3D printing with Acetal Filament

I recently got some experimental grade acetal filament for testing purposes from Stratum3D. Acetal, or Polyoxymethylene is an engineering plastic with the desirable characteristics of high stiffness and a low co-efficient of friction.

That low co-efficient of friction is where I think it can have a major impact on what can be produced with fused filament 3D printers such as the RepRaps we use. I have visions of moving parts with integral low friction bushings and other surfaces. It is not as low friction as PTFE but has better wear resistance.

The acetal was supplied in 1.75mm filament so the quick change extruder design on the Mendel90 came in handy and I was soon up and printing.

As the picture shows I had to experiment with a number of build surfaces; the advantage of this plastic being slippery in use is a disadvantage when trying to get it to stick to anything! The slight blue tinge to the picture is because it is taken through the acrylic enclosure that I added to our prototype Mendel90, acetal degrades if overheated giving off formaldehyde which you don't want to breathe.

Acetal also takes on moisture readily so I left it in the sealed bag with moisture absorption granules and made a small hole to get the filament out:

I tested extrusion onto:

• Clean Glass.
• ABS treated Glass.
• Plastic Primer treated Glass.
• Hairspray treated Glass.
• Masking tape on glass, Masking tape on glass+ hairspray,
• Kapton tape on glass with and without hairspray.
• High density cardboard (the stuff used as backing on cheap picture frames and cheap furniture).

Extrusion temperature was 225C with varying bed temperatures (these also influenced the temperature of the enclosure). 225C was as hot as I was prepared to go without risking the acetal degrading.

with the following results

1)  Clean Glass, ABS Juice treated Glass, Hairspray treated Glass, masking tape on glass. (40, 80, 130C) 

Insufficient adhesion at any temperature, everything extruded stuck to the nozzle making a nasty mess.

2)  Plastic Primer treated Glass

Adhesion at 80C was enough to allow part to build but warping forces rapidly overcame bed adhesion and the part ended up stuck to the nozzle. Lower temperatures than 80 gave insufficient adhesion. Higher temperatures saw the primer adhesion weaken quicker. 

3)  Kapton tape on glass with and without hairspray (130C).

Insufficient adhesion, same as 1, did not try lower temperatures.

4)  Masking tape on glass+ hairspray.

 Two cylinders, 12mm OD, 8mm ID, 0.2mm layer height, heated bed 130C

Sufficient bonding for the object to complete printing. Warping forces caused the edges of the bottom of the object to curl up and the first few layers to be distorted. Higher layers printed better. Inter-layer adhesion is strong.

Single wall cylinder, 20mm OD, 0.2mm layer height

Heated bed was set to 130C for first layer then dropping to room temperature after that. Sufficient bonding for object to remain on bed, sides warped up slightly. Drop in bed temperature and chamber temperature (doors to chamber were opened) cause severe warping and shrinking from the second layer onwards.

5)  High density cardboard. (130C, although cardboard is a good insulator so the top surface was not that hot)

Single wall cylinder, 20mm OD, 0.2mm layer height, 200% extrusion, heated bed 130C

Warping over the first few layers but remained stuck to the cardboard throughout the print. The 200% extrusion gave double thick, very strong walls. Printed object retained cardboard fibres following removal from the bed.

Single wall cylinder, 20mm OD, 0.2mm layer height, heated bed 130C.

Minimal warping due to leaving the heated chamber for 1 hour to get as hot as possible before printing. Even so the base did lift up around the edges. Excellent layer adhesion achieved.

Single wall cube, 30mm on side, 0.4mm layer height, heated bed 80C

As expected corners of cube warped more than the cylinder but did not rip the cardboard surface up with them. The cardboard stuck to the bottom was from where the object remained fixed to the surface throughout the print. This is probably because the warping began while the just deposited part of the shape was still hot. Layer adhesion was less strong possibly due to the increased warping forces due to the lower build chamber temperature.

Next...?

I want to try further build surfaces RichRap recommended Tufnol for nylon so I will try this as it should combine stiffness with "free" fibres like the cardboard. Also Andrew at Stratum3D is looking at the acetal formulation which may make it easier to print with.

I am going to print some replacement bushings for the lm8uus and install them on one of the prototype printers to do a wear test. Either PropsFactory's design that look like IGUS or Triffid hunter's design.

Ultimately incorporating this with ABS, PLA or nylon in a multimaterial print would open up a huge number of possibilities for complex objects.

A word from Stratum3D

The acetal filament used is being developed by Stratum3D Ltd, a new startup with 20 Years experience in Injection Moulding and plastics processing. They are aiming to bring a range of "Pro" engineering grade Filaments to market based on their knowledge and experience.  Follow them on Twitter - Stratum3D