Original Research Paper
3D printing of tuneable agglomerates: Strain distribution and effect of internal flaws

https://doi.org/10.1016/j.apt.2020.04.037Get rights and content

Highlights

  • Multi-coloured agglomerates with desired properties were achieved by 3D printing.

  • Evaluated printing accuracy, surface roughness and breaking behaviours.

  • The strain distribution over a random sphere agglomerate was plotted.

  • Effect of controlled internal flaws was studied on agglomerate breakage.

Abstract

The current study presents a novel and reliable method for producing 3D printed agglomerates with different colour distributions and material properties with 2-fold aims: providing feasible and accurate control on compression of agglomerates under different compression angles, and better tracking of individual particle position after agglomerate breakage. Multi-coloured agglomerates in cubic tetrahedral and random sphere shapes were printed with both rigid and soft bonds. The printed agglomerates were analysed thoroughly of their surface and structural properties including surface roughness and printing accuracy. The agglomerate breakage behaviours under static compression were analysed as a function of bond strength, loading rate and loading directions, with strain distribution plotted over the random sphere agglomerate structure. In addition, agglomerate structures with designed internal macro-voids in different positions and sizes were also created for breakage study, in an effort to better understand parameters governing the mechanical properties of agglomerates with cavities and voids which is inevitable in particle industry but poorly understood at present.

Introduction

Agglomerate breakage study is of great importance to particle processing optimisation, such as milling [1] and granulation [2]. Understanding agglomerate breakage mechanism can have an important role in powder process, storage and transportation [3], [4], [5], [6], [7], [8], [9]. There have been several extensive reviews on this topic published previously. Different influencing factors have been studied of their effects on agglomerate breakage such as impact velocity [10], [11], [12], [13]. Zheng et al. numerically studied the breaking behaviour of non-spherical agglomerates and found that higher impact velocity results in higher extent of agglomerate damage and breakage [12]. Increasing the impact velocity on a spherical crystalline agglomerates results in shorter impact events, more broken bonds and higher maximum wall force [13]. Another important factor with equal significance is impact angle [8], [14]. Oblique impact on walls has been most often used for analysing the effect of impact angle, of which the angle is defined as the angle between target wall surface and incident velocity (gravity direction). It has been demonstrated that impact angle can affect the position of clusters produced and thus influence agglomerate breakage pattern. The outcome is a complex interplay of speed and angle of impact, as shown by Moreno et al. [8].

Other structural parameters such as particle size and packing density have also been studied. Through microstructure and crush tests of ceramic agglomerates, Balakrishnan et al.[15] found that nanoparticle agglomerates showed much higher strength than micro-sized ones and the stress–strain curve transformed from brittle to plastic when particle size decreased below 50 nm. Different deformation behaviours of agglomerates consisting of varying primary particle size distributions were also studied [16], with the results suggesting that larger forces were required for breakage of homogeneous agglomerates, i.e. consisting of mono-sized particles. Particle packing density (dense or loose) can affect the agglomerate failure mode [17], [18], with meridian plane failure clearly shown on densely–packed agglomerates while progressive failure from the contact region is observed on loose structures.

The discrete element method (DEM), which has been widely used for numerical simulation of discrete matter, is an important tool in studying agglomerate breakage mechanisms by describing the inter-particulate contact behaviours [16], [19], [20], [21], [22]. The majority of agglomerate breakage studies to date focus on modelling various applications using numerical method of DEM. However, successful simulation or prediction can only be made when the input parameters are carefully selected and calibrated. Calibration methods through direct measurement of particle and bond/contact properties, or by performing experiments to measure material bulk properties, or a combination of both are necessary [23]. For the direct measurement method, the difficulty lies in accurate representation of real particle shapes, since most real life agglomerates exhibit irregular particle geometries [24]. For the experimental method, however, the unique orientation/structure of individual agglomerate makes it extremely difficult to replicate samples for agglomerate breakage study, and the destructive nature of such experiments results in unrepeatable data, which cause large variations in the experimental results [18]. It is therefore necessary to introduce a reliable calibration method that could provide accurate agglomerate shape reproduction and on-demand structure control.

Despite of the accumulated knowledge of relationship between agglomerate structure and strength, understanding agglomerates with special features such as internal voids or flaws remains a field of interest. Since such structures are commonly found in powder processing techniques such as spray drying. Eckhard et al. [25] simulated real spray dried granule structure with internal voids and correlated granule structure with fracture strength. Subero and Ghadiri [26] made agglomerates using glass ballotini as primary particles bonded together by bisphenol-based epoxy resin. In order to explore the effect of macro-voids agglomerate structure on agglomerate impact strength, the agglomerates were made with different levels of porosity by creating different numbers of the macro-voids. The particles were impacted at different impact velocities and angles. The shapes of the fragments were observed to elucidate the fracture patterns. They reported different patterns of breakage for agglomerate impact breakage obtained in their work, such as localised damage, fragmentation, multiple fragmentations with localised damage and disintegration. Although their work provides a novel method to create agglomerates with relatively well-defined structure, in order to study the effects of structure on agglomerate breakage, it is desirable to produce multiple “identical” test agglomerates with “well-controlled” structures with more accuracy, and then study their breakage behaviour in detail with the aid of mathematical models and experimental instruments.

3D printing has been introduced as an experimental tool for accurate realisation of agglomerate design [18], [27]. Polyjet 3D printing technique provides very high printing resolution of 42 µm XY directions, 16 µm Z direction (single material) and 30 µm Z direction (multi-material). The ability to print multiple materials with different mechanical properties and colours has also been taken as an advantage compared to other printing techniques. Initial work from our group has already demonstrated the feasibility of using 3D printing for agglomerate breakage studies [27], with the effect of printing layer orientation relative to loading direction studied. Further study investigated the effect of agglomerate structures including packing density, agglomerate shape and inter-particulate bond strength on agglomerate breakage [18]. The most recent study validated DEM simulations using an experimental study and found that some material behaviors such as non-linear materials and anisotropic bonds could be challenging for DEM, while multiscale experimental approach with the help of 3D printing greatly facilitate modelling of agglomerate deformation and breakage [28].

The current paper focuses on printing of agglomerates with different colour sections, so that the breaking particles could be better tracked after breakage. 3D printed agglomerates have been characterized in a thorough manner of printing primary material characterization, printing evaluation and surface analysis. Compression study was carried out under different loading speed and loading directions, with the strain distribution over 360° rotation of the whole random sphere agglomerates plotted. Finally random sphere agglomerates with designed internal cavities have also been printed and analyzed under compression study, providing meaningful insights in terms of agglomerate breakage in real particle industry.

Section snippets

Agglomerate design

Two types of agglomerate structures – a cubic shape with tetrahedral internal patterns (Tetrahedral) and a spherical shape with randomly packed internal structure (Random sphere) were used. Theses designs were structurally the same as published earlier [27]: Solidworks was used to design the tetrahedral structure by connecting 91 primary particles (4 mm in diameter) with cylindrical bonds (2.6 mm in diameter). Random sphere structure was designed in EDEM using “Particle factory” function. 120

Printing evaluations

By comparing geometry difference between CAD model and actual printed agglomerate (Fig. 3), the printing accuracy is very accurate among current 3D printing technologies, with very little difference between bulk geometry compared to CAD model. Through statistical analysis of the geometry difference from Table 3, the printing accuracy of multi-coloured agglomerates is high with printing error controlled within 1%. Tetrahedral-soft bond structure shows a printing accuracy of 99.4 ± 0.4%,

Discussion

The work presented here is the most detailed experimental investigation of the effect of agglomerate structure on agglomerate deformation and breakage conducted to date. The use of 3D printing to produce multiple, identical agglomerates where the structure can be controlled and the properties of the structure components systematically varied is a new approach. We have shown experimentally that a single agglomerate with a non-symmetrical structure has a distribution of possible strengths,

Conclusions

Agglomerates with controlled geometric structure, different bond strengths and multi-colours have been produced successfully by 3D printing technology. Surface and structural characterisations of 3D printed parts have further demonstrated the feasibility of using such technology to produce customised and well-controlled agglomerate structures for breakage studies, providing experimental guidance for DEM simulation. 3D printed agglomerates breakage behaviours have been investigated both

Acknowledgment

This research project was supported by International Fine Particle Research Institute (IFPRI), an ARC Discovery grant (DP150100119) and by Deakin University. Dr. Ruihuan Ge is much appreciated for his agglomerate structure design and general advice about agglomerate design and production. We would also like to acknowledge Ms Robynne Hall and Mr. Josh Tuohey in Deakin University School of Engineering for their 3D printing support.

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