April 03, 2020, 08:38:10 PM
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Topic: Formulating polymer/particle mixtures for 3D printing  (Read 194 times)

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Offline nastybyte

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Formulating polymer/particle mixtures for 3D printing
« on: February 28, 2020, 08:35:17 AM »
Hi,
i'm writing a thesis involving 3D printing and i'm trying to print different materials. I am curious about how to formulate polymers suitable for 3D printing - both filament and particle-loaded paste extrusion.

Some of the most interesting applications of 3D printing involve trying to stuff as much solid particles (metals, ceramics, etc.) as possible in the print material, but this leads to the obvious problem that the viscosity increases exponentially with the solids loading. Also using shorter-chain polymers decrease melt viscosity but is deleterious to the mechanical properties of the printed object. I'm curious as to what guidelines to follow when formulating a polymer mix for printing, for example i want to make a filament that is (1) flexible so it can be rolled onto a spool, (2) mechanically sturdy enough to print and (3) contains as much solid particles as possible.

In a first attempt i tried a polyvinyl acetate with a suitable melting point ~80 C with a 50% solids loading, but the melt viscosity was too high. I then added 10% dioctylsebecate plasticiser, this resulted in a filament too soft for printing, it just crumbled when being pressed into the extruder.

Aside from the formulation of the polymer binder i'm considering other strategies to try to force the paste out of a small nozzle. May it be worthwhile to coat the inside of the extruder head in PTFE to minimise friction/facilitate wall slip? Could a similiar effect be achieved by adding a surfactant to the print mixture? Could the viscosity of the paste be lowered by a surfactant (since the paste is made up mainly of particles, particle-particle interactions should be very important)? Can the mixture be formulated as to maximise shear-thinning (and avoid shear-thickening)?

Another interesting and by me unanticipated problem arised when i tried to extrude a paste of UV-curable elastomer liquid resin with 50% particles - even though i de-gassed the stuff in a vacuum it didn't stop oozing out of the syringe when i stopped the extruder screw. Is this related to relaxation of the fluid, or is it actually significantly compressible? I've found litterature about the free volume of polymers but it is a complicated subject and there seems to be very little information regarding compressibility.

Just throwing my thoughts out here to see if i get any response, i'll be happy to elaborate if anyone shows an interest!

Offline Enthalpy

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Re: Formulating polymer/particle mixtures for 3D printing
« Reply #1 on: March 02, 2020, 05:16:27 PM »
Hi,

to put more particle volume in a paste, I suggest to mix particles of very different size, like concrete is made: cement and water fill the voids in sand, sand fills the voids between stones, and you can still pour the mix with little cement and water in it. Just a continuous range of sizes isn't enough apparently: you need several classes of sizes, well separated, like 10* diameter ratio. I'd have more than two sizes. Not only should the melt flow better, the solidified plastic should also be less brittle.

Last time I put graphite or MoS2 in a thin liquid, the suspension already stopped flowing around 60% volume liquid, while in concrete the water and cement volume proportion is much smaller.

If you get useful results, tell us! It would be all-important to metal-matrix ceramic too
https://www.chemicalforums.com/index.php?topic=100877.0
Brittleness limits the proportion of ceramic presently.

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Non-newtonian liquids are known, for instance silicone oil is a catastrophic lubricant because its viscosity drops at high shear number. What compound can fit in a printable polymer, I dunno.

Or could you add a solvent that evaporates once the molten filament is deposited and still warm on the future part? Keeping in mind all the drawbacks of evaporating solvents: explosion, fire, toxicity, shrinkage...

Compressibility of cold and warm polymers is known from injection. Data is available in:
Kunststoff-Tabellen, by Carlowitz (in German and 89€, ouch)
typical moduli are like 5GPa, a bit less at warmth, so like I imagine 3D printing it shouldn't matter much.

I hate Ptfe against friction, as massive parts and as a plain coating. As a mechanical designer I had only trouble with it. It creeps endlessly, and already if it's lukewarm, its friction coefficient gets as bad as any other polymer. You get less problems from Pp and Pa.

The proper use against friction is to embed Ptfe powder in a resisting matrix. Most companies that sell plain bearings offer also the bearing material: Pi+Ptfe (expensive), bronze+Ptfe, of which you might make your nozzles. I let also make a coating of Ni+Ptfe, it was excellent even at huge contact pressure, and can be done in a hole too. For reasons I ignore, these compounds glide well even when warm.

If possible, I'd try a cylindrical or even diverging nozzle.

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