Investigation of bio-based and recycled printing materials for additive manufacturing

Additive manufacturing (AM) processes are becoming increasingly important alongside conventional processes. As a result, the consumption of materials is also increasing. The most widespread process in polymer AM is Fused Layer Modelling (FLM). Today, the FDM process often uses synthetically produced materials based on petrochemical processes. However, there is little knowledge about which bio-based and recycled polymer materials are suitable for sustainable polymer AM. The aim of this paper is to carry out investigations of eight selected materials, which are already commercially available, to gain insights into their suitability as materials for polymer AM. These materials are divided into four categories: conventional, recycled, bio-based and fibre-reinforced thermoplastics. The evaluation model consists of a point system in which the materials are evaluated according to various weighted criteria. For technical, economic and ecological evaluation meaningful criterions were developed and applied. Based on the evaluations, three two-dimensional strength diagrams were developed, from which the results of the materials, on two of the evaluations in each case, can be read. These results are combined in a three-dimensional diagram. This representation provides the ability to make a precise selection of bio-based or recycled materials for polymer AM.


Introduction
After the discovery of polymers as inexpensive yet robust materials in the 1950s, demand is still growing rapidly today [1].Polymers of all kinds are becoming increasingly popular because they proved to be cheap and easy to process [2].In addition, they could be used very flexibly for different purposes.Among other things, this material group is excellently suited for packaging, beverage bottles and housings, but polymers are also frequently used in the industrial sector.The low weight compared to metals and the weather resistance also contribute to the high popularity of the polymers [3].Due to the ever-increasing demand and relevance of ecological production and thus also the rapidly increasing relevance in the field of additive manufacturing, research is continuing in this area.New ways of using new materials or methods to reduce the environmental footprint are constantly being explored.In additive manufacturing, the use of bio-based and recycled materials could significantly improve the eco-balance [4].Therefore, the question arises which bio-based or recycled polymer materials are available on the market and which technical properties they offer?In addition, the economic and ecological properties are also of interest in order to be able to make a suitable material selection.

Additive manufacturing using polymers
The term AM technologies covers a large number of different processes and materials [5].The additive processes for processing polymers according to ASTM52900 can be classified into four types: • Additive manufacturing a 3D object by means of material extrusion (MEX) of plastic filaments: This includes, among others, the common manufacturing method Fused Deposition Modelling (FDM), which originates from the company Stratasys.• Vat polymerisation of photopolymers (VPP): a bath with a photopolymer is cured locally by a laser beam (e.g.stereolithography SLA) or over the surface with the aid of a projector or display (e.g.digital light processing).In general, subsequent post-curing is necessary.• Binder jetting (BJ) of photopolymers and curing of synthetic resins: Multi-Jet Modelling (MJM) are examples.In these methods, a liquid resin is applied, which takes on a solid form when irradiated with UV light.• Powder Bed Fusion (PBF) by sintering a powder material: one method in this category is called Selective Laser Sintering (SLS).In this process, the temperature in a heated building space is raised to just before the melting point of the powder.A short laser pulse is then sufficient to fuse and thus solidify the material at the desired location.The common feature of the different groups is the layer-by-layer production of the print models.In each of the methods, the component to be printed is built from scratch, by adding layers, hence the name of additive manufacturing.The method of FDM printing is probably the most common 3D production process [6].The possibility of producing inexpensive components without additional tools not only enables its entry into the professional environment, but also paves the way into the private households of many hobby developers.The necessary simple and robust 3D printers, which can work without lasers, UV light and heating, have contributed to the widespread use of this process.The easy handling of the plastic filament, which is wound as a wire on a spool, also contributes to the success of FDM.Today, there are many polymer materials on the market that can be processed by the open systems [7].Therefore, this study concentrated on this process.In contrast, closed systems are often represented on the market for the other processes, i.e. the manufacturers of the additive manufacturing systems also produce the printing materials.This often limits the selection of available materials for users.

Material selection for the material extrusion of polymers
Many different types of polymers can be used for material extrusion.These are usually thermoplastics that are melted in a nozzle during this process.Thermoplastics have a variety of properties and differ in terms of stability, weight, thermal conductivity, printability, price, environmental impact and other characteristics.The most commonly used materials for FDM include ABS, PETG and PLA.The materials relevant to this contribution are the bio-based and recycled printing materials, as well as composite fibre-reinforced (FR) printing materials, which usually consist of a mixture of bio-based and additives from conventional or natural fibres [8,9].After extensive research, eight commercially available printing materials from different original equipment manufacturer (OEM) were selected.In the table 1, filaments are listed with their brand name, the materials they are made of and the tensile strength.The market price was included in order to use it later for the economic analysis.For the cost comparison, the two standard materials PETG and PLA are taken as a reference.Pure PETG [10] is neither bio based nor recycled, it was only used for comparison with recycled PET [11].The pure PLA [12], which is a bio-based material, is also used for comparison with the recycled PLA [13].The two materials "Biofusion" [14] and "NonOilen" [15] are both based on PLA, which has been supplemented with additives and PHB respectively.PHB is a polyester that can be fermentatively produced from renewable raw materials.It belongs to the group of thermoplastic polyesters and is therefore deformable under heat [16].The material reinforced with carbon fibres provide a higher strength, as the fibres themselves have a higher strength and can therefore additionally strengthen the material [17].Thus, the material "GreenTEC Pro Carbon" with carbon fibre reinforcement should bring a significant improvement with a value of 65 MPa [18].Interestingly, the tensile strength with added wood fibres is only 40 MPa, which is worse than the conventional PLA [19].

Economic, ecological and technical evaluation model
The evaluation is based on the weighted point evaluation method.Various criteria for the three areas are defined for the evaluation.The materials receive a point score pi for each criterion, which is used to determine how well they meet the criterion.The maximum score is pmax=10.Due to the varying relevance of the criteria, a weighting factor wi is also included in the evaluation.Thus, a value per material can be determined for the ecological, economic and technical areas.In (1), this is demonstrated with the example of the technical point value.In order to plot the materials in the strength diagram, all values were normalized to a maximum of 1.This resulted in three values in the range 0.0 to 1.0 for each material.
The following criteria were determined with the help of data sheets [10-16, 18, 19] and the experience of the authors.The cost evaluation can be determined on the basis of the purchase costs with the help of the manufacturer's data (see table 1).Additional costs can be estimated, for example, if a change in nozzle diameter or nozzle material is necessary.Further criteria, in particular for ecological evaluation, were assessed on the basis of literature references [7][8][9]20].The technical evaluation is based on the values determined in the experiments and tests described in section 5 and 6.

Economic criteria:
Processing time: is of great importance in the economic viability of 3D printers.Depending on the material, printing can be done at different speeds.Some materials need a slower speed and are therefore much less economical.Time is weighted highest as the most important factor at 0.35.Costs of the filament: costs of the material can vary greatly.Since the material costs are lower compared to the processing time, they play a somewhat smaller but still important role and are weighted with 0.3.Change of nozzle: Most filaments can be printed with a standard 0.4 mm nozzle.For fibre-reinforced print materials, however, a nozzle size of 0.6 mm or larger is recommended to avoid clogging the nozzle.When changing the nozzle, the 3D printer stops and thus cannot produce.Due to the fact that the general conditions vary greatly, this criterion is given a low rating with an offset factor of 0.15.Nozzle wear costs: Pure polymers usually have a low abrasive effect and can therefore be used with cheaper brass nozzles.Fibre-reinforced polymers have a high abrasive effect and are therefore used with nickel-coated nozzles or nozzles made of hardened steel, which are much more expensive.Accordingly, the costs of the nozzle play a rather subordinate role here and are given a weighting of 0.2.

Ecological criteria
CO2 footprint of production: If the CO2 footprint of the production process is high, the material is rated lower.This criterion is weighted higher (0.3) as it plays an important role in sustainable production.Biodegradability: this criterion has a strong impact on the plastic waste that ends up in the environment.If a material can be biodegraded within a short time, this has a strong influence on how much plastic survives in nature.Therefore, this attribute receives the highest weighting of 0.35.Reusability: Once a material can be reused, there is an incentive to keep the raw material in the recycling cycle.However, it must also be considered that many products do not re-enter the cycle, but end up in residual waste and eventually landfill.This is partly due to the lack or absence of labelling of the material in the finished product.Due to the fact that, despite theoretical recyclability, printing materials are only partially recycled, this criterion is to be evaluated accordingly with a factor of 0.25 in the middle.Probability of rejects: This criterion refers to materials for which non-compliance with the corresponding requirements.It also increases the amount of waste produced from materials which can cause problems when printed.However, as the operator's experience increases, problems like these should be easier to control, so this criterion is given a multiplier of 0.1.

Technical criteria:
Dimensional accuracy: The criteria represents the difference between the actual and target dimensions and can be specified as a percentage deviation.If the deviation is too large, the printed object may become unusable.Since dimensional accuracy plays a major role, it is weighted with 0.25.
Layer adhesion: The adhesion between the individual layers is an important attribute for the stability of the printed object.It is largely responsible for the robust structure of the body.If the layer adhesion is not high enough, a component may its technical usability and thus cannot be used.For this reason, it is given a rating of 0.25, as is dimensional stability.Tensile strength: If the strength is higher, a correspondingly higher number of points is awarded.
In order to exclude errors caused by non-optimised parameters during printing, the manufacturer's specifications for the filaments are used for this evaluation.However, the tensile strength is only relevant for highly loaded application.For this reason, a somewhat lower weighting of 0.15 is used.
Temperature resilience: This criterion refers to the resistance of the materials to temperatures.However, temperature resistance only plays a role in extreme cases or for certain applications.For this reason, a somewhat lower multiplier of 0.15 is used.Printability: How well a material can be printed depends on various factors.For example, a susceptibility to warping of the material reduces the printability.Criteria such as poor layer adhesion also have a negative effect on the printability of the filament.Since poor printability can lead to increased problems during printing and prints may have to be restarted, a lot of time can quickly be lost.Therefore, the printability is weighted with 0.2 in the middle.

Experimental procedure
The technical evaluation included tensile testing, fracture surface examination and dimensional inspection.The FDM 3D printer "Original Prusa i3 MK3S+" from the Czech manufacturer Prusa Research was used.This 3D printer is characterised by a wide range of possible printing materials and easy handling [21] and is open to the use of materials from both OEM and other suppliers.The printing parameters, e.g.nozzle temperature, temperature of the printing bed, were taken from the manufacturer's specifications.In general, a nozzle diameter of 0.4 mm was used.For the FR materials a nozzle with 0.6 mm was deliberately used to avoid clogging by the fibres.The standard DIN EN ISO 527 was used for the specimens of the tensile test.The specimen was modelled in the cloud-based CAD programme Onshape [22].The dimensions are those of type 1A, which is presented in the second part of the standard.The tensile specimens were printed with 30%, 60% and 100% filling.After the tensile tests were carried out, the surfaces of the fractures could be analysed to gain possible insights into anomalies in the tensile tests and the fracture behaviour.The magnification was carried out using the scanning electron microscope (SEM) "JSM-6610LV" from the manufacturer Jeol.For this purpose, the samples were gold-plated.After gold plating, the samples were placed in the SEM and photographed at 40x and 500x magnification.For the examination of the dimensional accuracy, a sample was developed that contains a gradation of different dimensions.This allows the dimensional accuracy of the different materials to be compared with each other.The samples were to be used to measure the internal dimensions of eleven different apertures.A digital calliper gauge with a resolution of 0.01 mm was used as measuring equipment.

Test results
In the following, the test results are summarised and the results of the SEM examinations are added for explanation.Table 2 shows the most important measured values and findings for the materials examined.The measured tensile strength for PETG are significantly below the manufacturer's specifications.This significant deviation cannot be confirmed from other studies [23] and is probably due to incorrect 3D printing parameters.The measured tensile strength for rPLA deviate from the data sheet, but agree with the results of other studies [24].The fractures also show the characteristic surfaces for the two printing materials.Deviations in dimensional accuracy were 0.15 % lower for the rPLA material than for PLA.
For the materials rPET and PETG, the deviations are slightly better with a difference of 0.04 %.Specimens printed from the "BioFusion" material showed poor layer adhesion.This was confirmed by the breaking off of the edge layers during the tensile test and by examination with the SEM (see figure 1a).To ensure successful printing of this material, the parameters should be adjusted until the aforementioned problem is eliminated.Similarly, a deviation in tensile strength of -15.7 MPa (-12.8 %) from the data sheet was found here.Interestingly, when examining the surface of the test specimens made of "NonOilen", a fibrous structure was found in the case of laterally torn filaments (see figure 1b).
The filaments broken at 90° to filament strand had a smooth edge.AM using this material presented particular difficulties, as extreme warping occurred, causing the models to detach from the print bed.As a result, the print jobs had to be restarted several times.Finally, an enlargement of the support surface was able to counteract the problem.The "Dark Wood" and "Carbon" composites both had a significantly reduced tensile strength compared to the manufacturer's specification.The difference was -13.3 MPa (Dark Wood) and -22.6 MPa (Carbon), as shown in Table 2.One reason for this may be the strong occurrence of voids/air inclusions as detected in the surface examination (see figure 2).This reduces the actual cross-sectional area and weakens the material.Experimentation with different printing settings will most likely solve the problem.In addition, the Wood building material was found to have a large deviation in dimensional stability, averaging 3.76%.

Evaluation of the results
From the results, the strength diagrams can be derived according to ecological-economic, technicalecological and economic-technical projections.In the upper right corner in the point (1, 1) , also called "ideal value", would be placed a material that has achieved the full score for both one and the other axis.In the environmental-economic evaluation (figure 3a), the two materials rPLA and PLA stand out clearly.These are closest to the ideal value.The materials "NonOilen", "Carbon", "Dark Wood" and "BioFusion" are on a similar ecological level.However, these materials, especially "NonOilen", perform worse economically.The materials rPET and PETG perform best economically, but worst ecologically.The technical-ecological evaluation is shown in figure 3b.A cluster of the materials carbon, rPLA and PLA has formed in the upper right corner, which combines the best from an ecological and technical point of view.The material rPLA again proves to be clearly the best choice here.To the left of the cluster are the materials PETG, rPET, "BioFusion" and "Dark Wood", which together form a diagonal sloping down to the right.It is also noticeable that the materials rPLA, PLA, "Carbon", "NonOilen" and "Dark Wood" are on a similar ecological level.The figure 4a shows the projection according to economic/technical criteria.PETG is in the lead by a slight margin.It has achieved the highest score on the technical and on the economic axis.Just behind it come the materials rPLA, PLA and rPET, the latter having achieved exactly the same score and thus lying on top of each other.The "NonOilen" material performs worst in economic terms.This is mainly due to the poor printability due to the extreme warping and the long printing time.Technically, the material carbon is one of the front runners.The high filament costs, as well as the costs for set-up and nozzle wear, worsen the economic viability of the material.Printing with the filament "Biofusion" is technically less well positioned due to poor layer adhesion, but PETG is undercut by the filament with added wood "Dark Wood".In particular, the strong deviations in the dimensional stability analysis and the reduced tensile strength worsen the technical value of this filament.With the 3D calculator of "geogebra.org"it is possible to represent points in a three-dimensional space [25].Accordingly, the coordinates of the eight filaments could be entered into the system to convert the information from the three two-dimensional projection into a single 3D strength diagram (see figure 4b).A material achieving the ideal score in all three dimensions would be congruent with the upper right corner of the cube.The printing material rPLA scored the highest when all three areas were considered, placing it ahead of the starting material non-recycled PLA.The materials rPET and "Carbon" are in the midfield and "Dark Wood" is the worst.

Conclusion
The results in this contribution include an investigation of various commercially available 3D printing materials.For this purpose, materials were investigated which fulfil the criterion "recycled" and/or "biobased".The investigations of the technical criteria show that despite compliance with the manufacturer's specifications for 3D printing, the fibre-reinforced materials in particular show significant deviations in tensile strength.Some of these deviations are also found in the literature and can be explained, for example, by defects that become visible using SEM.With the exception of one material the dimensional deviations of the filaments are quite satisfactory.The overall comparison including all areas shows that the recycled materials rPLA and rPET can definitely keep up with the conventional materials.The investigations also show that the cost advantages of some recycled 3D printing materials available on the market can already be used today in combination with the ecological advantages.The use of bio-based and recycled materials for FDM offers a real opportunity to mitigate the impact of production on the environment.The reintegration of materials back into the recycling loop ensures a reduction in waste, while maintaining the same technical value over several cycles.In the case of composites, further research is needed to achieve high-quality results.

Figure 3 :
Figure 3: a) Strength diagram economic/ecological and b) Strength diagram ecological/technical.

Figure 4 :
Figure 4: a) Strength diagram economic/ecological and b) 3D strength diagram with three criteria.
of Engineering Against Failure Journal of Physics: Conference Series 2692 (2024) 012042

Table 2 :
Measured technical properties