Mechanical performance of L-shaped corner joints manufactured by 3D printing with a polylactic acid–wood composite

Abstract

Filaments were produced with eight different blends for use in Fused Deposition Modeling (FDM) printers by adding 10% and 15% linden and oak wood flours as support materials to a polylactic acid (PLA) matrix. Wood polymer L-shaped connectors were printed from the produced filaments in the FDM printer. These L-shaped connectors were fixed to the particleboard corner joints to produce L-shaped corner joint specimens. These specimens were subjected to cross-compression and cross-tension tests using special molds in a universal testing machine to determine the effects of wood flour, wood species, and additive ratio on cross-compression and tensile moments. Thermogravimetric analysis, differential thermal analysis, and differential scanning calorimetric analyses were performed to determine the thermal properties of the wood-polymer composites, while scanning electron microscope imaging was performed to determine their morphological structures. Additionally, the tensile strength of the composites was also determined. The results showed that the mechanical properties of the samples produced with different wood flour types and additive ratios were lower than those of pure PLA. However, in diagonal compression and diagonal tensile tests conducted using L-joint elements obtained from different wood species and printed on an FDM printer.

Full Article

Mechanical Performance of L-Shaped Corner Joints Manufactured by 3D Printing with a Polylactic Acid–Wood Composite

Yasemin Öztürk  , and Erol Burdurlu 

Filaments were produced with eight different blends for use in Fused Deposition Modeling (FDM) printers by adding 10% and 15% linden and oak wood flours as support materials to a polylactic acid (PLA) matrix. Wood polymer L-shaped connectors were printed from the produced filaments in the FDM printer. These L-shaped connectors were fixed to the particleboard corner joints to produce L-shaped corner joint specimens. These specimens were subjected to cross-compression and cross-tension tests using special molds in a universal testing machine to determine the effects of wood flour, wood species, and additive ratio on cross-compression and tensile moments. Thermogravimetric analysis, differential thermal analysis, and differential scanning calorimetric analyses were performed to determine the thermal properties of the wood-polymer composites, while scanning electron microscope imaging was performed to determine their morphological structures. Additionally, the tensile strength of the composites was also determined. The results showed that the mechanical properties of the samples produced with different wood flour types and additive ratios were lower than those of pure PLA. However, in diagonal compression and diagonal tensile tests conducted using L-joint elements obtained from different wood species and printed on an FDM printer.

DOI: 10.15376/biores.21.2.3248-3271

Keywords: L-Connected joint; 3D printer; Additive manufacture; Furniture

Contact information: Wood Products Industrial Engineering Department, Technology Faculty, Gazi University, P.O. Box 06500, Ankara, Turkiye; *Corresponding author: yozturk@gazi.edu.tr

Graphical Abstract

INTRODUCTION

Although the furniture industry is one of the world’s most important sectors, companies and scientists have been forced to seek alternatives due to the more efficient use of existing forest resources and the high cost of solid wood as raw material. In addition to composite materials produced using natural fibers, the production of wood-plastic composite materials has also become well established (Janíková et al. 2025). As a result of the parallelism between industry developments and technology, additive manufacturing methods have become increasingly popular. One of the most widely used additive manufacturing methods is Fused Deposition Modeling (FDM). A 2017 study conducted in Italy showed that 3D printing technologies are primarily used by accessory manufacturers (Murmura and Bravi 2018). FDM printers use filament, and a wide variety of materials are used for filament production. Among these materials, polylactic acid (PLA) and PLA-based filaments are the most preferred (Nampoothiri et al. 2010). PLA, which is considered an environmentally friendly material and a promising alternative to petroleum-based polymers, is being researched by scientists and companies to create wood-like products by incorporating wood and wood waste. A summary of some of these studies on the production of wood-reinforced filaments using PLA as a base material is provided in Table 1.

Table 1. Some Studies on Wood-Reinforced Filaments Using PLA

EXPERIMENTAL

Materials

Linden (Tilia rubra DC.) and sessile oak (Quercus petraea (Mattuschka) L.) wood flours were used as additives in the filament production. Wood flours that would be examples of hard and soft wood types were preferred in the selection of wood type. Total Corbion Luminy L175 brand polylactic acid (PLA) was used as matrix material, polyethylene glycol (PEG 6000) for dimensional stability and plasticize of the filament and polyethylene wax as a lubricant for the easy movement of the filament during the printing phase were used.

Melamine faced chipboard, 18-mm thick and 0.63 g/cm³ density was used for the pieces of L-corner joints which the L-shaped brackets would be fixed to 3.5 × 20 mm chipboard screws and 8 × 20 mm solid plastic dowels were used in fixing L-corner joint elements to each other.

Method

Wood flour (WL) derived from linden and oak was sieved using a 200-mesh (approximately 74 microns) stainless steel vibrating screen. Through this process, particles with a size of 74 microns or smaller were separated and selected for use in production. The selected WF was dried in an oven at 103 ± 2 °C until fully dried to eliminate all moisture content.

To achieve a more homogeneous mixture with WF, PLA in granule form was ground into powder using a polymer grinder. This process ensured that the particle size of PLA was similar to that of the WF, reducing the risk of phase separation in the composite. The powdered PLA was then dried in an oven at 55 °C for 6 h to minimize its moisture content. To enhance the dimensional stability of the filament and improve processability during 3D printing, PEG-6000 and polyethylene (PE) wax were used as auxiliary additives. PEG-6000 was added to improve the dimensional stability of the final filament, while PE wax facilitated the smooth flow of the filament during the extrusion process. Samples of PLA, WF, PEG-6000, and PE wax were weighed according to the ratios specified in Table 2, and mechanically homogenized in a Turbula mixer for 60 min to obtain uniform powder mixtures. In total, eight different filament formulations were prepared in this manner.

The prepared mixtures were processed using a twin-screw extruder with an 18 mm diameter and a length-to-diameter (L/D) ratio of 40, operating at 200 ± 5 °C. The extruded output was then pelletized using a granulation unit to form granules. In the final stage, the granulated biopolymer blends were processed in a single-screw extruder with a diameter of 16 mm and an L/D ratio of 25, and extruded into filaments with a diameter of 1.75 mm.

Table 2. Biopolymer Mixing Ratios

In order to determine the tensile strength of the obtained composite materials, samples were produced using the plastic injection method, while L-shaped connectors were printed on an FDM printer.

Fig. 1. Technical drawing of tensile sample in accordance with ASTM D638-22 (2022) Type-1 standard (mm)

Fig. 2. Tensile strength test

In order to determine the tensile strength properties of composite materials composed of different mixtures, tensile test specimens were produced in accordance with the ASTM D638-22 (2022) Type-1 standard (Fig. 1). The test specimens were produced by setting the first heating cell of an injection molding machine with 3 heating cells to 170 ℃, the second heating cell to 190 ℃, and the last heating cell to 200 ℃. Tensile strength tests (Fig. 2) were applied to the tensile test specimens extracted from the injection mold using a 50 kN Instron 5969 Universal testing machine. The loading rate applied in the tests was 4 mm/s.

Tensile test results were calculated according to Eq. 1, and the measurement areas of the tensile sample are specified in Fig. 3,

 (1)

where δ_t is tensile strength (N/mm²), F_max is the maximum load at break (N), a denotes thickness of the rupture area of ​​the sample (mm), and b is the width of the rupture area of ​​the sample (mm)

Fig. 3. Tensile test sample measurement areas

Fig. 4. Technical drawing of the L-shaped bracket

Table 3. Printing Parameters with a 3D Printer

The L-shaped brackets (LSBs) shown in Fig. 4 were printed with the printing parameters given in Table 3 by using filaments extracted with the mixing ratio given in Table 2. The specimens were prepared with a CCH brand Z23 model closed system 3D printer. The printing angle was 90 degrees, the fan operating at 100% power had an air volume of 9.43 CFM, and the extrusion flow rate was 30 mm/s. Images of the layers in the printer interface program during the printing process are given in Figs. 5 (a-b).

Fig. 5. Display of the layer in the interface program during the printing process (a and b)

For the corner joints, the melamine faced chipboard was cut into the dimensions specified in Fig. 6, and the LSBs were assembled.

Fig. 6. Corner joint with L shaped bracket (CJWLSB)

Then the samples of the corner joints (Fig. 7) were subjected to diagonal compression and the tensile tests were performed using special molds in the 50 kN Instron 5969 Universal testing machine.

Fig. 7. Diagonal compression (a), and tensile (b) tests of the corner joints

The samples printed on the FDM printer were tested without being assembled as well as being assembled, and the test results were calculated as specified in Eqs. 2 and 3.

A Hitachi STA7300 device was used for Thermo-gravimetric Analysis/Differential Thermal Analysis (TGA/DTA) and a Hitachi DSC 7020 device was used for Differential Scanning Calorimetric (DSC) analysis to determine the thermal properties of wood polymer composites. In the TGA/DTA analysis, the samples were heated up to 600 ℃ starting from 25 ℃ with a temperature increase of 20 ℃/min, and in the DSC analysis, the samples were heated up to 600 ℃ starting from 25 ℃ in a nitrogen environment.

To determine the morphological structure of the obtained wood polymer composite materials, samples were examined with a scanning electron microscope (SEM). Imaging was performed using Hitachi SU5000 brand SEM.

Statistical Analysis of Data

The statistical evaluation of the data was performed with the SPSS-22 package program. The normal distribution of the obtained data was evaluated according to the Skewness-Kurtosis method.

After the evaluation, the data showing normal distribution were used directly for statistical operations, and the normalization process was performed for the wood polymer L connection element tensile moment data that did not show normal distribution. The Levene test was performed to evaluate the equality of variances for the data showing normal distribution, and it was determined that the variances of the data groups were homogeneous.

The Duncan test was used to determine the difference between the groups for the groups with homogeneous variances.

RESULTS

Density

In Fig. 8, the density values ​​of 100% fill ratio (LSB) obtained with 8 different blends and pure PLA are given.

While the LSBs obtained with Lin10+PEG and Oak15+PEG mixtures had the lowest density value of 1.06 g/cm³, the highest density value of 1.26 g/cm³ was obtained from the LSBs produced from pure PLA.

The ANOVA test conducted to determine whether different mixtures influence the density of wood polymer L-joints is given in Table 5.

Fig. 8. Densities of the LSBs

Table 5. ANOVA Test to Determine Effect of Different Mixtures on Density of LSBs

As presented in Table 5, the p-value obtained from the analysis of variance (ANOVA) for the different mixtures was found to be above 0.05. This indicates that the differences between the groups were not statistically significant.

Effects of Different Mixtures on Diagonal Compression and Diagonal Tensile Moment of LSBs and CJWLSBs

Diagonal compression and diagonal tensile moment values ​​of LSBs produced with FDM printer using different mixtures and corner joints fixed using these LSBs are given in Fig. 9.

Fig. 9. Diagonal compression and tension moments of wood polymer L-joints and L-joint corner joints according to wood species and mixture ratios

The diagonal compressive moment of pure PLA L-joint corner joints was the highest compared to that of L-joint corner joints of all blends. Based on all wood-forget additives, the highest diagonal compressive moment was obtained in L-joint corner joints with Lin10 blend, while the lowest diagonal compressive moment was obtained in L-joint corner joints with Oak10+PEG blend.

Regarding wood-polymer L-joints, the ranking of L-joint corner joints with wood flour additives remained unchanged, and the diagonal compressive moment of pure PLA L-joints ranked first compared to L-joints of all blends.

The diagonal tensile moment of pure PLA L-joint corner joints was the highest compared to that of all L-joint corner joints of all mixtures. Considering all wood flour-added variables, the highest diagonal tensile moment was achieved in corner joints with L-joints using the Lin10 blend, while the lowest diagonal tensile moment was achieved in corner joints with L-joints using the Lin15+PEG blend.

The diagonal tensile moment of pure PLA L-joints ranked first among all mixtures in terms of wood-polymer L-joint tensile moment. Regarding wood-polymer L-joints, the ranking remained unchanged for both linden flour-added and oak flour-added mixtures.

Effects of Different Mixtures on Diagonal Compression Moment of LBSs and CJWLSBs

Diagonal compression moment values ​​of wood polymer L-joints produced with FDM printer using different mixtures and corner joints fixed using these L-joints are given in Table 6.

Table 6. Diagonal Compression Moment Values ​​of LBSs and CJWLSBs according to Wood Species and Mixture Ratios

As can be seen from the table, regarding CJWLSBs, the highest diagonal compressive moment was obtained in Lin10 blended CJWLSBs among the linden added variables, while the lowest was obtained in Lin15+PEG blended CJWLSBs. Among the oak added variables, the highest diagonal compressive moment was obtained in Oak15 blended CJWLSBs, while the lowest was obtained in Oak10+PEG blended CJWLSBs. Considering all variables, the highest diagonal compressive moment was obtained in CJWLSBs with Lin10 mixture, followed by CJWLSBs with Lin15 mixture, and the lowest diagonal compressive moment was obtained in CJWLSBs with Oak10+PEG mixture. The diagonal compressive moment of CJWLSBs made of pure PLA was the highest compared to that of CJWLSBs of all mixtures.

Regarding the wood polymer LSBs, the order did not change in the variables with linden flour additive, while in the variables with oak flour additive, the highest diagonal compressive moment was obtained in the LSBs with Oak10 mixture, while the lowest was obtained in the LSBs with Oak15+PEG. The diagonal compressive moment of the LSBs made of pure PLA was again in the first place compared to that of the LSBs of all mixtures.

The ANOVA test conducted to determine whether different mixtures have an effect on the diagonal compressive moment of the CJWLSBs is given in Table 7.

Table 7. ANOVA Test to Determine whether Different Mixtures Influence the Diagonal Compressive Moment of CJWLSBs

Different mixtures were effective on the diagonal compression moment of CJWLSBs, and the differences between the diagonal compression moment values ​​of the groups were important. The results of the Duncan homogeneity test performed to determine which groups have the differences between the diagonal compression moment values ​​are given in Table 8.

Table 8. Duncan Homogeneity Test

According to this grouping, increasing the wood flour additive ratio for both wood types and adding PEG to the mixture along with wood flour decreased the diagonal compressive moment values ​​of CJWLSBs. In addition, the diagonal compressive moment values ​​of CJWLSBs with linden flour additive were higher than those with oak flour additive.

ANOVA test was performed to determine whether different mixtures have an effect on the diagonal compressive moment of wood polymer LSBs and the result is given in Table 9.

According to Table 8, different mixtures were effective on the diagonal compression moment of wood polymer LSBs, and the differences between the diagonal compression moment values ​​of the groups were important. The results of the Duncan homogeneity test performed to determine which groups have the differences between the diagonal compression moment values ​​are given in Table 10.

Table 9. ANOVA Test to Determine the Effect of Different Mixtures on the Diagonal Compressive Moment of Wood Polymer LSBs

Table 10. Duncan Homogeneity Test

Based on this grouping, increasing the linden flour content and adding PEG to the mixture along with linden flour decreased the diagonal compressive moment values ​​of the LSBs. In the samples with oak flour content, increasing the flour content and adding PEG to the mixtures did not create a significant difference in the diagonal compressive strength of the LSBs.

Effects of Different Mixtures on the Diagonal Tensile Moment of Wood Polymer LSBs and CJWLSBs

The diagonal tensile moment values ​​of LSBs produced with FDM printers with different mixtures and corner joints fixed using these L-connectors are given in Table 11.

As can be seen from the table, regarding CJWLSBs, among the linden flour added variables, the highest diagonal tensile moment was obtained in Lin10 blended CJWLSBs, while the lowest was obtained in Lin15+PEG blended CJWLSBs. Among the oak flour added variables, the highest diagonal tensile moment was obtained in Oak10+PEG blended CJWLSBs, while the lowest was obtained in Oak15+PEG blended CJWLSBs. When all variables were taken into consideration, the highest diagonal tensile moment was obtained in Lin10 blended CJWLSBs, followed by Oak10+PEG blended CJWLSBs, and the lowest diagonal tensile moment was obtained in Lin15+PEG blended CJWLSBs. The diagonal tensile moment of pure PLA L-connector corner joints was the highest compared to that of all mixtures.

The ranking of wood polymer LSBs did not change with both linden flour and oak flour additives. The diagonal tensile moment of pure PLA LSBs was the first in the wood polymer LSBs tensile moment ranking of all mixtures.

Table 11. Diagonal Tensile Moment Values ​​of Wood Polymer LSBs and CJWLSBs Based-on Wood Flour Type and Mixture Ratios

ANOVA test was performed to determine whether different mixtures have any effect on the diagonal tensile moment of wood polymer CJWLSBs and the result is given in Table 12.

Table 12. ANOVA Test to Determine the Effect of Different Mixtures on Diagonal Tensile Moment of Wood Polymer CJWLSBs

As shown in Table 11, different mixtures were effective on the diagonal tensile moment of CJWLSBs. Differences between the tensile moment values ​​of the groups were significant. Results of the Duncan homogeneity test performed to determine which groups have the differences between the diagonal tensile moment values ​​are given in Table 13.

Table 13. Duncan Homogeneity Test

According to this grouping, the diagonal tensile moment decreased with the increase in the additive ratio in the CJWLSBs with linden flour additive and in the CJWLSBs with oak flour additive. Similarly, in PEG additives, the increase in the wood flour ratio decreased the diagonal tensile moment of the corner joint.

ANOVA test was performed to determine whether different mixtures are effective on the diagonal tensile moment of wood polymer LSBs and the results are given in Table 14.

Table 14. ANOVA Test to Determine the Effect of Different Mixtures on the Diagonal Tensile Moment of Wood Polymer LSBs

As can be seen from Table 13, different mixtures were effective on the diagonal tensile moment of wood polymer LSBs elements, and the differences between the values ​​of the groups were important. The results of the Duncan homogeneity test conducted to determine which groups have the differences between the diagonal tensile moment values ​​are given in Table 15.

Table 15. Duncan Homogeneity Test

Based on this grouping, while the increase in the linden flour additive ratio and the PEG addition were effective in reducing the diagonal tensile moment of the fastener in the LSBs with linden flour additives, both the increase in the wood flour additive ratio and the PEG addition were ineffective on the moment value in the ones with oak flour additives.

Deformations

The deformations that occurred in the corner joint and the wood polymer L connection element after the diagonal compression test are given in Figs. 10 and 11.

In CJWLSBs, after the diagonal compression test, wood polymer LSB deformations occurred in the rib part of the material. The deformations that occurred after the diagonal compression test applied to the wood polymer LSBs occurred near the screw hole and at the beginning or end of the rib. The deformations that occurred are shown in Figs. 12. and 13.