Revisiting the V-Process.

Only two decades old, vacuum molding remains flat in the U.S., yet holds promise for precise casting without much environmental worry.

In 1972, the first vacuum molding, or V-Process, plant was completed in Japan. Initially, the Japanese used the process, which employs a use of a vacuum to mold unbonded sand around a plastic-lined cavity, to reproduce detailed works of art in aluminum and bronze, including sculptures, statues and ornamental grillwork.

With the V-Process today, castings of all sizes and shapes, from a few ounces to tons, can be made in most metals, including aluminum, gray iron, ductile iron and steel. Since its evolution, the V-Process has developed into a process capable of producing industrial components.

While the process lends itself to prototyping, it is ideal for short and medium production jobs, as well.

V-Process molding technology is an alternative to conventional green sand and nobake molding. In some applications, it is also a cost-effective alternative to investment and plaster castings. The V-Process functions effectively as a diecast prototype process, and can be used in place of permanent mold and die castings where draft, size, quantity, complexity and metal-choice problems are present.

Although the V-Process was formerly proprietary, patents are currently expiring and foundries will no longer be required to pay licenses and royalties for its use. In the U.S., Roberts Sinto supplies equipment for the process.

How It Works

Two key characteristics differentiate the V-Process from conventional sand casting. First, a thin plastic film is heated to its deformation point and then vacuum-formed over a pattern on a hollow carrier plate. Second, the process uses dry, free-flowing, unbonded sand to fill the special flask set over the film-coated pattern.

The sand used contains no water or organic binders and is kept under a vacuum during molding. Permeability of the sand is not a concern, as in green sand, which is why finer sand can be used and improved finishes are achieved. As shown on the previous page, slight vibration quickly compacts the fine grain sand to its maximum bulk density. The flask is then covered with a second sheet of plastic film. The vacuum is drawn and the sand becomes rigid. Releasing the vacuum originally applied to the pattern permits easy stripping.

The other half of the mold is fashioned in the same manner. The cope and drag are then assembled, forming a plastic-lined mold cavity. Sand hardness is maintained by holding the vacuum within the mold halves at 300-600 mm Hg. As molten metal is poured into the mold, the plastic film melts and is replaced immediately by the metal. After the metal solidifies and cools, the vacuum is released. The sand, which literally drops away from the casting, is then cooled and reused into the system.

Process Advantages

The V-Process offers a variety of benefits, namely a smooth surface finish, excellent dimensional accuracy and zero draft.

A key benefit, zero draft eliminates the machining of draft to provide clearances for mating parts and assembly; provides consistent wall thickness for weight reduction and aesthetic appeal; allows simple fixturing for machining and inspection; and allows pattern construction to become more accurate and efficient. Other benefits of zero draft are that total tolerance range is for the actual feature, not the feature and draft; geometry/tolerance of the part is at its simplest form; draft does not use up tolerance; and design/drafting is less complex because calculations and depictions related to draft are eliminated.

Other advantages of the V-Process include:

* thin-wall capabilities;

* excellent reproduction of details;

* low tooling costs;

* unlimited pattern life. The pattern is only contacted by the plastic; therefore sand doesn't cause wear, reduce surface finish or open up tolerances;

* "user-friendly" patterns; easy revisions to patterns, no metal tooling; good for prototypes;

* fast turnarounds and short lead times.

Limits of the process from a casting standpoint are only what the film can draw over and into.

While the slower cooling that the process [TABULAR DATA OMITTED] requires can cause a coarser grain structure, castings don't exhibit as much gas porosity or as many inclusion defects other foundries generally experience because of the sand. However, the process has its own unique problems, such as film wrinkles (requiring extra grinding) or collapse of the cavity.

Equipment/Materials

The V-Process doesn't require conventional foundry equipment such as flask jackets, shakeout equipment, jolt machines, sand mullers, scrubbers, rollover machines, slingers, exhaust equipment, extensive sand laboratory equipment and hand tools such as slicks and trowels.

Equipment unique to the process, however, includes the film heater, pattern carriers, dry sand compaction units, and vacuum pumps, piping and special hollow, double-wall flasks. All patterns must also be vented to pull film.

In regard to consumable materials, V-Process foundries don't wrestle with new sand costs or binder consumption. Plastic film does become a new consumable material in contrast with conventional foundries.

Environmentally, there is virtually no sand disposal. But because of the absence of binders in the sand, there is more free-floating silica, primarily at shakeout. Dust collection is therefore more of a concern for V-Process foundries.

Patterns

Pattern equipment requirements for the V-Process differ from those in green sand and nobake molding. Using wood patterns is possible but not recommended. While the patterns are not touched by sand, they are still subject to changes in temperature and humidity.

The patterns must be vented or drilled to allow the plastic film to be drawn over the top, and this typically eliminates metal tooling. Cast epoxy or a machinable plastic component for patterns are usually best because these materials are stable and durable. They provide more accurate patterns because the pattern can be cut on a CNC machine using a customer-supplied IGES data base. The data base and CNC machining combination enhances accuracy, improves turnaround time and decreases cost.

Since the V-Process mold is covered with the plastic film, pattern stripping is performed smoothly. Straight walls draw as easily as tapered walls. There are two reasons - the plastic film provides a natural lubricity when stripping off the pattern; and the vacuum applied during the film-forming operation is released before stripping and the vacuum is then applied to the mold. This causes a slight expansion of the mold walls.

Draft elimination prevents excess machining cuts and can contribute to substantial cost reductions.

Dimensional Reproducibility

Due to the lack of pattern wear and the exacting conformity of the heated plastic film to the pattern, the dimensional repeatability is significantly higher when compared to green sand molding. Patterns must have a smooth finish since the V-Process will duplicate each and every defect.

V-Process molds are more rigid (no wall movement due to expansion) than green sand or [CO.sub.2] process molds and castings are consistently more precise in all dimensions.

Some casting configurations, though, do not lend themselves to the V-Process. For example, if there are narrow and deep recesses, the foundry will have problems. Generally, the depth of a hole or slot should not exceed its width or diameter when using the V-Process.

V-Process casting tolerances will sometimes eliminate machining in less critical areas. This is particularly important for wear resistant, high hardness parts where conventional finishing is virtually impossible except by grinding.