1.- Moldes en arena para fundición | Teoría y práctica | Procesos de manufactura.

Main ideas / lessons

• Definition of casting: Casting is a process where molten metal flows by gravity through an opening in a mold, then solidifies inside the mold cavity, producing the final object.

• Mold types (open vs. closed):

  • Open mold: Molten metal is exposed to the environment; described as not very useful for the intended application.
  • Closed mold: A mold with specific structural elements and interfaces used to control the metal and the solidification process.

• Key components of a closed mold:

  • Molding box (sand box): Retains sand; constructed in two parts (lower and upper).
  • Feeding system:
    • Pouring funnel (recommended to reduce waste and prevent molten metal from spilling during pouring).
    • Sprue: Channels molten metal downward.
    • Pouring channel (duct): Final duct that feeds molten metal into the mold cavity.
  • Riser (feeding riser / reservoir):
    • Acts as a reservoir of molten metal.
    • Helps compensate for shrinkage during solidification by feeding molten metal into shrinkage cavities.
    • In the lesson’s first exercise model, a riser is not used because it requires extensive calculations.
  • Molten metal cavity (mold cavity): The space that takes the shape of the casting.
  • Core (“heart”):
    • Used to create internal cavities in the casting.
    • Often includes features/supports (fastener/supports) for complex geometries; supports are planned here to allow safe core installation.

• Internal cavity example concept: To create an internal perforation, embed a core so it forms the interior cavity when metal solidifies around it.

• Parallel manufacturing workflow: Multiple tasks occur at the same time before metal pouring.

• Post-pour finishing steps: Solidify → remove mold → clean → inspect → remove gating/risers/edges → obtain final casting per engineering specs.

• Design requirements/parameters for good mold quality:

  • Strength (depends on grain shape, grain size, and binder).
  • Resistance to removal forces (pressures/forces when removing the pattern).
  • Permeability (lets gases escape; otherwise mold erosion and other defects occur).
  • Thermal stability (resists thermal shock when pouring molten metal at different temperatures).
  • Collapsibility (easy mold removal without cracking or damaging the mold/pattern/casting).
  • Cleanability (ease of cleaning to ensure quality).
  • Reusability (mentioned generally; in this exercise the mold is not reused).

• Green sand mold concept:

  • A sand mold made with relatively high moisture.
  • Provides good resistance, excellent collapsibility, and very good permeability.
  • Moisture enables adequate behavior for forming and gas escape during pouring.
  • Recycled sand can be used to make it economical.
  • Humidity control is critical because moisture can cause defects depending on molten metal and geometry.

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Methodology / process steps (detailed bullet format)

1) Establish foundational definitions and choose mold type

• Define casting as molten metal flowing into a mold cavity and solidifying in that shape.
• Select mold type:
  • Consider open molds (exposed molten metal) but note limited usefulness here.
  • Use a closed mold for controlled production.

2) Identify and assemble mold elements (closed mold)

• Build the molding box (sand box):
  • Two-part structure (lower + upper sand box).
• Configure the feeding system:
  • Use a pouring funnel to reduce waste and prevent spilling.
  • Connect to sprue to direct molten metal into the mold.
  • Continue to pouring channel/duct to feed the cavity.
• Decide on riser strategy:
  • Understand riser purpose: compensate for shrinkage by feeding molten metal into shrinkage cavities.
  • For the simple first model, do not implement a riser (avoids complex calculations).
• Define the mold cavity:
  • This becomes the external shape of the casting.
• If internal voids are needed, include a core (“heart”):
  • Core creates internal cavities.
  • Plan for safe core installation using supports/protrusions from the model.

3) Plan parallel activities before pouring

• Perform in parallel (recommended):
  • Manufacture cores (hearts)
  • Prepare sand (including additives/binders)
  • Place molten metal in the furnace so it melts while the mold is being built
  • Manufacture the pattern
• Then proceed to make the mold using the prepared pattern.

4) Pouring and solidification

• Once molten metal is ready and the mold is built:
  • Start pouring
  • Wait for solidification
  • Proceed to final casting recovery stages.

5) Demolding, cleaning, inspection, and finishing

• Remove the mold after solidification.
• Clean sand completely from the casting/pattern area.
• Cleaning and inspection:
  • Verify dimensions are within specified tolerances
  • Remove imperfections inherent to casting
• Remove non-product features:
  • Remove the gating system
  • If present, remove the riser along with gating
  • Remove extra edges not part of the intended piece
• Final outcome:
  • Obtain a fully finished casting according to engineers’ specifications.

6) Mold design requirements to ensure quality

• Ensure adequate mold strength:
  • Depends on grain shape, grain size, and binder.
• Ensure mold permeability:
  • Prevent gas buildup that causes erosion and defects.
• Ensure thermal stability:
  • Withstand thermal shock from molten metal temperature differences.
• Ensure collapsibility:
  • Mold should collapse easily to avoid cracking/damage during removal.
• Ensure ease of cleaning for quality results.
• Note reusability considerations:
  • In this exercise, the mold is collapsed/not reused.

7) Green sand mold selection and handling (materials/conditions)

• Use a green sand mold:
  • Sand with relatively high moisture
  • Offers resistance, collapsibility, and permeability.
• Reuse sand:
  • Use recycled sand when possible (economical).
• Control humidity carefully:
  • Moisture level can cause casting defects depending on molten metal and part geometry.

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Speakers / sources featured

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