The process used at Unicast to produce precision castings dates back thousands of years, yet remains the most precise of all casting processes available today for complex high quality components. Each step must be completed flawlessly in order to produce a successful casting. Good workmanship and an eye for detail is required to keep yields high and scrap rates low.

Wax Injection

Wax replicas of the desired part are produced by injection molding in precision tools. The replicas are called patterns and must be handled with care to avoid distortion or damage.

Wax Assembly

Smaller patterns are assembled onto a sprue using hot knife, stick-tite wax and other techniques. Larger patterns are gated with numerous runner bars to form a cluster, which both supports the pattern during shell building and provides proper metal flow and feeding during casting. Care must be taken to avoid any small holes, undercuts or defects in the assembly as mold defects will result.

Dipping

Sprues and clusters are dipped into a ceramic slurry mixture and coated with refractory sand. Repeated dips into first prime slurry and later backup slurry forms the layers of the shell mold. Care must be taken to drain excess slurry and blow excess stucco from the pattern during these operations to prevent mold defects. Shell layers must be dry before subsequent layers are applied, or shell failure may result.

Stuccoing

After dipping and draining of the patter in a slurry the assemblies are stuccoed with sand in either a fluidized bed or rainfall sander. Care must be taken to avoid contamination or mixing of different types of sand.

Dewaxing

Following a final drying period in controlled temperature and humidity conditions, the shell molds are rapidly transferred into either a flash firing furnace or a steam-heated autoclave. Shells with wax patterns inside must not be allowed to remain outside the controlled temperature environment of the shell room, or the wax may expand and crack the shell mold. Inside the machine, wax is melted from the mold and recycled for re-use.

Firing

Shell molds may be fired during flash-firing or post fired if an autoclaving technique is used. During the firing stage, all traces of organic wax or plastic SLA patterns are burned out, leaving a clean mold for casting. The shell mold composition will change slightly, as polymers used in the mold layers will also burn out leaving the mold somewhat fragile and porous. Porosity is a desired condition, and helps air escape during mold filling upon casting with liquid aluminum.

Casting

Molds are preheated to a precise temperature to prevent the metal from freezing prematurely during filling. Liquid aluminum is checked for chemical limits, degassed, prepared to a precise temperature, and then carefully poured into the mold. Ceramic filters and socks are used to filter the metal of oxides.

Knock-out

Knock-out and washout are used to remove ceramic shell from the casting. A hammer may be used on the riser head only to remove excess shell. Other delicate parts of the cluster are water blast to remove the shell without harming the soft casting. Parts may be pre-washed, followed by cut off on a band saw and possible re-washed to remove all of the shell mold.

Finishing

Castings have gates ground flush to the surface without damaging the casting. Parts are then inspected (visual, florescent penetrant, and X-Ray), possibly weld repaired, and heat treated. Solution heat treatment (T4) softens the parts for pressing or hand straightening. Subsequent precipitation hardening (T6) gives the castings their strength and hardness.

Inspection

Castings are dimensionally inspected and often re-inspected for cracks and surface flaws prior to shipment. Castings are serialized and marked, such that they can be traced to all manufacturing processes used at Uni-Cast. Documentation and manufacturing records for most parts, needs to be held for 7-10 years, in the event that our customer needs to retrieve important information.