Cost Impact of Complex Geometry in AM

How AM geometry cost differs from subtractive cost

Additive manufacturing removes material from the build envelope. A subtractive shop pays for chip removal, tool wear, and fixture time. An AM shop pays for powder or resin volume, laser energy, build time, support material, and post-processing. When a part has thin walls, deep cavities, or complex internal structures, the cost model shifts from material removal to material deposition.

A solid block of 30 by 30 by 30 millimeters costs more powder than a hollow lattice of the same outer dimensions. In SLS nylon, every layer that fills a cavity adds material. In metal powder bed fusion, every layer that fills a cavity adds powder that must be laser melted. If the cavity is small and enclosed, the laser must spend extra time filling it. That time translates to machine hours, and machine hours are a core cost driver.

Designers often assume that complex shapes are free in AM because there are no molds. The opposite is true in many cases. A shape that would cost one injection mold may cost three times more in AM if it requires a long build time and heavy post-processing.

Which features drive AM geometry cost

The cost of a part is not determined by its visual complexity alone. It is driven by how the geometry interacts with the machine. The main drivers are wall thickness, overhang angle, internal density, support volume, and surface area.

Thin walls below the process minimum cause failure. If a wall is too thin, it may not fuse. If it is too thick, it takes longer to print and may crack during cooling. A wall that is just thick enough to print is cheaper than a wall that is 20 percent thicker than necessary.

Overhangs above 45 degrees usually require support. In resin DLP or stereolithography, overhangs can be printed without support if the resin cures quickly. In metal PBF, overhangs need support. The support material must be added, printed, and removed. Removal of metal supports is manual and time-consuming. The more supports, the more cost.

Internal density matters. A 100 percent dense part costs more powder than a 50 percent dense part with the same outer shape. For functional parts, density must meet mechanical requirements. For display models or light brackets, a lower density or a lattice structure can reduce cost.

Surface area affects time in some processes. In resin, the build time is often fixed by the number of layers and the scan strategy. In PBF, a large surface area can increase the time the laser must spend on the part. A long, narrow part with many small features may take longer to print than a compact block of the same volume.

How design complexity changes build time

Build time is not just the time the laser or projector is active. It includes the time the machine spends heating, cooling, and moving parts. A part with many small features may require a slower scan strategy. The machine cannot scan too fast or the features will undermelt.

A part with a large flat bottom is usually faster than a part with many small holes. Holes require the laser to scan inward and outward. Each hole adds a small amount of time. A part with a large cavity is usually faster than a part with many small cavities. A large cavity can be filled with a standard hatch pattern. Many small cavities require the machine to change direction more often, which increases scan time.

Orientation changes the cost. A part that stands on a flat face is stable. A part that must be tilted to reduce overhangs may have a larger support area. A part that must be printed in multiple orientations to reduce warping may require more time and more supports.

How to reduce AM geometry cost without losing function

Reduce wall thickness to the minimum that passes inspection. Do not add material for aesthetics. A 1.5 millimeter wall in SLS nylon is usually enough for a non-load-bearing bracket. A 2.5 millimeter wall costs more powder and takes longer to print.

Replace solid features with lattices or ribs. A solid rib can be replaced with a lattice of the same stiffness at a lower cost. A solid boss can be replaced with a hollow boss with internal ribs. The part still works, but it uses less material and takes less time to print.

Increase overhang angles where possible. A 30 degree overhang may need support. A 45 degree overhang may not. A 60 degree overhang is easy to print. If the design allows a sloped feature, use it. If the function requires a vertical wall, accept the support cost or redesign the part to remove the wall.

Reduce the number of features. A part with 100 small holes costs more than a part with 10 larger holes if the holes are the same diameter. Merge features where possible. Remove features that do not affect function.

Use the process limit in your design. If the printer can print a 1.2 millimeter wall, do not design a 1.2 millimeter wall if the part does not need it. Do not design a 1.0 millimeter wall if the printer cannot print it reliably. Design to the process, not to the drawing limit.

How to write a clear RFQ for AM geometry cost

A vague RFQ leads to vague quotes. If you send a drawing without specifying the material, tolerance, finish, or quantity, the vendor must assume. The vendor will assume the most expensive option to protect its margin. The quote will be high. You will not know if the high price is because of your design or because of the vendor’s assumption.

A clear RFQ should include the part number, material, surface finish, tolerance, quantity, and delivery date. It should specify whether the part is for a prototype or production. It should specify if the part requires post-processing such as heat treatment, machining, or anodizing.

If the part has multiple materials, specify the material for each feature. If the part has a tolerance that is tighter than standard, specify the tolerance and the feature. If the part has a surface finish requirement, specify the finish and the area.

Example RFQ text:

Part number: BRK-100
Material: SLS nylon
Quantity: 50
Tolerance: +/- 0.1 mm
Surface finish: As printed, no machining
Delivery date: 30 days
Notes: Part has 12 holes of 5 mm diameter. No supports required on flat bottom face.

This RFQ gives the vendor enough information to price the part. The vendor can calculate the material, the build time, the support volume, and the post-processing cost. The quote will be closer to the actual cost.

How to compare quotes fairly

Do not compare quotes from different vendors using only the final price. The final price includes the vendor’s margin, overhead, and risk. Two quotes can be 10 percent apart for different reasons.

Compare the cost drivers. Ask each vendor to break down the quote into material, build time, support, and post-processing. If one quote is 15 percent higher because of more support material, the difference may be due to a different build orientation. If the difference is due to material, the vendor may have used a higher grade material.

Compare the design revision. If you sent the same drawing to three vendors, the quotes should be based on the same design. If one vendor changed the orientation, the cost will change. If one vendor added a feature that is not in the drawing, the cost will change.

Compare the quantity. A quote for 10 parts and a quote for 100 parts are not directly comparable. The per part cost for 100 parts is usually lower because the setup cost is spread over more parts. If you need a fair comparison, ask for a quote for the same quantity from each vendor.

Use a table to compare quotes.

| Cost driver | Vendor A | Vendor B | Vendor C |
| Material | 12 percent | 10 percent | 14 percent |
| Build time | 8 hours | 10 hours | 9 hours |
| Support material | 5 percent | 8 percent | 4 percent |
| Post-processing | 15 percent | 10 percent | 18 percent |
| Total | 100 percent | 100 percent | 100 percent |

This table does not show the actual cost. It shows the relative cost. It helps you see where the cost is coming from. If Vendor B has a higher build time but a lower support cost, the difference may be due to a different build orientation.

How to verify cost before release

Before releasing a part for production, verify the cost with the vendor. Ask the vendor to run the part through the machine planning software. Ask for a simulation that shows the build time, the support volume, and the material usage. If the simulation shows a cost that is 20 percent higher than the quote, the vendor may have missed a cost driver.

Ask the vendor to identify the most expensive feature. The most expensive feature is usually the one that requires the most support, the most build time, or the most post-processing. If the most expensive feature can be redesigned, the cost can be reduced.

Do not release the part without a cost review. A cost review takes 30 minutes. It can save thousands of dollars on a production run.