Aluminium vacuum investment questions.

Hi,
I'm just starting this adventure and could do with a little help answering a few questions. As indicated in my introduction thread, I intend to cast aluminium parts for tethered cars. A bit of a niche hobby but very interesting. I have bought all the necessary equipment and I am quite proficient in 3D printing the parts. Now for the questions:
1/ Is it okay to use scrap 6082 aluminium?
2/ After the burnout, what temperature should the flask be for pouring?
3/Looking at the images, should these cast okay, or do they need more sprues?

There will be many more questions.

 

Thanks, Kelly.
If this is vacuum casting, is there a need for vents?
The gearboxes have little mass, with the largest cross-section around the large holes at about 5mm. Both the single and triple are intended to be cast.

 

Yes, I appreciate that I may need to go to a 100mm flask, rather than squeezing the three into a 70mm flask. That would enable a thicker sprue and a more realistic gap between the parts.

Are you using 3D printed patterns? If so, what resins are you finding to be the best and, could you share your burnout schedule? Your help is much appreciated.

 

I vacuum cast 1/3 scale Harley finned cylinders in aluminum with no vents. I'll see if I can find photos of the setup, but some of the internet wizards might be able to find the thread from the old site.

 

This is the lost wax vacuum cast setup I made for the Harley cylinders.
I injected the wax to form a washer composed of the fin and a bore with a male/female protrusion that was the same space as the fin thickness of .070". Then I rolled up some pierced metal and wrapped it with duct tape to pour the investment. I don't remember the burnout temps but followed the recommendation of the material. The cover was cast to hold the pouring cup with a taper and the investment itself and placed on the stainless can. I pulled the vacuum with a Harbor Freight air siphon and as I remember maintained about 15" . As I remember there is a silicon bead on the lid to insure the seal.

 

No Kelly, it is another stalled project because the cylinders and heads were originally a single casting. Some of the Harley lore mentions high foundry loss rates since it required a very complex mold. This lost wax was my first foray into that process with the goal of a cast iron cylinder and head. I made wood patterns but was never able make a silicon mold for the wax.
Then I was distracted by something else.
I didn't mean to threadjack Mike.

 

No problem, it's all very interesting stuff. Cylinder heads are a bit beyond me at the moment, but with a bit of luck and some sound advice, I will get there. That is something a 3D resin print should be very good for. I have cheated and bought a furnace/vacuum all-in-one machine and a burnout kiln. I'm still trying to find the correct pour temperature of the flask - hold that thought! I've literally, as I am writing this, just received a phone call from SRS, the manufacturers of the investment plaster I am using and they suggest starting at 450C and working down in 50C stages till I get an acceptable surface finish.

 

SRS are a UK company that specialises in investment powders. The one they recommend for 3D printed patterns is Cadcast. "SRS Cadcast is designed primarily for investing and burnout of CAD/CAM rapid prototype resin and resin/wax patterns." The website is: https://www.srs-ltd.co.uk/products/investment-powder/cadcast
I was also sent a very useful sheet on mixing, flask sizes, melting points and flask temperatures.

 

I'm assuming the vacuum is from the bottom of a round flask. If that's the case, I think the sprue base might be a little deeper and larger around. I've never watched a vacuum casting being done. I'm not sure if the draw down can be faster than the pour. You would be a better judge of that than I. Until I knew the draw down rate that would be a concern for me. That and you'll have a lot more metal than you'll need for the casting. A bigger sprue base will give it somewhere to go.
I've only used centrifugal casting machines. I have three, but almost all my gold and silver casting has been done with a broken arm Kerr. Try not to boil the metal, and skim befor casting.
Fredo

 

I will be using perforated flasks that draw through the sides and base. Bearing in mind one or two previous replies, I have thickened the sprues on the latest prints and will have to see if this helps. My workshop time is somewhat limited as I am a 24/7 carer for my wife so progress and thread updates will not be very regular.

 

I haven't investment cast aluminum but have been curious about using it for couple of years. I have done searches of the web and there is very little on investment casting aluminum. I have investment cast a lot of small bronze pieces using 3D printed "waxes" in the same size range that you are attempting.

I've also wondered about investment casting zinc alloys. I've sand cast ZA-27 a number of times and find it easy to cast, probably easier than aluminum. You might consider the zinc alloys as another option to aluminum if the casting will serve as a bearing. It's heavier, but much better as a bearing material than aluminum.

I use vacuum casting, but with the vacuum from the bottom of the flask. I use "straws" of wax or PLA or plastic rain gutter guard mesh to give a path for vacuum to about 3/4 or the way up on the wall of the flask. I'd like to try perforated flasks but I have quite a few without perforation and to replace them would be quite an investment. I have never investment cast with wax, only castable resin and PLA.

It is impressive how well bronze flows when vacuum casting, even with very small sprues. You'll just have to experiment with how small of a sprue you can use with aluminum.

In my experience, the burnout schedule will be controlled by the type of castable resin you use, size of flask, and possibly the investment. I have used several types of castable resin, all with bronze, and the maximum temperature and time was determined by the resin. It's been a while, but my recollection is that one type burned out at 150 or 200 degrees hotter and had a longer burnout time than the Siraya Cast Purple that I've been using for a while. Only the final temperature at the time of pouring would be determined by the metal alloy. Too hot and you will possibly have shrinkage and surface finish problems.

I use SRS Classic investment, mostly because many vendors won't ship to Alaska or only ship via UPS second day. I previous used Kerr Satin Cast 20 with similar results until it stopped being available. Something specifically formulated for castable resin might work better, but I use what I can get.

I'm interested in hearing what you learn.

 

Hi Dick, thanks for the response. The prints shown at the start of this thread have both been made with Siraya Tech Cast Purple but I also have some of their True Blue to try. Apparently, it has a little more wax in it and can be a bit more difficult to dial in. However, I have done a test print, at the same settings as the Purple, and it has come out really well. I now need to do a fourteen-hour print for the next set of waxes. I will update as soon as I am able. Cheers, Mike.

 

Dick Morris, Taking a closer look at your avatar, are there a couple of small UV joints along with the coin? If so, these are similar to some of the components in the tethered cars we are running and I would be very interested to learn how you made them.

 

Yes, they are low speed U joints to extend a valve stem on a 1/8 scale locomotive.

The first of my investment casting starts at about the middle of this page and there is some sand casting before that. Follow it through several pages and you'll find a number of posts on my investment casting efforts.

https://www.chaski.org/homemachinist/viewtopic.php?t=98849&start=24

 

Fascinating read, thanks. So you don't think the UV joints will take 40,000 rpm our little engines are turning out?