Metal casting is the process of pouring molten metal into a shaped space so that it will cool and harden in that form. For many items, this process can be less expensive than machining the part out of a piece of solid metal. Even though the idea behind it seems singular and simple, there are many ways foundries cast objects. How they make a metal casting mold will depend on the metals used, the size of the run, and the shape, symmetry, and complexity of the casting.

Foundry:

Facility that produces metal castings and offers related services such as mold-making, melting, pouring, degassing, heat treating, surface cleaning, and other finishing operations. 

CASTING PROCESSES

Sand Casting, mold casting, precision casting, pressure casting, Plaster mold casting, Evaporating casting, investment casting & Die casting.

Centrifugal casting:

What is Centrifugal Casting?

Centrifugal casting is a process that delivers castings of very high material soundness, and thus is the technology of choice for applications like jet engine compressor cases, petrochemical furnace tubes, many military products, and other high-reliability applications.

In the centrifugal casting process, molten metal is poured into a spinning die. The die can be spinning either on a vertical or horizontal axis depending on the configuration of the desired part. Ring and cylinder type shapes are cast vertically; tubular shapes are made with the horizontal centrifugal process. Either process may be used to produce multiple parts from a single casting.  External structures and shaping can be cast in place to significantly reduce post-processing including machining or fabrication.  

Because of the high g-forces applied to the molten metal in the spinning die, less dense material, including impurities, “floats” to the I.D. where it is subsequently removed by machining. Solidification is managed directionally under pressure, from the O.D. to the I.D., avoiding any mid-wall shrinkage, leaving a defect-free structure without cavities or gas pockets.

Centrifugal casting – When to use it:

  • Symmetrical parts that can rotate on an axis
  • Sand Casting (Static casting) material properties are inadequate
  • Centerline shrink is an issue using other casting processes
  • Limited I.D. features
  • Large parts, up to 135,000 lbs./61,350 kg or more
  • Net-shaping: Some tooling cost is often justifiable when significant finishing is required

Die casting:

Casting method in which molten metal is injected into the mold (or die) under pressure. Two hardened dies are pressed together to form the mold cavity. Once the injected metal has cooled, dies are separated and the casting ejected. Die casting can achieve high dimensional accuracy, intricate detail and smooth cast surfaces that require minimal additional machining. Dies are expensive to produce, making them more suitable for high-volume runs. Ferrous metals are rarely used as an injection material.

Net-Shaped Centrifugals:

Net-Shape Centrifugal Technology provides measurable benefits compared to fabrications, forged parts, or conventional castings.

Near-Net Shaped Centrifugals combine the cost advantages of a product that has desired profiles cast in with the metallurgical and thru-wall structural integrity that only the centrifugal process delivers.

Net-shaping can be readily added to centrifugally cast components through the use of sand or ceramic split dies. True net-shaping at investment casting levels of O.D. Either net shaping or near-net shaping can very dramatically reduce post processing costs while delivering the advantages of centrifugal casting.

Why to use Net-Shaping as an alternative to fabrications:

  • Simplify: Reduces number of components
  • Cost savings: Eliminates welding
  • Cost savings: Eliminates post-weld heat-treat
  • Quality: Improves quality of difficult to weld material
  • Speed:  Increases speed of production

Why to use Net-Shaping as an alternative to forged parts:

  • Cost reduction: Reduces milling
  • Quality: Improved dimensional stability of a centrifugal
  • Speed: Shorter lead times / Increased speed of production
  • Design choices:  Increased variety of alloy choices

Why to use Net-Shaping as a alternative to conventional casting:

  • Shaping: Benefit of a static sand casting while eliminating mid-wall shrink
  • Welding: Smaller grain size compared to static sand casting improves weldability
  • Tooling: Mid-range tooling and casting pricing provides value for mid-short runs
  • Cost Savings: Compared to conventional centrifugals, reduces amount of metal to be poured, heat treated and machined away

Investment casting:

Casting method typically used for intricate pieces that require a high degree of accuracy with minimal machining. It can be used to create products with smooth surfaces and no parting lines. Due to high setup costs, investment casting is most suitable for high-volume production.

The steps of investment casting are as follows:

  1. Use an injection mold to form wax patterns in the shape of the final product;
  2. Coat the wax patterns in ceramic to form disposable ceramic molds;
  3. Heat ceramic molds to melt and drain wax;
  4. Pour molten metal into the ceramic molds;
  5. Remove ceramic molds to reveal the solidified metal casting;
  6. Removing any gates, risers or other excess metal from the finished product. Investment casting is also referred to as lost-wax casting.

Sand casting:

Casting method characterized by the use of sand as the mold material. Molding sand is typically mixed with a bonding agent such as clay and moistened with water or other liquid to create suitable mold strength and plasticity. The mold cavity is formed by compacting sand into a mold box (or flask) around the pattern. The pattern is then removed from the newly formed mold cavity. Once molten metal has been poured and allowed to cool, sand is removed to reveal the final casting. Finished surfaces are not as smooth as with other methods, and additional machining, including the removal of gates and risers, is typically required. sand casting is one of the most common methods used by foundries; it can be used for both short- and long-run productions.

What is Sand Casting?

Sand casting as a technology has been around for millennia, and is selected as a preferred method to produce shaped parts that weigh less than a pound, to very large parts. The process is versatile and cost effective, even for low volume runs because of tooling cost. Nearly any part configuration that can be made using another casting process can be reduced to a pattern and created as a sand casting.

Sand Casting Process:

In Sand Casting, a pattern of the desired finished part including the metal delivery system (gates and risers) is constructed out of hardwood, urethane, metal or foam. Sand containing bonding material to retain its shape is packed around the pattern. The pattern is removed from the bonded sand, leaving a cavity in the mold that is in the shape of the part.  Molten metal is poured into the cavity and the metal solidifies. The sand is removed though a shakeout process. Other cast attachments, including the metal delivery system, are trimmed leaving the desired part.

Internal passageways, including intricate structures, can be included in the sand castings by adding cores during the molding process. This makes sand casting a popular selection for pump and valve applications. Sand castings would typically be at least partially machined before use.

Sand casting – When to use it:

  • Shaped or non-symmetrical parts
  • Large parts
  • Components with internal structures
  • More liberal tolerances

HPLT Castings

What is HPLT Casting?

Is an alternative to traditional sand casting processes, The technology is suitable for shaped parts, including those with internal structures.

The HPLT Casting Process

Similar to traditional sand castings in up-front tooling and concept, sand with a bonding material is packed around a pattern of the desired part to create a mold. The pattern is removed leaving a cavity in the mold. Using HPLT process technology, metal is introduced to the mold in a way that minimizes turbulence.  This low turbulence helps prevent inclusions and other casting defects that can develop using traditional casting processes. After solidification, the sand is broken away, gates and risers are removed, and the finished part remains.

Parts manufactured using the HPLT process can still feature intricate passageways gained by coring in the same fashion as other static casting processes. This process can generate castings that are free of defects, and is often specified for applications that do not allow weld upgrade. Products utilizing HPLT technologies can be produced using nearly any copper, ferrous, or nickel-based alloy up to 6,000 lbs. (2,721 kg) ship weight / 12,000 lbs. (5,443 kg) pour weight. 

HPLT Casting – When to use it:

  • Shaped or non-symmetrical parts
  • Large parts
  • Internal structures
  • More liberal tolerances comparable to static casting technologies
  • Very strict weld repair or “no weld” limits

Vacuum Casting

Vacuum is used when part detail and control of exposure to oxygen is critical. Vacuum casting at Wisconsin Centrifugal is performed using a centrifugal process. In addition to the advantages of casting in vacuum, the inherent high metal integrity delivered by centrifugal casting is realized, including directional solidification, absence of porosity, and net-shaping. Vacuum centrifugal casting provides products with very high reliability, often used in aerospace and military applications.

Ingot (AOD Refining)

This refining process produces steel with low carbon properties (.01% or lower) and low silicon levels, which, in turn, creates a metal that has better weldability (can reduce weld electrode use by over 50%), and is more corrosion resistant, malleable, and heat resistant. 

Vacuum Heat Treat

Air contains elements that can cause undesirable results, such as discoloration, scale, or contamination, during the heat-treating process.  By heating treating the metal in a vacuum, these reactive elements are removed from heat treating chamber and heated to the appropriate temperature to achieve desired properties. As in the case with traditional heat treating, the metal is quickly cooled to below 400 degrees but in the case of vacuum heat treating, an inert gas such as Argon in used.

Patternless Molding

Patternless molding is a process where the desired shape is machined directly into the sand mold.

Patternless Molding is very flexible because the shape is stored as an electronic image and converted to a machining language for actual production.

Minor changes can be made quickly and efficiently within the electronic world with a high level of confidence that all resulting dimensions will be accurate. Changes can be made at any time up until the mold cavity is cut. Time to market for any size program, but especially low volume and prototype, is cut in half.

Why select Patternless Molding?

  • Turnaround time is much less than for new pattern development
  • No pattern storage, maintenance, or tracking
  • Cost effective solution for R&D projects
  • Quick modifications for shrink factors related to different alloys being poured to the same design
  • No tooling wear or repair
  • The product is still manufactured using standard production processes

METALS

Alloy:

Combination of metals which may contain other non-metal elements. Alloys are typically produced to achieve desirable material properties related to strength, hardness, corrosion resistance, conductivity, melting point and cost.

Aluminum:

Non-ferrous metal that is notably lightweight and corrosion-resistant. Its low melting point makes aluminum highly castable. Aluminum is commonly alloyed with copper, zinc, magnesium, manganese and silicon.

Carbon steel:

Steel that contains 0.12-2.0 percent carbon and up to 10.5 percent alloy content. Carbon steels are often categorized as either high carbon or low carbon. High carbon content increases hardness at the expense of ductility, and vice versa. Carbon steels do not include stainless steels.

Cast iron:

Group of iron alloys that contain approximately 2-4 percent carbon along with 1-3 percent silicon and other trace elements. Most cast irons, with the notable exception of malleable cast iron, are brittle. White cast iron and gray cast iron are widely used for their castability, machinability and resistance to wear.

Ductile iron:

A modern iteration of cast iron; ductile iron has superior ductility and impact resistance. Ductile iron is made using small amounts of magnesium or cerium to manipulate carbon into spherical formations that won’t crack under stress. Also referred to as ductile cast iron, nodular cast iron, spheroidal graphite iron and spheroidal graphite cast iron.

Ferrous alloy:

Metal alloy that has iron as its main constituent.

Non-ferrous alloy:

Metal alloy that is not ferrous (does not contain iron in significant amounts).

Stainless steel:

Steel that contains 10.5-30 percent chromium. The high chromium content provides stainless steel with natural corrosion resistance. Chromium oxidizes to form a non-reactive barrier that protects the internal structure. Numerous grades of stainless steel incorporate alloy ingredients such as nickel, molybdenum, titanium, aluminum, copper, nitrogen, sulfur, phosphorous and selenium. 

Steel alloy:

Steel that has been alloyed with elements in addition to carbon to achieve desirable properties related to strength, hardness, and resistance to wear and corrosion. Common ingredients include manganese, nickel, chromium, molybdenum, vanadium, silicon and boron.

Steel casting:

Specialized form of casting that involves carbon and alloy steels.

OTHER FOUNDRY PROCESS TERMS

Annealing:

Form of heat treatment where metal is heated to and held at a high temperature, then cooled at a controlled rate. Annealing is used to alter chemical or physical properties, especially hardness.

Case hardening:

Process for hardening ferrous alloys so that surface layers are made substantially harder than interior or core materials.

Casting drawing:

Engineering drawing that shows the final shape of a part to be cast. It includes all information related to dimensions, tolerances, machining and any other data necessary to determine foundry procedures.