This is a class of casting processes that use pattern materials that evaporate during the pour, which means there is no need to remove the pattern material from the mold before casting. The two main processes are lost-foam casting and full-mold casting.
Lost-foam casting is a type of evaporative-pattern casting process that is similar to investment casting except foam is used for the pattern instead of wax. This process takes advantage of the low boiling point of foam to simplify the investment casting process by removing the need to melt the wax out of the mold.
Full-mold casting is an evaporative-pattern casting process which is a combination of sand casting and lost-foam casting. It uses a expanded polystyrene foam pattern which is then surrounded by sand, much like sand casting. The metal is then poured directly into the mold, which vaporizes the foam upon contact.
Patterns used in evaporative castings must be precisely made since they define the shape of the part to be made within a sand casting cavity formed by packing the pattern in sand. Such patterns are consumed during the evaporative casting process, and a large number of patterns are required for evaporative casting since each part requires a new pattern. As a result, economies and improvements in the production of the patterns can have a substantial effect upon the cost and quality of the end product produced by the evaporative casting process.
A previously known method for making such patterns comprises assembling a plurality of previously configured mold parts including mold wall surfaces that define the pattern forming chamber. The mold walls are separated apart from housing walls of each mold part to form a heat exchange cavity therebetween. The mold parts also include couplings for connecting the heat exchange cavities with fluid heat exchange such as cold water and hot steam. Assembly of the mold parts forms a pattern making tool.
To prepare expanded foam patterns with such parts, beads are formed from polystyrene or other materials such as polymethyl methacrylate (PMMA) or polyalkylene carbonate (PCA) and a thermal expanding agent such as pentane or the like. The beads, originally about the size of salt grains, are heated in a large container to a temperature of about 215° F. to pre-expand the grains and form beads of a density of about 0.9-1.9 pounds per cubic foot. The pre-expanded pellets are then transferred to fill the mold cavity defined by the mold walls. The pellets are then heated for a limited time depending upon the degree of fusion desired and the composition of the pellets, for example, generally from a fraction of a minute to several minutes, by steam introduced to the heat exchange cavities at a temperature of about 215° F. This heating step will expand, soften, distort and merge the expanded beads together to form a unitary foam pattern shaped by the mold walls. The expansion process is then stopped by running cold water through the heat exchange cavities. After separating the mold parts, the pattern is removed from the mold.
In such a process, temperature variations along the mold wall can have a profound effect upon the quality of the part produced. The degree of expansion of the pellets depends upon the temperature and the time duration at which the temperature is maintained. Accordingly, dimensional stability of the pattern and the surface quality of the pattern are improved when the temperature of the mold cavity surface is consistent throughout the cavity. However, when the mold wall includes recessed wall portions or various wall thicknesses, temperature along the mold surface may be inconsistent throughout the stages of operating the mold. When parts of the mold wall surface are recessed away from the heat exchange chamber, the resistance to heat energy transfer from the heat exchange medium can interfere with consistent heat exchange to the foam beads forming the pattern. Moreover, while thin mold wall portions may be desired for efficient heat transfer, a thicker mold wall portion will absorb more heat to reach the same temperature even though the heat exchange medium contacting it has a substantially constant temperature. Furthermore, a greater amount of heat energy may need to be transferred to a mold wall part which defines a mold wall cavity portion for a relatively thick portion of the pattern to be formed.
The problem of inconsistent mold wall temperatures is particularly acute in molding tools using mold cores since the cores typically do not include heat exchange passages. Accordingly, previous expanded foam patterns may require assembly of two or more expanded foam parts by gluing or the like to complete a single pattern. Such assembly interferes with the integrity of the pattern and the quality of the pattern surfaces.
Moreover, the quality of the pattern formed has a substantial effect upon the part to be cast using the pattern in an evaporative casting process. Improved surface finish and dimensional stability in the pattern can substantially reduce if not eliminate certain machining operations otherwise necessary to satisfy the molded part's required specifications. Each such machining operation which can be eliminated represents a substantial savings in energy, labor and tooling costs which must be incurred in order to produce a finished part after evaporative casting.
In order to overcome the problem of temperature variation, a previously known attempt for improving the pattern casting method included venting the core. In a hollow vented core or mold, vent passageways are provided in the mold wall forming the mold cavity so that the heat exchange chamber is coupled for fluid communication with the mold cavity. The vent typically includes restricted slots to prevent expansion of the foam material into the vent or the heat exchange chamber during molding. Before the foam material is introduced into the cavity, the vent passages permit the heat exchange fluid medium to enter the cavity and contact both sides of the mold wall defining the mold cavity to enhance uniformity of the heat transfer to the molded material. Although such venting provides greater uniformity in temperature throughout the mold cavity, foam material can expand into the slots of the vent passage during the process. The bulges of foam into the vent cause the part to deviate from the part specifications. Furthermore, they cause interference between the foam pattern and the core or mold wall surfaces and cause scoring of the pattern during removal of the core or the pattern from the mold. Such interference can cause substantial damage to the surface finish of the molded foam part and thus, to the cast workpiece formed with the pattern. As a result, a large number of unacceptable patterns are produced by such a method.
Another process for making foam patterns without the difficulties of vented cores or mold walls comprises the use of hollow cores or mold walls through which the heat exchange medium can penetrate into deeply recessed portions of the mold wall. Although such hollow openings permit the heat exchange medium to contact the mold walls, the restrictions to flow caused by dimensional limitations of such channels can cause a substantial variation in temperature between the recessed mold wall and contiguous mold wall portions. Accordingly, the heat exchange medium must remain in the heat exchange cavities for longer periods of time in order to permit the temperature at the bottom of the recess to reach the temperature of the remaining mold wall portions. Such time delays in the pattern forming process can substantially reduce the production output of the foam patterns and thus the output of the evaporative casting process. As a result, the number of tools to produce the patterns must be increased in order to provide a satisfactory production rate. Furthermore, the variation in expansion rates throughout the mold cavity affects the surface consistency and dimensional stability of the pattern.
The inability to uniformly transfer heat has also been recognized in the art of injection molding, where the molten material, usually plastic, is heated before introduction into the mold chamber. In such a process, the material must be heated to a molten state in order to be transferred from its source through delivery conduits and within the mold cavity so that it remains capable of conforming to the mold cavity shape. In order to maintain enough heat in the fluid during transfer between the source and the cavity, U.S. Pat. No. 4,034,952 to Stewart discloses a hot plastic injection bushing surrounded by a heat pipe to maintain all parts of the tubular bore at the same temperature as an externally controlled electrical heater.
U.S. Pat. Nos. 4,338,068 and 4,387,762 disclose a variable volume heat pipe for use in injection molding. U.S. Pat. No. 4,500,279 discloses a hot manifold system employing heat pipes adjacent runner channels to provide continuous heating of the runners during injection molding. Such injection molding apparatus require heat pipes to transfer sufficient heat energy to maintain the high temperatures, often 400°-500° F., necessary to keep the mold material fluid so that a fixed amount of molding material can be transferred in its molten state to completely fill the mold cavity.
On the other hand, foam pattern molds do not require the high temperatures required for injection molding. Moreover, the foam material is transferred without reaching a molten state. Rather, the expandable foam pellets are heated to increase the volume of the pellets within the confined space of the mold chamber. As a result, the time during which heat is supplied rather than the ability to maintain high temperatures is required in foam pattern making.
Furthermore, injection molded parts can be removed from the mold whenever the molding material solidifies. Once injection material has filled the mold cavity, the application of heat is terminated. The material solidifies once the heat energy is no longer input to maintain the molten state of the material. As a result, previously known injection molding apparatus do not address the problem of maintaining consistent temperature variation throughout all parts of the mold during the curing process as is required in the cooling phase of making expanded foam patterns.
From the foregoing, the skilled artisan will appreciate that the teachings of the previously known injection
Last Updated ( Thursday, 15 October 2009 07:55 )