|
Bronze Craft has been making sand castings since 1944, and company oral history claims that the foundry which Bronze Craft originally acquired, was making iron castings for the American Civil War. So, we come from a long line of New England Foundry professionals.
Despite the passage of time, the basic steps in making a sand casting remain the same. The basics steps are:
- Pattern Making. Patterns are used to create an impression in the sand mold. The sand mold is made by packing sand around the pattern. Generally, a sand mold is made in two sections. The top section is called the cope and the bottom is the drag. The pattern will define the plane on which the two sections (cope and drag) meet. The plane between the top and bottom is the mold parting line. The mold parting line plane is indexed by alignment pins positioned by the pattern and molding flask.
When the pattern is removed from the molding flask, the cope (top) sand mold and bottom (drag) of the mold are placed top over bottom on the sand plane parting line. Dimensions measured across this parting plane of the sand mold have the highest probability of variation. This is why sand casting parting line dimensions for small castings carry a tolerance of, + or- .015" on features measured across this plane. Large casting patterns are generally made from wood. Patterns made at Bronze Craft are metal or stereo lithography which have tolerances of + or - .005". Dimensionally accurate castings must start with precise patterns.
- Cored Castings. If the casting has an internal shape or hollow, a core box is used to make a sand form which creates the inside geometry of the casting. A core box is foundry tooling which allows sand to be packed into a shape which forms the inside of the casting. The core box provides appendages or core prints from the sand shape, which allows the core to be suspended in the mold. The void between the core and mold becomes the hollow casting.
- Molding. Sand molds are made from sand, clay, and water in proportions and materials appropriate for the type of metal being poured. The molding sand used at Bronze Craft is fine, due to the smooth casting finishes required for the products we make. The sand is packed around the pattern which is held in place by cope and drag flasks. The pattern is withdrawn from the sand after the cope flask is removed. If hollow castings are being made, the core is placed in the mold and the cope flask is placed over the drag flask. Alignment pins on the flasks insure that the cope and drag areas of the casting match. Sometimes this type of molding is referred to as "match plate molding".
- Melting and Pouring. These processes prepare the molten metal alloy composition and bring it to the proper temperature before transfer to the molds for pouring the castings. Bronze Craft uses induction melting in push up crucible furnaces with an insulated crucible pouring rig.
- Shakeout and Cutoff. After cooling, the castings are removed or "shaken out" of the sand. We remove the sand from the castings with a blast booth before cutting off the castings from the gating (mold plumbing) system. The sand removal before sawing off the castings reduces silica exposure to our workers.
- Inspection and Gate Removal. Castings which pass inspection are then processed for removal of gates and parting line by grinding.
- Secondary Operations. Other processes such as machining, polishing, plating or painting may occur before shipping the finished casting.
While the basics of sand casting remain the same, the use of technology in the process continues to grow. Patterns and core boxes are made by CNC and various rapid prototyping methods today. Foundry schedules and customer relationships are software managed with more information available and better levels of management productivity.
|
