In part I of this article, we wrote about 3D Metal Printing for Under $3000 – or 3D Printing at Home. In this article, we will document the sintering part of that process. Sintering will convert our 3D printed metal parts consisting of a metal powder plus polymer binder into pure metal. At a high level, this process consists of the following steps:
- Forming a Mold
Each step is described in detail below.
Before we begin, the obligatory digression: What am I building? I have always been fascinated with watches. Simply put, I want one that says “Made in Australia,” and I received a challenge. The challenge came by way of a watch maker web site, Otto Frei, a prominent watch and jewellery supplier in the United States.: It says:
A blogger on watchuseek.com expands on that to say:
“No sane right” and “should not handle” sound like perfect invitations to do just that and more: develop a digital, advanced manufacturing process to make a watch end-to-end. This article will be part of building that process – see Summary and Conclusion. So far our parts look something like this – shown here is a square watch case. The material is Filament by Virtual Foundry.
Note how the copper filament is able to reproduce the most intricate level of detail. We observe screw holes and a groove for a water proofing seal. All features are printed directly and without machining, drilling or cutting. Alas, the parts don’t look very metallic – and that is because they are made of metal powder held together by a polymer binder. This is what we will fix today. We will work with the watch ring that will hold the crystal of the round variant of our watch.
We can trivially program other shapes. For a triangular shaped watch…
Alas, we will continue with the round design. For completeness, shown below is the digital specification of the above part. The utility function “drill-holes” takes any shape and a list of holes to “drill into it.” I call it drilling holes without shavings…
We then define our watch crystal ring as the ring without holes, plus holes applied:
The actual crystal ring is defined in a style specific module . In this case, the style will be “round,” so our basic shape will be that of a cylinder…
Step 1: Forming a Mold
This step is about encasing our part in a support structure. The reason for forming a mold is simple: During our process, we will firstly remove the polymer binder and then sinter the metal powder into a coherent shape. The mold holds our part in suspension in the interim. This prevents the part from slumping. Shown below is the small metal casing filled with a slurry – embedded within the slurry is our part. The slurry sets hard as concrete within minutes of mixing it with the prescribed amount of water. The casing is a simple metal fencepost cap obtained at Bunnings, a local Australian hardware shop, turned up-side-down. The slurry is also referred to as Magic Black Powder.
Step 2: Candling
Candling removes residual moisture from the slurry that makes up the mold. I use a simple Breville toaster oven with digital temperature and timing control as that saves energy over the use of a kiln for this part of the process.
Steps 3 & 4: Vaporising & Sintering
Vaporising is about removing the polymer binder, leaving behind only pure metal grains. Sintering fuses the metal grains at just below melting point. Both steps are performed in a kiln. Vaporising takes place at 350 degrees Celsius for 3 hours and sintering at 950 degrees Celsius for 40 minutes. We don’t have separate images here but if you are curious what metal looks like at about 1000 degrees, it looks like this:
For the sintering step, I removed the “kiln furniture” shelf from the kiln and replaced it with a ceramic plate from the kitchen. The reason for that is that experience has taught me that metals, e.g. silver, will leave impressions on the otherwise porous surface of the shelf which transfer onto objects being fired in the kiln subsequently. The ceramic plate has a smooth, glazed surface that does not suffer this problem. Nonetheless, I recommend brushing the plate surface with kiln wash to assist in removal of the hot casing from the plate as otherwise it will bind.
Step 5: Quenching
Quenching is about cooling the part and “blowing away” the mold. This is where I picked up the glowing part by hand, using but a pair of welding gloves and threw it into room temperature water. Don’t use a plastic bucket here. I used the pot from a rice cooker and had extra water ready on stand-by. The effect was akin to a miniature explosion, a bang, rapid boiling, and a flurry of white emanating from the mold disintegrating on contact with the water.
As I poured additional water into the pot, the sintered copper part became visible inside the casing.
After rough cleaning of the part, we have this:
At this point, close your kiln and allow it to cool off gradually. This prevents temperature shock to your kiln furniture. The part is now ready to be transferred to a tumbler for cleaning and polishing. If you do not have a tumbler, some steel wool or fine grit sand paper will suffice. I use a LORTONE Rotary Tumbler, made in the United States, that has never let me down. Below is the final part, cleaned and polished.
Summary and Conclusion
Shrinkage, Relative Density & Annealing
Removal of the binder during the vaporising step, appears to have reduced the part from a diameter of 44mm to a diameter of 41mm. Hence the final part has a volume 93% of the original, 3D printed part – which corresponds to the ratio of copper and binder in the filament. The advertised metal content of the filament is 91.5%.
The astute reader might ask why 93% shrinkage was observed rather than 91.5%? We need to be mindful that while the polymer binder has been removed, metal powder sintering works based on fusing neighbouring grains of metal in the powder. Only liquefaction would yield 100% density by melting the grains of metal fully into one another. Hence our material exhibits a density relative to cast copper of 98%.
The mechanical properties of powder sintered (laser or kiln) materials will differ from the mechanical properties of forged materials. In particular tensile strength and mean fatigue will be different. Refer to this study for details. Generally held opinion is that a laser optimised sintering process gets one to within a factor of 2-10 of forged metal tensile strength. All fusion deposition methods build stresses into the resultant materials due to irregular cooling and other factors (also true for plastics). Annealing is one method used to counter such effects which means that laser sintered parts can benefit from an addition post processing step – annealing. What is particularly interesting about the Virtual Foundry method is that the whole part is held in a suspension while it is sintered which means that all stresses from fusion deposition printing are released, similar to annealing — while the quenching process is analogous to that of forged metals, yielding both hardening and strengthening.
I have not performed laboratory tests comparing tensile tests to laser sintered parts.
From a production point of view, shrinkage is not a problem. Parts can simply be pre-scaled to suit in the 3D printer software or indeed in the digital model once the relevant coefficient is known.
We also observe that all of the described steps are amenable to simple pick-and-place robotics. Indeed removing the part from a heated kiln is arguably safer for a robotic arm than a human one. The mold nearly removes itself & post processing can be delegated to a tumbler. There again, pick-and-place will work. We required the following capabilities: 3D printing, an oven, a kiln and a tumbler. Additional items where trivially sourced from either the kitchen or the local hardware shop. The almost comical triviality was deliberate.
Overall process complexity has been deferred to a) composition of the capabilities and b) the digital logic specifying the part.
We are on the road to “Made in Australia.“