Sand casting patterns are made from wood, plastic (polyurethane board or castable epoxy and urethane), metal (aluminum or cast iron), and, more recently, 3D-printed polymer. Which one your casting gets depends mainly on how many castings you buy a year and on the molding process your foundry runs.
Polystyrene foam is machined into patterns for one-off and very low-volume castings. Wax, plaster and silicone rubber appear in lists of pattern materials too, but they are less usual choices; wax serves chiefly investment casting with its wax patterns.
Wood Patterns
Wood is the right pattern material when you buy about 50 castings a year or fewer and the foundry molds in green sand or air-set (no-bake) sand. It is the cheapest pattern to build, and an engineering change is easy to make on it.
The drawback is dimensional movement: wood absorbs moisture as the humidity changes and swells out of dimension, and after long storage it dries out, splits and cracks.
Best Wood for Sand Casting Patterns
For most sand casting patterns the best wood is white pine or sugar pine: soft, straight-grained and easy to carve, so the pattern is cheap to build, though it dents and wears fastest. Mahogany is the better pick when a wood pattern has to hold its dimensions over repeated runs, because it is more stable and harder-wearing. Harder woods such as maple, cherry or birch go into small loose pieces and thin projections that take the most sand abrasion.
Plastic and Polyurethane Patterns
Plastic patterns are the right choice for low to medium volumes, up to about 5,000 castings a year, when you want a pattern as easy to machine as wood without its moisture movement. The usual form is polyurethane board, machined and glued up like wood.
Castable epoxies and urethanes are poured to shape for patterns and, especially, core boxes, and a worn urethane core box can be re-cast over a weekend.
Metal Patterns
A pattern needs to be metal once annual volume passes about 5,000 castings, or when the foundry’s molding process is too hard on softer materials. Metal is the costliest pattern to make and to revise, and it gives the best dimensional stability and the longest life.
A metal pattern is often cast from a wood master, which has to carry a double shrinkage allowance: one for the metal pattern as it cools, and one for the casting the pattern will make.
Aluminum Patterns
Aluminum is used mainly for match plate and cope and drag patterns on automated molding lines.
Cast Iron and Steel Patterns
Cast iron is used instead of aluminum where molding pressure or heat is too much for aluminum: high-pressure molding such as vertically parted flaskless lines, and shell molding and core processes that cure the resin with heat. Iron is the standard ferrous pattern metal; whole patterns in steel are rare.
3D-Printed Patterns
A 3D-printed pattern makes sense when you need the pattern fast, when the design is still changing, or when an old pattern has to be replaced:
- Lead time is short: a pattern prints in hours or days, which suits prototypes and short runs.
- The design is still changing: a revision means editing the CAD file and printing again, faster and cheaper than reworking a wood or metal pattern.
- The old pattern is worn out or lost: worn pattern equipment can be reverse-engineered and reprinted.
In one documented high-pressure green sand application, a pattern printed in a stronger polymer ran up to 30,000 cycles with almost no measurable wear.
Printing the sand mold itself skips the pattern altogether, but every mold is printed, so it becomes uneconomical beyond a few dozen castings.
How to Choose a Pattern Material
Pick the pattern material by annual volume first, then check it against the molding process your foundry runs; design changes and casting size can move the choice. Core boxes follow the same rules, from wood and plastic up to metal for high production.
Production Volume
Annual volume picks the starting material, moving from wood at the lowest volumes through plastic to metal as volume rises:
- About 50 castings a year or fewer: wood.
- Up to about 5,000 a year: plastic, usually polyurethane board.
- Over 5,000 a year: metal, aluminum or iron.
If you expect volume to grow, start in polyurethane board rather than wood.
Molding Process
The molding process sets a minimum hardness for the pattern:
- Green sand molded by hand, and air-set (no-bake): wood, plastic or a printed pattern.
- Automated match plate and cope and drag molding: usually aluminum.
- High-pressure vertically parted molding: cast iron.
- Shell molding and heat-cured cores: cast iron; shell cores need an iron core box.
Settle which foundry and process will run the part before you order the pattern.
Design Changes
While the design is still changing, keep the pattern in wood, plastic or a printed pattern, and move to metal once it is frozen.
The foundry measures the first castings off a new pattern, and small corrections to the pattern are common, sometimes over several rounds. Those corrections cost least before the pattern goes into metal.
Casting Size and Weight
Large, heavy castings usually get wood or plastic patterns, because they are seldom bought at volumes that justify metal and are mostly molded in no-bake sand.
Large-format 3D printing now offers a plastic replacement for big wood patterns, with more strength and dimensional stability; polyurethane board has usually been used for small and medium work.
For a large casting bought only now and then, a polystyrene pattern machined when each order comes in avoids storing and maintaining big pattern equipment between orders.