Post Processing for FDM Parts

FDM is most suitable for cost-effective prototypes with short leading time. As layer lines always present on FDM parts, post processing are applied to get a smooth surface. This process can also increase strength and mitigate anisotropic behavior of FDM parts. We will summarize the common FDM post processing as following:

Post processingFinishToleranceSpeedApplied area
Stand support removalLowLowHighALL FDM thermoplastics
Dis-solvable support removalMediumLowHighALL FDM thermoplastics
SandingHighMediumLowAll FDM thermoplastics
Cold weldingLowLowHighABS
Gap fillingLowMediumMediumAll FDM thermoplastics
PolishingHighMediumLowAll FDM thermoplastics
Priming & PaintingHighMediumLowAll FDM thermoplastics
DippingHighLowHighABS or PLA
Epoxy coatingHighLowHighAll FDM thermoplastics
Metal platingHighMediumHighAll FDM thermoplastics

Support Removal

Support removal is the first step of post-processing in FDM, this process can be separated into 2 categories, standard and dis-solvable. It is a mandatory requirement without surface finish improvement.

Standard support removal

Tool kit: needle-nose pliers, dental pick set.

Process: most support materials can be removed easily form FDM parts, for features hard to reach, dental picks and needle-nose are used to achieve. Proper orientation and well support structures can reduce support aesthetic impact greatly.

Support removalAdvantageDisadvantage
StandardNever alter overall geometries

Very quick
Cannot remove layer lines, striations, or blemishes

Affect parts aesthetic once leave excess materials or marks

Dis-solvable support removal

Tool kit: solvent-safe container, solvent, ultrasonic cleaner.

Process: FDM parts are placing in a bath of appropriate solvent until support material dissolves. The support structure are in HIPS associated with ABS, PVA associated with PLA, Hydro-Fill associated with ABS and PLA. For dissolving in water, any non-porous containers can work. For HIPS/ABS parts, a 1:1 raito of limonene and isopropyl alcohol process will for rapid support removal. For PVA or Hydro-Hill can dissolve in plain water simply.

Ultrasonic cleaner can speed up the dissolving of soluble support materials, once saturated with dissolved support materials, change the solvent solution. If an ultrasonic cleaner is not available, warm solvent will also speed up dissolving time.

Support removalAdvantageDisadvantage
Dis-solvable support removalFor complex geometries

Smooth surface in support contact area
Improper dissolving will result in leaching and warping

Cannot remove layer lines, striations, or blemishes

Result in small divots or holes once soluble materials leak onto FDM parts

Sanding

Tool kit: 150, 220, 400, 600, 1000, and 2000 grit sandpaper, tack cloth, toothbrush, soap, face mask.

Process: After support structures are removed or dissolved, sanding process can be used to smooth parts surface and remove obvious blemishes, such as blobs or marks. The first sanding of  sandpaper grit depends on layer height and printing quality. For 200 microns or lower layer height, 150 grit is recommended to apply. For 300 microns or higher layer height, 100 grit is recommended to apply.

We recommend to wet sand FDM parts from start to finish, in order to prevent friction and heat build-up from parts damage. The parts should be cleaned with a toothbrush soapy water.

Surface finishesAdvantageDisadvantage
SandingExtreme smooth surface

Good preparation for painting, polishing, epoxy coating
Not suitable for parts with 2 or less perimeter shells

Difficult for intricate surface

Impact overall accuracy

Cold Welding

Tool kit: acetone, foam applicator

Process: Once parts volume exceeds the printer maximum volume, we always separate the design into small sections and assemble together after printing. For PLA or other materials, Bond-O or appropriate glues can be applied to assembly. For ABS, acetone welding can be used to assemble multi-part together. The mating surfaces require to be brushed with acetone, then hold together until the most acetone evaporates. At this time, these two parts are chemical bonded together. Increase the acetone contact surface will increase the joint strength, it can be achieved by interlocking joints.

Surface finishesAdvantageDisadvantage
Cold weldingAcetone will not change surface color

Joint area with ABS properties will simplify and uniform further finishing
Less strength in joint area

Excess acetone will impact final finishes and tolerances

Gap Filling

Tool kit: Epoxy resin, auto-body filler, ABS filament & acetone

Process: After sanding or support dissolving process, there will be gaps emerge on parts surface. This is inevitable once parts layers incomplete in reason of tool-path constraint. Small gaps can be filled easily with epoxy, this doesn’t require additional processing. Large gaps or hollows can be successfully with auto-body fillers, this requires additional sanding once dry. Auto-body filler is an excellent filler, it can be easily sanded and painted once solidify. ABS part gaps can be filled with ABS filament and acetone, which can chemical react with ABS parts and seep into voids on the surface. Once gaps emerge on parts surface before sanding, fill gaps with Bond-O or epoxy then sand. This will reduce process time to achieve a smooth surface greatly.

Surface finishesAdvantageDisadvantage
Cold weldingEasily sanded and primed

No surface discoloration
Discolored patches on parts

Require additional sanding

Impact overall accuracy

Polishing

Tool kit: Plastic polishing compound, 2000 grit sandpaper, tack cloth, toothbrush, buffing wheel

Process: Plastic polishing can be applied to achieve a mirror-like surface finish. Once parts are sanded up to 2000 grit, wipe excess dust and clean parts with toothbrush in warm water. After dry parts, apply buffing wheel or micro-file cloth and plastic polishing compound to produce a long-lasting surface shine.

Surface finishesAdvantageDisadvantage
PolishingMirror-like surface finish

Very cost-effective
Impact tolerance

Impact primer or paint adhesion

Priming & Painting

Tool kit: Tack cloth, toothbrush, 150, 220, 400 and 600 grit sandpaper, aerosol plastic primer, topcoat paint.

Process: Once FDM parts are sanded properly, priming should be finished in two coats with aerosol primers. Aerosol primer can provide even and thin coverage without any details obscuration. After priming is completed, we can start painting process. This process can be finished with acrylic paints and brushes, while airbrush or aerosol can provide a smoother surface. If necessary, some section of parts can be masked with tape to preserve undercoat color. Remove the mask and polish the paint once all paint layers are complete, in order to create multiple colors on parts surface.

Surface finishesAdvantageDisadvantage
Priming & PaintingProfessional detail

Flexible visual appearance
Alter parts tolerance

Increase cost

Dipping

Tool kit: Solvent-safe container, solvent, eye hook or small screw, drying rod or rack.

Process: Fill container with appropriate solvent, in order to accommodate FDM parts and the solvent. Normally, acetone is used for dipping ABS, MEK or THF for PLA. Once finish preparation, submerge the entire FDM parts into solvent for a few seconds with the wire. Shake parts to facilitate drying and no solvent pools left on the surface.

Surface finishesAdvantageDisadvantage
DippingQuick process

Reduce safety risks
Alter parts tolerance

Long dipping will result to deformation

Epoxy Coating

Tool kit: Epoxy resin, foam brush applicator, mixing container, 1000 grit or higher sandpaper.

Process: As FDM parts are sanded and cleaned fully. Mix the appropriate ratio of resin and hardener as instructions, apply smooth revolutions to minimize air bubbles number. Add the first epoxy coat with a foam applicator, and try to minimize pooling one surfaces or details. For optimal finish, slight sanding with fine sandpaper should applied to remove any imperfections, than apply a second epoxy with same procedure.

Surface finishesAdvantageDisadvantage
Epoxy coatingLess impact on tolerance

Protective shell
Surface layer lines are still visible

Too much epoxy will result dripping

Metal Plating

Tool kit: Electroforming solution, sacrificial anode, conductive paint or acetone&graphite, powder rectifier, conductive screw, non-conductive vessel, lead set, non-conductive gloves and protective eyewear.

Process: Proper metal plating requires high knowledge of materials, for superior finishes with chroming, the professional workshop is the best option.

It is very important to keep FDM parts surface as smooth as possible before plating, any irregular and layer lines will fail plating process. Firstly, we should coat parts with a thin layer of high quality conductive paint, then dry this conductive fully, and sand as necessary. Insert a screw or eyehook into parts inconspicuous surface, this is the cathode and connected to the rectifier negative terminal. Attach the copper anode to the rectifier positive terminal, fill the vessel with copper electroforming solution to cover FDM parts and copper anode. Turn on the power rectifier, set power to 1-3 volts, start plating until parts are fully coated. Finally power off the rectifier and remove parts to dry with microfiber towels, coat parts with a metal lacquer from corrosion as dry.

Surface finishesAdvantageDisadvantage
Metal platingIncrease shell strength

Tight tolerance

Excellent surface vision
Very expensive

Electrical injury