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Thread: Help with investment casting small intricate parts

  1. #1

    Help with investment casting small intricate parts


    My project:

    Last cast (almost perfect fill, but bad surface with lots of needle-like structures):


    First cast (almost good surface only on the ribs/edges):

    first cast.jpg



    I chose the lost wax/investment casting process to try to achieve smooth surfaces & sharp details on the cast pieces. The wax patterns look perfect but I am getting really bad looking casts. I started experimenting with casting only a few months ago and I am making my way through two industry books on the process, however I already tried most tricks to achieve quality so I fear I'm stuck:

    - Casting out of Aluminum. Started with Aluminum 6061, then tried Zamack, and have Aluminum 535 and 356 on order to see if casts are better. I am also open to using vacuum cast stock.
    -- My supplier for Zamack:
    -- My supplier for 356 alloy:
    -- My supplier for 535 allow:

    - I tried both the UltraVest investment (box casting) and SuspendaSlurry (shell casting), both from Ransom & Randalf. I found that for small parts, the ceramic mold surface quality appears to be the same, but I am still experimenting with UltraVest. Obviously box casting is way easier so I would rather do that.

    - I characterized the temp. profile of my kiln and found that going over 1000 F cracks the box molds. I am now doing burnout at 600 F and taking up to 800 F for mold pre-heat before casting. Prev. attempts at pre-heating to 1400 F crack the molds really bad and still give horrible cast surface. UltraVest in particular is good up to 1400.

    - I got de-gassing tablets and I am degassing the melt & pouring an ingot before re-melting for the cast into pattern.

    - I also got flux and likewise I apply it & pour ingot before final cast.

    - I got a water de-ionizer and a large vacuum chamber for making the ceramic molds, to ensure the highest quality.

    - I leave a large mass of metal in a pouring cup above the mold to ensure there is a lot of gravity pushing down into the mold to get details

    - Lastly, I got ceramic blankets and I wrap the crucible, taking only a few seconds to transfer it to the pouring station to ensure the metal doesn't loose too much heat.

    - I also wrap the molds in the ceramic blanket after pouring, to ensure slower cooling.

    - This month I am working on taking tons of measurement samples to understand the heating and cooling curves of the kiln, the molds, and the metal. I will make Excel graphs when I am done collecting this data, but I doubt this will help that much.

    The books I am reading are:
    • Precision Investment Casting by Edwin Laird Cady (Reinhold Publishing)
    • Investment Casting edited by Peter R. Beeley and Robert F. Smart (the Institute of Materials)


    I am documenting my process on youtube:

    Any pointers here? I am not really sure where to go next.

    Thank you!

  2. #2
    How did you gate these and did you build vents and/or risers to allow air to escape? I have never used ceramic slurry but Ultravest has a specific burnout schedule and you have to follow it pretty closely. I've never experienced any flask casts break like what you are experiencing - that looks odd to me.

  3. #3
    Moderator DavidF's Avatar
    Join Date
    Jan 2012
    Wilmington DE
    I have used the ultravest quite a bit and rarely seen any trouble with the molds cracking.
    Your burn out should be ramped up in stages up to 730c. You will get better surface finishes at lower flask temps especially with aluminum. Flask temp of 230c is my go to. You really need vacuum assist if block casting aluminium, it works wonders for surface finish and assists the molds fill.
    A calm sea does not make a skilled sailor...

  4. #4
    Senior Member Zapins's Avatar
    Join Date
    Jun 2010
    CT, Hamden
    Either vacuum assist or centrifugal casting would make this extremely easy. I only have experience with centrifugal which I use for jewelry casting. I've even cast tiny plants with paper thin leaves and they came out perfectly. So that is definitely the way to go IMO

  5. #5
    Senior Member
    Join Date
    Mar 2014
    Charleston, West Virginia USA
    I know nothing about investment casting, but watching the Box Casting video I noticed a couple things. The color of the crucible looks like you were pouring bronze. If it is aluminum, I'd say it was way too hot. The other thing is, you have too much flame exiting the vent. The furnace will be too rich, causing gas in the melt. Turn the propane down and you won't have to de-gas.

  6. #6
    Wow, can't believe the quality of the responses! I had no idea that DIY "vacuum assist" existed - I thought I was limited to either tiny vacuum casting setups that can fit at most a jewelry ring, or giant setup like the ones used for casting titanium turbines for airplanes.

    Now I have a clear plan of attack:

    * Put together a vacuum casting rig using the same technology as, but for much larger box casts (from 4x4" to 24x24"). It looks like I don't need a vacuum chamber capable of not melting around the crucible like I thought, I just need a machined iron base large enough to put my flask(s) on. Perhaps I can even use the small one on etsy: I already have 2 vacuum pumps.

    * Careful with feeding the forge, it's getting too much gas. Revisit supplier instructions on how long it's supposed to take to melt aluminum, and use minimum temperature & feed that can still achieve that.

    * Ensure all experiments going forward use more vents than necessary (is this still a requirement with vacuum?) and post on here for help with designing gating systems until I learn the best way to do that.

    * Pay extra attention for heating ultravest in the kiln, setting alarms/timers meticulously and taking temp. samples at smaller intervals. I have a manual kiln. Going to revisit the ultravest schedule, which I have on all of my computers as a PDF already.

    Thanks again!

  7. #7
    Administrator Site Admin
    Join Date
    Dec 2005
    Huatulco, Mexico
    You need a pyrometer. Estimated times of melting can vary from minutes up to an hour. They mean nothing. To pour quality castings you must pour at correct temperatures.

    When I die, Heaven can wait—I want to go to McMaster-Carr.

  8. #8
    Moderator DavidF's Avatar
    Join Date
    Jan 2012
    Wilmington DE
    spend some money on some good perforated flasks. You wont need any special venting if using vacuum assist. If you cant use vacuum assist arrange your spruing for a bottom up pour with venting out the top...
    A calm sea does not make a skilled sailor...

  9. #9
    @Rasper: I got an IR thermometer that goes to 1382 F: Do I still need a pyrometer? I did see the pyrometer page before I created a thread.

    @DavidF: I will get some of these of various sizes: However, I don't understand what the guy in your video used for the outside. I see solid wall flasks, but nothing with a welded on fitting for the vacuum hose. I also see some gaskets, but I don't know what the whole assembly looks like. Am I supposed to DIY the outside part in a sheet metal shop? I can do that, but maybe there's a way to get the whole thing?

  10. #10
    I did an inventory of the parts I want to cast. While many are small, some are large. This is somewhat off-topic, but would you recommend using shell casting for large parts and flask/box casting for small parts? If so, do I use a similar vacuum casting procedure for the shells, just have to build a large flask?

    Average size, a small part (3.5" diameter flask will work):

    upper arm shoulder.png

    Similar (6" diameter flask will work):


    Long part (3.5" flask will work):


    Large part, tall (3.5" flask might work):


    Large part, wider (6" x 10" flask might work):


    Very large part, tall (needs a custom flask?):


    Very large part, wide (needs a custom flask?):

    chest base.jpg

    I am getting these tomorrow, if that looks good:

    3.5 diam x 5 h
    4 diam x 10 h
    4.5 diam x 5 h
    2.5 diam x 10.5 h -- same as above --
    6.2 diam x 10 h -- same as above --

    These will have to be custom:

    6.5 diam x 14.5 h ?
    12 diam x 5 h ?

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