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Protobuild Casting Study by Steven Adler, A3DM



Rapid Prototype machines and CAD are now in the mainstream of the fine jewelry industry with precious metal casters encountering a new model material with unique properties. Unlike the traditional hand carved or injected waxes, these new materials require special care in order to maintain the surface quality achieved by these advanced micro-jet and laser technologies. Unfortunately, in many cases either through error or inexperience, a great deal of detail and surface finish has been lost or damaged in the investment and loss wax casting process.

When using an additive layer fabrication system like the RapidToolmaker to produce jewelry parts, the device places many droplets of dense material on each layer to form the outer perimeter of the object shape. Within the dense object perimeter droplets are placed in a matrix or honeycomb that supports the perimeter. This inner matrix requires far less time and material to deposit yet supports the dense perimeter surface. As each layer is added, air and other gases from the cooling material are trapped within the honeycomb body of the part. On completion there may be 5% to 30% of the part volume containing a gas of some type. The problems that arise in investment casting this semi-hollow structure are due greatly to the current process jeweler's use for casting the existing materials. For many years casters have used various methods to remove air bubbles that adhere to the surface of a wax model. Casters used vibration, vacuum, and chemical agents to nearly eliminate the bubbles in today's modern factories. NONE OF THESE METHODS WORK with this new material without some risk of distortion. Therefore, casters need to take new measures to insure a perfect finished product.

Listed below are the key elements that need to be addressed.

 I. De-Bubble Wash

The use of Isopropyl Alcohol as a drying agent with any of the commercial de-bubble washes should be avoided at all cost. Alcohol will dissolve the build material surface and make well defined features on the part muddled and vague. Isopropyl Alcohol may be used sparingly with small swabs, buffs, or absorbent materials to smooth surfaces and to remove any minor stepping prior to flask investment

II. Gate and Sprue

 Many designers over the years have left the decision of where and how to gate a model for efficient casting. By placing the burden on the caster this insured a fully developed model. This new material and semi-hollow structure is fragile and handling any delicate model can be a game of Russian Roulette. For this reason many designers add a small cylindrical "nub" to the part surface. This allows your caster to easily attach wax wire without damaging a valuable model surface using a wax pen.. Even an overheated wax wire can damage the model when applied too quickly.

III. The Silent Sucking Sound

In the "normal" investment process, the investment is vacuumed twice. The first time to remove the large pockets of air from the slurry in a mixing bowl and then again after the slurry has been poured into the flask. It is this second vacuum that causes most of the problems for build material. Since the model has trapped gases within it's structure, the internal gases are drawn from the model and create blisters on the surface of the model as the lighter gas escapes into the slurry. When the vacuum is completed, the remaining water and slurry rush into displaced volume and leave surface blisters. Thus, the results after burnout and casting will not resemble the model off the machine

IV. Old Meets New (Solution Number 1)

 Using a swab carefully clean the models and smooth any minor stepping that the machine may have left behind using Isopropyl Alcohol. Whenever, possible us a small flask and tree dedicated to casting this type of material. First mix and vacuum the investment in the mixing bowl. Once the slurry is smooth, gently paint the models with a thin layer of the slurry using a small artist brush. Water Color brushes seem to work best. The action of your brush will deflate any surface bubbles from the model. Once the model is covered, and before it has dried completely, assemble the flask and gently pour the investment into the flask.


 This is an old "craft" method that was popular before vibration and vacuum steps became so commonplace. The only downside of this method is the greater risk that the investment fracture in the burnout process due to excess water vapor. A longer melt out is preferred.

V. Constructing the Barrier (Solution Number 2)

This method is similar to the above method except instead of painting with investment slurry, you may use a substance to create a barrier. Using a Water Color brush gently coat the part with Mineral Oil or diluted Polyurethane. After applying the barrier, some clients apply vacuum to the tree before investment is poured. This process, in theory, will infuse the honeycomb areas with a liquid that would not be displaced by a vacuum of the full flask. The advantage to this method over the previous is that you can vacuum the second time and that the flask is less likely to fracture in the burnout process. There is an increased risk depending on the volume of the model that blisters will appear using this method due to the vacuum pressure being applied



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