Plastics gain importance as patternmaking material.

Plastics Gain Importance as Patternmaking Material

Wood is still widely used to fabricate master patterns for metal castings, and metal certainly retains its place as the pattern material of choice for high-volume tooling such as that used in automotive production foundries. Increasingly, however, plastics are being selected as an alternative pattern material. Once a master reverse is constructed, plastic patterns can be duplicated and repaired quickly and easily They also can save time and money.

The use of castable urethanes, epoxies and plastic composites as pattern stock has advanced rapidly during the last decade. This increase is largely due to quick turn-around time and labor savings, but it also can be traced to the development of more versatile plastic compounds better suited to foundry patternmaking requirements. The use of sand coolers by foundries has also furthered the use of plastic by keeping sand temperatures cool enough to effectively run plastic patterns.

Patternmakers use plastics because they can be compounded to fit a wide range of pattern and corebox requirements. Plastic tools can be cast to finished dimensions, eliminating the need for much expensive handwork. The more complex the pattern shape, the greater the advantage for opting for plastic construction materials. Production time for plastic tools can be measured in hours or days while the same tooling in metal could take weeks or months.

There are some disadvantages to using plastics for patterns. They generally include a surface hardness that is considerably less than metal and, thus, more prone to shop damage and wear. Also the flexural, tensile and compressive strengths of plastic tooling are less than half that of most metals, and working temperature limitations are much lower.

Characteristics such as memory, low weight, elasticity and resistance to wear and corrosion make plastics ideal for many pattern applications. They are available as castable liquids and in solid shapes suitable for machining, and as adhesives, fillers for surface repairs, seals and gaskets.

Unlike metal patterns that require a great deal of expensive fabrication equipment and considerable handwork, plastic materials also can be formed to near net shape using much less expensive equipment.

Two commonly used plastics are the epoxies and the polyurethanes. Though different chemically, they enjoy considerable utility because of the variable physical characteristics that can be built into them. They also accept a number of inexpensive fillers such as glass roving and spheres and wood and metal powders that add rigidity, strength, abrasion resistance and impact strength. Many plastics are physically superior to wood in many respects and less costly than metal.

The urethanes find more utility than the epoxy resins because they are equal to metals in some applications in addition to being lighter, and more easily worked. Unlike epoxies, urethanes usually don't require surface coatings, are simpler and quicker to use and are better heat transfer agents.

General Classifications

Polyesters - Used mostly where precise dimensional accuracy is not critical, polyester resins cure rapidly and are used extensively as filleting and as filler material to correct pattern surface irregularities. Relatively inexpensive, their high shrinkage coefficient is a major disadvantage for some applications.

Epoxies - Epoxy resins are versatile. Their advantages as tooling materials are a low shrinkage rate, high compressive strength, dimensional stability and good resistance to wear and corrosive environments. They are widely used with fillers such as glass roving, wood, glass beads and aluminum. Since they make excellent adhesives, epoxy tooling can be repaired with epoxy glue quickly and inexpensively. Their disadvantages are that they are heavy and brittle.

Urethanes - Polyurethanes make excellent patterns because they form permanent and rigid shapes with low shrinkage, dimensional stability, high compressive and tensile strength and significant durability. They can be drilled, machined, sanded or milled, stored indefinitely, are impervious to moisture, most acids and oil and can easily be repaired.

The addition of various filler materials, like aluminum and aluminum oxide powders, cast iron granules and glass beads, results in suitable compounds for many pattern and corebox applications. The cure time, shrinkage and physical properties depend on the thickness of the plastic sectioning and the type and amount of hardener used to set the plastic. Paramount to its use, however, is the consistency of the physical properties over long periods and, when carefully compounded, the uniformity of the material from batch to batch.

Some plastics can be colored, which enables color coding of coreboxes and loose pieces. Where appropriate, they also can be metalized either by bonding or electroplating to improve heat dissipation, enhance fine pattern details and improve release properties.

In summary, plastics for foundry tooling has grown because they offer the shortest turn-around time after production of a master pattern, simple pattern duplication, light weight tooling, simplified pattern modification and repair, and the ability to use existing pattern equipment for masters, all of which add up to significant cost savings.

PHOTO : Reflecting the increased interest in plastic as a patternmaking material, the Cast Metals Institute includes segments on the use of plastics in its patternmaking courses.