Week 8: Polymer AM

Additive Manufacturing - Principles and Applications

• This course covers advanced understanding of polymer laser sintering.
• The course will cover recapping laser sintering, factors affecting part properties (pre-process, in-process, and post-process), scientific understanding of the process, material requirements, structure of final parts, and characterisation techniques.
• These sessions cannot cover everything; literature searches are recommended for a deeper understanding of the topic.

Re-cap of Laser Sintering

• Parts are built by selectively scanning and sintering cross-sections of powdered material.
• The process involves powder supply, a powder bed, scanning mirrors, and a laser.
• A diagram illustrates the process.

Why are we interested?

• Laser sintering enables complex designs without support structures (polymers).
• Parts can be assembled in one build step.
• Post-processing requirements are minimized.
• The process is time-saving.
• Down-facing surfaces have better surface finishes.
• Parts can have relatively high mechanical properties and stability.
• There's a full build volume.

Factors affecting part properties

• Pre-process factors: material choice (type and morphology), fillers (type and level), and thermal history.
• In-process factors: build orientation and part bed temperature.
• Post-process factors: finishing methods.

Material choice

• Traditionally, Nylon-12 is common, including fillers.
• Elastomers, PEEK, and polypropylene are also used.
• Choosing the right material is crucial for the desired properties of the part.
• Consider the application and the required properties of the part when selecting material.
• The properties might differ in comparison to injection molding.
• Some materials exhibit more susceptibility to in-process variations.

Morphology

• Powder deposition is critical.
• Issues to consider include low density, poor accuracy/surface finish, over-exposure of part areas, particle size, particle shape, and powder agglomeration.
• Particle size and distribution: Size and distribution affect powder packing and porosity. Average particle size (Nylon-12) is ~50 microns, with a range from ~10-90 microns. Larger particles result in poorer surface finish and lower density, while smaller particles could be susceptible to static, leading to poor deposition. Small particles are susceptible to nitrogen flow within the chamber (affected are the laser window; larger particles less so).
• A range of particles can assist with denser part beds and higher mechanical properties.
• Fine particles may aid the flow. Shape varies, with grinding sometimes leading to 'cornflakes' (illustrated). Smooth, spherical shapes are usually preferred.

Powder agglomeration

• Powder can clump together before or during the build process.
• Static build-up, often arising from sieving, can be an issue.
• Additives help flow but impact properties.
• Moisture absorption can occur.
• Drying may be needed to reduce issues.
• How much drying is affected by the material type.

Fillers

• Fillers impact mechanical properties, often with compromise.
• Excess filler reduces properties (ratio matters).
• Homogeneity of mixing is important.
• Mixing is simple but maintaining good distribution is difficult.
• Complex mixing methods (e.g., co-extrusion followed by grinding) offer improvements but can affect particle shape.
• Inclusion impacts resultant properties (e.g., glass beads in Duraform GF).
• Build parameters for filled materials are often similar to standards. The resultant part is a composite of filler surrounded by the base material.
• Young's Modulus increases with glass content (e.g., by 157% between 0 and 50%).
• Elongation at Break decreases with glass content (e.g., by 84% between 0 and 50%).

Thermal history

• Laser sintering is a thermal process.
• Unsintered materials are in a heated bed for a long time, affecting molecular weight.
• A decrease in melt flow rate and an increase in molecular weight occur over successive builds.
• Virgin and recycled materials are often combined to reduce cost, but virgin provides greater repeatability, and recycled can give increased properties (notably in elongation at break).
• Thermal conditioning can be used to improve elongation at break.
• Orange peel surface is a concern post material re-use.

In-process Factors

• Build orientation affects anisotropy, although less so than some other processes (e.g., FDM).
• Different orientations may produce differing mechanical properties.
• The discussion involves planning for this influence during real-life applications.
  • Build orientation affects mechanical properties (e.g., X, Y, and Z axes have different values for tensile strength, flexural strength, and compressive strength).
  • Part bed temperature: pre-heating of the build area is important, but uneven temperature distributions exist.
  • Edge temperatures often remain cool.
  • System upgrades can mitigate these issues.
  • Uneven part bed temperatures affect density, potentially decreasing it, in part bed temperature of 178°C (at identical energy density), which differs by roughly 4%.

Processing parameters

• Energy density is defined as Laser Power/(Scan Spacing x Laser Scan speed).
• It highlights energy input, but does not consider part bed temperature or layer thickness.
• Higher ED can lead to increased ductility.
• In some materials, scan speed affects properties differently from laser power.
• Volumetric energy density is just beyond the energy needed to fully melt a layer of material to achieve the best properties possible.

Characterisation methods

• Techniques assess materials and parts, including: DSC, MFI, HSM, PSA, and TGA.
• For example, DSC is a thermal analysis technique to measure heat flow differences between a sample and a known reference, relating to time and temperature.
• Different samples are used to determine necessary energy input.
• MFI is used to measure material viscosity, observing the amount of material that flows through a specific-size orifice under load in a given time. High melt flow rates correspond to low viscosities and low molecular weights.
• HSM is a microscopy technique applied to a heated base (with variations), to measure particle flow behavior (e.g., temperature at necking, agglomeration time).
• PSA assesses particle size and shape (e.g., sieving, optical microscopy, and laser diffraction).
• TGA measures sample mass at various temperatures, providing insights into physical or chemical changes (e.g., oxidation, vaporisation).
• Other techniques such as tensile and compression testing, and surface profilometry are also used.

Scientific understanding

• Laser sintering involves melting powder using a laser.
• In reality, several factors are more complex.

Material requirements

• The majority of laser sintering materials are based on Nylon-12.
• Nylon-12 is suited well since there is a large window between material melt and crystallisation temperatures.
• Bed temperature must stay in a molten state for uniform stress distribution during cooling.
• A narrow melt range allows higher bed temperatures.
• Other (amorphous polymers) and elastomer materials have lower mechanical but good dimensional properties.

Sintering behavior

• Several models predict sintering and related mechanical properties.
• The Frenkel model offers a suitable approach to many aspects.
• Viscosity is a key parameter, but the model's complexity is a consideration.
• Surface tension and temperature dependency also matter but are challenging to assess accurately for powders.

Structure of parts

• Parts have regions with varying melting proportions.
• Some particles are fully melted, crystallised, or have un-melted cores.
• Partial melting occurs due to insufficient energy input, differing with particle size.

Other techniques

• Factors in different processes (e.g., High Speed Sintering), such as energy input methods, may vary as will the presence of ink for some methods.
• Analysis of techniques used in other areas of additive manufacturing is recommended.

Content

• The material, sintering behavior, structure of parts, and characterisation methods are key course topics.