Hollowing Parts for 3D Printing

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Hollowing Parts for 3D Printing

Written By: Ron Luther, Manufacturing Specialist - Oct 11, 2023

Although 3D printing does offer significant design freedom when compared to traditional manufacturing, that doesn’t mean that it isn’t possible to optimize designs prior to print. Hollowing is one of the most common optimization methods and comes with its own set of best practices in order to strike a balance between strength, weight, cost and stability.

What is hollowing?

Hollowing or coring is the process of modifying a part by removing internal material; the primary benefits of hollowing a part are reduced cost, weight, and print time. Dimensional accuracy can also be improved - parts with a uniform wall thickness also cool more evenly, reducing the buildup of stress in specific locations that can cause warping and deformation. A well designed 3D printed part is one that uses the least material possible to meet the strength requirements of the intended application. 

 

Although hollowing a part can increase complexity of the geometry, there are no added manufacturing costs for added complexity in a 3D printed part - in fact, costs typically drop.

Preparing a part for 3D printing

There are a few things to watch out for when hollowing parts in preparation for 3D printing.

The first consideration when hollowing a part is the strength of the part - will it still be able to meet the loading and bending requirements for the application? If too much material is removed, the part can be significantly weaker than a completely solid part, but if care is taken to add appropriate reinforcement along points of stress, strength can be maintained while removing redundant material. After material has been removed, the part should be analyzed (either through simulation or physical testing of a printed prototype) for loading and bending. Based on this analysis, we can add walls and ribs (beam elements) to stiffen the part in the direction of bending/loading.

Ribs added to hollowed part to resist bending in one direction
Perpendicular ribs added to hollowed part to resist bending in two directions

The part on the left has ribs to resist bending in one direction. On the right, ribs are perpendicular to resist bending in two directions.

When hollowing a part, be sure that there is a drain hole - material will be trapped inside a fully enclosed hollow part unless a hole is added that allows for the removal of the powder, resin or breakaway support structures inside the part.  (Check out our design guide for more technology-specific tips here.)

drain hole added to 3d printed part in order to remove support material from internal cavity

The automatic shelling/hollowing tools may generate sharp internal corners. In general, abrupt transitions become points of stress concentration under load, so they are best avoided. Smooth transitions can be more self-supporting, which allows for more orientation options and reduces the need for supports. Once a part is hollowed, it should be reviewed and any sharp corners should be smoothed or rounded over using a fillet tool.

internal features with sharp inside corners are prone to damage
adding fillets to internal corners increases strength

Case study

In this example (a jig), we want to use less material in order to reduce weight, material consumption and print time:

unhollowed jig

We will first decide on wall thickness - depending on the process and material selected, the minimum wall thickness that is suitable for our application will vary. Absolute minimums are defined in our design guidelines, but thicker walls may be required for mechanical strength. For example, in an FDM process (which has more weakness concentrated along the layers lines), the shearing strength of the part in the z axis is going to be proportional to the surface area of bonding from layer to layer. For a process like SLS, which produces a more isotropic part, thinner walls will not exhibit the same mode of failure. When in doubt, review the material datasheets for an in-depth overview of mechanical properties.

2mm wall thickness
3mm wall thickness
4mm wall thickness
5mm wall thickness

After hollowing, we can observe that many of the functional features could potentially shear off. To avoid shearing, we can add generous fillets to the bottom of these features where they connect to the base. Adding these rounds will result in a significant improvement of the shear strength of these features.

section view of internal corners

Because this part will be under some load from multiple directions, we have decided to further reinforce it with ribs in two axes. In this instance, the added ribbing is much shorter than the outer walls, since the full height was not required to resist bending in that axis - this conservative use of material achieves an additional reduction in material required.

hollowed part with ribs added for reinforcement

Finally, there is always the option of filling the hollow part with another material of choice, such as a two part resin or epoxy. It could be lower cost, or have material properties that are desirable. This is also a good way to end up with a solid part while avoiding the warping and material cost that comes with printing solid.

hollowed 3d printed part filled with resin to increase strength after printing

Cost Comparison

Reviewing the above part, you can see how the same part, hollowed to a 3mm wall thickness, can result in significant cost reductions:

3D Printing Process

Cost (Solid)

Cost (Hollowed)

Print time (Solid)

Print time (Hollowed)

Fused Deposition Modelling (FDM)

$253.12

$125.06

4 hours

3.5 hours

Stereolithography (SLA)

$513.14

$233.87

9 hours

8 hours

Selective Laser Sintering (SLS)

$192.23

$147.56

n/a*

n/a*

MultiJet Fusion (MJF)

$359.35

$169.28

n/a*

n/a*

PolyJet (printed glossy)

$796.66

$411.38

3.5 hours

3.5 hours

Direct Metal Laser Sintering (DMLS)

$1963.83

$905.12

n/a*

n/a*

*batch 3d printing process

Technology breakdown

Hollowing is applicable to all of our additive technologies, although each technology has different constraints and mechanical properties, which must be taken into account:

 

Selective Laser Sintering

SLS sinters at a very high temperature, so hollowing can be useful to avoid heat buildup and deformation in geometries with variable thickness. That said, large hollow parts with very thin walls (below 4 mm) can actually be more likely to warp, especially over large surfaces or open-ended enclosures. Reinforcing beams or ribbing are recommended in those situations, and a drain hole should be present to remove powder.

 

Stereolithography

Since it is very heat-sensitive, SLA can be very sensitive to parts with very thick features - in addition to reducing cost, hollowing resin parts can significantly improve their dimensional accuracy. A drain hole should be present to drain any trapped resin.

 

Fused Deposition Modeling

Another heat intensive technology, FDM can warp when very thick parts are produced, especially in areas of uneven thickness. An escape hole should be present to remove breakaway or soluble support. Another option (unique to the FDM process) is to use a sparse infill on the part - rather than coring the part, the print software can be used to automatically produce a sparse-filled interior, where instead of printing the part solid, the interior is partially filled with a repetitive pattern, like a honeycomb or grid-like lattice. This technique maintains the object's strength and stability while reducing the overall material usage.

 

PolyJet

Since the biggest driver of costs for Polyjet parts is raw material costs, reducing material usage will significantly reduce price. It is important to note that hollowing the part will have no impact on the cost if the hollowed area is completely filled with support, so orientation and overhangs do matter here. Similar to stereolithography, an escape hole should be present to remove support gel.

 

MultiJet

Hollowing of parts is critical in MJF to produce a high quality part - MJF exhibits substantially more warping than SLS does, due to the larger disparity between the temperature of the powder in the build and the temperature at which the Nylon sinters, so  controlling wall thickness is critical to prevent warping. Walls thicker than ¼ inch are very likely to deform or resolve with an ‘orange peel’ finish. As with SLS, a drain hole should be present to remove powder.

 

Direct Metal Laser Sintering

DMLS sinters at a very high temperature, and requires rigid supports to help prevent warping. Hollowing parts does reduce cost and weight, but metal supports can be very difficult to remove so hollowing must be reviewed by a technician to ensure that it is possible to fully remove all support. A substantial escape hole should be present to remove powder and any trapped supports.

Need Help?

The above is a very simple example of how and why to hollow parts for 3D printing. If you’re looking to order parts where complex optimization might be required, but aren’t sure about the best route to take, feel free to reach out to us at sales@forgelabs.com, and we will be more than happy to lend a hand and make recommendations about how to optimize your part geometry for production.