Presently engine manufacturers use a well known sand casting process to solidify iron into engine parts, e.g. engine blocks, cylinder heads, valve covers, etc. In sand casting, a cavity is formed in foundry sand using a pattern of the engine part. The sand is mixed with a binder to retain its shape. Molten iron is poured into the cavity. The sand holds the iron until it solidifies into the shape of the engine part.
The pattern typically has separate upper and lower portions. The upper and lower pattern portions fit together to form the outside shape of the engine part. The upper portion is positioned in an upper half of a flask—an open-ended metal box. The sand and binder mixture is poured into the upper half and compacted around the upper portion, thus taking its shape. Similarly, the lower portion of the pattern is positioned in the lower half of the flask. The sand and binder is poured into the lower half and compacted around the lower portion, thus taking its shape.
The upper and lower portions of the pattern are removed from the sand. The binder holds the sand in place, thus leaving the imprint of the pattern in the sand. In the upper half, holes are cut through sand for pouring iron into the cavity and for air to escape. The flask is assembled to form the mold of the engine part.
Molten iron is poured into a pour hole to fill the cavity in the sand created by the pattern. After the iron solidifies, the flask is disassembled. The engine part and sand are separated. If needed, the engine part is machined or ground to final dimensions and shape.
In the typical sand casting of engine parts, it is often desired and sometimes necessary to have a hollow portion in the engine part. Hollow portions reduce the cost of manufacturing and sometimes are needed for proper operation of the engine. For example, hollow portions must be formed in the engine block for the pistons to operate. The hollow portions, or cylinders, may be formed when the engine block is cast or they may be drilled or machined at a later time. Forming the cylinders when the engine block is cast will generally reduce manufacturing costs compared to machining the cylinders at a later time.
To form a hollow portion during casting, a core or casting core is positioned inside the cavity in the sand. The core will have the desired size and shape of the hollow portion. The core may rest upon sand in the flask. The flask is assembled to form the mold. Molten iron is poured into the mold. The iron solidifies around the core leaving the desired hollow portion.
After the iron solidifies, the flask is disassembled. The engine part and sand are separated including any sand and other core residue in the hollow portion. After the sand is removed, the hollow portion may need machining and grinding to remove imperfections formed during casting.
The “cores” or casting cores are typically made from sand and binder mixtures in core boxes. A core box forms a cavity, having the size and shape of the hollow portion desired. The sand and binder mixture is blown into the core box cavity through a hole in the core box. The core box has other holes to let air escape. The binder is a type that, once cured, causes the sand to maintain a “solid” shape or form of the cavity in the core box. After curing, the core may be removed and positioned inside the mold for an engine part. Core boxes generally have two or more parts, which separate to remove the core.
While necessary to produce the core, the core boxes increase the manufacturing costs of engine parts. For example, in engines using hydraulically actuated fuel injectors, the engine must supply high pressure oil to the fuel injectors for proper operation. While tubes and hoses may be used to supply the oil, a high-pressure oil rail is commonly cast into the cylinder head of the engine. The high-pressure oil rail is an elongated, narrow cavity running the length of the fuel injectors in the engine head.
A core or casting core is typically used to cast a high-pressure oil rail, or cavity, in the cylinder head. For example, as described in U.S. Pat. Nos. 5,119,881; 5,197,532; and 5,333,581, which are incorporated herein. Generally, the core is comprised of an inner and outer portion. The inner portion is used to add structural support to the outer portion of the core. The structural support is necessitated by the oil rail's length. The core's inner supporting portion typically is a steel tube. The core's outer portion is typically made from a sand and binder mixture. To form the core, the steel tube is positioned inside a split-mold core box. The sand and binder mixture is poured around the steel tube, taking the shape of the core box. The sand and binder mixture cures around the steel tube. The core box is separated to remove the finished core.
The existing core-making process described typically results in imperfections. A typical sand core, made with this process, has voids on its surface because of the spacing between the sand grains. In addition, the core box forms parting lines where parts of the core box meet. The parting lines also form voids and/or projections on the core. These voids and projections on the sand core are undesirable. During casting of the engine parts, the voids result in iron penetration into the core. Iron penetration results in parting line fins, veins, and other imperfections on the surface of the high-pressure oil rail. Also, the projections on the sand core create voids on the surface of the high-pressure-rail in this case.
In high-pressure oil rails, these imperfections must be removed to avoid interference with the hydraulic flow. The imperfections also must be removed to keep pieces from breaking off and entering the fuel injectors during engine operation. To remove imperfections, the oil rail must be ground or machined. Other engine parts experience similar requirements when sand cores are used. This grinding and machining increases the manufacturing costs of engine parts.
Moreover, the nature of a sand and binder mixture does not permit the removal of the parting lines or voids between the sand grains. Any attempt to grind or machine the core surface will result in the removal of too much material and a misshaped core. Worse yet, if too much pressure or force is used, the core will break or crumble. In practice, a refractory coating (core wash) is applied to the surface of the sand core. However, the refractory coating does not completely eliminate the inter-granular penetration of molten iron and may result in core wash related defects.
Accordingly, in the manufacture of engine parts from cast iron, there is a need to have a core without parting line voids, without inter-granular voids, and without core wash related defects.