A normal circumstance is that a customer will place an order for a quantity of castings which they feel will fulfill their needs on a short term basis. This number is usually rounded to an even number for convenience. A foundry will endeavor to fill the order with the exact quantity requested, however, the shipment quantity will usually vary from the ordered quantity. To explain this situation one needs to look at the casting process in terms of its impact on quantities processed through the foundry:
Castings per tree; Multiple parts are usually assembled on a single “tree” or mold. For small parts this can easily range to 50 pcs or more per tree. The number is determined by considerations of handling the assembled tree and subsequently the resulting shell and by feeding requirements of the specific part.
Finished cast weight; The weight of the part becomes significant especially when any type of non-standard metal is being cast. The preference is to cast all of an order in one or sequential batches. With particularly small quantities, attempting to melt small batches can present metallurgical control problems.
Breakage; The investment process is noteworthy by its use of very fragile materials to the point of casting the metal. Wax patterns and tree assemblies along with the basic ceramic shell will not forgive the slightest bump. The common situation is the loss of one or more parts from a tree with the remaining tree being cast successfully but with a low part count.
Past History; Substantial process records are maintained in an attempt to identify potential problems as to specific geometry or metal type. Anticipation of either high or low internal scrap will change the quantity started for a specific part.
Scrap; Although reasonable care is taken to minimize process variables, an unexpected combination of variables can yield unacceptable scrap castings.
In an attempt to assure that a customers’ order quantity is achieved, it is common practice to start 10% more than the order quantity at the front end of the process. It is also common practice to consider an order complete at 90% of the order quantity if substantial losses were incurred during processing of the order. The point to remember is that communication with Nova Precision is key to understanding.