Although 3D printing technology is advancing rapidly and home 3D printing is becoming both increasingly accessible and reliable, it will likely be a while before metal printing catches up with plastic Fused Filament Fabrication technology. That said, there is a way to fabricate metal parts from some sets of 3D FFF designs. In this blog post, I will describe a technique I have been using with some success to produce high quality metal parts from 3D prints.
Metal Clays and ‘Lost HIPS’ Molding
Metal clay is essentially a very low-tech approach to powder metallurgy. Metal clays are a combination of atomized metal powder and organic, water soluble binders. When soft, metal clay can be worked like a regular ceramic clay, dried to a hard yet brittle state and finally sintered in a conventional kiln to produce a solid metal piece. The first metal clays were silver compounds but today metals such as bronze, brass, copper and steel are also readily available.
High Impact Polystyrene (HIPS) is a standard FFF filament, though definitely less popular than PLA or ABS. My experience with HIPS as a general printing filament is quite good, it is easy to print with and can be printed very successfully on a Elmer’s glue coated, heated borosilicate glass plate. An advantage of HIPS is that it is soluble in Limonene, a solvent derived from citrus fruit rinds. As organic solvents go, limonene is about as safe as you can find, it is used medicinally for heartburn and GERD. There is anecdotal evidence of it making a good margarita mixer…
Putting 3D printing with HIPS, metal clay, limonene as a solvent and finally kiln sintering together, we come up with the ‘Lost HIPS’ technique for creating metal parts from 3D prints.
Step 1 : Create a 3D Mold
The beauty and power of CAD/CAM is the ability to define, manipulate, visualize and refine 3D parts numerically prior to actually creating the physical part. For our purposes, it is straightforward to take a 3D object definition in an STL file and create a mold for that part by performing a boolean difference operation between the 3D part and a rectangular cubiod (i.e. block). For this post, I used the ‘Sun Medallion’ design I found on Thingiverse. I then used OpenScad to create the cuboid and perform the binary difference to create a mold of the medallion. I tweaked the mold along the way to strengthen the connections of the arms to the central solar disk but the design is still quite obviously that of Hank Dietz.
When printing a mold, try to find a good middle ground between a mold that is physically strong enough to work with but contains a minimum of HIPS material. In ‘Lost HIPS’, all the mold material has to be dissolved by the limonene – so less is definitely more.
HIPS is soluable in acetone as well, which means it can also be vapor polished in the same way as ABS. I find vapor polishing to be helpful in smoothing the surface of the mold and sealing up any small holes or creases that may be left in the mold after printing – particularly when printing with thin layers.
Step 2: Fill the Mold with Metal Clay
At present, I am using FastFire Bronze Clay as it is relatively cheap (~$200/kg) and easy to work with though I have found it very sensitive to sintering temperatures. I have also worked with PMC+ and it is easier to work with and very forgiving with respect to sintering temperatures but it is expensive (~$1500/kg).
When filling the mold, I have had the best luck painting the mold with water containing just a tiny amount of dish soap. The water will cause the clay to form a thinner slurry next to the mold (much like ‘slip‘) and the detergent acts as a surfactant to help insure the slurry covers the entire base of the mold. NB – do not use much water/detergent solution in the mold, as making the clay runny has lead to poor results for me. I just paint the surface of the mold with a brush and that is it. I usually put down a first layer of clay with an emphasis on insuring all the corners, nooks and crannies are filled and then fill the rest of the mold. I use an old credit card to scrape off excess clay.
Once the mold is filled, I let it stand for a day to dry and sand the whole thing with a 200 grit sanding sponge to remove any excess clay. It doesn’t take much sanding to get to a point where the finer features of the mold are visible again. Finally, I wanted to make the sun medallion into necklaces for my daughters, so I added a loop to the back of the piece. To make the loop, I used three pieces of HIPS together and placed a bit more clay over the HIPS and onto the back of the medallion.
Step 3: Lose the HIPS
Once the clay is dry, place the mold into a container of limonene and let the solvent do its work. I use a glass container with a flourinated plastic lid that I found at Bed, Bath and Beyond (don’t forget your coupons). Limonene is a solvent and will attack non flourinated plastics, though plastic gas cans and many consumer plastics are flourinated these days. The more HIPS in the mold, the longer it will take to remove the material so expect anywhere from overnight to a couple days to get all the HIPS removed. Fortunately, the metal clay does not appear to be nearly as sensitive to limonene as does the HIPS, so a couple days in a limonene bath does not appear to effect the clay.
Once the bulk of the HIPS is gone, I soak the piece in fresh limonene for a couple hours to get rid of the rest of the mold material and then dunk it in acetone for a minute or two. The acetone serves two purposes. First, it removes any gooey HIPS /limonene emulsion from the surface of the piece and second, it is a drying agent so after just a few minutes in air the piece is dry and can be worked a bit before sintering.
Step 4: Make Repairs to the Clay Piece before Sintering
In its current state, the dried clay can be worked just as green clay can be worked. I will typically sand off visible printing artifacts (i.e steps between layers), fill any voids with fresh clay and file off any excess material from the piece. At this point as it is much easier to add/remove the clay material compared to post sintering. I also find it helpful to use the water/detergent solution again to paint the surface a couple times to get a smoother finish. The metal clay will absolutely reproduce every detail in the printed mold, so it you want a smoother aesthetic look – now is the time to take off the rough edges.
Step 5: Burn out HIPS and Binder then Sinter
Once you are happy with the appearance of the piece, it is time to sinter. I have a Paragon Caldera kiln which I love. I did not get the digitally controlled version which I would suggest strongly for anyone looking to purchase a kiln. I find the difference between a beautiful finished piece and an under-fired or over-fired piece to be just tens of degrees F. Thus I end up having to watch my kiln closely as it finishes its ramp to insure it gets into the right temperature range and holds that range long enough to fully sinter the piece.
Pretty much any material other than silver needs to be fired in an anoxic (i.e oxygen free) environment. For firing metal clays, someone far more clever than I figured out that one could easily create a locally oxygen free environment by burying the piece in carbon granules during firing. This process works spectacularly well. I will not go into the details here, there are plenty of references online.
I use a ceramic container for firing. Firing in a stainless steel vessel leaves lots of black oxide in my kiln whereas the ceramic fiber pot leaves no residue whatsoever. Having sintered with both, I also expect the pot to outlast a stainless vessel as well.
I typically rest the piece on a piece of fiber kiln paper and then put the piece on the paper into the container filled with an inch or two of acid washed carbon granules. I do not cover the piece but ramp my kiln to 400F and leave it there for an hour to burn off any remaining HIPS and the binder in the metal clay.
After burning off any organic compounds left on the piece, I put another piece of kiln paper over the top of the piece and fill the container with carbon granules to within an inch of the top. I put the lid on the container and ramp my kiln to 1450F and leave it there for an hour. I then turn the kiln off and crack the lid to cool the piece quickly.
Step 6: Cleaning the Piece
It can take several hours for everything to cool to a point where it can be touched. In particular, the vessel and carbon will hold heat well.
Once everything has cooled, I remove the piece from the carbon granules and clean it. I use a Dremel tool with a wire brush to take the black scale off the surface of the piece and then use a bath of Picklean to remove the rest of the oxidation. It may take a couple Picklean baths to really get the piece cleaned up but it is the only way I have found to get all the little details in the piece bright and shiny.
I have created a number of other designs as well, below is a Tudor Rose extracted from a Thingiverse design. What is interesting about this design is that there are regions in which upper layers overlap lower layers and if your printer does a decent job of bridging and you can force the metal clay into the mold, you can get a fairly intricate design which would be hard to fabricate using other means – like straight up stamping.
Though there are a number of steps involved with this process, for folks with more engineering talent than artistic talent this provides a way to create some gorgeous pieces simply by ‘turning the crank’. After a few practice runs, I have found the ‘Lost HIPS’ process to be fairly straightforward.
Next I will probably try to fabricate some structural pieces using steel clay. I have tinkered with steel clay once early on and I expect to have similar success with that material as well.