Tips for Designing 3D Printed Parts

How 3D Printing Works:

3D Printing is a type of additive manufacturing in which a part is constructed layer by layer. The 3D printers used in the Innovation Station are modified Replicator 2s, made by MakerBot. They use an extrusion process called Fused Deposition Modeling (FDM) to create parts. PLA plastic in wire form is pulled into the printer head where it is heated to 230°C and extruded from a nozzle. This molten plastic is positioned on the build surface based on instructions the printer receives from a program that slices CAD models into layers. The following video is a good introduction to 3D printers and how they work. Introduction Video

Introduction Video

Dimensional Constraints:

As in all manufacturing processes 3D printing has constraints that must be met in order to print parts correctly. The constraints of the Innovation station are as follows. The bolded constraints are especially important:

  • Maximum part size = 10.5 x 5.5 x 4.75 in or 266 x 140 x 120 mm (L x W x H)
  • Minimum surface area resting on build plate = 0.295” x 0.295” or 7.5 mm x 7.5 mm
  • Minimum height = 0.1” or 2.54 mm
  • Minimum resolution (X/Y axis) = 0.01575” or 0.4 mm
  • Minimum resolution (Z axis) = 0.00787” or 0.2 mm
  • Minimum wall thickness = 0.0197” or 0.5 mm

Dimensional Changes from CAD model to Printed Part:

Plastic expands and contracts as it is heated and cooled, and this thermal expansion/contraction causes dimensional variation in parts. Dimensional variation is also caused by the discrete nature of filament deposition, with the thickness of each layer corresponding to the diameter of the extrusion nozzle. To create dimensionally accurate parts, adjustments must be made to CAD models. Outer features expand and inner features shrink as shown in the pictures below.


Example of dimensional variations from modeled diameter

Two formulas have been identified for modifying the dimensions of hole diameters. To use these formulas, the x value is your desired diameter (for example 4 mm) and the y value is the adjusted diameter for your CAD model (in this case 4.34 mm). Use the vertical hole formula if the axis of the hole is parallel with the Z axis of the build plate and the horizontal hole formula if the axis of the hole is parallel with the X or Y axes. Both of these formulas are in millimeters.

If your part requires dimensionally accurate holes, use these equations.

  • y = 1.0155x + 0.2795 vertical
  • y = 0.9927x + 0.3602 horizontal

Reducing Build Time:

The cost of plastic filament and the running costs of 3D printing are low. Because of this, the real cost of 3D printing is the part build time. To allow more users to be able to use the 3D printers in the Innovation Station, there is a 3 hour limit for print jobs. There are various things that you as a designer can do to reduce the build time of your parts.

  • Decrease the size of parts as much as possible: Any unnecessary material used in a part is just wasted time.
  • Scale down parts: If the part does not need to be full size for what you are doing, scaling the part down will significantly reduce build time.
    Warning: When scaling down parts it is imperative to ensure that all structures remain greater than 1mm. This is a commone error when scaling down downloaded models.
  • Place parts close together on the build plate: The closer the parts are to one another on the build platform, the shorter the print job because the printer head does not spend as much time traveling between parts for each layer.

Limits of the 3D Printers:

Because parts are built layer-by-layer, each layer must be supported by something underneath it. Features that are not directly supported by underlying layers or the build platform are called unsupported overhangs and examples can be seen in the pictures. Without any supports these features fail to print correctly as seen in picture of the printed part below.

To build these parts successfully, you can choose to use the “Supports + Rafts” profile when printing your part. This profile will add support structures to your part automatically while printing. After you receive your part, you can remove the support structures. An example of a 3D printed cat with supports and rafts and then with the supports and rafts removed can be seen below.

Rafts and Supports are extremely difficult to remove and so it is recommended to not use them unless absolutely necessary. For example, if overhangs are supported by angled supports, they can be self-supporting, as shown below. It is good practice to use angled supports of 45 degrees or more for the best quality of parts.

Tests performed with the 3D printers in the Innovation Station indicate that much smaller angles can be used with success, although some additional “whiskers” might have to be trimmed after the part is retrieved. As can be seen from our tests of varying angles, 22.5 degree angled overhangs print successfully every time for any length and are the smallest recommended angle. Smaller angles such as 15 degrees successfully print but leave whiskers on the angled edge and also droop significantly if the overhang is too long.

Bridges are features that are supported on both ends but require the printer head to cross a gap in the middle. Supports can be used in these situations but it can be difficult to remove the support structures from the parts.

If designers opt out of supports, filament will be laid between the ends of the bridge with no support in between. This lack of support can cause problems for bridges of certain dimensions. As seen in the picture, bridges shorter than 36 mm long print with drooping of 0-0.5 mm, bridges 36 to 60 mm long print with drooping of 0.5-2 mm, and bridges longer than 60 mm long print with significant drooping of 2-5 mm. Parts must be designed accordingly depending on the need for the bridge.

Lettering

Personalized parts frequently include lettering. This lettering comes in two forms, embossed and engraved text. Engraved text has the letters slightly indented into the part whereas embossed text sticks out of the part. Embossed text will always lead to failed parts because the small stroke width of the letters is well below the minimum structure size of 1mm. The text will fail to resolve and lead to a failed or rejected part. Engraved font is highly recommended unless the stroke width is greater than 4mm.

Part Orientation on the Build Plate:

Once your part is created, it must be oriented on the build surface. Part orientation greatly influences the quality and the material properties of parts. Some best practices for part placement and orientation include:

  • Positioning the part on the build plate:
    • Center parts on the build surface: The closer a part is to the center of the build plate the less it will warp because the build surface is leveled most accurately at its center. In addition, the heated build surface is cooler on the edges which increases warping near the edge.
    • Place parts directly on the platform: Make sure that the part is positioned on the build plate itself. You can check this by clicking on the “Move” button in MakerWare and making sure the Z position is 0.
    • Place parts close together: When parts are positioned close together on the build platform, the printer head can move more quickly between parts, reducing build time. Arranging parts too close together can ruin multiple parts if one of the parts detaches. Parts should be placed 5-15 mm apart.
    • WARNING: Building multiple parts in the same build can cause other parts to print incorrectly if one of the part fails to print correctly. It is best to limit the number of parts per build and submit multiple jobs.
  • Selecting an appropriate orientation: Because 3D printers print parts layer-by-layer, the parts are weakest in the Z axis because layers can sometimes separate from one another. Also, the interior of parts are filled with a 10% dense honeycomb to reduce material use and build time, which also effects the material properties of parts.
  • Avoiding stair-stepping: Place curves or sloped surfaces so that they are printed in the XY plane. Curves printed in other planes have a stair-stepping effect. This stair-stepping effect can most easily be seen in parts with really shallow angles as shown in the picture below.

  • Avoid tall, thin features: Parts with small surfaces areas attached to the build surface are likely to become detached during printing due to the forces and moments exerted on them by the printer head. The taller the part, the larger the moments (or torques) it will experience at its base, which means the more likely it will detach while printing.

Post-processing Parts after Printing:

Some parts require post-processing after printing. If you printed your parts with rafts and supports they must be removed before your part will be useable. Using needle nose pliers to pry off supports can be effective. Metal spatulas have also been used successfully to remove rafts. It is best to add all holes to parts in CAD software prior to printing. Drilling holes into 3D printed parts can cause parts to collapse if nuts are tightened onto the holes because the interior of the parts is composed of a honeycomb structure to reduce material usage and print time.