3D printed parts from non-commercial printers are almost never dead smooth, except perhaps for flat faces located against the heated bed. (But note that the technology is improving and there are commercial printers that now offer near injection moulded finishes at a price.) 3D printed parts are also porous with small voids throughout the print. There are several techniques you can use for improving the surface finish, reducing the porosity and increasing strength by improving inter-layer adhesion.
- Use a scrap piece to test the smoothing process and avoid turning your 3 hour print into an unrecognisable lump of goo.
Solvent Vapour Smoothing
This is my preferred technique because it is quick and produces a nice even finish. Some folk recommend this simply to reduce the possibility of printed part de-lamination (splitting between printed layers).
If you have any really rough spots then initial smoothing with relatively fine cut (smooth or dead smooth) files or > 400 grit abrasive paper will be useful followed by a gentle water scrub with a nylon brush to remove dust and any abrasive. The wall thickness on most parts is usually about 1 mm so you must take care not to remove too much material. Then dry the part.
The best solvent that I have found for PLA is ethyl acetate. While you may be able to purchase this in small quantities, it is often a primary solvent in nail varnish remover, and it is readily synthesised from ethanol and acetic acid. Like many solvents ethyl acetate is highly flammable (flash point of -4°C) but it has a comparatively low toxicity and it is denser that air (so it tends to stay in the heating container).
Never heat ethyl acetate or other low flash-point solvents with an open flame. Use a non-sparking electric hot plate or a water bath for heating, and ensure your work space is well ventilated.
The solvent vapour smoothing process is straight forward. Find an appropriately sized glass (see through) heat proof container with a lid that will fit the model with some height clearance. Cover any surfaces of the model that you do not want smoothed with masking tape or Vaseline. Suspend the model from a non-critical part such as a hole with non-insulated wire. Solvent vapour may cause wire insulation to soften and fuse with your printed part.
Place about 2 ml of ethyl acetate in the container, put the lid on and heat it by placing the container in a bath of boiling water or on a cold electric hot plate. The ethyl acetate will boil at about 70°C and you will see a vapour condensation line rise up the sides of the container. Once this layer is high enough to immerse your model, turn off any heat source, remove the lid and hold your model in the vapour for just a few seconds (5 maximum) keeping it off the sides of the container. If the solvent has entirely evaporated and the condensation line isn’t high enough then add another 2 ml of solvent. On immersion below the vapour line the surface of the part will instantly turn glossy due to the condensed solvent vapour. Don’t hold the model in for more than 5 seconds to prevent drips and runs of condensed solvent. Remove the model, replace the lid and suspend the model from the wire in a well-ventilated space away from ignition sources and dust to dry. Do not be tempted to touch the part unless you want an indelible copy of your finger prints. If the surface finish isn’t quite right after drying then repeat the process.
With appropriate care you can use a hot air gun to smooth your printed part. This can also be a useful technique for removing warp from printed parts and to activate a plastic surface for gluing.
Note that flame can also be used but the exposure time is extremely short and critical, and the flame must burn clean with no carbon. A butane lighter is not the best way to do this.
Heat the part slowly by progressively decreasing the distance between the part and the hot air source while moving the part or the hot air source around. When the surface becomes glossy remove from the heat or move to another area.
If the part is warped then once it has softened from the application of hot air you will need to apply and hold gentle force to straighten it while it cools. Note that warp implies a problem with the print layout due to stress cause by uneven cooling contraction on upper layer. You may want to revisit the part design, printing orientation, or consider using a raft to prevent warp in the first place.
- Warp is caused by stress due to differential cooling contraction on upper layers. It occurs around the edges of the part in contact with the bed (print platform). Where possible use a symmetrical layout and relatively thin walls of constant thickness. Added support, a clean heated bed, and increased ambient temperature can reduce warp. And make sure that the bed is clean and level.
You can use a solder iron to remove high profile imperfections, repair unbound filaments, fill void cavities with some scrap filament, or make holes and indentations. In essence you will be manually 3D printing but without axial constraints. A temperature controlled iron is best set at or slightly above the filament extruder temperature. Work with a gentle wiping action running a clean soldering iron tip over the area of concern to sculpt the original print. Take care to avoid inadvertently contacting other areas as the damage will be immediate and potentially severe.
An appropriate paint can also improve the surface finish and seal porosity. The challenge here is to find a spray paint with a resin compatible solvent base. Non-compatible paint will either not provide even coverage (it will tend to form beads), will dissolve the print, or will readily chip off after drying. I have used several formulations of PlastiCoat to good effect.
Make sure the part is clean and follow the paint manufacturer’s application directions. A number of thin coats are preferable to a single thick coat to prevent runs and drips.
I have yet to experiment with vacuum dip painting but this is on my list of things to do. I figure that this will provide a much better seal than regular spray painting.
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