Prototyping for direct metal casting: data to castings in 12 hours. (Product Innovations).

One of the hurdles facing today's foundries as they try to grow market share is lead time. Customers want sample cast parts today to fit into their product designs for form, fit and function. If the foundry can't deliver, then the customer will turn to a fabricator for a quick solution.

Z Corp., Burlington, Massachusetts, has developed one lead-time solution for casting--ZCast. The ZCast system utilizes the firm's Z810 or Z406 three-dimensional printers and a self-developed plaster powder and aqueous binder material (ZCast 500 for aluminum and ZCast 1200 for gray and ductile iron) to build "sand-like" molds, mold segments, cores and core segments that look and act similar to nobake molds.

In operation, the prototyping system deposits the binder onto the powder, following the path set forth in the CAD file of the solid mold and/or core design. The binder activates an adhesive in the powder to glue it together in the shape of the mold and/or core layer by layer (horizontally). No hard tooling is required to produce the molds and/or cores and off-the-shelf ink jet print heads are used for the prototyping system.

The basic operating principle of three-dimensional printing was developed by the Massachusetts Institute of Technology (MIT) in 1989 for the production of a variety of prototype products using ceramics, plastic and metal. One of the original objectives of the MIT project was to create molds for metalcasting.

To produce a cast prototype with the ZCast system, a foundry can select between one of two approaches--the direct pour or shell method (or the foundry can utilize both). The direct pour method allows the foundry to incorporate the gating system design with the electronic mold design so the prototyping system produces the entire mold package at once. The second method--shell--involves designing, printing and using prototyped mold inserts and/or cores in conjunction with traditional foundry tools and processes to create a mold. Typically, with the shell method, a foundry will produce a basic nobake mold, develop a traditional gating system and then insert the complex inserts and cores into this basic package to produce a complex casting.

With the prototyping system, cores are produced with a hollow channel through the center to allow for better venting. The wall thickness of the core depends on the size and geometry of the core, and is generally selected to be as thin as possible while still maintaining adequate strength and durability for routine handling.

For most cast prototypes, the mold creation is a straightforward process. A uniform shell is created enclosing the part geometry. While traditional mold design requires draw and a closely controlled parting plane, the only requirement for three-dimensional printed molds is that the unfused powder can be removed from enclosed areas in the mold before casting. As a result, the parting plane is selected with the goal of being able to remove powder conveniently, not for draw reasons.

Once the mold and/or core is designed, it is built on one of the prototyping system printers. The possible build envelope is 16 x 20 x 24 in. However, molds and/or cores can be built in multiple pieces and then assimilated into one package outside of the prototyping system.

After building, the mold must be dried in a convection oven at 200C for a few hours (length is dependent upon size). The mold must he completely dry to avoid problems during casting. Then, the mold surface can be treated using mold wash or other traditional foundry techniques to improve surface finish. The mold and/or cores then can be poured with aluminum or iron and the castings can proceed to cooling and shakeout.

Once the design of the component and its gating system is completed, this prototype process can deliver prototype castings in days. The mold preparation work, post treatment and pouring will add only 8-12 hr to the production time of the mold and/or core, making 12-24 hours from CAD to metal achievable for a wide variety of designs.

The prototypes produced via this technique thus far have established the following data:

* minimum mold wall thickness--3 mm;

* accuracy--0.38 mm ([+ or -]0.015 mm);

* surface finish--to 200 microns untreated (100 with mold wash);

* minimum core size--8 mm;

* mold material cold tensile strength--285 psi;

Two examples of prototype components cast with prototype molds are a valve body and a manifold. The valve body had dimensions of 33 x 24 x 12 cm and required 3 hr and 58 min. of build time for its mold, 4 hr of oven time to dry the mold and had a material cost of $400. A manifold that measured 28.3 x 17.5 x 10 cm had a mold build time of 3 hr and 32 mm, an oven time to dry of 4 hr and a material cost of $450.

One casting customer that has used the system for ductile iron manifold prototypes is Wescast Industries, Inc., Brantford, Ontario, Canada. A pilot operation for the ferrous prototyping system, Wescast was able to deliver cast hot-end exhaust system prototypes to customers in as little as four days that met all mechanical property requirements and could be tested for form, fit and function.

This prototyping process is built to become a seamless component in traditional casting production. Metal pouring for molds can be conducted in virtually any aluminum foundry of any scale with no special preparation or handling. It is typical for a foundry to produce the mold set, oven dry it and then have the part poured on the spot for a few hundred dollars.

Select No. 150 at www.moderncasting.com/info