Optimizing Material Selection for Large Scale 3D Printing: A Step-by-Step Approach

The correct material for your specific additive manufacturing application improves the print’s characteristics significantly. CEAD Group not only develops and supplies large scale printing solutions, but also advices organisations with using the right material. This article walks you through the process of selecting the right material for your 3D print. The number of available materials is constantly increasing, how do you determine the best material for your application?

Thermoplastic versus thermoset materials

This article is aimed at large scale 3D printing. For medium and desktop size printing, both thermoplastic materials and thermoset materials are used. While for large format additive manufacturing (LFAM) thermoplastic materials are the only viable option.

Thermoplastic polymers can be melted and reshaped multiple times. Unlike thermoset polymers, this type of polymer does not crosslink bonds. Thermoplastic polymers stick together through polymer chain entanglements, ‘Van der Waals forces’, and hydrogen bonds. This characteristic makes thermoplastic polymers the perfect material for sustainable applications since it can be recycled and printed again.

1: Select the right type of polymer for your print

The first step in the process of selecting the right material is selecting the right base polymer. As always, the application of the print influences the choice for the right polymer. We distinguish 3D prints between molds and tooling, structural end-parts, non-structural end-parts and research.

Let’s take the example of 3D printing for molding. In case the print is used as a mold for another process, it is important to consider the temperature to which the mold will be exposed. Selecting a polymer that has good dimensional stability at the intended use temperature is the right step. Materials like ABS, PC and PEI are used in mold applications, each with their own use temperature limit.

Additionally, other environmental aspects should be considered. In case of an application that involves water, think of a printed boat, a material should be selected that does not deteriorate when exposed to water. Polyolefins like PP and HDPE are known for their low water absorption. Also, polymers that absorb a little moisture, like PETG, can still be stable in the expected operating temperature range.

A final consideration is the exposure of the print to chemicals that can attack the polymer material. These are particular cases, like in industrial seals, or aerospace applications where contact with aggressive hydraulic fluids can occur. Polymers with aromatic backbones like PPS, PEEK and PEKK are known for their excellent resistance to chemicals

1.1 semi-crystalline vs amorphous

The two primary families of thermoplastic polymers are semi-crystalline and amorphous polymers. When cooled down from a molten state, semi-crystalline polymers form areas where the polymer chains orient themselves in an ordered fashion and form crystallites. These crystallites are surrounded by randomly oriented amorphous material. The amount of material that is in the crystalline state after being processes is called the degree of crystallinity (DoC).

In general, polymer materials that possess the two following characteristics are semi-crystalline. Firstly, they should have a constant repeat unit order in their backbone. Additionally, their backbone side groups should be relatively small. Examples of polymer materials that meet these two properties are: PP, PLA, PA6, PPS, PEEK.

On the other side of the spectrum are amorphous materials. These polymers do not form any crystallites when cooling down. Additionally, their polymer chains remain randomly orientated, as opposed to the crystallized structures of semi-crystalline polymers. Polymer materials which can have the monomers in their backbone randomly ordered, or that have relatively large side groups are generally amorphous. Examples of amorphous properties are ABS, PETG, PC, PEI.

Processing semi-crystalline polymers vs amorphous polymers

In order to illustrate more on the processing differences between amorphous and semi-crystalline polymers, two examples are given:

In large format additive manufacturing, semi-crystalline polymers are slightly more difficult to process. Upon cooling down of a print, the degree of crystallinity in the material is changing. Because the crystalline fraction of the material has a different density than the amorphous fraction, this results in internal stresses in the material which causes the printed structure to deform and warp.

Also, when applying a freshly printed polymer bead of material on an already printed layer, the interface of the new and old bead both need to become molten to allow enough polymer chain mobility to achieve proper layer bonding. The crystallites in semi-crystalline material require a lot of extra energy to be molten compared to amorphous material. This extra energy required in the bonding process can only be added by the newly printed bead, which should therefore be printed relatively hot.

1.2 elastomeric materials

Another type of thermoplastic materials is that of elastomeric materials or elastomers in short. Elastomers are a type of polymer that have the ability to stretch and return to their original shape. This is comparable to a rubber band.

This group of elastic plastics refers to rubbers: natural or synthetic substances that have the capability to integrate the elasticity of rubber into a printed object. Elastomers generally have a high flexibility and thus a low stiffness. While CEAD’s pellet extruders are able to print elastomeric materials, they print almost exclusively with thermoplastic polymers.

2: Type of grade

After the selection of the right base polymer, you may find that a variety of grades of the same polymer exist. Suppliers often offer several different grades, optimised for certain use cases or processes. Two factors should at least be considered when selecting a polymer grade:

Viscosity

The viscosity of the material at the melt temperature is critical for successful LFAM. When the viscosity is too low (watery) the material will not keep the shape of the extrusion bead but will flow away, and no structure can be printed. When the viscosity is too high (peanut butter) the polymer chains in the bead will not have sufficient mobility to achieve proper bonding to the previous layer of material.

Crystallisation speed

As already mentioned, semi-crystalline materials are more difficult to process. Grades that crystallize fast (being designed for processes which a short cycle time and thus high rate of cooling) will quickly achieve a degree of crystallinity close to what is maximum achievable upon cooling down.

This is disadvantageous while printing, because a high degree of crystallinity means that a lot of extra heat needs to be added in the following layer to melt all the crystallites in the previous layer and to achieve proper layer bonding. Slow crystallizing grades are preferred in LFAM.

3: Reinforced vs virgin materials

The third step in the material selection process consists of the choice between fiber reinforced or non-reinforced (virgin) material. The addition of fibers to the material affects the printability of the material and the performance of the printed part. The presence of fibers helps to improve the dimensional stability of the printed material. The shrinkage and warpage effects are also reduced when using reinforced materials. This comes at the cost of a slightly increased melt viscosity.

It is worth noting that reinforced materials can have a wear effect on the barrel and screw. You therefore must use a wear resistant barrel and screw when printing reinforced materials. CEAD’s pellet extruders are always designed to print reinforced and non-reinforced materials.

We can distinguish 3 kinds of reinforcing fibers: glass fibers, carbon fibers and bio-based fibers.

Glass fibers

Glass fibers provide a cost-effective reinforcement for additive manufacturing resins. They are the middle ground between price and performance and give a significant improvement of stiffness to the printed structure.

Another characteristic can be found in the low thermal conductivity of glass fibers. They have a lower thermal conductivity than carbon fibers. Glass fiber prints therefore cool down a bit slower compared to similar prints with carbon fibers.

Carbon fibers

The largest improvement of mechanical properties is provided by the addition of carbon fibers in materials. Carbon fibers have excellent strength and stiffness, combined with a lower weight than glass fibers.

Additionally, carbon fibers have a low Coefficient of Thermal Expansion (CTE), which helps to reduce the thermal expansion of the complete printed structure. This is very advantageous for applications such as molds that are used at elevated temperatures.

Carbon fibers are the most expensive of the reinforcing fibers and their use should be justified by the field of application of the printed part.

Bio-based fibers

The final type of fiber used in additive manufacturing is classified as bio-based fibers. These fibers originate from renewable sources and are often biodegradable. Bio-based fibers are excellent for sustainable purposes, but, these types of fiber provide less strength in the printed object compared to glass and carbon fibers.

Like most bio-based materials, the maximum process temperature before degradation occurs is limited and the material should be dried extensively.

4. Additives

Once the right fiber reinforcement has been chosen, the selection process can be concluded with the ultimate step: the consideration of material additives. Additives are commonly used throughout the polymer industry and are added to polymer grades for specific purposes. Commonly used additives are:

• UV stabilisers: these additives slow down the degradation of the polymer when exposed to sun light. Carbon black is known to be a good UV stabiliser.
• Heat stabilizers: these additives reduce the polymer degradation when exposed to elevated temperatures, for instance during the pelletizing process and the 3D printing process.
• Flame retardant: these additives limit the flammability of the polymer. Commonly used in transport applications where a certain degree of flame retardancy may be required.
• Anti-bacterial additives: for applications where the print is submersed in water. Anti-bacterial additives prevent the growth of bacteria onto the print surface.
• Coloring additives: just like filament, thermoplastic polymers can also be colored according to wish. Material suppliers do so by mixing the polymer with a batch of material that has a concentrated number of coloring pigments added to it, called a master batch.

Usually, suppliers of 3D print material have a selection of grades with certain additives available. Additives increase the cost of 3D print materials, so carefully select which additives are needed for your application.

CEAD advice for additive manufacturing materials

CEAD has extensive knowledge of the materials used in additive manufacturing and has good contacts with most material suppliers active in the industry. We can support you in selecting the optimal material for your AM application using the Flexbot or pellet extruder series.