The pattern is usually made by injecting wax or plastic into a metal die. These mated multi-cavity tool, depending on production quantities and complexity of the part. A heat-disposable pattern is required for each casting. These disposable patterns have the exact geometry of the required finished part with allowances made to compensate for volumetric shrinkage in the pattern production stage and during solidification of metal in the ceramic mold. The pattern carries one or more gates which are usually located at the heaviest casting diction. The gate has three functions: to attach patterns to the sprue or runner, forming a cluster/tree to provide a exit for pattern material as it melts upon heating; to guide molten metal entering the mold cavity in the casting operation; and to ensure a sound part by feeding the casting during solidification.
Patterns are fastened by the gate to one or more runners and the runners are attached to a pouring cap, Patterns, runners and pouring cap comprise the cluster or tree, which is needed to produce the ceramic mold. The number of runners per section and their arrangement on the pouring cap depend on alloy type, size and structure of the casting.
The Ceramic Shell Mold Process 型壳的制造
The ceramic shell mold technique involves dipping the entire cluster into a ceramic slurry, draining it, then coating it with fine ceramic sand. After drying, this process is repeated again and again, using progressively coarser grades of ceramic material, until a self-supporting shell has been formed. The shell may be from 3/16 to 5/8 in. thick The coated cluster is then placed in a high temperature furnace or steam autoclave where the pattern melts and runs out through the gates, runners and pouring cap. This leaves a ceramic shell containing cavities of the casting shape desired together with a suitable running and feeding system
The ceramic shell molds are fired to burn out the last traces of pattern material, to develop the high temperature bond of the ceramic system and to preheat the mold in preparation for casting. Because shell molds have relatively thin walls, they can be fired and be ready to pour after only a few hours in the furnace.
The hot molds may be poured utilizing static pressure of the molten metal heat, as is common in sand casting, or with assistance of vacuum, pressure and/or centrifugal force. This enables the investment casting foundry to reproduce the most intricate details and extremely thin walls of an original wax or plastic pattern. Melting equipment employed depends on the alloy. For nonferrous alloys, gas fired or electric crucible furnaces are usually used. High-frequency induction furnaces are most commonly used for melting ferrous alloys.
After the poured molds have cooled, the mold material is removed from the casting cluster. This is done by mechanical vibration and chemical cleaning. Individual castings are then removed from the cluster by means of cut-off wheels and any remaining protrusions left by gates or runners are removed by belt grinding. The casting is then ready for secondary operations such as heat treating, sizing, machining and for whatever inspection is specified.
The products’ dimensional accuracy can be up to ±0.3mm/100mm; surface roughness Ra up to 12.5-1.6μm; weights of casts from several grams to dozens of kilograms; local wall as thin as 0.5mm.