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 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 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 melting and depositing thin layers of 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.

Many 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 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 may have a deviation of 0.1-0.2 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 (0.008 in) larger in diameter than desired, and outside dimensions in the xy plane a minimum of about 0.2 mm (0.008 in) 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 via the slicing 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 the guidelines from Stratasys to maximize the accuracy of the resulting print. It is suggested that the linear tolerance for the STL file be at 0.001" or smaller, and the angular tolerance be 1°-2°.

General clearance guidelines

Static parts that are printed separately and assembled:

• Clearances of at least 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 applied to either mating surfaces, or may be halved and applied to both mating 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) may require extra machining steps to provide easy motion.
• This large clearance is especially important when printing complete 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 typically not filled completely with plastic in order to save expensive materials, but are filled with a pattern of internal supports. The density of this internal fill can be specified for a print job, with more dense fills 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 F370CR printer in the SDL, the minimum thickness of any feature that can be reliably printed is about 1.5 mm. If hollowing a solid body, a minimum wall thickness of 2.5 mm to 3 mm will reliably produce a good part. The maximum dimension of the build envelope of the F370CR is 14"(x) x 10"(y) x 14"(z). Larger parts can be broken up into pieces and bonded together after printing, see bonding guidelines from Stratasys.

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 strategically as 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 lot 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 post-process a part.

Material 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 materials that can be printed with the F370CR are (click on the material name to open the material data sheet)

• ABS-CF10 is a strong, stiff and lightweight carbon fiber-infused thermoplastic suited for jigs, fixtures, tooling and production parts. 
• ABS-ESD7 is a durable and electrostatic dissipative material suited for end-use components, electronic products, industrial equipment, and jigs and fixtures. It is suitable for assembly of electronic components.
• ABS-M30 is 25-75% stronger than the standard ABS material. It also provides realistic functional test results, smoother parts, and finer feature details. ABS-M30 is the default material in the SDL.
• ASA is an all-purpose prototyping material with exceptional UV stability. ASA is a especially well suited for outdoor, commercial, and infrastructure-related production parts. ASA does not degrade with prolonged outdoor use. 3D parts are accurate, stable, and very durable.
• Diran 410MF07 is a durable, nylon-based thermoplastic that demonstrates exceptional toughness and resistance to hydrocarbon-based chemicals. Parts have a smooth, low-friction texture, which makes it perfect for non-marring, factory-floor tooling where durability is required.
• FDM Nylon-CF10 delivers high strength, a high stiffness-to-weight ratio, and chemical resistance, allowing for strong, geometrically complex parts. FDM Nylon-CF10 can be used for machined jigs and fixtures, and is used in industries such as transportation, service bureaus, machine shops, and industrial equipment. 
• FDM TPU 92A is an elastomer material that is ideal for prototyping highly functional, large, durable, and complex elastomer parts.
• PC-ABS is a production-grade thermoplastic that offers the superior strength and heat resistance of PC with the flexibility of ABS. PC-ABS material is commonly used in the automotive, electronics, and telecommunications industries.

Note that not all of these materials are available for printing in the SDL at any given time. The default material in the SDL is ABS-M30, with resonable stock on hand. Other materials may be ordered if necessary for a design project, or may be left over from another project. Always ask before assuming that a material is, or is not, available to use. Delivery of a specific material may take several weeks, so make any special material needs known to your instructor as soon as possible.

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 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 (ABS, for example, melts in contact with acetone and rehardens as the acetone evaporates) or using a thin layer of cyanoacrylate adhesive (Crazy Glue).

Threads are easily accommodated in FDM parts by either using threaded inserts made specifically for this purpose, or 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 cyanoacrylate adhesive.

See the Stratasys FDM Design Guidelines for other hints and solutions.