INVESTMENT CASTING DESIGN
Size and Weight. Part size and weight are the most critical factors in determining part cost because mold capacity is limited by both size and weight. The more pieces that can run on a mold, the lower the part cost. Unnecessary mass should always be removed by correct engineering practices to reduce part weight.
Number of Gates. When possible, parts should be designed so that a single gate can feed the part. This will generally yield more pieces per mold and reduce the pour weight per mold.
Castability. If the design contains features that will raise scrap or rework rates (and the piece price), the Signicast Estimating Engineer will recommend design modifications to keep the piece price down.
Alloys are listed from least to greatest cost:
1. Carbon and Low Alloy Steels
8620, 4140, 1020, 1035, WCB are best choices
2. 17-4PH and 400 Series Stainless Steel
17-4PH & CA-15 (410) are most frequently poured
|Do not specify 416 unless necessary for machinability|
3. 300 Series Stainless Steel
|CF-8 (304), CF-8M (316) are best choices|
|Do not specify CF-16F (303) unless necessary for machinability|
4. Tool Steels
|Costs vary widely depending on grade|
5. Nickel Base Alloys
|Alloy C and X are best choices|
6. Cobalt Base Alloys
|All grades pour well – Choice is based on end use and cost|
Signicast builds gate grind fixtures for virtually all parts produced that require gate grind. When possible, the part should be designed so the gate can be put on a flat surface rather than a curved surface. Gate grind tolerances are listed from least to greatest cost.
- Break-off witness (same alloys)
- .010 - .025 inch high witness
- Flush grind
Gate Witness Examples
Heat treatments are used to control hardness, mechanical properties, corrosion resistance, and surface treatments. Unless otherwise indicated, all alloy steels are quoted with a normalize or anneal. All 300 series stainless steels and 17-4PH are quoted with a solution anneal. A cost savings.
Straightening adds cost, but sometimes cannot be avoided. The actual straightening costs are often dependent on the tightness of the tolerance specified, so do not specify tighter flatness, straightness, roundness, or other requirements than you actually require. (Signicast typically straightens parts using die sets that will be included in your tooling price.)
Signicast performs a 100% visual inspection. However, if dimensional verification is required, a sampling plan and process control is less expensive than 100% gaging of each part. Fixed gages such as "go-no-go" are less expensive to use than gages with dial indicators. (Important: Whenever custom gages are used, there should be identical gages at Signicast and at the customer's plant.)
All sample investment castings are non-destructively tested in order to establish first article quality. However, this is generally not required on production commercial investment castings; doing so will add costs. The following non-destructive tests are listed from least to greatest cost:
- Magnetic Particle
- Fluid Penetrant
Chemical Certification can be provided at no extra cost. Mechanical Property Certification adds to cost, so it's best to consider a combination of Chemistry and Hardness Certification instead. Marking each casting with heat number also adds to the cost. Investment casting in lot numbers may be a good tracability option for commercial parts.
Tooling types are listed in order of least expensive tooling, but highest piece price.
- Manual – For prototype or low volume production work
- Semi-Automatic – For parts that are too large or fragile for fully automatic tooling
- Fully Automatic
More cavities means higher tooling cost, but lower piece price. Configurations that do not allow metal cores to be drawn must be treated in one of the following ways:
- Collapsible Cores – Lowest piece price, higher tooling cost
- Loose inserts – Best for low volume parts
- Multipiece Wax Assemblies – Best for certain configurations, but tolerance control suffers
- Soluble Cores – Requires additional die and labor
- Pre-Formed Ceramic Cores – High cost for specialized shapes
To download our complete Design Guide, Click here.