Hi everyone I am going to share with you my method of turning a 3d printed part directly into a aluminum casting. I learned this practice when I needed to make a custom shifter for my race car and a 3d printed plastic one just would not hold up to stress. It was out of my price range to have it CNC machined so I took matters into my hands and came up with this. Traditionally known as investment casting this process has been a black art to some people and viewed as something not feasible to your standard DIY person due to the extravagant tools needed. Well as I have proved here it does not have to be a expensive investment to get good results! I quickly realized the opportunities of this process could be endless so I thought I would share it with the rest of you.
- 3d printer
- PLA, ABS, or much preferred wax based filament like Moldlay filament "price varies".
- Bag of silica sand from a local material supplier "$6"
- Tub of Plaster of Paris from art store "about $15"
- 3 buckets with flat bottoms for better mixing, 1 bucket to mix solids, 1 bucket with pre-measured water for the actual mixing, and 1 bucket with water for clean up.
- Good sized container with any type of sand, this is for mold support and safety in case the molten aluminum spills. "and it probably will"
- Working oven
- A way to melt your preferred metal, I used my homemade foundry furnace fired by a $40 propane burner I purchased from ebay.
- Aluminum scraps to melt.
- Cupcake pan or some sort of ingot mold to pour extra molten metal into.
- Orbital sander or some type of tool that makes strong vibrations.
Step 1: 3d Printed Parts, Sprue, and Vents.
Start out by printing your parts like you normally would. If you need a part to be accurate then you will have to account for extra shrinkage of the material you are casting with, in my case this is aluminum so I scaled the part about 2% larger and since the metal is shrinking the holes will actually get larger while the part gets smaller, all the holes about .040" or 1mm undersized at a minimum which allowed for shrinkage and imperfections. I plan to drill out all the holes since they have to be a precise fit anyway. These values change depending on many factors but 2-3% shrinkage for aluminum is a safe range to start with. You can also print with as little infill as your comfortable with since these parts are not going to take any load and it will melt out faster with less material.
I also printed out the sprue and vents which also help with securing the part in the tub of investment. I learned the hard way that these plastic parts have a lot of buoyancy in plaster investment so it is nice to have them held securely. From my experience it is hard to put too many vents on your part, I will put a vent on every high point and larger sized vents at the end opposite of the sprue. the sprue is the large pyramid shaped piece you see in the picures which is where you will pour the molten metal. If there is a long flat section on your part then put a vent every couple of inches. The vents will allow the metal to flow into areas that would otherwise be taken up by air that cannot escape, these vents also will allow metal to be pulled into thicker sections of the part as the molten aluminum is solidifying and shrinking. Vents can either be designed into the part, or printed after the fact and glued with hot melt glue. I prefer to glue them on later just in case I decide to add or subtract the amount of vents used. If your part to cast is small enough, about grape size, vents may not be necessary as your sprue will provide enough room for the air to escape as your pouring. The location and size of these components is a science in itself, but as a general rule of thumb use a nice large sprue to aid with pouring and metal shrinking, and use alot of vents.
Step 2: Securing the Part in Your Tub.
For a tub I used a standard plastic drywall mud pan since it has a nice taper to the side walls it is easy to release the mold after it is poured. I made a temporary wall from cardboard so it takes less plaster to fill the mold, you do not want any un-necessary volume of plaster if you can help it because it will just take longer to dry and it will be more prone to cracking. I try to keep a solid 1/2" to 1" of plaster material all the way around the part if I can. As you can see in my examples I made of couple of my vents level with the top of the drywall pan so I can glue pop-sickle sticks to them and serve as a way to hold the part secure while I pour the plaster and wait for it to dry. I use hot glue to hold all the parts together since it will melt out cleanly and it is easy to peel off of the mud pan later on. The wood sticks are removed once the investment has dried and can be reused if you want.
Step 3: Mixing and Pouring the Investment.
In this step I will show you my method to mix the investment, this is a simple method for a typical home workshop without a vacuum pump or any fancy dedicated investment casting equipment. I know you can buy professional investment pre-made on the internet and probably get better results than I do with this home-brew version. but I will say that I have tried it and for a non-professional like myself I could not notice a difference so take that for what its worth. I used your standard hobby grade plaster along with some fine silica sand from a local material supplier to keep everything cheap and easy. For the investment mixture I use 2 parts plaster to 1 part silica sand, I have heard of people using a 1:1 mixture, but I tried this and had issues of the investment being very brittle when its dried and just crumbling apart on me. Again, this is what works for me. The purpose of the silica sand is to help the mold break apart easier and help with the plasters resistance to high heat.
Investment powder mixture:
2 parts of plaster
1 part of silica sand
I start with 8 cups plaster and put that into a dedicated dry solids bucket, then 4 cups of silica into the solids bucket and mix that well so there are no clumps or anything. Do NOT add water yet.
Now when we mix the investment powder with water. This will be 2 parts of our powder mixture to 1 part water. You want to use cold water so it will slow down your curing process.
2 parts investment powder
1 part cold water
This is a time critical step, so make sure you have all your tools and casting close by and ready to go. Add powder to the water, not the other way around. So I got my final mix bucket with 6 cups of cold water in it already, "12 cups of investment powder to 6 cups of water" Pour the powder into the water and start to stir it making sure you scrap the bottom and keep breaking up any clumps as your mixing, keep on mixing for a solid 2 or 3 minutes. It should be a nice creamy consistency not clumpy or thick. Once fully mixed I ran a orbital sander against the side of the bucket to help coax all the air bubbles to the top then give it a quick final stir before pouring into the mold. Do not wait to pour after stirring because all the sand will tend to fall to the bottom. While pouring into the mold I try to pour along one of the sloped sides to prevent formation of air bubbles if at all possible and pour till it overflows somewhere, then when the mold is full I run the orbital sander on the side of the mold for 30 seconds to a minute to remove bubbles and get the investment into all the little details and make sure to top off the mold after its settled. At this point you have done all you can and now you leave it completely untouched for a few hours to set.
Step 4: Example of Too Much Plaster, Not Enough Water.
This is a example of a failed experiment with investment mixture. This was far too much powder to the amount of water used.
Step 5: Drying the Mold and Burn-out Procedure
This step is by far the most tedious and time consuming part. This is also where I am most likely to mess up my molds as I have learned from experience. If you have access to a kiln then I highly recommend that, if not you can follow along and do it my way!
First remove the molds from the mud pan, at this point the mold is still very wet and heavy. I start the drying process in my kitchen oven when the wife isnt at home "thats the important part ;) " put the molds on a tray just in case the plastic drips and start at low heat, bump it up 50 degrees every hour or so till your about 350 degrees. At this point the plastic starts to droop and melt. You dont want to get much hotter than this when in your house or it will really start to smoke and stink, trust me on this, or not, but I learned the hard way. Once at 350 degree's I lit the foundry furnace that is located outside, and let it heat up at a low setting. As you can see I stacked a bunch of foundry bricks to make it taller and allow me to fit both the molds at once. After my foundry is up to a temperature of about 500 or so I put both molds in there and the plastic started to melt out, I slowly turn up the temp every hour. Like I said this step takes patience and lots of it. I keep raising the temp for about 6 more hours making sure I see them starting to glow red meaning its very hot over 1000 degree's. If you heat it too fast it will crack, too slow and it takes forever. Even with a kiln the professionals will burnout their investment castings of this size for 14+ hours. I have noticed cracks of different sizes in the exterior of almost all my molds probably due to this burnout step but it has not caused me any major issues yet, so dont let that deter you from continuing on. Once the mold has been thoroughly dried and burnt out I remove it and use compressed air to blow any residue out of the mold, this is important step to make sure all the carbon is removed from the mold so you can get all your fine details. I have ruined some nice molds because I have failed to remove all the residue from them. Handle gently and be careful because these are red hot chunks of plaster so you want some really nice flame proof gloves, and the debris you are blowing out with compressed air will also be hot.
Step 6: Bury the Molds in Sand to Insulate and Protect.
After cleaning out the molds I bury them in a big container of sand. I use cut-up pop cans to cover the openings to make sure no sand will get inside the mold. These thin scraps of aluminum will easily burn apart when the molten aluminum makes contact with it. I make sure that the sand is moist but not too wet, you want it to stick together when you pack it down, when working with molten aluminum you do not want anything to be WET this can be a dangerous situation where water turns to steam very fast and can cause blow backs and small explosions. This sand enclosure is to make sure the small cracks in the molds do not blow open and leak molten aluminum on the ground, and it also supports and insulates the molds so they dont cool down too fast.
So I first make a nice flat base of sand to set the molds on, I will then lay the flat pieces of aluminum over the top of the molds so I do not accidentally shovel sand into them. Then slowly use a small garden shovel to surround the whole thing with sand. You can blow any small pieces of sand out of the way if you need to you just need to be careful not to get too much in there. Leave all aluminum pieces and the round pop can in the sand the entire time, you can pour the aluminum right into it and I havent noticed any issues.
Step 7: The Melt and Pour!
I will add a safety disclaimer here, if you have not worked around molten metal before please do some research first because I cannot possibly touch on all the dangers of this. Obviously it is very hot and liquid metal can flow into some strange places if spilled. So please do this outside and wear steel toe leather boots and no bare skin visible including a face shield.
Since my foundry was still piping hot from the mold burnout it made quick work of my favorite part of this project... Melting the metal! I used a bellhousing from a Ford Mustang, smashed it into pieces with a hammer "way faster and more fun than using a saw" then I melted it down. Once its melted I make sure to keep the burner on for a little bit longer till the material is up to pouring temp. Make sure you melt more than enough metal, it will take more than you think and it really sucks to come this far only to have the whole project ruined by whats known as a partial pour. Im not really scientific when it comes to this, I heard from a old timer that you should pour the aluminum once its hot enough to NOT stick to a steel rod when dipped in. So once its that hot, I make sure to skim the surface to remove the dross or slag. I use some steel bailing wire wadded up to do this and it works great leaving nothing but beautiful shiny molten aluminum on top. Pour the metal as soon as you can after skimming the dross, when you pour make sure you have a solid stance and steady hand as you want to pour fast and deliberate, most importantly do not stop pouring even if you miss the target slightly just readjust and keep the metal flowing until you see it come out of the vents. Then pour the remainder in your cupcake tin or whatever container you have to make ingots out of.
Step 8: Breaking Apart the Mold
Let the molds cool for a while, I let mine sit for about a hour and still used some pliers to remove the molds because they're still very very hot! This is the fun/nervous part for me, just start smashing the plaster apart to see your perfectly molded part hidden inside that mess! Cut off the sprue and vents with a hacksaw and the rest looks amazing, the detail is crazy you can see every layer of the 3d print, now I wish I would have put more detail into the model like some text maybe. Be careful as this hobby is very addicting and the uses are literally endless! Good luck and be safe. Thanks for reading.
Runner Up in the
3D Printing Contest 2016