LECTURE PRESENTATION
Reading Assignment
- 12.1 Introduction to Expendable Mold Casting
- 12.2 Sand Casting
- 12.3 Cores and Core Making
- 12.4 Other Expendable Mold Processes – Multiple use patterns
- 12.5 Other Expendable Mold Processes – Single use patterns
- 12.6 Shakeout
- 12.7 Summary
Recommended
- Casting Hit or Miss Engine (video)
- Investment Casting Process (Suburban Tool video)
Outline
Introduction
Factors to consider for castings
- Desired dimensional accuracy
- Surface quality
- Number of castings
- Type of pattern and core box needed
- Cost of required mold or die
- Restrictions due to the selected material
Three categories of molds
- Single-use molds with multiple-use patterns
- Single-use molds with single-use patterns
- Multiple-use molds
Sand Casting
- Sand casting is the most common and versatile form of casting
- Granular material is mixed with clay and water
- Packed around a pattern
- Gravity flow is the most common method of inserting the liquid metal into the mold
- Metal is allowed to solidify and then the mold is removed
Patterns and Pattern Materials
- Wood patterns are relatively cheap, but not dimensionally stable
- Metal patterns are expensive, but more stable and durable
- Hard plastics may also be used
Types of Patterns
- One-piece or solid patterns are used when the shape is relatively simple and the number of castings is small
- Split patterns are used for moderate quantities
- Pattern is divided into two segments
- Match-plate patterns
- Cope and drag segments of a split pattern are permanently fastened pins and guide holes ensure that the cope and drag will be properly aligned on reassembly
- Cope and drag patterns
- Used for large quantities of castings
- Multiple castings can occur at once
- Two or more patterns on each cope and drag are possible
Sands and Sand Conditioning
Four requirements of sand used in casting
- Refractoriness-ability withstand high temperatures
- Cohesiveness-ability to retain shape
- Permeability-ability of a gases to escape through the sand
- Collapsibility-ability to accommodate shrinkage and part removal
Size of sand particles, amount of bonding agent, moisture content, and additives are selected to obtain sufficient requirements
Processing of Sand
- Green-sand mixture is 88% silica, 9% clay, and 3% water
- Each grain of sand needs to be coated uniformly with additive agents
- Muller kneads, rolls, and stirs the sand to coat it
Sand Testing
Moisture content, clay content, compactibility
Properties of compacted sand
Mold hardness, permeability, strength
Standard testing
- Grain size
- Moisture content
- Clay content
- Permeability
- Compressive strength
- Ability to withstand erosion
- Hardness
- Compactibility
Sand Testing Equipment
- Permeability tester
- Sand Mold hardness tester
Sand Properties and Sand-Related Defects
Silica sand
- Cheap and lightweight but undergoes a phase transformation and volumetric expansion when it is heated to 585°C
- Castings with large, flat surfaces are prone to sand expansion defects
- Trapped or dissolved gases can cause gas-related voids or blows
- Penetration occurs when the sand grains become embedded in the surface of the casting
- Hot tears or crack occur in metals with large amounts of solidification shrinkage
- Tensile stresses develop while the metal is still partially liquid and if these stresses do not go away, cracking can occur.
Making Sand Molds
Hand ramming
- Used when few castings are to be made
- Slow, labor intensive
- Nonuniform compaction
Molding machines
- Reduce the labor and required skill
- Castings with good dimensional accuracy and consistency
- Sand slinger uses rotation to fling sand against the pattern
- Jolting is a process in which sand is placed over the flask and pattern and they are all lifted and dropped to compact the sand
- Squeezing machines use air and a diaphragm
- For match plate molding, a combination of jolting and squeezing is used
Alternative Molding Methods
- Stack molding
- Molds containing a cope impression on the bottom and a drag impression on the top are stacked on top of one another vertically
- Common vertical sprue
- Large molds
- Large flasks can be placed directly on the foundry floor
- Sand slingers may be used to pack the sand
- Pneumatic rammers may be used
- Sodium Silicate-CO2 Molding (video)
- Molds and cores can receive strength from the addition of 3-6% sodium silicate
- Remains soft and moldable until it is exposed to CO2
- Hardened sands have poor collapsibility
- Shakeout and core removal is difficult
- Heating makes the mold stronger
- No-Bake, Air-Set, or Chemically Bonded Sands
Shell Molding
Basic steps
- Individual grains are sand are precoated with a thin layer of thermosetting resin
- Heat from the pattern partially cures a layer of material
- Pattern and sand mixture are inverted and only the layer of partially cured material remains
- The pattern with the shell is placed in an oven and the curing process is completed
- Hardened shell is stripped from the pattern
- Shells are clamped or glued together with a thermoset adhesive
- Shell molds are placed in a pouring jacked and surrounded by sand, gravel, etc. for extra support
- Disadvantages
- Cost of a metal pattern is often high
- Design must include the gate and the runner
- Expensive binder is required
- Advantages
- Amount of required material is less
- High productivity, low labor costs, smooth surfaces, high level of precision
Other Sand-Based Molding Methods
Cores and Core Making
Other Expendable-Mold Processes with Multiple-Use Patterns
Plaster mold casting
Rubber-mold casting
Rubber-mold spin casting
Expendable-Mold Processes Using Single-Use Patterns
Investment casting
One of the oldest casting methods
Products such as rocket components, and jet engine turbine blades
Complex shapes
Most materials can be cast
Investment Casting
- Sequential steps for investment casting
- Produce a master pattern
- Produce a master die
- Produce wax patterns
- Assemble the wax patterns onto a common wax sprue
- Coat the tree with a thin layer of investment material
- Form additional investment around the coated cluster
- Allow the investment to harden
- Remove the wax pattern from the mold by melting or dissolving
- Heat the mold
- Pour the molten metal
- Remove the solidified casting from the mold
Advantages and Disadvantages of Investment Casting
- Disadvantages
- Complex process
- Can be costly
- Advantages
- Complex shapes can be cast
- Thin sections can be cast
- Machining can be eliminated or reduced
Evaporative pattern processes
Pattern is made of polystyrene or polymethylmethacrylate
Pattern remains in the mold until the molten metal melts away the pattern
If small quantities are required, patterns may be cut by hand
Material is lightweight
Evaporative Patterns
Metal mold or die is used to mass-produce the evaporative patterns
For multiple and complex shapes, patterns can be divided into segments or slices
Assembled by hot-melt gluing
Full-mold process
Green sand is compacted around the pattern and gating system
Lost Foam Process
Advantages of the Full-Mold and Lost-Foam Process
Sand can be reused
Castings of almost any size
Both ferrous and nonferrous metals
No draft is required
Complex patterns
Smooth surface finish
Absence of parting lines
Lost-Foam Casting
Molds can be sectioned to allow undercuts, etc.
Shakeout, Cleaning, and Finishing
Final step of casting involves separating the molds and mold material
Shakeout operations
Separate the molds and sand from the flasks
Punchout machines
Vibratory machines
Rotary separators
Blast cleaning
