Expendable Mold Processes

Definition: Expendable molds are single-use molds designed for casting processes.
Types of Patterns:
- Multiple-use patterns with single-use molds.
- Single-use patterns with single-use molds.
Materials Used:
- Various binders are combined with refractory materials, such as sand, plaster, or ceramics.

Characteristics:
- Most widely used casting process.
- Among the least expensive casting methods.
- Low tooling costs.
- Minimal restrictions on size, shape, weight, and complexity of pieces.
- Nearly any metal can be cast, with titanium (Ti) as an exception.

Strength Addition:
- The addition of 3-6% sodium silicate to sands can enhance the strength.
Dimensional Tolerance:
- This method offers excellent dimensional tolerance and accuracy in production.
Mold Behavior:
- Stays soft and moldable until exposed to CO2.
- Limitations include hards sands that have poor collapsibility, creating challenges in shakeout and core removal.
- Heating increases the strength of both the mold and the material.

Process Description:
- Sand coated with thermosetting plastic resin is applied to a heated metal pattern.
- The resin cures, allowing segments to be stripped from the pattern and assembled.
- Once metal is poured and solidifies, the shell is broken away to reveal the finished casting.
Advantages:
- Faster production rates compared to sand molding.
- High dimensional accuracy and smooth surfaces.
Limitations:
- Requires expensive metal patterns.
- The cost is increased due to the use of plastic resin.
- Size limitations typically range from 30 g (1 oz) to less than 10 kg (25 lb), with a mold area of less than 0.3 m² (500 in²).
- Thickness limits range from 0.15 to 0.6 cm (1½ to ½ in.) depending on materials.
- Expected tolerances are around ±0.005 cm/cm or in/in, with a draft allowance of ±½ degree.
- The surface finish can be around -4.0 microns (50-150 μin.) RMS.

Description of Process:
1. A heated pattern is positioned over a dump box filled with resin-coated sand.
2. The box is inverted, allowing the heat to form a partially cured shell around the pattern.
3. The box is righted, and the pattern with the partially cured sand is placed in an oven for further curing.
4. The shell is then stripped from the pattern.
5. Matched shells are joined together and supported in a flask for pouring.

Components:
- Seal Ring, Cavity Ring,
- Loose sand between grooves and other structural components.

Various core-making techniques such as dump-type core box, baking core halves, and assembling cores through adhesive methods.

Strength requirements before and after hardening, smooth surface, minimal gas generation when heated, adequate permeability and refractoriness, and collapsibility.

Incorporation of vent holes, connection of cores to outer surfaces, and the use of core prints and chaplets (small metal supports) to enhance stability and integration in the final casting.

Injection of wax or plastic into a die to create a pattern.
Patterns are gated to a central sprue for efficient casting.
Patterns are coated with a ceramic slurry, and refractory grain is sifted on the coated patterns.
The pattern is melted out after curing, and the mold is prepared for metal pouring.
The mold is filled with metal via gravity, pressure, vacuum, or centrifugal methods.
Mold material is subsequently broken away from the castings.
Castings are separated from the sprue and gate stubs are ground off for finishing.

Advantages:
- Superior surface finish, high dimensional accuracy, unlimited intricacy potential, capability to cast nearly any metal, and elimination of flash or parting line issues.
Limitations:
- High costs for patterns and molds, potentially high labor costs, and limitations on maximum size.
Commonly Cast Metals: Includes aluminum, copper, steel, stainless steel, nickel, magnesium, and various precious metals.
Size Limits: As low as 3 g (oz) but typically less than 5 kg (10 lb).
Thickness Limits: Ranges from as thin as 0.06 cm (0.025 in.) to less than 7.5 cm (3.0 in.).
Typical Tolerances: 0.01 cm for the first 2.5 cm (0.005 in. for the first inch) and 0.002 cm for each additional cm (0.002 in. for each additional inch).
Surface Finish: 1.3-4 microns (50 to 125 in.) RMS.

Process Steps:
1. The pattern is made from a foamed plastic (like polystyrene) and includes essential components like sprues, runners, and risers.
2. The pattern is dipped into a ceramic material and dried, then surrounded by loose sand.
3. Molten metal is poured into the mold, vaporizing the pattern and allowing for venting through the sand.

Advantages:
- Design flexibility with limitless shape and size capabilities, capability to cast most metals, no drafts or flash required (eliminating parting lines).
Limitations:
- High pattern costs for small quantities, and low-strength patterns that can easily be damaged or distorted.
Common Metals: Cast using aluminum, iron, steel, nickel alloys, copper, and stainless steel.
Size Limits: From 0.5 kg to several thousand kg (1 lb to several tons).
Thickness Limits: As small as 2.5 mm (0.1 in.) with no upper limit.
Surface Finish: Typically ranges from 2.5-25 microns (100-1000 μin.) RMS.

Control of mold shape, liquid flow, and solidification plays a crucial role in determining properties of castings.
Each process presents its unique advantages and disadvantages, and the optimal method should be selected based on product shape, material type, and desired properties.