Intro: The Making of a Star
So recently my Mother in law passed away from Motor Neurone Disease. It was particularly aggressive and it took her in less than a year. In my Wife's family (of 7 children) they had a tradition of celebrating success by the 'successful one' having their meal served on a 'Star plate'. My mother in law served selflessly in her local church and school and so I thought it only fitting to create her a star in her memory.
While I have done some casting before, I am anything but a pro and rather than a how to as such it is more the story of a journey in creating the star and the weird and wonderful ways that I tried along the way.
Step 1: 3D Printing and Taking a Mould - Silicone RTV
So the go to for the best definition of a cast tends to be the lost wax method. While I have successfully used the lost ABS method (whereby a 3d print is set in a kind of plaster and then put a fire/ kiln and burnt out leaving a void in a perfect negative) I thought that I would try the lost wax.
Firstly I designed the star in Fusion360 and then printed it out on my 3d printer. I knew all along that the text would be a challenge but little did I know exactly how much of a challenge.
For my first try at making a mould I tried the Silicone RTV and detergent method.
In this method you take a tube of 100% silicone caulk and squirt it into a bucket of water that you have laced with ALOT of detergent. You then gather the Silicone together with your wet hands kneading it into a ball.
The next step is to take your item being cast and pressing it into the silicone.
Leave the silicone overnight and the next day pry your object having been cast from the mould.
When I tried this with the star being so big (180mm across) I used 2.5 tubes of silicone. After it had all been kneaded together all looked well. I left the mould overnight and the next day I pulled the 3d print from the mould. Initially the mould looked good but after closer inspection it looked like pockets of water in the mould. Perhaps it was that I hadn't kneaded the silicone enough. Also the mould seemed a little fragile and seemed to tear quite easily.
Many people have had success with this method but I think at such a large mould that it was pushing it to its limit.
Step 2: Proper Silicone
Still keen on the idea of taking a mould, I tried the 'proper' route to go down and bought some 2 part silicone resin.
After searching for way too long, I found a New Zealand supplier that seemed to be the cheapest. I say the cheapest but it wasn't by any means cheap... 1 pound of it cost me approx $70 NZD including delivery and tax.
When the package arrived I was disappointed to see just how little 1 pound of silicone was and so I needed to build a 'dam' around the 3d print to try and minimise the use of the silicone.
The silicone I used here was called mean green from Sculpture And Moulding Supplies (SAMS) in Auckland New Zealand.
The 3d printed part was glued to a piece of plywood that had been coated in polyurethane the night before and then the dam built out of cardboard. (Hot glued to the plywood)
When I started pouring the silicone into the dam over the part it became evident that the dam had not bonded properly with the hot glue around the entire perimeter and the silicone started to leak.
It was action stations trying to bog the holes with paper towel and hot glue.
Eventually we stopped the leaks and had just enough silicone to complete the mould.
The silicone was left overnight and the next day I remoulded it to find that the bottom was not set. I am not a pro here but it seems that any place that had been in contact with the polyurethane plywood had not set.
I left the tacky side in the open air for a couple of days and eventually it did set.
If only the story ended happily here...
I don't have any photos of it but paraffin wax was cast into the mould.
It was a nightmare, the surface finish was pitted with tiny holes and it shrunk incredibly such that even though I had a decent looking mould that the wax part would not be suitable for lost wax casting :(
Step 3: Gelatine and Glycerine Moulds
Appalled by the fact that I had spent such money on silicone my Father got in touch with me telling me of a recipe he had seen on the internet for a castable mould material consisting of Gelatine, Water and Glycerine.
I was very apprehensive but the next day I bought the materials from town and brought them home after work.
The recipe on the internet called for 4g of Gelatine totally dissolved in 20g of hot water and then adding 24g of Glycerine, mixing well and then letting to set. Best of all this magic goop promised to be reusable, by heating in the microwave it would turn to liquid again for re-use.
The magic goop lived up to expectations and as it stands I have a batch of 40x the original recipe stored in the fridge that I have melted and moulded several times already.
I'm not confident that wax would not melt it so I once again moved on and this time back to more traditional methods.
This 'rubber' is well worth playing with. It is comparatively cheap, I don't think particularly toxic, re usable, Moulds seem to be washable in water (I expect there would be limits to this).
I have added a link below to where I found the original info on this material.
Step 4: Traditional Sand Casting
Disappointed by the tries before in mould making on the way to a part I moved back to traditional greensand casting.
Wanting the best results I could achieve I purchased approximately 30kg of greensand from Metcast Supplies (a foundry supplies company in Auckland New Zealand).
Ramming the part up in the flask I was initially very happy with how the part looked, The text did break away a little so try again I thought....
Step 5: Greensand Casting, a Brief Rundown
For greensand casting, you take a positive pattern of your part and place it on a board.
Talc or similar is then powdered over the part to try and prevent the sand from sticking to the part.
Fine sand is then sifted over the part to try and achieve the best finish possible.
Unsifted sand is then placed over the part in a wooden frame called a flask.
The sand is firmly rammed down over the pattern and then any additional sand that is proud of the flask is scraped off to form a flat surface.
The flask is tipped upside down and top flask is added. Once again the surface is dusted with talc to prevent the sand from sticking to the sand on the bottom and the pattern.
The top section is also filled with sifted sand atop the part followed up by unsifted sand, once again ramming the sand tight.
A path for the molten metal to enter the mould is either cut afterward or formed (see the aerosol can in the second picture)
Note the hole on the bottom of the pattern in the upside down mould... by screwing a wood screw or similar into this hole and then lightly tapping it with a screwdriver the pattern will (hopefully) come free from the sand causing as little breakage of the sand and loss of definition as possible.
Step 6: The Night of the 25th of June
So its now 2227 hrs on the 25th of June and the Instructables casting contest will probably be closed by the time I wake and have to go to work tomorrow.
While I had hoped that practice would make perfect even after 10 odd star prints with different text combinations just would not come out clean.
As a last ditch attempt to pull this off I have compromised to a single large C instead of the text Christine that I had originally intended. Wherever possible all of the faces of the text have been filleted and chamfered to give me my best chance of a clean cast.
As you can see from the last 'step', I did do one cast already but the text was shallow from sand breakaway and then barely visible once cleaned up.
Unfortunately with my commercial greensand I have been having an issue whereby the rammed sand is 'delaminating' and coming away in big slabs.
Metal of choice will be a bronze(ish) alloy. (Somewhere along the way my bronze, aluminium bronze and brass ingots got muddled up and I can't yet tell the difference between them)
I have added an 'action shot' of me moving the crucible which shows my furnace in the background.
Also included is a photo of my 'casting tools' including my homemade tongs, pouring shank and burner along with my safety gear.
Hopefully this one comes out well, out to check on the results....
Step 7: The Results
So the results...
Photos from left to right are
First straight after the cast was pulled from the sand
Second after a quick wire brush
Third, after the sprue has been cut off and I have spent 10 minutes with a grinder and flap disk.
The surface quality has been impacted most likely from my sand being too wet, I was having issues with the commercial greensand that I had purchased so I had returned back to my homemade which has a bigger grain size too.
I would have loved to have shown you a mirror polished brass star but unfortunately not quite there just yet.
This will likely be the final version and with many hours of sanding and polishing it will look quite good.
Hopefully you have enjoyed this casting project journey with me.