Producing foam patterns with ventless molds.

Producing Foam Patterns With Ventless Molds

Recently developed, ventless molding of foam patterns for the production of metal castings represents another refinement of the evaporative pattern casting process.

Using ventless molds to produce foam patterns for the evaporative pattern casting (EPC) process is a relatively new technology, but one that has been applied successfully in a variety of applications.

Patented in 1986, ventless molding was developed specifically to improve the surface of the foam pattern and, thus, the final casting. The process also improves density and bonding strength.

Actually, the term "ventless" is a misnomer, according to Hal Clarke, president, Foseco-Morval, Inc (FMI), the patent holder and largest user of the process. "There are vents in the tooling, but they are so small that they are barely visible," he said. "The idea is to keep them smaller than the foam beads to prevent plugging the vents."

The primary difference between this newer molding method and conventional vented tooling techniques is the control of the steam during the bead expansion and fusion stage of the process.

Evacuation of the steam at the end of the molding cycle also differs significantly from the more traditional methods.

Foam Molding

Simply put, producing foam patterns involves filling a metal mold with a prescribed amount of pre-expanded polystyrene beads. This is usually accomplished with a fill gun or guns with a venturi air stream which aids in filling the mold completely. Heat is forced through the mold, usually in the form of steam. The heat causes the pentane inside the bead to expand each bead against one another.

As the beads "blow up" and contact the tool's surface--which also has been heated--they flatten out and fuse together, forming the desired shape. The temperature of the tool surface is critical in determining the pattern's surface finish.

"If you don't put the right temperature into the bead, you don't get good expansion and bead bonding," Clarke said. "If the temperature of the tool isn't correct, you don't get the good pattern finish.

"The temperature controls everything. If the tool's surface is too hot because you need to get the heat into the bead, you can get poor pattern surface. If it's just right for the surface but not hot enough for the bead, you get poor fusion."

In the case of conventional vented molding methods, the tooling must have enough vents to allow the air used in filling and the air already in the mold cavity to escape. The vent area should be about 2% of the total tool cavity surface area. Vents usually are made from a capped tube that has two small slots in its working face to allow air or steam flow.(*)

After the mold is filled with beads, the molding cycle is initiated as steam enters an internal cavity on the back side of each mold half. This heats the tool's surface, helping to flatten the beads as they contact the mold. If extended cycle times are required to heat the mold, this can result in burning or collapse of the pattern.

The same steam that warmed the tool is then forced through the vents to expand and fuse the beads together to form the pattern. The steam is then exhausted through the vents, the mold halves cooled with water, then opened and the foam pattern is ejected. Generally, a shot-to-shot time of 90 seconds or less is considered normal.

Ventless Molding

Ventless molding of patterns for the EPC process differs from conventional vented molding methods because it uses two separate steam systems. The steam that goes into the backup cavity only heats the wall of the tool. It is isolated in that area and is vented out after being used.

A second system, which directs its steam directly into the molding cavity, is used solely for bead expansion and fusion.

The separate systems allow for improved control of steam temperature, pressure and moisture content, according to the developers of ventless molding of EPS. "It's a matter of controlling the steam, where it goes and the relative condensation points within the mold chamber," Clarke explained.

Larry Crabtree, Foseco-Morval manager of Engineering and Technical Sales Support, and co-holder of the ventless mold patent, has worked with foams for 26 years, and specifically with foundry foam patterns for more than 20 years. He explained that molds with vents can create water mark defects on the surface which translate themselves to the final casting.

"The very marks caused by the vents themselves are objectionable to some casting users," he said. "In that case, it is more of an appearance factor than anything else. But if you can avoid them, you're over just one more hurdle."

Crabtree includes improved interior fusion, which leads to strong stable patterns, as an important benefit of the ventless technique. "We find shrinkage factors are easier to control as well," he added.

"In terms of density, we are achieving densities of 1.6 lb/[cubic feet] [plus or minus] 0.05 for patterns used for aluminum castings and 1.3 lb/[cubic feet] [plus or minus] 0.05 for iron castings," Crabtree said. "Some new developments in standard EPS materials are enabling us to go to even lower densities."

In terms of wall thickness, Crabtree explained, "Our preference is to maintain a thickness of 4.5 mm. There have been several projects where we have achieved walls as thin as 3 mm. It all hinges on the design and capability of molding a particular style of part. Generally, our experience has been that two beads wide is not enough to fill the mold cavity properly."

Regardless of molding technique, Crabtree insists that the ultimate quality of an EPC casting is a function of the quality of the tool used to mold the foam pattern.

"Tooling quality runs hand in hand with the quality of the finished product, from both the surface finish and dimensional stability and raw dimensions," he said. "Generally, cast aluminum molds are adequate for parts that are not dimensionally demanding. For complex, dimensionally demanding parts, we recommend molds cut from billets."

Control is Key

Clarke emphasizes that while Foseco-Morval possesses the patent for ventless molding for EPC patterns, the company uses both vented and ventless molding techniques, depending upon the part and customer needs.

"That's the thing you have to remember," Clarke said. "We produce the patterns and if the pattern results in a scrap casting, nobody's happy, nobody wins. We feel the additional control afforded by ventless molding in most cases offers the foundry an improved opportunity to produce high-quality castings."

PHOTO : A robot controlled, five-station glue machine at FMI's Kitchener, Ontario plant allows the

PHOTO : firm to provide complete multi-part foam patterns.

PHOTO : Larry Crabtree, Foseco-Morval manager of engineering, displays a pattern made in ventless

PHOTO : molds. He designed his first EPC pattern 22 years ago.