Explainer: Infill Density in Metal AM Parts

What is metal AM infill in PBF builds

Metal AM infill refers to the internal structure of a part printed using powder bed fusion. In a PBF build, layers of metal powder are sintered into solid material. The infill density determines how much of the internal volume is filled with solid metal.

A part with 100 percent infill is solid throughout. A part with 50 percent infill has an internal lattice or pattern that fills half the volume. The rest is air. This internal structure is invisible from the outside but defines the part’s mechanical behavior.

The key terms here are infill density and infill pattern. Density is the percentage of solid material. Pattern describes how that material is arranged. Common patterns include cubic, gyroid, and triangular. The pattern influences how stress is distributed. A gyroid pattern, for example, distributes force in multiple directions. A cubic pattern is simpler but may be weaker in certain loading scenarios.

Infill is not just a setting in a slicer. It is a design variable. Changing the infill percentage changes the part’s mass, its resistance to deformation, and its cost. It also affects the post-processing steps required to remove support material and clean the part.

How infill percentage affects strength

Strength in a metal part is not a single number. It depends on the type of stress applied. A part may have high compressive strength but low tensile strength. Infill percentage changes how the part handles these different loads.

Higher infill density generally increases strength. A part with 90 percent infill will be stronger than a part with 40 percent infill under most conditions. The solid metal provides a continuous path for force to travel. When the infill is low, the internal voids can act as stress concentrators. Under cyclic loading, these voids can become the starting points for cracks.

For static loads, the difference may be less dramatic than for dynamic loads. A bracket that holds a fixed weight may tolerate lower infill. A turbine blade that spins at high speed will not. The fatigue life of a part drops significantly as infill density falls below a certain threshold. That threshold depends on the material, the pattern, and the load direction.

Engineers must test parts to find the right balance. A standard test may involve bending a bar to failure and measuring the strain. Another test may apply a cyclic load for a set number of cycles. The results show how infill affects the part’s performance. If a part fails at 80 percent infill but passes at 100 percent, the design must be revised.

How infill percentage affects weight

Weight is one of the most direct benefits of lower infill. In aerospace and automotive applications, mass is a critical factor. Every gram saved can improve fuel efficiency or payload capacity.

In PBF, the metal powder is sintered into solid material. If the infill is low, less metal is used. The part weighs less. The build time may also decrease because the machine does not have to deposit as much material. However, the time saved is not always proportional to the weight saved. The laser still scans the same area. The difference is in the number of passes and the power applied.

A common design approach is to use a solid outer shell with a lower density core. This reduces weight while maintaining surface quality. The outer layers are critical for dimensional accuracy. The core can be a lattice structure that is lighter. This approach is common in structural brackets and housings.

Weight reduction must be balanced against strength. A part that is too light may deform under load. The design must ensure that the infill density is sufficient for the expected loads. Engineers use finite element analysis to simulate the part. The simulation shows stress concentrations and deflection. If the deflection exceeds the allowable limit, the infill density must be increased.

How infill settings impact cost and lead time

Infill percentage has a direct impact on material cost. More infill means more powder is consumed. Powder is expensive. A part with 100 percent infill will consume significantly more powder than a part with 50 percent infill. The cost difference can be substantial, especially for large parts.

Build time is the second factor. A part with 100 percent infill requires more laser passes. The build chamber is occupied for a longer period. This increases the cost per part. A part with lower infill can be built faster. This allows a service bureau to produce more parts in the same time.

Lead time is also affected. If a part has a complex infill pattern, the slicing time may be longer. The machine setup may require more time. These factors can extend the lead time. For low-volume production, lead time is often more important than cost. A part that takes two weeks to build may be less valuable than a part that takes one week.

Buyers should ask service bureaus about infill options. Some bureaus offer standard infill densities. Others allow custom settings. The cost structure should be transparent. A part with lower infill may cost less, but the cost difference should be clear. The buyer should also ask about the impact on post-processing. Lower infill may require more support removal. This adds labor cost.

A worked example: A structural bracket

Consider a structural bracket used in an industrial machine. The bracket connects two frames. It must hold a static load. It will not experience high cyclic stress. The material is a common alloy steel.

The first version of the bracket was designed with 100 percent infill. The part was strong but heavy. The machine was designed for a fixed load, but the extra weight added stress to the mounting points. The bracket was heavier than necessary.

The engineer revised the design. The outer shell was kept solid. The core was changed to a gyroid pattern with 40 percent infill. The part was rebuilt. The weight was reduced. The static load test passed. The part did not deform.

This example shows how infill can be used as a design tool. The change did not require a new material. It did not require a new process. It only required a change in the infill setting. The result was a lighter part with the same functional performance.

The key was to test the part. The engineer did not assume that 40 percent infill would work. The part was tested under the expected load. The results confirmed that the infill density was sufficient. This process should be repeated for any part where infill is used as a design variable.

How to specify infill for service bureau quotes

When requesting a quote from a metal AM service bureau, infill specification is critical. A generic request for a metal part will not yield an accurate cost. The buyer must define the infill density and pattern.

The quote should break down the cost by infill. A part with 100 percent infill will cost more than a part with 50 percent infill. The difference should be itemized. This allows the buyer to see the impact of infill on the total cost.

The buyer should also specify the mechanical requirements. If the part must withstand a specific load, the infill density must be high enough to meet that requirement. The service bureau should be able to confirm that the infill setting meets the requirement. If the requirement is not clear, the infill density should be increased to ensure safety.

Infill is a design decision. It is not just a cost-saving measure. It is a way to optimize the part for its function. A well-designed infill can reduce weight, save material, and lower cost. A poorly designed infill can lead to failure, rework, or increased cost. The buyer must work with the service bureau to define the infill setting based on the part’s requirements.

Common mistakes in infill selection

One common mistake is using the default infill setting without considering the part’s function. The default setting may be 100 percent infill. This is safe but not always efficient. It may not be the right choice for every part.

Another mistake is reducing infill to save cost without testing. The part may pass a static test but fail under dynamic load. The infill density must be matched to the expected loading conditions. A part that will experience vibration or impact requires a higher infill density than a part that only holds a static weight.

A third mistake is ignoring the infill pattern. Two parts with the same infill density can have different strengths if the patterns are different. A cubic pattern is simpler but may be weaker in certain directions. A gyroid pattern distributes stress more evenly. The pattern should be chosen based on the load direction.

The final mistake is not documenting the infill setting. If a part is successful, the infill density and pattern should be recorded. This documentation is essential for future production. It ensures that the next part is built the same way. It also helps in troubleshooting if a later part fails.

The infill setting is a critical part of the design. It must be selected carefully. It must be tested. It must be documented. These steps ensure that the part performs as intended.