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Introduction to 3D printing

Authors: Kari Karwedsky, PTC, and Colm Prendergast, Mentor for FIRST Robotics CompetitionTeam 1965

3D Printing Background

As with all manufacturing processes it is important to understand the limitations of the materials and equipment that you are using when you are doing your initial designs in PTC Creo. The 3D printing process is no different. The following sections will introduce you to a 3D printing workflow and will identify some of the potentials issues that you may encounter during the process. At each step along the way useful tip-and-tricks will be shared to make the process more predictable. Like any manufacturing process, when you are producing a lot of parts, yield and repeatability are important.  Maximizing Yield and Repeatability makes any manufacturing process more efficient and predictable. If you have good yield and repeatability your process for making parts will be more predictable and will allow for minimal use of materials. The yield is really important when time is critical and money is limited, you cannot afford to waste either time or money on badly formed parts. Repeatability is also a big factor. If you have multiple copies of the same part to produce, it is important to know from one run to the next whether a part will be built correctly. Not understanding whether your design is repeatable can be very costly in terms of time and money, not to mention the frustration that this can bring. Especially, given that 3D printing can be slow. Run lengths of 10+ hours are not unreasonable. Poor yield and repeatability can easily result in adding days and weeks to build schedules. Both of these factors are really important during build season.
Many of the recommendations that are presented here were learned by FRC Team 1965 as a result of creating all of the parts necessary to build our Tank Drive System. The actual print time taken to build these parts was approximately 400 hours. However, if our process had 100% yield and repeatability the print time would have been 266 hours. In other words, the whole process took almost twice as long as necessary. We ran into issues with both repeatability and yield during this process. Hopefully, the lessons that are shared here will help you avoid some of these issues.
Please note that all of the 3D print work and the examples presented here were completed using a MakerBot Replicator 2 and PLA plastic. Since the majority of consumer/prosumer 3D printers that are available to FIRST teams use the same FFF (Fused Filament Fabrication) technology the lessons presented here are applicable to many different 3D printers.

How does FFF 3D-Printing Work?

The 3D print process using Fused Filament Fabrication (FFF) or similar technologies is remarkably simple. At the core is an extruder that heats plastic (in this case PLA) and pushes a melted bead unto a build platform. The plastic then cools and hardens to form a shape. In effect the extruder operates like a hot glue gun. The extruder is moved in the X and Y directions across the build plate to create a 2-D layer, the height of which can be controlled. The build plate is then lowered (away from the extruder) to create more layers which progressively build up a 3D shape. The following video illustrates this process.

From this there are several things that impact the quality of a build. The first is the layer height. Currently, most available printers can create layers with a minimum height of 0.1mm. This will result is smooth surfaces. However, print times can become very long. For example, a print using a 0.2mm layer height will take half the time as a print using a 0.1mm layer height. All of the extruder movements are controlled using stepper motors or some kind of servo. The movement precision of these motors will limit the accuracy and fine detail of your final print. The layer width is also important to consider. A cross section of a MakerBot extruder is illustrated below. Note that the extruder nozzle is 0.5mm (19.7mils) wide.

When plastic is extruded through the nozzle onto the plate it will form a bead that is also 0.5mm wide. Since the plastic is molten when extruded it can spread out when deposited on the plate and form a wider bead. This must be considered when designing parts with tight tolerances.

3D Printing Work Flow

The 3D Printing Work Flow that we use is composed of 7 steps.  These steps will be described in more detail in the next several blog posts.
Step 1: Design your part using PTC Creo.Step 2: Export Model from PTC Creo in .stl format.Step 3: Prepare Model and Generate g-code file (or equivalent) for the printer.Step 4: Prepare and Level the 3D Printer Build Plate.Step 5: Load printer filament.Step 6: Print Model.Step 7: Clean up printed model.