Intro: 3d Printing, More Than Just Plastic
Modern technology has allowed us to do truly amazing things.
With some freeware software, a Reprap 3d Printer (or similar) and a basic foundry furnace a person can create truly amazing things previously only possible by people with thousands of dollars of equipment.
In this Instructable I will detail how it is possible to go from a 3D printed object to one in metal.
The 'Lost ABS' method used here is similar to the lost wax method used by jewellers and sculptors for many years.
For me the metal of choice was Aluminium (or some alloy thereof) but the technique could be applied to other metals possibly such as Brass, Copper or even Iron.
This Instructable has been created from my own archived photos so unfortunately some steps are photographically missing but I will describe the steps nonetheless.
You will need:
A 3D Printed Object (ABS or PLA)
Plaster of Paris
A Metal Can to use as a Flask
Fine Sand (Sifted Play Sand may well do)
A Fire to burnout the Mould
Means of Melting Metal. (Easier than you think)
Step 1: Step 1; the 3D Printing of Course
I did not come up with idea of "Lost ABS" casting and printing, there are a number have done it before me. Detailed here however are my experiences.
The 'Thing' in question that I printed can be found here and is not my own design but one that I thought would look cool off Thingiverse.
My printer was a Reprap Prusa I3, Single Plate with a 0.3mm nozzle and 0.3mm layer height.
The infill and wall thickness settings don't seem to make a great difference to the end product.
While the majority of people that use 3D printing for casting seem to use PLA, I chose to use ABS so that I could use vapour smoothing.
Vapour smoothing uses a small no flame heat source to vaporise acetone in a container. The acetone vapour then condenses on the part to smooth out the layer detail and give the part a more smooth glossy look.
There is an excellent description of the technique here.
On my part the smoothing had some effect but I had to stop it before the bottom could be smoothed because it was starting to over erode the details at the top of the part.
Step 2: Step 2; the "Investment"
The next step, unfortunately I do not have photos of and is called investment.
I have therefore drawn up a sketch to describe this step.
First step here is to attach a 'Sprue' as can be seen in green atop the print. The Sprue provides the path for the molten metal to flow into the mould.
In my case I used a candle with the end heated in a flame the stuck onto the base for this purpose but anything that will burnout cleanly can be used.
Other options here might be a printed Sprue, Hot Glued on EPS Foam or similar.
A mix of (POP) Plaster of Paris and Fine Silica Sand is mixed up (experiment with ratios but I suggest around 30% by volume) and this is then poured into the 'Flask'.
In my case the flask was a piece of 4 inch galvanised pipe with some cling wrap over the bottom to stop the mixture pouring out the bottom.
The Print and Sprue is then lowered into the Investment mixture taking care to avoid bubbles and ensuring that the top of the Sprue protrudes out of the investment mixture.
In the ideal world one would use a vacuum pump to evacuate any bubbles but I did not have one and taking some care I was pretty happy with the end result.
While adding Silica Sand to the mix does make this step a bit more difficult, a number of previous try's without the sand had resulted in the POP cracking and the moulds being spoiled.
It is well worth experimenting with ratios of POP to Sand here to see what works best for you.
Step 3: Step 3; the "Burnout"
While the "Pro's" use temperature controlled furnaces and kilns with fancy ramp up's and such I simply used my household wood burning fireplace for this step.
The temperature controlled way may well give better results but with a bit of practice simple means like this can be very effective.
The POP mixture sets hard surprisingly quickly and approx. 20 minutes after the mix first set it was in the fireplace.
Rapidly overheating could result in 'steam explosions' which can cause flying debris or cracking, as a result I have found the fireplace more effective than my furnace for this step. In the fireplace the temperature seems to come up slowly having a similar effect to a 'proper' burnout kiln/ furnace with a temperature ramp.
Initially the POP was seen to be sweating as the water dried out, then the Flask was upturned and Wax from the Sprue and the ABS from the print dripped out, the remaining plastic burnt away leaving a clean negative mould.
The fireplace door was closed for the most part and the Toxic ABS fumes were vented up the chimney, only a small amount escaped into the room when the door was opened for photos or inspection.
In the ideal world a wood fire outside would be better or alternatively PLA is a bit 'Greener'.
Step 4: Step 4; Hot Metal... Casting
While Metal Casting might seem out the reach of most people this is not as much the case as you might think.
A Foundry Furnace may be as simple as a metal bucket of coals and a vacuum cleaner on blow, with a stainless steel container as a crucible to hold the metal.
Initially some dry sand was poured into the bottom of the Metal Bucket, the still hot mould was then placed atop this with the Sprue hole of course pointing up.
A Tin can with both ends open was placed atop the flask and this was 'sealed' with a ring of child's play dough (thanks to my Wife for this great idea), the sand was then poured in some more until the level was part way up the Tin can.
The purpose of the Tin can is to provide 'head pressure' for the cast and to provide a supply of Aluminium for the Casting to draw from as it shrinks as it cools.
A general rule of thumb for Aluminium moulds is that they should be approx. 3% oversize to allow for shrinkage.
There are plenty of good Instructables and YouTube information on the topic of metal casting and Safe Practices for doing so.
Step 5: Step 5; the Anticipation and the Reveal
It is necessary to leave the Casting and Mould for 10 to 15 Minutes at least for the Aluminium to solidify.
While it is still hot take the Casting (still in the Flask) and plunge it into a bath of water. The rapid temperature change causes the POP to crack and fall apart.
Some tapping of the flask and a bit of scratching and the part should be able to be removed relatively easily.
After cleaning up the part with a scrubbing brush/ wire brush or similar, the Sprue can then be cut off.
Note in the finished part it quite clear what parts were and were not smoothed and the layer detail evident.
The only defect that I could see on the final piece was at the very tip, my suspicion is that it was caused by either a bubble in the POP or by a little remaining material.
In the future I would tip it upside down or squirt it gently with a little compressed air to blow it out after the burnout stage(taking extreme care not to temperature shock the mould).
Step 6: Step 6; the Possibilities, Endless...
I experimented with the technique of 'Lost ABS' as a means of creation of parts for my CNC mill.
Unfortunately my 3D printing results were a bit poor an non consistent so I sold the printer.
In the future I may well either contract out my 3D printing or buy another if necessary.
By combining these 2 technologies or any number of technologies you are limited only by your imagination.
While Aluminium was used here, other metals (non metals too?) may well be able to be used. Some of the specific steps may well have to change but some may not.
Hopefully I have demonstrated her how you need not necessarily have the best workshop or the most expensive gear to achieve some pretty amazing results.