3D Printing Design Guide

How does 3D printing work?

Every 3D printer builds parts based on the same main principle: a digital model is turned into a physical three-dimensional object by adding material a layer at a time. This where the alternative term Additive Manufacturing comes from.

3D printing is a fundamentally different way of producing parts compared to traditional subtractive (CNC machining) or formative (Injection molding) manufacturing technologies.

In 3D printing, no special tools are required (for example, a cutting tool with certain geometry or a mold). Instead the part is manufactured directly onto the built platform layer-by-layer, which leads to a unique set of benefits and limitations - more on this below.

The process always begins with a digital 3D model - the blueprint of the physical object. This model is sliced by the printer's software into thin, 2-dimensional layers and then turned into a set of instructions in machine language (G-code) for the printer to execute.

From here, the way a 3D printer works varies by process. For example, desktop FDM printers melt plastic filaments and lay it down onto the print platform through a nozzle (like a high-precision, computer-controlled glue gun). Large industrial SLS machines use a laser to melt (or sinter) thin layers of metal or plastic powders.

The available materials also vary by process. Plastics are by far the most common, but metals can also be 3D printed. The produced parts can also have a wide range of specific physical properties, ranging from optically clear to rubber-like objects.

Depending on the size of the part and the type of printer, a print usually takes about 4 to 18 hours to complete. 3D printed parts are rarely ready-to-use out of the machine though. They often require some post-processing to achieve the desired level of surface finish. These steps take additional time and (usually manual) effort.

A brief history of 3D printing

• The sci-fi author, Arthur C. Clarke, was the first to describe the basic functions of a 3D printer back in 1964.

• The first 3D printer was released in 1987 by Chuck Hull of 3D Systems and it was using the "stereolithography" (SLA) process.

• In the 90's and 00's other 3D printing technologies were released, including FDM by Stratasys and SLS by 3D Systems. These printers were expensive and mainly used for industrial prototyping.

• In 2009, the ASTM Committee F42 published a document containing the standard terminology on Additive Manufacturing. This established 3D printing as an industrial manufacturing technology.

• In the same year, the patents on FDM expired and the first low-cost, desktop 3D printers were born by the RepRap project. What once cost $200,000, suddenly became available for below $2,000.

• According to Wohlers the adoption of 3D printing keeps growing: more than 1 million desktop 3D printers were sold globally between 2015 and 2017 and the sales of industrial metal printers almost doubled in 2017 compared to the previous year.

3D printing: beyond the hype

So where is 3D printing today? Is the hype over? Well, maybe but...

The hype of the previous years was based on the idea of widespread consumer adoption. This was (and still is) a misleading interpretation of where the technology actually adds value.

3D printing today has found very specific roles in the world of manufacturing. The inflated expectations of the previous years have given their place to an increased productivity. Many aspects of the technology are now mainstream and adopted by both professional and hobbyists.

Of course, 3D printing is an evolving technology. Every year new 3D printers are released that can have a significant impact on the industry. For example, HP launched their first 3D printing system relatively late (in 2016), but it proved to be one of the most popular industrial 3D printers already by 2017.

Benefits & Limitations of 3D printing

It is important to understand that 3D printing is a rapidly developing technology. It comes with its unique set of advantages, but also lags behind traditional manufacturing in some ways.

Here we summarize the most important benefits and limitations of 3D printing, taking into account the pro's and con's of all 3D printing technologies currently available. Use them to understand where 3D printing stands today and where it is headed in the near future.

• Geometric complexity at no extra cost

3D printing allows easy fabrication of complex shapes, many of which cannot be produced by any other manufacturing method.

The additive nature of the technology means that geometric complexity does not come at a higher price. Parts with complex or organic geometry optimized for performance cost just as much to 3D print as simpler parts designed for traditional manufacturing (and sometimes even cheaper since less material is used).

• Very low start-up costs

In formative manufacturing (think Injection Molding and Metal Casting) each part requires a unique mold. These custom tools come at a high price (from thousands to hundreds of thousands each). To recoup these costs identical parts in the thousands are manufactured.

Since 3D printing does not need any specialized tooling, there are essentially no start-up costs. The cost of a 3D printed part depends only on the amount of material used, the time it took the machine to print it and the post-processing - if any - required to achieve the desired finish.

• Customization of each and every part

Have you ever wondered why we buy our clothing in standardized sizes? For the reasons we just mentioned, with traditional manufacturing, it is simply cheaper to make and sell identical products to the consumer.

3D printing though allows for easy customization. Since start-up costs are so low, one only needs to change the digital 3D model to create a custom part. The result? Each and every item can be customized to meet a user's specific needs without impacting the manufacturing costs.

• Low-cost prototyping with very quick turnaround

One of the main uses of 3D printing today is prototyping - both for form and function. This is done at a fraction of the cost of other processes and at speeds, that no other manufacturing technology can compete with:

Parts printed on a desktop 3D printer are usually ready overnight and orders placed to a professional service with large industrial machines are ready for delivery in 2-5 days.

The speed of prototyping greatly accelerates the design cycle (design, test, improve, re-design). Products that would require 8+ months tp develop, now can be ready in only 8-10 weeks.

• Large range of (speciality) materials

The most common 3D printing materials used today are plastics. Metal 3D printing finds also an increasing number of industrial applications.

The 3D printing pallet also includes speciality materials with properties tailored for specific applications. 3D printed parts today can have high heat resistance, high strength or stiffness and even be biocompatible.

Composites are also common in 3D printing. The materials can be filled with metal, ceramic, wood or carbon particles, or reinforced with carbon fibers. This results in parts with unique properties suitable for specific applications.

Limitations of 3D printing

• Lower strength & anisotropic material properties

Generally, 3D printed parts have physical properties that are not as good as the bulk material: since they are built layer-by-layer, they are weaker and more brittle in one direction by approximately 10% to 50%.

Because of this, plastic 3D printed parts are most often used for non-critical functional applications. DMLS & SLM though can produce metal 3D printed parts with excellent mechanical properties (often better than the bulk material). For this reason, they have found applications in the most demanding industries, like aerospace.

• Less cost-competitive at higher volumes

3D printing cannot compete with traditional manufacturing processes when it comes to large production runs. The lack of a custom tool or mold means that start-up costs are low, so prototypes and a small number of identical parts (up to ten) can be manufactured economically. It also means though that the unit price decreases only slightly at higher quantities, so economies of scale cannot kick in.

In most cases, this turning point is at around 100 units, depending on the material, 3D printing process and part design. After that, other technologies, like CNC machining and Injection Molding, are more cost effective.

• Limited accuracy & tolerances

The accuracy of 3D printed parts depends on the process and the calibration of the machine. Typically, parts printed on a desktop FDM 3D printer have the lowest accuracy and will print with tolerances of ± 0.5 mm. This means that if you design a hole with diameter of 10 mm, its true diameter after printing will be something between 9.5 mm to 10.5 mm.

Other 3D printing processes offer greater accuracy. Industrial Material Jetting and SLA printers, for example, are able to produce parts down to ± 0.01 mm. It is important to keep in mind though, that these results can only be achieved after optimisation for specific features in a well-designed part.

Metal 3D printed parts for critical applications are often finished via CNC machining or another process after printing, to improve their tolerances and surface finish.

• Post-processing & support removal

Printed parts are rarely ready to use off the printer. They usually require one or more post-processing steps.

For example, support removal is needed in most 3D printing processes. 3D printers cannot add material on thin air, so supports are structures that are printed with the part to add material under an overhang or to anchor the printed part on the build platform.

When removed, they often leave marks or blemishes on the surface of the part they came in contact with. These areas need additional operations (sanding, smoothing, painting) to achieve a high quality surface finish.

How to select the right 3D printing process

Selecting the optimal 3D printing process for a particular application can be difficult. There are often more than one process that are suitable and each of them offers different benefits, like greater dimensional accuracy, superior material properties or better surface finish.

For this reason, we have prepared decision making tools and generalized guidelines to help you select the right 3D printing process.

Generally, there are three main things you always need to consider:

• The required material properties: strength, hardness, impact strength etc.

• The functional & visual design requirements: smooth surface, strength, heat resistance etc.

• The capabilities of the 3D printing process: accuracy, available print volume, layer height etc.

If you are still unclear about your part process, then contact us enquiry@best-prototypes.com for our evaluation and recommendation.