Introduction

3D printing, or fused deposition modeling (FDM), is relatively new and very useful method for modeling, prototyping and producing mechanical components. Most traditional manufacturing techniques are subtractive, that is, portions of a solid are removed through machining, cutting, drilling, etc., to form the finished part. FDM is an additive technique, where material (mainly plastic) is deposited in thin layers to slowly produce the finished part.

Successfully using FDM technology requires that the characteristics of the printing process be kept in mind during the entire design/print/use cycle. This document provides an overview to the process and describes several techniques that are essential to produce high-quality, useful components via FDM.

Overview of the FDM Process

FDM printing begins with CAD solid models of the part. Once the geometry is finalized, the part is saved in .STL (stereolithography) format. This format contains only geometric information for the part. The .STL file is then processed through software that positions the part within the printer volume, "slices" the geometry into horizontal layers in the z-direction and determines the paths (in the xy plane) the print heads will take during printing. This file is then sent to the printer where the parts are produced by depositing thin layers of melted plastic, slowly building up the parts, usually over several hours.

It is important to keep the entire FDM process in mind as you design, position, slice, print and finally use your parts. Below are some hints and considerations that must be taken into account to produce successful parts.

A lot of the tolerances inherent in FDM printing originate with the width of the print head. In the discussions below, PHW will refer to the print head width. All of the discussions below assume the use of common ABS plastic as the printing material.

Printing Accuracy and Tolerances

The accuracy of the printing process in the horizontal, xy, plane is controlled by the print mechanism and the accuracy of the stepper motors, and is usually excellent unless the mechanism is misaligned. The tolerances in the xy plane are influenced mainly by the width of the print head (PHW, typically 0.4-0.5 mm) and its temperature, which influences how much the line of plastic line spreads as it is deposited and cools. This tolerance is important when producing parts that must fit together with other components. Any part produced via FDM may have a deviation of 0.2 mm in any direction - this is normal for FDM printing. Due to the melting/cooling aspect of the process, the printed volume is always slightly larger than the designed part, that is, internal features such as holes are smaller while external features are bigger. Generally, allowing one-half of the PHW as a print tolerance for each dimension in the xy direction will produce useful components. Therefore, specify holes in the xy plane a minimum of about 0.2 mm larger in diameter than desired, and outside dimensions in the xy plane a minimum of about 0.2 mm smaller than desired.

The accuracy of the process in the vertical, z, direction depends mainly on the thickness of the layers. This is often referred to as the resolution of the printer, and can be controlled with the nozzle and sometimes software. Tolerances in the z direction are determined by the layer thickness, but the x/y tolerances must also be accounted for in features that occur in xz or yz planes. The most straightforward way of doing this is to print at a resolution equivalent to the PHW (usually the standard resolution of the printer) and to follow the tolerance guidelines above.

When saving your files in STL format, refer to these guidelines from Stratasys to maximize the accuracy of the resulting print:

• CAD to STL - CATIA
• CAD to STL - SolidWorks

General clearance guidelines

Static parts that are printed separately and assembled:

• Clearances of 0.15 mm (0.006 in) are recommended between all touching surfaces. Note that this is a radial clearance for round features, diametral clearances are twice this value.
• Smaller clearances of 0.1 mm (0.004 in) may fit too tightly for easy assembly, but provide the opportunity to fit the parts precisely with extra machining steps
• Larger clearances of 0.2 mm (0.008 in) are easy to assemble, but may not provide precise alignment
• These clearances may be halved and applied to both touching surfaces when both are produced via FDM. Parts that are to be used with metal parts (such as holes for fasteners, recesses for nuts, etc) need to have these clearances applied to the printed part.

Parts that move with respect to each other, whether printed as an assembly or assembled separately:

• Clearances of 0.3 mm (0.012 in) are recommended between all touching surfaces. Note that this is a radial clearance for round features, diametral clearances are twice this value.
• Smaller clearances of 0.2 mm (0.008 in) will often require extra machining steps to provide easy motion
• This large clearance is especially important when printing part assemblies in order to leave room for support material within the joints

Sizes of Printed Parts and Features

The smallest features that can be successfully printed are closely related to the tolerances of the printer discussed above. Printed parts are produced with a "skin" that is usually about as thick as twice the PHW, so a typical skin would be about 0.8-1.0 mm thick. Skin thickness can typically be changed (usually by integer values of the PHW) for any given print for parts requiring thicker skins. Parts that consist of solid volumes are not filled completely with plastic in order to save expensive materials, but are filled with a honeycomb pattern of thin (one PHW) internal supports. The density of this internal fill can usually be specified for a print job, with about 10% being standard and higher values leading both to greatly increased print times and material use.

The smallest features that can be successfully printed are one PHW thick. The next smallest feature is two PHW thick, then three and four PHW thick. Once thicker than 4 PHW, featured are filled between the outer skins with a honeycomb support pattern. The smallest circular holes that can be successfully produced are about PHW in diameter, remember to apply the print tolerance guideline above.

With the printers available in the SDL, the minimum thickness of any feature that can be reliably printed is about 1 mm. If hollowing a solid body, a minimum wall thickness of 2.5 mm to 3 mm will reliably produce a good part.

Material Support While Printing

As layers of soft, melted plastic are deposited, they need to be supported as they cool, by either the printing platform, by layers of material that have already been deposited, or by a sacrificial material deposited specifically as a printing support. Geometric features that either protrude from or recess into the main portion of the part need to be carefully oriented when printing. The professional quality printers available in the SDL take a lost of the previous guesswork out of material support, at additional cost of support material. The support material can be broken away and/or dissolved in a special heated, ultrasound bath for perfectly clean parts. This necessary step adds 4-12 hours to the time to process a part.

Identifying Markings on Parts

Labeling parts with identifiers that are printed directly into the part is a convenient way to identify the parts during assembly. It is also critical to use such labels for identification purposes when the printer is used in a shared environment. Letters and numbers can be embossed into a part easily during design and automatically print into the part. Characters that are a minimum of 4 PHW high and embossed one PHW deep into the part are easy to print without supports, easy to read (and fill, if necessary, for better visibility), and usually can be positioned so as to not interfere with the function of the part. Use of sans-serif fonts is highly recommended for ease of printing and readability.

In the SECS Senior Design lab, all parts to be printed should be labeled. Minimum suggested text size on the top or bottom build plane of a FDM model is 16 point boldface. Minimum suggested text size on vertical walls is 10 point bold. A good format for the part label is this format:

GN-PN-YYMMDD

where

GN = 2-digit group number (eg: "02", "12", etc),
PN = part number, for design group internal use
YY = year
MM = month
DD = day

For example, the label 04-23-141104 on a part tells us that the part belongs to design group 4, it is their part number 23, and was submitted for printing on November 4, 2014.

Material Strength Considerations

FDM works by laying down thin lines of material in the xy plane, and depositing multiple layers of these lines in the vertical (z) direction. As plastic is deposited along the print head paths in the xy plane, the molten plastic must partially melt the plastic it contacts in both in the xy plane and the layer below. As the plastic cools, the new line adheres to the existing plastic in both its own layer and in the layer below. It is very important to understand, and always keep in mind, that FDM does not result in homogeneous, isotropic material properties. The strength of any portion of the part highly depends on its orientation with respect to the printing directions. Generally, sections are stronger in xy planes than in the z direction, and shear stresses commonly "delaminate" a part by separating the printed layers. Therefore, it is always best to orient parts so that planes of high shear stress do not occur parallel to the xy printing planes.

The material properties of ABS plastic are

Young's Modulus (E) = 246-410 ksi, average about 300 ksi (2 GPa)
Ultimate Strength = 3830-7750 psi, average about 5300 psi (36 MPa)
Poisson's Ratio = 0.08-0.46
Delamination (between layer) shear strength = 4500 psi

Parts that need to be rigid and stiff should be made of ABS. Nylon-12 is roughly twice as strong as ABS, and is also far more flexible, making it ideal for living hinges when printed with very thin sections. Like all polymers, the material properties of both nylon-12 and ABS are highly dependent on strain rate, temperature, orientation and many other factors.

Parts that must support significant loads, and/or parts that will slide and wear on each other, can be reinforced with metal in order to provide greater strength and better wear properties. For example, rods can be inserted into internal recesses along the length of protruding features, greatly increasing the shear strength of the feature. Metal tubes can be installed over protruding features to increase strength and provide better wear surfaces if, for example, those features were used as axles. Parts that must slide with respect to others should be faced with smooth metal in order to minimize the wear of the relatively soft ABS plastic. A lot of friction and wear can be alleviated by the use of simple machine washers glued into recesses along wear surfaces.

Depending on the orientation of the applied loads, reinforcing metal can simply be pressed into a printed recess, or it can be glued in place by wiping the metal with acetone before application (the ABS melts in contact with acetone and rehardens as the acetone evaporates) or using a thin layer of cyanoacrylic adhesive (Crazy Glue).

Threads are easily accommodated in FDM parts by designing and printing hexagonal recesses into which standard hex nuts can be inserted. These recesses can be sized to provide a press fit or the nuts can be glued in place with cyanoacrylic adhesive.