3D Printing Primer

Introduction

Are you interested in 3D printing? Interested enough to buy a 3D printer perhaps? The subject seems simple on its face, but the devil is in the details. Many manufacturers make a variety of types of 3D printers. Whether you are a basement-bound maker or a business in need of prototyping capabilities, you may be overwhelmed with the options available to you (and perhaps teasing you on Kickstarter). This report is intended to be a practical overview of the array of diverse methods and approaches which have fallen under the broad label of “3D printing.”

In the past few years, there have been an ever-increasing number and variety of commercial 3D printing systems offered by both nascent startup companies and well-established corporations. The marketing and advertising associated with these offerings usually does little to educate the consumer in the basic principles used. Because of this, consumers are often overwhelmed with the options becoming available to them.

This report is not intended to promote or endorse one 3D printer (or type of 3D printer) over any other make or model. Rather, it is meant to emphasize the strengths and drawbacks of the currently available technologies, to help narrow the field of selection prior to shopping for a specific unit. It also emphasizes open source and do-it-yourself (DIY) approaches when available. The goal is to help non-specialists find a foothold for future knowledge development, though specialists might find a few bits of new or useful information as well.

As the field has been developing rapidly (and shows little sign of slowing down now), any survey of 3D printing will likely be subject to rapid obsolescence. With that in mind, the material contained within this report should be sufficiently generalized to provide a useful introduction to the relevant methods for the foreseeable future, providing a basis of understanding for the new and as yet unavailable methods that are right around the corner of discovery and commercialization.

What’s In a Name?

Before diving into the details of each method, it is worth taking a moment to address the confusion created by the (not entirely unjustified) use of technical jargon in both popular and scientific literature, as the terms rapid prototyping, additive manufacturing, and 3D printing have been used almost interchangeably in the public discourse. This has caused some amount of confusion and debate, with many newcomers asking; “What’s the difference between these terms? Is there one?”

While arguments can be made to draw distinctions between the terms used, 3D printing appears to be the current popularization of what once was called rapid prototyping and what academics have now come to describe as additive manufacturing (AM). Using Google Trends, Figure P-1 shows the search history for these three terms, in a sort of linguistic popularity contest from 2007-2014.

Figure P-1. A Google Trends result for “3D printing,” “rapid prototyping,” and “additive manufacturing”

As you can see, “3D printing” overtook “rapid prototyping” as a search term in 2011, while “additive manufacturing” is beginning to catch up to RP as well. Why the long decline of rapid prototyping as a phrase? As a basic critique, the word rapid is entirely subjective, and a fully functional part that you intend to put into service is not a prototype. The word prototype tends to have some connotation of “just for show” or “not fully functional,” a stigma that 3D printing technologies seek to transcend.

The term additive manufacturing can be contrasted with subtractive manufacturing (SM), a concise description of milling, machining, grinding/polishing, and other approaches in which material is removed selectively from a solid chunk. Additive manufacturing is the preferred term used to describe 3D printing in academic, industrial, and governmental circles.

The future of 3D printing can be seen in these terms. There is already substantial discussion of hybrid or mathematical manufacturing processes, which seek to combine additive and subtractive processes with more developed monitoring/control systems. These theoretical systems will behave something like a craftsman: making small changes to the current creation, assessing the results of each step, and correcting for errors or issues along the way. That being said, these advanced methods might not be in widespread use for a number of years and traditional machining methods still have much relevance.

Organization and Content of This Report

This report is not a fully detailed timeline of the development of 3D printing technologies (see Wohler’s Report for that). Instead, it attempts to generalize the types of methodologies used in 3D printing systems, to help the semi-technical reader identify and recognize the technologies that enable the 3D printing products marketed to them.

Because there are many ways to “3D print” an object, we are primarily concerned with the distinctions between the different methods used. If you have watched the recently released documentary Print the Legend, you will be aware of both the commercial and consumer markets and a few of their primary players, but you might not know why you would want to choose one equipment manufacturer over the other, beyond some limited assertion of quality (e.g., “This one makes ‘better’ parts!”). This report divides common AM methods into three generalized approaches: lithography-based methods (Chapter 1), robot-controlled extrusion methods (Chapter 2), and powder-bed methods (Chapter 3). Approaches that do not fall into these groups are discussed in their own right.

Each chapter on a given printing method begins with a general overview of the operating principles employed, followed by a discussion of the materials and methods currently available in commercial and consumer products, in each case addressing the issue and logic of support structures. Following the overview is a discussion of the benefits, limitations, and upkeep required of each system. Each chapter closes by identifying primary suppliers of commercial and consumer equipment are identified, including current estimated prices.

TL;DR

TL;DR stands for “Too long; didn’t read.” In this report, a TL;DR sidebar attempts to capture one or more quick, untechnical key points for those attempting to speed-read this document.

After covering each type of printing method, Chapter 4 outlines unique approaches and materials that are different enough from the previously described broad categories to warrant a separate discussion. And finally, Chapter 5 offers some general guidelines for using your 3D printer, including downloading printable designs, creating your own design, or scanning an object to clone.

Problems and Concerns Across AM Methods

All available additive manufacturing methods share several common issues and limitations, which anyone comparing 3D printers for purchase or use should be aware of before making a decision. These problems are less relevant if you are considering only 3D printers within a certain method, but they are pretty critical when comparing different printing methods.

Surface Quality

Because most 3D printing methods involve the deposition of layers, layer height has a significant impact on the final surface quality, with thick layers creating an exaggerated terrace effect in some systems. High resolution prints require thin layers, but using thin layers requires more layers for a given object, and more layers means longer printing time. This is where the trade-off between printing resolution/quality and print time arises in most methods. The largest and most detailed print jobs can take a lot of time to produce. The final quality of the surface and the print time needed to obtain it are always at odds.

Anisotropy

As the majority of current 3D printing technologies involve some type of layer-by-layer process, the mechanical properties within the build plane are typically not the same as those measured in the build direction (“normal” to the build plane). In practice, this means that choosing several orientations for printing a part might result in parts with significantly different physical properties. For some applications, this anisotropy can be a serious technical problem, in that the traditionally produced design might need to be reengineered with the capabilities and limitations of the 3D printer’s output in mind.

Consistency

Consistency is another significant concern for all 3D printing methods. In general, obtaining a similar part every time a particular design is printed requires not just the same design manufactured using the same process, but also the same printing orientation and the same support structures. Sharing the orientation and support structure data developed for the manufacture of a particular design is not yet standard procedure, and different operators might handle a given design in different ways.

In practice, this means that you might get a slightly different result from different job shops or even the same shop, if a different machine operator happens to be running the 3D printer when your design comes up for printing again. This issue represents one of the primary stumbling blocks for trusting and using 3D-printed parts in industrial applications.

Support Structures

Support structures are often critical to achieving an accurate and successful 3D print, but the reason for their use might vary from one process to the next. As most printing methods rely on a single material, many supports are generated to be weak structures that can easily break away from the object of interest. Delicate or filigree parts that require supports are likely difficult to clean up after printing, necessitating the use of a sacrificial/dissolvable support material in addition to the primary part material, if possible.

In some processes, particularly those in which the supports are made of the same material as the final part, the removal of supports can be a substantial headache. As such, successful use of 3D printers requires skill and experience in the design of supports as well as the design of the parts themselves.