose, first the concepts of thermal conductance and time constants are studied.”

Time constant maps obtained by the heat accumulation scheme using the concept of
overlapping cells.

Thermal conductance, equivalent to the reciprocal of thermal resistance, is a structure’s measure to conduct heat, while the time constant of the transient thermal response is studied to quantify the heating/cooling rate. The team also divided the design in their experiment into overlapping cells, so as to increase the possibility of detecting the heat accumulation zones.

High time constants were recorded close to overhang surfaces, and so the researchers discovered that heat accumulation for design features depends a lot on the nearby local geometry, and that “purely geometric design guidelines of prescribing a limiting overhang value might become insufficient for preventing problems associated with local heat accumulation.”

“The computational advantage offered by the proposed method enables development of a physics based topology optimization method which would be beneficial for designing precision AM components,” the researchers concluded. “Next step for this research is to combine the developed method with density based topology optimization by penalizing design features which are prone to heat accumulation during each iteration.”

Co-authors of the paper are Rajit Ranjan, Can Ayas, Matthijs Langelaar, and Fred van Keulen. The team will publish an additional paper on their heat accumulation detection method’s integration within a TO framework.

Discuss this research and other 3D printing topics at 3DPrintBoard.com or share your thoughts in the comments below.