Wax-Powder Sintering for Lost Wax Casting

Well the title basically sums it up. I searched around on the forums and wiki to see if this idea had been suggested before, but the closest I found was plastic-sintering and hot-wax extruders. If this has already been researched, I'd appreciate someone pointing me in the right direction.

I'm intrigued by the possibility of powder-bed printing, largely because it maintains its own support structure, and it seems that the potential resolution is higher in powder-bed systems (such as most of the commercial 3d printers use).

One of the troubles with sintering is that it generally takes a high-power laser to melt materials of any consequence. Wax has a very low-melting point, and seems to be a readily-available material worldwide. It even seems to come in powder form, most usually for waxing shuffleboard tables or ski equipment (though those are not always genuine wax / paraffin).

If we used either a relatively low-wattage laser ( higher than a laser pointer, but lower than one traditionally needed for metal or plastic sintering ) or a light focused with mirror-and-lens optics (such as a cheap halogen bulb behind a magnifying glass), we could do wax-powder-bed sintering of objects. Another option is to use a soldering-iron type of heater, and in order to focus the heat, one could use an elliptical reflector, centering the heating element in one of the ellipse's focal point, and use the second focal point as the sintering target.

Once the object is fabbed, the wax model could be packed in a box with sand or clay, drained of the wax, and filled with whatever metal alloy the fabber had on hand. Lost-wax casting is common all over third-world countries, and if a fabber needed to make an automobile, motorcycle, or bicycle part, taking the wax model to any metal artisan with a forge should enable him to have his part for very low cost.

Advantages of wax-powder sintering:
* Low cost and ready availability of material, even in 3rd world countries.
* Can be used to create models for fabrication of high-strength cast-metal parts.
* Operation with the potential of being simpler to tune than an extruder-based system.
* Powder-bed would provide its own support material for more complex parts.
* Would be low-temperature enough to use the diode laser from an inexpensive DVD burner.

Disadvantages of wax-powder sintering:
* Getting consistently-fine source material could be a tuning problem.
* A laser-based system incurs its own cost, intricacies, and dangers.
* No opportunity for multiple materials, such as ones capable of extruding conductive polymers for embedded circuitry.
* Sintering generally produces porous surfaces, though lost-wax casting often produces a rough surface anyways, particularly when sand is used for the mold material.


Like I said, I'd appreciate any feedback on this idea, particularly as to where it's been explored before. I sincerely doubt I'm the first person with such an idea, but it seems very very attractive to me as a way of making a reprap that can produce parts to cast into itself.


Thanks for reading!

--clint

Edited 1 time(s). Last edit at 10/14/2008 03:55PM by HanClinto.

Hi there HanClinto!

Indeed, such a thing has been attempted before, and it does work to an extent on our Sinterstation's. However I wasn't here when it was actually done, so my knowledge of it is shaky. If you like I can shoot some emails off and I'll let you know exactly what was attempted.

Procuring Wax powder of uniform consistency is quite easy, there are many suppliers. Unfortunately I don't have any on hand, or I'd throw some in my dev Sinterstation and get melting.

The problem with laser sintering on low powers is the extremely long scan time that it necessitates. Different area's of the sintered material will cool at different times and rates, leading to a very good acquaintance of mine, curling/warping. What you need to find is the glass transition temperature of the wax powder. A set of heaters that are heating the part bed to *just* under this temperature is what you'll need to accomplish this. The sintered part should then stay flat in its bed, allowing you to roll the next layer of material. The heaters are completely necessary, and are far more integral to a successful build than laser power alone.

proto: Fantastic, thanks for the information! That's extremely helpful information.

I totally forgot about the possibility of warping in my brainstorming -- it didn't occur to me that wax would have that problem, but what you're saying makes total sense.

So the build chamber would essentially have to be an oven that was kept slightly below the transition temperature. In that sense, it's sounding unlikely that such a precisely-controlled oven would be any easier to create or calibrate than an extruder-based system.

Thanks for entertaining the idea.

Again, Waxes aren't my area, so its entirely possible curling is not as large an issue as it is with Nylon. It seems that any heat related fab process does need to take it into account however.

And while it has been attempted, I'm sure not that much effort has ever been put into it. Its a question of economics when using these machines, there's far more profitable ways to utilize them than lost wax processes.

Controlling an oven like this is not as difficult as it sounds. You need an IR sensor, some heaters, and a computer to control the duty cycle. The calibration work would be quite a challenge though.

Im only too Happy to offer any suggestions, schematics or info on any type of laser sintering

... if you want to make lost-cast moulding-forms from wax then it would be easier to mill them with a dremel atached to the Z-axis.

As wax is an extremely easy to mill material you can do this even with the Darwin-mechanics ...

Viktor

Viktor: Well, I was thinking that we could have all of the benefits of positive-space modeling if we "grew" the wax rather than machined it, since the possibility space of valid objects is so much larger for positively-built objects (especially if they have a support structure, as in a powder-bed).

Hi clint,

... i made some tests with laser-sintering different pastes and powders with a diode-laser outputting until 5Watt and a small focus of maybe 100 microns.

At home i have a 1-Watt-diode-laser with a focus-spot of 60 microns, so it's nearly the half energy-density, but is enough to cut 0,5mm thick sheets of plastic or sinter 0,4mm thick layers of plastic-powder.

With most materials it's difficult to hold the shape/form of the melted area - when the powder-particles melt, they coagulate together and form a sort of blob-tray thicker than the powder-layer with an empty groove along the sides.

So you have much work to bring low-power-lasersintering to work.

Here milling is much simpler and cheaper and the accuracy/surface-finish is much-much better, than with sintering from powder either ...

Viktor

Viktor: Gotcha' -- that makes sense, thanks! I also hadn't thought about the congealing of the powder particles increasing the thickness of that layer -- that's very very interesting.

Phew. I'm constantly learning how 3D printing is a delicate issue any way you slice it.

Thanks for the help!

--clint

Hi Clint

I came across this thread over the summer while I was designing and building just such a printer as a research project in college. I used an IR laser running at about 1.5 Watts to sinter a mixture of Candelilla wax and carbon powder into solid objects. I hadn't thought of the thermal warping issues until I was well into the process of developing a workable print powder and began seeing them in my sintering samples.

Some waxes were less susceptible to thermal warping and other powder properties definitely came into play as well. My early paraffin powder formulations were somewhat "sticky"-- the ratio of carbon powder to wax was such that the wax particles still had some surfaces exposed (not covered in carbon powder) to each other, causing particles to stick to each other. This led to rampant clumping problems, but it did prevent warping because freshly sintered wax regions were held in place by this "stickiness." Other formulations with waxes with higher melting points yielded "drier" powders that did not clump and also did not restrain warping/curling. Amongst these waxes, some warped more than others and some were also more brittle than others. Candelilla wax was the best compromise--little clumping, little warping. I obtained Candelilla and other waxes for testing from Strahl and Pitsch.

Some of the objects that I printed definitely exhibited warping and I agree with proto that a heated build volume would help with warping, and likely with print speed too. I put all of my work on the RepRap Wiki (fixed link 1/26/12)-- there are some more details about the print powder, print hardware, lost-wax casting, and the laser.

--Andreas

Edited 1 time(s). Last edit at 01/26/2012 10:24AM by AndreasBastian.

Andreas:

Wonderful, thanks so much for posting this followup -- it's great to read about your results!!!

Andreas - I agree with HanClinto - Great work and a very good writeup.

Hey thanks for sharing this wonderful writeup! IR laser running at about 1.5 Watts to sinter a mixture of Candelilla wax is a great idea inded.

Sintered Bushes

Hello Clint,

Rapid Pattern Based Powder Sintering is a new rapid tooling technique proposed by the manufacturers. It is an integration of three techniques: powder sintering, lost wax casting and rapid prototyping. The main advantages of the RPBPS technique compared with other investment casting techniques are low investment and production cost, short production cycle and a variety of materials of products. There are some key factors to be kept in mind when you negotiate, just select and bid for the right casting company. This aims at managing you with those so that you don’t down prey to unfortunate mishaps during investment casting foundry.

Mangaliron: That's exciting! Very glad to hear an update on this, and to hear that a process is becoming standardized and growing in popularity / availability!