Packing density in SLS 3D printing is the percentage of fused powder in a build versus the total powder in a build. The fused powder is the parts printed. If you were to print an entire build as one sintered block, your volumetric packing density would be 100% and your mass packing density would be slightly lower (due to “buffer powder “around the addressable build).  While few people ever have a reason to do this, Formlabs engineers have managed to print an entire, solid build during the stress-testing of Fuse SLS 3D printers. 

Mass-based packing density, which is shown in PreForm, is the mass of sintered parts divided by the mass of all powder required. This includes all the powder required to complete the build, including “buffer” powder around the addressable build volume. 

Volume-based packing density is the CAD volume of parts after they’ve been scaled to account for shrinkage*, divided by the addressable build volume. Volume-based packing density is most commonly used in packing software, including Materialise Magics, HP, and EOS. PreForm also includes volumetric packing density so that it’s easier to do a one-to-one comparison between Formlabs and competitors. 

*PreForm scales models to account for shrinkage so that printed parts are dimensionally accurate. 

The best way to determine throughput is the quantity of parts in a given time period. This relates to packing density because more parts packed into a build means more finished parts in a given time period, or higher throughput. 

For example, packing a lot of small parts around large ones will maximize efficiency, making it possible to print more parts in the same amount of time. Therefore, a high packing density can increase throughput as more parts can be printed in the same amount of time. 

Some manufacturers may obfuscate throughput by pointing to other metrics, such as the volume of powder processed per hour instead of the number of sintered parts. Powder processed doesn’t equal throughput, as not all powder is being sintered into parts in a build. Ultimately, the number of parts produced in a given time period is the best indicator of throughput. 

Refresh rate is the ratio of fresh powder to used powder. Fresh powder is mixed with used powder and loaded back into the hopper of the printer, providing material for the next print. The used powder acts as the support for parts in a print batch and is reclaimed during sifting in Fuse Sift and Fuse Sift X1. 

Mass is used for refresh rate calculations. Used powder is measured by mass, and fresh powder is added by mass. Powder cartridges, scrap, and reclaim are all tracked on a scale, with operators measuring mass. Volume-based density, therefore, doesn't connect to refresh weight and powder usage. 

PreForm calculates a number of useful statistics related to powder consumption and build volume utilization, displaying these values in the Summary flyout. Here, we clarify what these values represent and how they are calculated. 

Summary Flyout Categories and Descriptions

Given a sintered model density (⍴_s), there are three key volumetric representations that can be used for sintered powder calculations, all of which serve different purposes. They include pre-shrinkage, post-shrinkage, and powder consumed. The mass is always consistent between these three representations.

Pre-Shrinkage Volume

The pre-shrinkage volume (V_s,pre) is defined based on the print-scale corrected models (or the models after PreForm compensates for the thermal shrinkage of a part). Pre-shrinkage volume is a key metric for determining how efficiently the build volume is used, as this volume is what’s used for volumetric packing density calculations. This is because all of the collisions, packing, etc. are calculated based on this print-scale correction representation.

Post-Shrinkage Volume

The post-shrinkage volume (V_s,post) is defined based on the final model volumes, but most importantly, it is used to calculate the mass of sintered powder. Note: this is the only volume representation that can be used with the specified sintered density (⍴_s).

Powder Consumed or Powder Use Volume

The powder consumed or powder use volume (V_s,use) is defined as the volume of unsintered powder required to create the sintered models. This is a key metric for powder usage estimates, as this is the volume of powder you need to input into a printer to sinter into the models in the scene. 

This is calculated based on the mass of sintered material and the unsintered powder density (⍴_u):

V_s,use = ms / ⍴_u

Mass

The mass of sintered material (m_s) is relatively straightforward, with the only important caveat being that the sintered mass must be calculated using the post-shrinkage volume, since ⍴s is defined based on the final model volume.

m_s = V_s,post ∙ ⍴_s

The unsintered powder is straightforward in terms of converting between masses and volumes, given the density of unsintered powder (⍴_u). The only key distinction is powder contained within the build volume and powder contained within the build chamber.

Build Volume

The volume of unsintered powder contained within the build volume (V_u,BV) is used to calculate the volumetric packing density. This matches how other slicers, such as Materialise Magics and Autodesk Fusion with Netfabb, calculate volumetric packing density.

This is defined based on the build volume area where models can be placed (A_BV) up to the active build height (h_s), subtracting out the volume of the pre-shrinkage models:

V_u,BV = A_BV ∙ h_s - V_s,pre

Note that orange peel armor is included in the Build Volume sintered powder estimate.

Build Chamber Volume

The build chamber volume powder consists of the powder contained within the build chamber, but outside the usable build volume. This includes the volume of powder consumed in preprint (h_pre), postprint (h_post), and around the edges of the build volume by the powder boundary (A_BC):

V_u,BC = (A_BC - A_BV) ∙ h_s + A_BC ∙ (h_pre + h_post)

Mass

All unsintered mass calculations are simply handled using the unsintered powder density (⍴_u):

m_u = (V_u,BV + V_u,BC) ∙ ⍴_u

Cost per part is a mass calculation as powder is priced and consumed by the kilogram, not the cubic centimeter. In PreForm, users can enter their refresh rate and powder cost, as powder discounts are available for bulk orders. Once this information has been input and models added to a build, a cost per part will be calculated based on how much powder a part will require.

If the sintered parts removed from a build are the same percentage of mass as the refresh rate, it’s easy to replace them with new, unaged powder, as the mass of parts removed is equivalent to the mass of new powder that needs to be added. Powder management stations, Fuse Sift and Fuse Sift X1, monitor reclaimed powder and dispense and mix used and new powder automatically for easy workflows and reduced material waste.

Depending on parts in a build and how they are oriented, it may be desirable to age powder. In PreForm, this can be done by moving parts up in a build to create a buffer of what will be unsintered powder below the parts. 

While volume-based packing density is often referenced in marketing materials and in packing software, using mass to determine packing density is more accurate and relevant when it comes to determining refresh rate, the amount of powder required for printing, and cost per part. 

PreForm shows mass-based metrics in addition to volume-based ones, making it easier to determine how much powder is required for printing, ensuring print success, as well as delivering accurate refresh rate and cost per part information.