Design for 3D Printing

Additive manufacturing or "3D Printing" has done wonders to bring affordable and high-quality prototyping and small-run manufacturing lots to within the grasp of consumers. However, 3D Printing has not yet reached the phase where one can simply hit print. Unlike 2D printing, the designer must be aware of the capabilities and limitations of both the machine and technology that are being used.

The goal of this tutorial is to serve as a guide to the designer. We will demonstrate good design practices and tools for the most common type of home 3D printer; Fused Filament Fabrication (FFF). in FFF the part is created by pushing molten material out of a nozzle. The nozzle traces out a set path and then in-fills the material. The resulting part will have small ridges and look as if it is formed by laying one spaghetti layer on another. FFF is by far the most common entry level printer. Models include MakerBot's Replicator, RepRap Mendel, UP3D, Dremmel, and Cubify.

One of the major challenges of 3D printing is creating the digital model in which we can produce a part. While there are multiple programs, there are only three basic types of programs; Mesh Based, Direct Solid Modeling, and Parametric Solid Modeling. For the purpose of this tutorial we will demonstrate the use of a freely available, open source Parametric Solid Modeling program called FreeCAD.

In this tutorial we demonstrate:

1. Creation of the solid geometry
2. The use of constraints to get the part to look (and measure) how we want it
3. Manipulating the part for 3D printing
4. Saving the part as an STL.

For Mobile users click here.

While we demonstrate the use of FreeCad, there are several other programs that can be used to generate the geometry. By Class they are:

Parametric

Parametric modelers, like FreeCAD, use constraint based models in order to design a robust part. While these programs typically have a higher learning curve, they offer the most robust methods of creating a model. The dimensions of the part may be altered in order to quickly modify the solid. Commercial programs include Solidworks, Catia, Pro-E, and AutoDesk Inventor. Open Source alternatives include: FreeCAD, NaroCAD, and HeeksCAD.

Direct Solid Modelers

Direct solid modeling programs focus on speed of part creation over robustness. Modifying the shape or size of your part in these programs is typically difficult and may lead to problems later on. Freely available programs include: AutoDesk 123D, Design Spark, and PTC Creo Elements.

Mesh Based

Mesh based modeling programs allow you to directly manipulate the mesh. These programs can easily create 3D shapes. However, manipulating the shapes will require you to move each individual point on the part. For highly complex shapes, this manipulation can be time consuming when multiple parts are required. Commercial programs include Rhino3D and Maya3D. Open Source/ Freely Available programs include Blender3D.

Once a 3D model is generated we need to take into consideration the material and the method of fabrication. To do so, we examine the part for sides and parts that can be difficult to print. These aspects include:

1. Wall and feature thickness. The minimum wall thickness that printers can reliably print is about 1mm (.04").
2. Holes: Holes typically come out undersized. If holes need to be exact, please consider drilling them afterward
3. Warping, For long thin parts some warping may occur as the part cools. It may be desirable to and ribs or other thick sections to prevent warping
4. Overhangs or steep angles may be difficult to print with support material. Angles <35 degrees from vertical (see image) can be easily printed by while angles up to 55 degrees can be printed with difficulty and angles exceeding 55 degrees can sometimtes be printed depending on feature size.

In our case, the model came with several overhangs. We will now need to edit these parts to allow for printing. And create a STL file that can be uploaded to the printer. You can watch this in action above or if on mobile please click here.

The selection of materials is another important aspect of 3D printing. However, it may not be as important as one initially thinks. The majority of materials used in FFF are thermoplastic polymers that exhibit similar mechanical properties. Table 1 contains a list of commonly used materials, their recommended extrusion temperature, platform temperature, and the bulk mechanical properties. When selecting a material for printing it is important to recognize that not all printers are created equal and unless a specific material is needed, it is advisable to select a material the prints well in the machine that you select. The exception to this rule being when extreme material properties are needed, such as the extreme flexibility of thermoplastic polyurethanes (e.g. NinjaFlex).

With a material in hand and an stl file created and modified to print we are ready to send our part out for fabrication. There are several services that are available with very short lead times that will do this for you.

Often times when printing with FFF printers the layers will not fully adhere to each other. This can cause problems with strength and/or appearance. Or it may desirable to bond to parts together that could not be printed otherwise.

If the part is printed using ABS an easy fix is to use a glass syringe with methyl ethyl ketone (MEK), or acetone to create a solvent bond between the surfaces. A clamp can then be placed on the outside of the part to apply the pressure necesarry to form a bond. Be sure to read the handling instructions for acetone or MEK prior to use. And please use it in a well ventilated space.

If the part is printed using PLA, superglue or other epoxies generally do a fair job in creating a lasting bond.

If holes were used, check the holes for the correct dimensions. If the holes are undersized, a drill bit can be inserted to enlarge the hole. Often, this can be done simply by turning by hand.

3D printing is a wonderful tool that is revolutionizing the way we create. However, it is not foolproof. Care must be taken to both design for the strengths of 3D printing, as well as design around its limitations. This tutorial has demonstrated the use of FreeCAD to go from concept to model and then demonstrated redesigning the concept for print-ability. We also discussed addtional aspects to consider when designing. In review, a filament based 3d printable part should

1. Avoid the use of overhangs and steep angles. Instead use slopes less than 45 degrees from verticle, or consider spliting the part into two pieces.
2. Consider that warping may occur during cooling. To prevent severe warping on long thin parts, Consider adding ribs or thick sections
3. Holes are often undersize. Oversize holes prior to printing or use a drill bit to enlarge the hole after printing
4. Many of the different materials have similar strength and stifness properties. Don't forget to consider the other properties such as chemical bonding, or smoothing when selecting a material.

Resources

FreeCAD