Chemically bonded self-setting sand mixtures are referred to as No-Bake or Airset Molding Processes. Since the induction to the foundry industry, furan and urethane binders have gained wide acceptance for the production of molds and cores for castings. The advantages, regardless of which resins are used, are based upon control of the setting times by means of addition of specific amounts of catalyst. This feature combined with high strength and a desirable range of hot properties for use with various alloys presents the opportunity for great flexibility in mold making.
To produce a typical mold using no-bake binders, the pattern is covered to a depth of four to five inches with no-bake sand mixture. The sand is then allowed to set for a period of time at room temperature before the pattern is removed from the mold. The depth of the no-bake binders will vary depending on the size and weight of the casting. Also, the set time will vary depending on the depth of the no-bake binder. The simplicity of the chemical system usually makes quality control less complex and results are more consistent.
Airset molding allows for a simplified sand mix, good flow ability of the sand mix. Uniform hardness throughout the mold. Exact dimensional control.
A limited bench life for the sand mixture. High sand temperatures are more critical. Patterns must be maintained in good condition.
- Range of Alloys
|Chemically Bonded Molding|
|No-Bake or Airset Molding|
|Typical dimensional tolerances, inches||± .005″|
|Relative cost in quantity|
|Relative cost for small number|
|1/2 lb to tons|
|Thinnest section castable, inches|
|Relative surface finish|
|Relative ease of casting complex design|
|Relative ease of changing design in production|
|Range of alloys that can be cast||Unlimited|
Parting Line Influence
When Parting lines are considered, very close tolerances are difficult to obtain. A parting line absorbs fractions of inches per inch. A foundry is doing well to hold a parting line to 0.015 inch. Additional measurement is added to the casting tolerance.