[Jewelry making - Article 00381] - Re: [Orchid] CAD/CAM grown model woes

[Thomas C]

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Richard

    This specific problem you describe sounds like an incomplete
    burnout. I would guess that the caster may have consulted the grower
    but did not follow instructions in detail, and if the instructions
    are not followed in detail, the casting will fail. 

    The different resins have different castabilities, And several are
    very castable"Invision HR, Perfactory", We have found no ash content
    in Invision HR resin using a full burnout process. Some are castable
    but with a bit of work around(Viper). We cast primarily Invision HR
    resin. Of note is that we often cast hand carved waxes right along
    with the resin with equal success. 

    There are very specific steps to take in casting the materials. A
    caster must want to change some steps to have a successful casting of
    resins. Most failures occur because the caster will not adapt the
    process to the material, they insist on using methods that work with
    wax, but just wont work with the newer materials. Skipping any of the
    steps will definitely result in failure. 

    It took us a bit of trial and error to dial in casting of resins.
    From experience, the biggest problem is the investment and then the
    burnout. I will briefly cover some of the primary details of the
    process. 

    Sprueing: 

    Sprueing is the most overlooked part of casting in many cases, A
    smooth transition from the sprue to the piece to be cast is
    essential. The piece needs to sprued at its heaviest section, general
    sprueing techniques apply here. 

    **Viper models are castable but need a extra blowout sprue to assure
    full cleanout of the cavity, there is the possibility of ash in viper
    models. Prior to casting, the cavities are blown out with compressed
    air to make sure all Viper residue is gone** 

    Investing : 

    The choice of investment is one of the most critical steps in the
    process. We have tried many investments and many processes. These
    steps work consistently in our production. We use "Doc's Plaster"
    from PM West for all resin casting, it works for gold silver and
    platinum we follow the directions as specified by the manufacturer.
    We use the rubber flasks as supplied by PM West for investing. For
    smaller castings we use a yoghurt cup that we cut the bottom out of.
    We do not use a metal flask, all castings are done "flaskless" Its OK
    to use a debubblizer solution. 

    Yes, the investment costs more than regular investment, but getting
    the casting right every time is worth it. 

    Burnout: 

    We burn out with an electric oven with a programmer. ( Gas ovens
    naturally have enough oxygen to get a good burnout) We have modified
    the oven by drilling four holes into the kiln at floor level, equally
    spaced. We drilled about 1/4 inch holes. To increase the ability of
    the oven to truly burn out the resin, the oven needs more oxygen. 

    To create the proper burnout, We use an old aquarium bubbler as an
    air supply. We attached about a foot of quartz tubing to the plastic
    hose and insert the quartz tubing into one of the holes we drilled.
    This is probably the most critical step in burnout, electric ovens
    do not have enough oxygen circulation for a good burnout. Heat alone
    will not fully burn the resin. Our burnout schedule is ramp to 300F
    hold 1 hour, ramp to 600F hold one hour, ramp to 1500F hold for
    three hours, ramp down one hour to 1100F or your desired casting
    temperature. You can actually do a three hour or so burnout if your
    in a great rush, but in general we recommend the above. The lower
    temperatures are very important and should not be rushed Casting we
    cast with a centrifugal casting machine or for platinum the
    traditional upright machine.

    We cast many ounces of Gold and Platinum every week for large
    accounts and have a very high success rate, and failure is usually
    based on traditional casting problems such as sprueing, too cold
    metal, Flask temperature, etc not the resin. 

    The models can be molded, but unless you need a mold there is really
    no need to make one. The casting of resins is a more scientific
    approach, but easily achievable. 

    This has been very brief, but gets the process pointed in the right
    direction. Again the right investment and the proper high heat
    burnout with air itroduced into the kiln are the primary factors in
    casting resins. 

    Contact us with any questions. 

Thomas Cavagnaro, G.G.
Cadsmithing, LLC, A Service Bureau
CAD-CAM, Casting in Gold and Platinum for the Trade
480 632 1595
Fax 480 632 1598
cadsmithing.com
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