To Print or Machine: How the Process Can Impact Your Prototype

Prototyping is a crucial step in any development process, it’s important to validate your design outside of a CAD environment as there are some things that can’t be checked in a computer, or are just easier to evaluate with parts in hand. For most human scale items this leads to a decision between two major…

3d printed forms to test the ergonomics

Prototyping is a crucial step in any development process, it’s important to validate your design outside of a CAD environment as there are some things that can’t be checked in a computer, or are just easier to evaluate with parts in hand. For most human scale items this leads to a decision between two major technologies; machining or 3d printing. Historically this has been a fairly easy decision as 3d printing materials were limited to being brittle, expensive, and fairly rough, however as the technology and materials improve it becomes more tempting to use 3d printing in place of a machined part.

When deciding it is important to evaluate the strengths and weakness of each process as they pertain to cost, timeline, geometry, and material properties. Let’s look at 5 instances when 3d printing is a great option and 5 times it might be better to choose a machined part.

Use a 3d print if you need a:

  1. Highly featured part: A great benefit of 3d printing is that it usually costs nothing to add extra detail. Asking a machine shop to produce a complex array of ribs will invariably lead to higher cost, however in 3d printing those ribs will actually reduce the amount of material used, thereby reducing cost. Features like internal sharps and extruded logos/text can also be added without the extra cost that would be incurred in a machined part.
    Example of how solids must be cored for injection molding

    Figure 1 Adding the ribs in the upper image would be very costly for machined parts, but is actually cheaper for printing

     

  2. Visual model: To sell an idea to investors or future clients it is beneficial to have parts that look just like the final product will, even if it doesn’t function. 3d printing can be great to achieve this as the part can be made exactly as designed without the compromises necessary for a machined part. 3d prints can also be easily finished in a variety of colors and textures using paint techniques to match a final tooled part.

    Figure 2 Surface finish options for a 3d printed part, this is a painted texture similar to one that would be applied in mold

  3. Guess and Check: It’s not un-common to make a part that fits with another piece that already exists, often without CAD data to reference. In these instances 3d printing can be a great to tool the check that the fitment between the designed part and the existing part will match as intended and iteration can occur very quickly to check geometric tweaks.
  4. Ergonomic evaluation: 3D printing is great for ergonomic evaluation as it is quick and fairly cheap to test multiple configurations and minor iterations of an idea until the correct feel is achieved. For items like the contoured handle of a kitchen utensil, machining may be extensive and costly as curved surfaces require a large number of finishing passes, conversely printing a curved surface takes just as much time as a straight one.
    3d printed forms to test the ergonomics

    Figure 3: multiple variations of parts can be printed for ergonomic evaluation without adding significant cost

  5. Assembly mockup: In complex systems it’s one thing to design in a computer; it’s another entirely to be able to assemble in the real world. 3d printing can be very effective at mocking up the volumes of different components to make sure fasteners are accessible when putting together a dense assembly.

Machine if you need:

  1. Tight Tolerance: The most important reason to choose machining over 3d printing is to get parts are accurate to the design. Even on high end printers you cannot expect much greater than 0.05mm (.002”) accuracy, and that is often not sufficient to check fitment for precise assemblies. Most competent machine shops should be able to easily produce prototypes to within +-.025mm (.001”) tolerance and even tighter where required.
  2. Material Properties:
    • Strength: 3d printing materials have come a long way from the brittle gypsum Binder Jet printers, but it is still difficult to find material that will match a processed material as well as machining parts from a block of the native material.
      Brittle 3d printed parts

      Figure 4 brittle failure of a 3d print (garyhodgson.com)

    • Elastic Modulus: One of the best cases for prototyping is to evaluate the “feel” of a product, things like buttons and catches can be particularly personal. One can calculate the opening forces for a latch fairly easily, but without parts in hand it is difficult to directly correlate that force to a “feel”. However you can’t simply print the part, as snaps & buttons rely on the flexural modulus of the material to provide the spring force, so they must be prototyped out of materials with the same modulus as the production material.
    • Isotropic Properties: Most 3d printed parts have fairly distinct lateral lines that result from the layer by layer nature of the process. While these are often largely cosmetic there is a difference in material properties depending on the orientation of these lines, so if you need equal strength in all directions it is best to use a machined part as the base material will be homogeneous.
      Close up view of 3d printed part showing layers

      Figure 5 lateral striations in the direction of print are natural starting points for print failure

    • Process evaluation: If your prototype relies on the properties of a thermoplastic material, like those typically used in injection molding, then some 3d printing technologies will let you down. SLA and other resin based print technologies result in a thermoset material (meaning they don’t melt), so if you are trying to ensure accurate pullout strength for a thermal threaded insert, then it would be best to machine a prototype from the intended material.

      Figure 6 thermal insert installation in thermoplastic material “assembly magazine.com”

  3. Large volume: Not only is 3d printing material costly, but the machines that can print large parts are even more expensive, which usually means a higher per volume cost as vendors need to amortize their investment. So if you have large, simple parts to make, it’s often cheaper to machine, particularly if you can use a low cost prototyping material such as 40lb machinable foam.

There are obviously exceptions to all these cases, but in the current landscape of 3d printing technology as it applies to prototyping, keeping these strengths and limitations in mind should lead to a prototype that provides all the validation necessary to move forward with production investment.  It should be noted that no one process is the perfect answer, mixing both processes can work to optimize cost for example; using a 3d printed model for ergonomics and show, but with a machined piece to evaluate the snap feel. Be sure to let your prototyping vendor know the use case for the final part; most companies will have either process available and will be able to steer you in the right direction based on your needs.

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