Rapid Production Tooling
IV. RAPID PRODUCTION TOOLING FOR PRECISION SAND CASTING
The advent of chemically bonded sand has brought a new term and a new capability to the world of the foundry. The term is precision sand casting. As the name implies, the process yields castings with finer surface finish, more intricate detail, and significantly higher-dimensional accuracy than previously possible with conventional green sand casting. Chemically bonded sand can replicate a surface quickly, accurately, and economically. This enables RP&M-generated tooling solutions that can satisfy especially time-sensitive cast-metal requirements.
The compatibility of chemically bonded sand with SL ACES patterns is enabling the production of as many as 1000 castings from a given configu-ration. This unique tooling approach has already been successfully applied for both short-run prototype and long-run production requirements.
In its simplest form, the chemically bonded sand approach involves a sand mixer that coats very fine sand particles with a catalyst. This operation is accomplished in isolation from a second mixing operation that coats similar sand particles with a binding agent. Then, the catalyst-coated sand particles are brought together with the binder-coated sand particles in a high-speed mixing cone. Here, the two types of coated sand particles come in contact in a continuous stream. The output of the mixing cone is directed over a pattern set. The combined sand mixture is then tucked and hand compacted against
the pattern, which is held by a rail set in the X–Y plane.Figure 9shows the mixing cone, with the resulting stream of mixed binder- and catalyst-coated fine sand particles being directed onto an ACES pattern.
The catalyst-to-binder ratio establishes the available ‘‘working time’’
of the sand before it takes an initial set. When first mixed, the sand is very fluid and is easily directed into the cope (top) and drag (bottom) pattern boxes.
After a given amount of time, based on the sand volume and the catalyst/
binder ratio, the mold will exhibit sufficient strength to allow inversion and pattern withdrawal without damaging the cured sand mold, provided reason-able care is exercised.
Once fully prepped, the two mold sections are closed against one another and clamped with sufficient force to withstand the hydrostatic pressure of the
Figure 9 Precision sand-casting mold formation.
molten metal during the pouring operation. The binder holding the sand parti-cles together, at the interface between the cast metal and the precision sand mold surface, is subsequently broken down by the high temperature of the molten metal. This results in a loose sand envelope adjacent to the casting.
This ‘‘thermal debinding’’ facilitates the removal of the casting and its associ-ated gating system from the mold with minimal stresses being imposed on thin or delicate cast sections.
Fortunately, the precision sand-casting process can produce quality sand castings quickly. For many simple configurations, it has already been demon-strated that it is possible to close the mold, pour molten metal, cool down, and remove the solidified metal casting within 1 h. Obviously, larger and more geometrically complex parts may take somewhat longer.
Although the thermal debinding mechanism does greatly assist casting removal, it does, of course, destroy the mold. In that sense, precision sand casting is similar to investment casting: both processes provide one casting per mold. However, it may take 5–10 days to create an investment casting shell from a single pattern. Furthermore, the pattern itself is eliminated in the investment-casting process. With precision sand casting, the same pattern may be used over and over again, and the mold can be produced in a matter of hours.
Virtual pattern making is not a term of the future, it is a fact now. The ability to utilize the skill sets of a journeyman pattern-maker to guide the construction of precision sand tooling through the computer is becoming less rare. Designing a pattern in a CAD environment employs procedures similar to those used in conventional pattern making; specifically, determining the parting surfaces, establishing the core prints, defining core boxes, and so forth.
The tooling for the precision sand case study shown inFig. 10.involves a single impression cope and drag plate with its associated core boxes. All tooling components were modeled in solid CAD, including major portions of the gating systems.
From the solid CAD model, the primary parting surface is defined and the CAD model is split. Part features that will be formed by secondary cores are identified. Appropriate core prints are also CAD modeled for the respective cores. The core print, as well as with the core itself are extracted from the model as a single entity. This is illustrated inFig. 11.
At this point, a core box can be modeled around the core and core print.
This process is repeated until all cored areas are described. The sand mold, with all its cores in place, can be simulated in the computer. Finally,‘‘molten metal’’ can be ‘‘poured’’ in the computer simulation, and the resulting ‘‘cast-ing’’ can be ‘‘extracted’’ and ‘‘inspected.’’
Figure 10 Rapid tooling for precision sand casting, including the cope and drag plate and associated core boxes.
Figure 11 Core and core print extracted from CAD model.
Once the design has been achieved, the individual components are gen-erated on an SLA using the ACES build style. All tooling components are currently built using a 0.004-in. (⬃100 µm) layer thickness for maximum surface resolution and accuracy. Nonetheless, the components still require some benching prior to mounting and assembly, in order to eliminate ‘‘stair-stepping’’ on inclined surfaces. When thinner layers or advanced techniques such as ‘‘meniscus smoothing’’ become available, the improved surface qual-ity of inclined or compound curved surfaces will greatly reduce the amount of benchwork. In turn, this will further accelerate the entire process.
Precision sand casting requires no external packing, pounding, or tamp-ing. Consequently, the fine sand/binder/catalyst mixture can be molded against an ACES part with very little abrasion. As a result, there is almost no degradation of the ACES patterns during the sand-filling, mold-curing, or pattern-extraction steps. A seal coat of paint applied in a light color is sug-gested to further aid in the visual inspection of abrasion on the active tooling surfaces.
The ACES patterns have proven to be extraordinarily robust when used in a production mode. Some configurations have yielded over 1000 precision sand molds without any signs of wear. Obviously, care must be used in mold-ing, pattern extraction, and general handling to allow for the reduced strength and impact resistance of cured epoxy resins relative to either aluminum or steel tooling. Experience to date indicates that tools fabricated in this manner certainly require care in their use, but, of course, this is true for any precision tooling.
For the case study described herein, the sequence of events and the time required to develop ‘‘Precision Sand-Cast Rapid Tooling’’ is listed. Note that this total elapsed calendar time includes not only the first article production casting, but weekend time as well.
1. Generating a solid CAD model of the casting from 2D 5 days customer data
2. Solid CAD modeling of the tooling, as well as associated 10 days engineering
3. Building the ACES patterns/core boxes 10 days 4. Bench finishing and assembling the tooling components 5 days 5. Producing the ‘‘first article casting’’ and performing QA 5 days
inspection
Total calendar time from customer 2D data arrival until 35 days delivery of the first article casting
The development of the practices and procedures needed to extract the greatest amount of time from the process while still delivering quality castings at a favorable cost continues. By combining the technologies of CAD, RP&M, and precision sand casting, it is now possible for customers to receive aero-space quality castings in quantities from 1 to 1000 in a time frame that would have been considered utterly impossible just 5 years ago.
Without question, the manipulation of digital data to produce tooling is the wave of the future. The prospect of being able to generate tooling with a computer-controlled additive system is truly fantastic. The word ‘‘precision’’
in the term precision sand casting takes on additional significance when aug-mented with the capabilities of solid CAD modeling and an accurate RP&M technique such as SL. The applications that can be addressed with these tech-nologies appear limited only by our collective imaginations.
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