Understanding Materials for Pattern Construction
When you are looking into the tooling required to produce your casting, you will find a variety of materials are available to use in the construction of your pattern and core boxes. Pattern materials will definitely play a part in the overall cost of your tooling. The number of castings you purchase annually, as well as the molding/coring process used by your chosen foundry source are linked closely to the type of pattern material used. Let’s examine several types of commonly available pattern material to understand the effect on tooling costs.
Polystyrene: For extremely low-volume casting production, patterns made from machined polystyrene are commonly used. These can be used in the lost foam casting process, which destroys the pattern during the metal-pouring stage. In addition, some foundries have successfully made air-set molds using polystyrene patterns to make several parts before the pattern is worn out. Core boxes can also be made from polystyrene for the air-set process. This can be ideal for large castings that are only purchased periodically since the patterns can be machined as needed. This avoids the maintenance and storage requirements that would be associated with traditional pattern equipment.
Wood: If a casting has very low volume, perhaps 50 pieces or less per year, then the purchase of a wooden pattern may provide suitable production castings. Wood is most appropriate for use in green sand and air-set molding and core processes. The initial investment for pattern equipment will be relatively low, and any engineering changes can be easily performed, but this material has several drawbacks that need to be considered. Wooden pattern equipment is not as robust as other materials, so you can expect to encounter a fair amount of pattern wear. Wood also has the tendency to absorb moisture due to changes in humidity, which can cause the pattern to swell and exhibit dimensional instability as a result. Conversely, wooden patterns can dry out after long periods of storage, causing them to split and crack, which also causes dimensional problems. While wooden pattern equipment is common, you should be aware that the pattern will wear out quickly, or at the very least, require a higher level of pattern maintenance to keep it in usable condition.
Plastics: Plastic tooling is often used for castings with annual volumes up to 5,000 pieces. Within this material category are a number of choices for the pattern buyer to consider. Several manufacturers produce polyurethane boards that can be used as a direct replacement for wood. The material is available in various densities for different foundry applications and has the benefit of being more moisture- and heat-resistant than wood. The material can be glued together to form thick sections and can be CNC machined or worked by hand. This type of material is used quite often for low- to medium-volume patterns with great success, and if the annual demand for your casting increases, you will have much less pattern maintenance to contend with.
Also within the plastic pattern category is the family of castable epoxies and urethanes used for patterns and core boxes. Any castable shape can be produced with these materials and they also offer the ability to provide a quick turn-around time for the patternmaker and foundry. In order to avoid lost production time, foundries have taken worn out urethane core boxes and had them re-cast over the weekend, putting them back into production on Monday morning.
Metal: For castings with annual volumes over 5,000 pieces, the pattern material of choice would be metal; either aluminum or iron. Aluminum is most often used in the production of matchplate and cope and drag patterns used in automated molding processes. Iron pattern equipment is most common in high pressure molding processes such as vertically-parted molds and in the shell molding and core processes that employ heat-cured resins. Some foundries use iron patterns that have been plated in nickel to improve the wear and sand release characteristics in high-volume casting production. Metal pattern equipment will be costlier to produce and will be more expensive to revise for engineering changes, but the superior performance and dimensional stability will be of the highest degree.
It’s important for the casting buyer to understand the complete process involved in the manufacturing of their pattern equipment. To improve your process knowledge, work with your foundry team to schedule a visit to the pattern shop and learn what they have to offer. CS