Six 3D Printing Processes for Prototyping: Which is Right for You?

Comparing cost, material strength, surface finish and suitability for functional testing.

A new product will only be innovative for so long, and an innovative design immediately loses its edge if a competing product beats it to market.

That’s why it’s crucial to choose the right prototyping process at each stage of product development.

3D printing, an additive manufacturing process that uses 3D CAD models to build physical objects layer by layer, is a frequently leveraged prototyping technology. Compared to more traditional prototyping processes, 3D printing can build more complex parts.

Even if manufacturing will eventually require a more traditional process such as injection molding, 3D printing is still useful in the early stages of development when prototypes are produced in small quantities.

In addition, building parts in thousands of thin layers enables design engineers to create highly complex geometries that cannot be produced with more traditional processes.

These days there are so many 3D printing processes available for prototyping that it can be difficult to decide between them. Here’s a look at the range of 3D printing technologies used for prototyping.

1) Fused Deposition Modeling (FDM)

(Image courtesy of Proto Labs.)

(Image courtesy of Proto Labs.)

FDM involves melting and re-solidifying thermoplastic resin (ABS, polycarbonate or an ABS/polycarbonate blend) in layers to form a finished prototype. 

FDM parts tend to be stronger than parts from other 3D printing processes, however, they can sometimes be porous and have a pronounced stair-stepping or rippling texture on the outside finish.

2) Stereolithography (SL)

(Image courtesy of Proto Labs.)

(Image courtesy of Proto Labs.)

Stereolithography uses a CNC laser to create parts from a pool of UV-curable liquid resin. 

Each time a layer is finished, the part is lowered in pool to allow the next layer of liquid to be solidified. SL yields an excellent surface finish, one of the best for an additive process.

Tony Holtz, technical specialist at Proto Labs, described the three standard surface finishes the company offers for SL:

“One is the unfinished part that leaves the dots or nubs from the support structure that the part is built off of. The second is a natural finish, where we sand those supporting structures down. Our standard finish is where we lightly sandblast the part with a very fine grit to give it a uniform look.”

Besides the three standard finishes, there are several secondary options for having parts finished so they can represent the production parts, including chrome plating, painting or applying soft touch finishes. Since SL parts are still relatively low in strength, selective laser sintering is good alternative for functional testing.

3) Selective Laser Sintering (SLS)

(Image courtesy of Proto Labs.)

(Image courtesy of Proto Labs.)

Selective laser sintering works similarly to SL, but rather than working on a liquid from the top down, SLS uses a CNC CO2 laser to fuse powdered material from the bottom up. 

The strength of SLS parts is better than SL, but still typically lower than those produced through traditional manufacturing processes such as machining. The strength also depends on where the heat resistance is coming from.

“If your heat resistance is coming directly down on top of the part, you’ll have comparable resistances, whereas if you’re coming from the side of the part, breaking into the layers, then the heat deflection may be a bit less,” said Holtz.

On the other hand, SLS can also be used as a production method, which makes it all the more attractive for prototyping.

The surface finish on SLS is a bit rougher than SL, but some additional finishing can offset that.

“We can apply a light bead blast finish to get rid of more residual powder and give it a more consistent look so the top surface and side profile blend out,” said Holtz. “That way, you don’t see the thick layer lines that you would in other materials, such as FDM.”

4) PolyJet (PJET)

(Image courtesy of Proto Labs.)

(Image courtesy of Proto Labs.)

PolyJet uses a print head that sprays layers of photopolymer resin, which are then cured, one after another, using UV light. 

The material is supported by a gel matrix which is removed after the part is completed. The layers in PJET are very thin, enabling superior resolution compared to many other methods.

5) Digital Light Processing (DLP)

(Image courtesy of Proto Labs.)

(Image courtesy of Proto Labs.)

In DLP, a solid is digitally sliced into layers that are then sequentially projected onto the surface of a liquid photopolymer bath. 

These ultraviolet projections harden a layer of liquid polymer resting on a moveable building plate. Similar to SL, the build plate moves down in small increments with each projected image.

Once the part is complete, the remaining liquid polymer is drained from the vat. As with SLS, a significant advantage of DLP is that it can be useful in low-volume production runs, particularly for small, highly detailed parts. However, it is less suitable for larger parts, especially those requiring smooth finishes.

6) Direct Metal Laser Sintering (DMLS)

(Image courtesy of Proto Labs.)

(Image courtesy of Proto Labs.)

Direct metal laser sintering is an ideal method for making metal prototypes. The process is similar to SLS, but uses metal powders instead.

“Unlike the previous processes that give you similar but not identical material properties, DMLS is going to give you near-identical material properties compared to CNC machining aluminum or stainless steel because it’s real metal powder that’s being fused together,” said Holtz.

For that reason, a number of secondary operations can be performed on parts made with DMLS, including drilling, slotting, milling and reaming.

Although porosity is often a concern in 3D printing, it is less of an issue for DMLS.

“When you’re fusing those materials together, it’s in layer thicknesses of a thousandth of an inch,” said Holtz. “A normal resolution is about 30 microns and a high resolution part is 20 microns, so there isn’t much spacing to create porosity. With DMLS you’re creating a part that exceeds 98 percent part density.”

Compatible finishing procedures for DMLS parts include anodizing, electro-polishing, hand polishing and powder coating or painting.

Comparing 3D Printing Processes

Below is a chart comparing the seven processes covered in this article according to price, strength, surface finish and suitability for functional testing:

In addition to these four factors, there are several other questions worth asking before you begin prototyping.

“When a customer comes to us and they don’t know how they want their part produced, there are a few different questions we ask,” said Holtz. “What type of quantities do you need right now? What quantities will you need in a month or a year from now?”

These questions are important not just for the prototyping phase, but also going forward into production.

“Typically, we tell a customer that 1 to 50 parts is cost beneficial for 3D printing, said Holtz. “CNC is good for 1 to 200 parts. Injection molding is anywhere from 25 to 10,000-plus parts.”

Check out this article for an overview of the advantages and disadvantages of 3D printing, CNC machining and injection molding.

For more information, visit the Proto Labs website.

Proto Labs has sponsored this post. It had no editorial input into this post. All opinions are mine. –Ian Wright