Acoustic behaviour of 3D printed bio-degradable micro-perforated panels with varying perforation cross-sections
Introduction
Environmental concern pertaining to materials circular economy leads to the development of bio-degradable materials for various applications. Most of the materials currently being used in sound absorption (SA) and sound transmission loss (STL) applications like glass wool and foams are toxic and pose severe environmental and health issues. These materials leach toxins and are non-biodegradable, resulting in disposal related issues. Stec and Hull [1] investigated a detailed quantitative estimation of fire toxicity of glass wool, stone wool, expanded polystyrene, phenolic, polyurethane and polyisocyanurate foams when subjected to different fire conditions.
Infante et al. [2] identified that glass fibres are potentially more carcinogenic and cause cancer to humans as compared to asbestos on fibre to fibre basis. Baan and Grosse [3] in their review highlighted on the carcinogenic hazards of man-made mineral and vitreous fibres like glass fibres, continuous glass filament and rock/slag wool during their manufacture, use and disposal as waste. Napier and Wong [4] reported that polyurethane foams emitted toxic gases like Hydrogen cyanide, isocyanates, urea, halogenated compounds and alkenes when heated above 220 °C. Paabo and Levin [5] focussed on the review of various methods for estimating toxicity of gases while burning polyurethane foam and their physiological, biochemical and lethal effects on humans.
Hence, there is a need to develop biodegradable material for SA and STL applications. Most of the researchers used biodegradable resin and natural fibres for developing the eco-friendly green materials for SA and STL applications. They analyzed the influence of constituent elements on acoustic performance with and without micro-perforation. Very few researchers investigated the effect of cross-section and functionally graded (FG) perforation variations on acoustic performance. Acoustic performance can be enhanced by altering the porosity and its pattern through the panel thickness. However, most of the Micro-Perforated Panels (MPPs) are made with perforation with constant circular/square cross-section throughout the thickness. Nowadays, with the advent of FFF based 3D printing (3DP), realization of MPPs with varying cross-section and functional gradation through the thickness is quite possible and feasible [6], [7], [8], [9].
Pioneering work in the field of acoustics of MPPs has been carried out by Dah-You Maa [10], [11], [12], by applying the acoustic principles formulated by Rayleigh [13], Ingard [14], [15], Crandall [16] and Zwikker [17] for thin panels. According to the model proposed by Maa [10], [11], [12], for MPP with end correction, incident b > r, SPL < 100 dB with end corrections, the acoustic impedance of MPP is defined as,where
1 < x < 10.
From Eq. (1), it is clear that ZMPP varies linearly with panel thickness. The impedance of panels increases linearly with thickness. Further, Maa had taken the assumption that the diameter of holes should be nearly equal to the thickness of panels (Eq. (25) [11]) and only panels with submillimeter holes can act as good absorbers. Following these assumptions, thick MPPs with perforations of variable cross-sections (Like micro horns as suggested by Randeberg [18] and tapered ones as suggested by Sakagami et al. [19], [20]) needs to be used for achieving better structural-acoustic properties (MPP impedance matching with that of air). Importance of substituting thin, limp panels which do not have sufficient strength or stiffness for practical application by thick ones have been highlighted by Sakagami et al. [19], [20].
Poly Lactic Acid (PLA) is the most widely exploited bio-degradable material having comparable physical, mechanical, thermal and acoustic properties compared to other materials used in the polymer industry [21]. PLA is an easily mouldable and 3D printable popular material which is most widely used due to its better surface finish, dimensional accuracy and suitable printing conditions. Mosanenzadeh et al. [22] analyzed the acoustic behaviour of porous membranes with a bimodal PLA foam having pores of varying size fabricated using a blend of PLA and Poly Ethylene Glycol. Chin et al. [23] studied the absorption characteristics of MPP made from kenaf fibre and PLA composite through conventional machining methods (mixing, drilling, and hot-pressing).
Huang et al. [24] presented a design guideline for FG acoustic black hole beams prepared through 3D printing technology. Gao et al. [25] designed a micro-helix PLA MPP using 3DP for sound absorption applications. Deshmukh et al. [26] designed periodic foam microstructures using a digital light process-based 3DP. Akiwate et al. [27] developed honeycomb core sandwich material with MPP facing and analyzed the effect of cross-sectional variation of perforation on the acoustic performance. Mamtaz et al. [28] investigated natural fibre reinforced granular materials influence on absorption behaviour.
Othmani et al. [29] explored sugarcane waste-based material as acoustic materials. Liu et al. [30] investigated the absorption characteristics of a layered MPP prepared by 3DP. Meng et al. [31] analytically investigated structurally-efficient honeycomb sandwich having perforated faceplate. Influence of different structures was investigated and compared with finite element models for validation. Liu et al. [32] used statistical methods to find geometric characteristics through experimental absorption data for perforations of any shape. Herdtle et al. [33] modelled tapered holes in MPPs and concluded that tapering had a significant effect on their dynamic resistance. Randeberg [18] worked on micro horns and found that the higher the variation in their radius with height, the more bandwidth they can absorb and better the viscous dissipation of sound. Temiz et al. [34] obtained end correction coefficients in MPPs numerically for different end geometries and validated them experimentally.
Qian et al. [35] produced MPPs using MEMS technology and analyzed their acoustic performance. Liu et al. [36] showed that MPP could be analyzed using the transfer impedance approach. Sakagami et al. [19], [20] studied MPPs with tapered perforation for sound absorption applications. Akiwate et al. [37] investigated the acoustic performance of 3D printed tubes with reverse flow resonators. Mosa et al. [38] worked with double layer MPP with non-homogeneous perforations to get better absorption. Both at higher and lower frequencies, the absorption bandwidth was enhanced by varying the inter-plate distance and back air gap, respectively. Qian et al. [39] developed a finite element model of thick MPPs having tapered holes and validated it experimentally. Jiang et al. [40] analyzed acoustic parameters of thick MPPs with stepped holes and compared them with experimental values.
In the present scenario, a significant amount of research work is being pursued in developing high-performance, sustainable metamaterials for building MPPs from natural resources. Nonetheless, the realization of geometrically complex perforations, associated time and costs are the issues to be looked into. PLA is a non-toxic, bio-degradable and recyclable polymer with known thermal history. Moreover, its mechanical properties and ease to work with are comparable with other polymers. PLA can also be blended with many materials to enhance its properties. However, very little work has been done in the field of the acoustic behaviour of 3D printed PLA with different perforation variation, which is the focus of this work. Further PLA is the best material in terms of dimensional accuracy, surface finish and aesthetic appeal with regards to 3D printing which defines its scope predominantly. An excellent numerical model developed will facilitate better analysis, validation as well as proper utilization of resources during experimentation.
Section snippets
Materials
PLA (Fig. 1) is used as the material to prepare the specimens with different cross-sections. PLA granules of 3052D grade are procured from Nature Tech India Ltd. Chennai, Tamil Nadu, India dealer of NatureworksLLC, Thailand. It has a molecular weight of 1,16,000 g/mol. Its vicat softening point and degradation temperatures are 66.2 °C ± 1.4 °C and 336.7 °C respectively. These granules are preheated to 80 °C for 6 h for removing moisture if any. Technical specifications of PLA are listed in
Experimental details
MPPs having different perforation cross-sections are tested for sound absorption and sound transmission loss characteristics in an impedance tube.
Sound absorption coefficient
Influence of perforation cross-section variation on sound absorption coefficient obtained through the experiment is depicted in Fig. 6. It is observed that the influence of the perforation pattern on the sound absorption coefficient is significant. The coefficient of the sample with 8 mm constant diameter perforation is significantly lower as compared to the perforation variations. Wide perforations transmit sound energy uninterruptedly due to wider air column, which reduces the friction and
Conclusion
Geometrical variations of perforations influence the dissipation of sound (absorption coefficient and frequency bandwidth). From the sound absorption coefficient results as presented in this work, it is found that the DC perforation registered better performance compared to the constant 1 mm diameter perforation pertaining to both peak absorption coefficient and bandwidth. All other perforations depicted less peak absorption coefficient amid comparable values for constant diameter 1 mm. The
Acknowledgement
The authors acknowledge the support and research facilities provided by the Mechanical Engineering Department of NITK Surathkal and School of Mechanical Engineering, VIT Chennai during this work.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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