How to improve Aesthetic and Mechanical Properties of FDM 3D Printed Parts

Article by Stratasys Ltd. created/updated June 21, 2019

Finishing adds Value to your FDM 3D printed parts

3D printing technology breaks the rules of conventional manufacturing. It allows engineers to build complex geometries, create massive parts, consolidate multiple part components and design solely for form, fit and function rather than for the manufacturing process. Finishing/Post-processing enhances what a 3D printer can do, improving both aesthetic and mechanical properties, from providing impeccable, near injection molded finishes, to meeting tight tolerances and achieving added durability and chemical resistance.

This article is intended for users of Stratasys FDM printers or anyone researching the capabilities of FDM parts for post-processing. It explains the primary finishing operations available for FDM-printed parts, grouped by finishing objectives and methods.

Contents
  1. Smoothing
  2. Meeting Dimensional Tolerance
  3. Assembling Components
  4. Coating

Smoothing

Sanding

Plastic parts made with FDM can be sanded by hand or with orbital sanders to remove the stair-stepping effect that is inherent to the process. Finishing experts use a variety of grits to smooth the surface to the desired aesthetic.

Sanding FDM parts

Hand Sanding and Orbital Sanding FDM parts

Materials and Applications

Sanding is the most common form of finishing for parts produced with FDM because it is a simple way to remove layer lines without affecting mechanical properties. Finishers must also sand parts smooth before applying coatings, such as primer, paint or plating to ensure a smooth surface finish. All FDM materials can be sanded to a certain degree, but consult with an additive manufacturing project engineer to find the best grit for your application.

Design Guidelines

Depending on the amount of material that will be removed with sanding, it may be necessary to adjust the part design to include supplemental material. Also, keep in mind that it is difficult to reach small undercuts, overhangs and holes, so you may need to implement an alternative smoothing method.

Vapor Smoothing

Vapor smoothing addresses stair-stepping on the surface of parts built with FDM by slightly melting the outer surface with a solvent. A part is dipped into a vapor chamber for a few seconds, which causes the plastic to liquefy, then the part is immediately dipped in a cooling chamber to stop liquefaction. The result is a smoother, shinier part. Media blasting, defined in the next section, may be used after vapor smoothing to deliver a matte finish that is often preferable for secondary filming, coating and plating operations. Vapor smoothing also preserves dimensional integrity and can help eliminate the inherent porosity of FDM parts for liquid or gas-holding applications.

Vapour smoothing

Vapour smoothing

Materials and Applications

Vapor smoothing is a fast way to achieve a surface finish of 32–63 microns on standard ABS and ABS-M30, when costs or time prohibit slower, hand smoothing methods. It is often selected to prepare the surface for electroplating or investment casting or for liquid-holding geometries, such as bottles or cooling lines in molds. However, after vapor smoothing, the surface is not completely flat and may require additional operations in order to be paint-ready.

NOTE: Stronger FDM thermoplastics like Polycarbonate (PC), PPSF, ULTEM 9085 and ULTEM 1010 cannot be vapor smoothed because of the chemical reactions with solvents.

Design Guidelines

Make sure the material is an ABS-based FDM thermoplastic as the process can alter other plastics’ mechanical properties. The changes to the part are insignificant (no more than 0.0009 inch/0.023 mm) so there is no need to include extra material. However, we recommend avoiding small feature details, such as text or sharp edges that may become distorted through this process.

Media Blasting

Media blasting is a faster alternative than sanding to smooth the surface of a part and it is often an easier way to reach small features or internal channels. Media blasting is performed with a spray gun and plastic media that is blasted on a part’s surface at up to 30 psi.

Sand blasting

Sand blasting a PC-ISO part

Materials and Applications

Media blasting is often used to remove the sheen from a vapor-smoothed surface if a matte surface finish is required. All FDM materials can be media blasted, but we recommend consulting with an expert on the best media size and abrasive level for your specific application.

Design Guidelines

Although media blasting removes material, it is usually very minimal and does not affect the overall dimensional accuracy.

Mass Finishing/Smoothing

Mass finishing is most commonly found in the metals industry to process and polish metal parts, but there are automated solutions available that use vibratory methods to apply the same technique to plastics using softer media. The parts are placed in a vibratory unit filled with either ceramic, plastic, synthetic or corn cob media, in which the machine rotates for several hours until the media burnishes the surface.

Processed ULTEM 9085 airplane wing

Processed ULTEM 9085 airplane wing

Materials and Applications

Mass finishing is a great option for finishing the more durable FDM materials, including polycarbonate (PC) and ULTEM materials. A major advantage of mass finishing is the ability to smooth multiple parts at once—a good option for uniformly finishing a low-volume production run. Relatively aggressive ceramic media is typically used with ULTEM 9085 parts, while less aggressive ceramic media is used with PC.

Design Guidelines

Mass finishing works best on round parts, such as duct work, with minimal sharp edges and corners. Basing media choice on your design is critical. Small parts require small media, and highly detailed or featured parts require complex media shapes such as a star shape. Be aware that manufacturer descriptions of desired finish, such as “brushed” or “satin,” describe results for metal parts, not plastic. Project engineers choose media based on material, geometry and desired surface finish. Also, mass finishing smooths the material from the outside surface, removing 0.0015–0.003 inches from the surface, so depending on the required dimensional accuracy, you may want to design your parts with thicker outer walls.


Meeting Dimensional Tolerance

Machining

CNC machining is typically used to mill a part out of a block of material. But with complex, additive manufactured parts, machining can function as a secondary operation to achieve very specific dimensional tolerances that cannot be met with 3D printing alone. It’s also often used to add threads for inserts.

Fuel tank simulation

Lockheed Martin fuel tank simulation built in 10 sections, hot air welded together
and machined to the design’s critical dimensions

Materials and Applications

All FDM materials can be easily machined using operations such as drilling, tapping, sawing, turning and milling.

Design Guidelines

Machining does not change the mechanical properties of a 3D printed part, but when designing the part, adjust the part geometry to include extra material that will be removed during the machining process.


Assembling Components

Inserts

A wide range of metal inserts like nuts, bolts and washers can be implanted during or after building a part with FDM. A common approach is to use a machining process to size a hole and then heat the insert and press it into the part. This is called heat staking. The heated plastic flows into the insert knurls and ridges and as the plastic cools, it solidifies to resist torque and pull-out.

Since FDM builds parts one layer at a time, hardware can also be inserted during the build. The machine is stopped at a certain layer, the insert is embedded and then the build resumes from that point.

Metal inserts

Metal inserts

Materials and Applications

Inserts are commonly added to functional prototypes or end-use parts to fasten or attach to other components in a larger assembly. High performance FDM thermoplastics, such as PPSF and ULTEM resist the melting point with heat-set inserts, but embedding smaller mid-build inserts is possible. Discussing how and where the part will be used with a project engineer will help you determine the appropriate material for the insert.

Design Guidelines

You can either include a built-in cavity in the design file to account for an insert or ream a hole, which can be more accurate, but also more time consuming. If you are adding hardware during the FDM build process, there is an additional guideline. The FDM machine can only be paused on the Z axis so the part should be oriented in the X/Y plane in order to place the insert in the cavity.

Hot Air Welding

Hot air welding allows you to create parts larger than the machine build platform. A CAD design is split into separate parts with dovetail joints which are built on separate 3D printers and then welded together after printing. A hot air welding tool is slowly drawn along the joint to melt the filament, which then fills the seam. An advantage of hot air welding is that it does not add any foreign material to the part— it uses the same model thermoplastic. For example, instead of gluing flame-retardant ULTEM material with epoxy that could potentially be flammable, it is bonded with ULTEM, preserving the overall mechanical properties of the part.

Hot air welding

Hot air welding a seam with FDM thermoplastics

Materials and Applications

Engineers use welding to assemble parts that are too large to fit on one machine build platform by sectioning the design, building separate pieces and joining them together. Sectioning and welding is also used to save time. Project engineers can strategically section parts to eliminate excessive amounts of support structure by cutting overhanging features from the part and building them separately.

Design Guidelines

For best results, parts should be sectioned in non-load-bearing areas and on thicker walls of the part with dovetail joints to optimize overall strength. An additive manufacturing project engineer can help you decide where and how to section the part based on the geometry, optimize of strength, build-time and costs of large parts.

Bonding

Bonding is another way to connect multiple sections to create large parts. Additive manufacturing project engineers usually use two-part epoxies because they exhibit excellent mechanical strength and are easy to use. The epoxy components are mixed and then applied with dispensers, brushes or infiltration. Cyanoacrylate, also known as Super Glue, is a popular fast-curing adhesive applied for light-duty bonding applications and repairs.

Bonded 3D printed design

Urbee is an Eco-friendly vehicle that was 3D printed in sections and bonded together

Materials and Applications

Some chemically resistant FDM materials will not adhere with some solvents or glues. Consult with an additive manufacturing project engineer to determine the best joining process and bonding chemicals for your part material.

Design Guidelines

See design guidelines for hot air welding.


Coating

Sealing

FDM parts are naturally porous right off of the machine which presents an obstacle for containing gases and liquids. The solution is sealing the part with a water or chemical resistant epoxy coating. Vapor smoothing also seals part surfaces, but is limited to applications no higher than atmospheric pressure. Two-part epoxy brushed onto the surface of a part generates an airtight seal and is resistant to many chemical agents. The other option is immersing FDM parts in epoxy resin and using a vacuum to infiltrate the epoxy to create a watertight seal and resistance to chemical agents and high temperatures.

Epoxy sealed valve

Epoxy sealed intake valve for the University of Minnesota,
College of Science and Engineering’s Formula SAE vehicle

Materials and Applications

Epoxy coating and infiltration enhance FDM parts for many applications from prototype to end-use,including cooling lines for molding tools, fuel-holding, intake manifolds, ductwork and more. All FDM materials are compatible with either method and maintain an airtight seal up to a pressure of 65 psi (448 kPa) after being treated.

Design Guidelines

Avoid intricate features and internal channels which can be difficult to reach with coating. If your part needs to meet tight dimensional tolerances, we recommend epoxy infiltration which does not affect dimensions.

Electroplating

Electroplating deposits a thin layer of metal, such as chromium, nickel, copper, silver or gold, on a part’s surface. The electroplated coating gives the appearance of production metal and provides a hard, wear resistant surface with reflective properties. Prior to plating, FDM parts need to be sanded smooth and sealed with either a vapor smoothing process, solvent dipping or paint to aid chemical adhesion.

Electroplated FDM part

Electroplated FDM manifold

Materials and Applications

Plating is often applied for cosmetic reasons, but also increases strength and surface durability. In tests, the tensile strength of an electroplated FDM test bar increased 10–12 times and the results of the flexural tests showed an increase of 21–24 times. The added strength makes electroplating a good fit for automotive applications. The decorative value also benefits home fixture and appliance applications. ABS-M30, ABS and ABSplus have been tested for electroplating.

Design Guidelines

In order for the metal to properly adhere to the part’s surface, it must be extremely smooth. An additive manufacturing project engineer will work with you to optimize FDM build orientation to minimize layer lines. You will also need to offset additional thickness that comes with electroplating in the CAD model.

Priming and Painting

FDM parts can be primed and painted to produce attractive conceptual models, functional prototypes and end-use parts. Prior to painting, FDM parts should be polished with vapor smoothing, application of body filler and sanding.

Painted FDM part

Thomas Jefferson statue replica at the Smithsonian Museum, built using FDM and painted bronze

Materials and Applications

Painted 3D printed parts are ideal for concept and display models or applications with virtually any cosmetic requirements. All FDM materials can be painted, but ABS and ABS-M30 generally require the least smoothing effort.

Design Guidelines

If your part is going to be painted, it needs to be as smooth as possible, and that starts with design. Choose a build orientation that will generate a smooth surface, but keep in mind that orientation also affects strength. Project engineers will often choose the smallest tip size and thinnest slice to create a smooth surface while maintaining strength. Holes, undercuts and cavities must also be taken into consideration, since they are difficult to reach when sanding, priming and painting.

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