Metal casting is a process that produces shapes (castings) by filling voids in molds with liquid metal. Cast metal products offer exceptional strength and durability, making them useful for heavy machine applications in industries related to farming, automobiles, railroads, home appliances, building architecture, ship building, oil and gas extraction, water processing, forestry and mining – to name just a few.
Qualifying a supplier foundry and ordering metal castings can be a time-consuming ordeal – but it doesn’t have to be. Costly blunders can be avoided by understanding the quotation and casting process; clear and thorough communication with the supplier foundry will result in a well-designed and economical final product.
Requesting a Quote
The purpose of requesting a quotation for a casting is to establish the optimal balance of purchased casting cost, quality, and delivery time. While it is tempting to automatically select the lowest price quoted, doing so can lead to delayed delivery or a lower quality casting – both of which can be more expensive in the long term.
A savvy customer must weigh all of the provisions of the quotation; including tooling requirements, tolerances, and finish allowances, as well as any exceptions taken to drawings, specifications, and processing requirements. Other factors, such as reduced machine work, improved delivery terms, and supplier foundry experience and reliability, are particularly important to determine the actual cost versus value of the casting. The following information should be included in a quotation request:
- Preliminary casting design
- Material and inspection requirements
- Actual or estimated casting weight
- Pattern (if available)
- Production and delivery schedules
A cast metal part should be designed to take full advantage of the casting process. For efficient and cost-sensitive production, casting design should select for the simplest production methods that will result in the desired physical properties, net shape demands, and cosmetic appearance.
Drawings detailing the exact dimensions of the part required should be included with any request for quote; the number of castings to be produced (length of run) should be clearly stated. Order quantity will have a significant impact on price, with longer runs corresponding with a lower cost per casting.
Castings are shipped with un-machined, as-cast surfaces unless otherwise specified. To take advantage of the casting process, the supplier foundry should know which surfaces are to be machined and where datum points are located. The acceptable dimensional tolerances must be indicated when a drawing is provided. Tolerances are normally decided by agreement between the supplier foundry and customer.
Close cooperation between the customer's design engineers and the supplier foundry is essential to optimize the casting design. All casting designs should be checked by a foundry professional prior to production, in order to determine the feasibility of the casting project.
Industry standard specifications provide the casting customer with the tools necessary to establish criteria for almost any casting application. These specifications do not preclude special requirements that the customer's technical staff members may require. Variations from standard specifications can result in misunderstandings, higher costs, and disqualification of potential supplier foundries. If exception is taken to a provision in the main body of a specification requirement (as opposed to taking exception to a supplemental requirement of a specification), the resulting casting cannot be held to compliance with those specifications.
The exact metal alloy to be cast should be specified using an internationally recognized standard such as ASTM. There are generally two types of metals that castings are produced from: ferrous (iron-based) and non-ferrous (not iron-based). Steel is the most common casting metal, however other metals may be used depending on desired physical properties, including stainless steel, malleable iron and ductile iron; aluminum and copper are the most common non-ferrous metals used in casting.
Mechanical properties may be verified by the use of test bars cast either separately or attached to the castings. The mechanical properties obtained represent the quality of the metal, but do not necessarily represent the properties of the castings themselves, which are affected by solidification conditions and rate of cooling during heat treatment, which in turn are influenced by casting thickness, size, and shape. In particular, the hardening ability of some grades may restrict the maximum size at which the required mechanical properties are obtainable.
Minimum Section Thickness
Green sand is bonded with clay or bentonite and tempered with water, then used to create molds for casting
Every casting has a minimum thickness that is determined by its strength and rigidity. Designing castings thinner than the specifications may make the project un-castable. To be successful, a castings design must allow liquefied metal to fill the mold in the thinner sections.
Liquefied (molten) metal cools at an exceptionally fast rate. It may cool too quickly to enter thin sections that are far away from the mold’s gate. As a general rule, designs should not have areas that are thinner than of 0.25 in (6 mm), when conventional processes are used. Investment casting allows for a greater level of freedom and wall thickness can drop to as low as to 0.030 in (0.76 mm).
Draft and Core
The term “draft” refers to the taper on the vertical faces of a pattern. Draft is necessary to extract the pattern from the mold without disturbing the mold walls – ignoring the limitations of these angles will cause the mold to tear when it is removed from the casting.
Several factors will affect how much draft is required for each casting: the manufacturing process, the casting size, and whether the molding is performed by hand or machine will all change the required degree of draft. Less draft is required for castings that are machine molded. Castings produced with green sand molds, however, will require more draft than usual. The general rule of thumb is to allow 3/16 in. of draft per ft. (approximately 1.5 degrees).
A core is a hollow space created within a casting by a piece of molding sand. Cores are necessary when the spaces or cavities cannot be created with the pattern alone. The use of cores may eliminate the need for draft, however their use should be limited as they add to the cost of the casting.
Three factors affect the minimum diameter of a core that can be placed in a casting: (1) the thickness or depth of the area in which the core will be placed, (2) how long the core is, and (3) the casting method that the supplier foundry will employ.
Areas in the casting that are less accessible make it more difficult to remove cores and affect the economic feasibility of the projects. Castings should always be designed with openings large enough to allow for the removal of cores.
Poured molds before shakeout
After production, all castings should be tested to ensure that they meet the requirements of the specification. In many applications, testing is mandatory; additional tests may be required to ensure that material specifications or other general requirements are met. Testing and acceptance criteria must be clearly communicated and agreed upon prior to production. Generally, more rigorous terms of testing will increase the overall cost. To remain cost-efficient, the terms of testing and acceptance should be based on the precise requirements of each application.
Soundness of metal components refers to the level of freedom from impurities and/or discontinuities such as sand inclusions, slag inclusions, macro porosity, and shrinkage. It is impossible to produce a defect-free casting – only castings with defects of varying degrees. The acceptance or rejection of castings is determined by examination of parts (in accordance with internationally recognized standards such as ASTM) based on customers' formal engineering requirements; acceptance and rejection criteria are defined by an agreement between the supplier foundry and the client prior to production. These criteria affect both the casting cost and the production processes used.
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Casting Production and Delivery
An abrasive grinder is used to remove flashing and other imperfections prior to shipping.
A cooperative effort is essential to the successful outsourcing of a metal casting project – the supplier foundry should be involved from the early stages of design through to manufacturing and product delivery.
Once the supplier foundry reviews a request for quote, they will work alongside clients to evaluate all costs and determine product goals. Good planning pays off for both the foundry and the client; foundry experts evaluate the project prior to production, and will recommend the best materials and methods for achieving the most economical solution.
A request for quote should be made well before the required delivery date. Due to the complexity of the metal casting process, project times can run up to 24 weeks leading up to full production and delivery.
Reliance Foundry delivers quality, precision castings in a clear time frame. Contact our sales team now to request a quote.
- Poured molds before shakeout: OKFoundryCompany, CC BY 2.0, via Flickr
- Engineering detail with casting: OKFoundryCompany, CC BY 2.0, via Flickr