Casting, Moulding & Powder Processing Flashcards
What is Casting
Which types of Metals can be cast
Process in which a volume of material (metal) flows by a force into a mould, the metal then solidify in the desired geometry.
Ferrous and Non-Ferrous Metals (All Metals)
Describe an Open Mould
Draw and label an Example of an Open Mould
A Container in the shape of the Desired Product,
Describe a Closed Mould
Draw and label an Example of a Closed Mould
Container that Contains an Upper Cope, Lower Drag, a gating system, and a riser.
This allows for a more complex geometry to be cast.
Describe a Closed Mould with a Core
Draw and label an Example of a Closed Mould with a Core
Container that Contains an Upper Cope, Lower Drag, a gating system, and a riser.
The Mould also contains a core held up by Chaplets
This allows for a more complex geometry to be cast, with a hollow section.
What are the 3 Considerations need to be made about the Design of the Chaplets inside a Closed Mould containing a core.
Chaplets must be of the same or similar material as the metal being cast
Must Hold the Core in Place for the duration of the cast (Not too Small)
Must Melt and fuse Completely into the poured metal (Not too Big)
A Manufacturer can design a mould that doesn’t use chaplets, why is that design better?
Chaplets cause casting defects and weak spots in the casting
Explain why the Upper Chaplet in a Mould must be Stronger then the Bottom.
it has to counteract the pressure of the molten metal and any gases that may push upwards during the casting process
Why is a Riser Required in a Closed Mould
What design considerations must be made when designing the riser
Cast Metal Shrinks as it solidifies, the riser fills gaps that have formed due to metal shrinkage.
Molten metal in riser must solidify more slowly then the Metal inside the Cavity
Riser must have a larger volume-Surface Ratio
What are the Positives to casting
Creates Relatively complex geometries
Creates external and internal shapes
Can create both Net and Near-Net Shapes
Can Produce Large Parts Easily
Can be suited to Mass Production
Describe what Net and Near-Net Shapes are
Net Shapes are products which require little-to-no additional machining
Near-Net Shapes, require additional machining to reach the final product
What are the negatives to casting
Doesn’t work with High-Melting point Materials.
Poor Dimensional Accuracy and Surface Finish
Excess material needs to be used , separation and recycling processes are required.
Highly Dangerous Machines, Conditions and High Temperature Molten Metals. Mean that alot of safety precautions are required.
Explain the Positive and Negatives to Permanent Moulds and Expendable Moulds
Permanent -
Mould can be reused many times to produce many castings
Higher Production Rates
Less Complex Geometries can be cast (needs to be an open Mould)
Expendable -
More Complex Geometries can be cast.
Sacrificial Mould must be destroyed to remove casting.
Manufacture time limited by the time to create the mould.
Explain what Temperature must a furnace heat the metal until.
Heat beyond the melting point
Heat of fusion to convert from solid to liquid
Then Heat to raise molten metal to desired pouring temperature
What are factors that determine success in Casting
Pouring Temperature (needs to be above required temp)
Pouring Rate (Slow enough to not wear away the mould internal surface, fast enough so it can fill the whole cavity before solidifying)
Turbulence - (irregular variations in flow speed and direction of molten metal)
Label the cooling curve
What is the Chvorinov Rule
A casting with a Higher Volume-Surface Ratio solidifies _________________
more slowly
What does the Mould constant depend on
Mould Material
Thermal Properties
Pouring Temperature Relative to the melting point
2 Design Considerations that need to be Considered
- Riser Must Solidy after casting
- Riser volume must be 3 times the shrinkage in the casting.
Both need to be considered independently, both will produce a minimum length, the higher minimum length is then selected
Describe the Pro’s and Con’s of using Wood , Metal and Plastics as a Mould Pattern Material
Compare using Small Grains of Sand/Silica , and larger grains of sand/silica within an Expendable Mould
What are the 4 Desireable Mould Properties
Strength - Maintain Shape and Erosion Resistant
Permeability - Allow Hot air and gasses to pass through voids in sand
Collapsibility - allowing the cast to shrink without cracking (withstand thermal stresses from cast Shrinkage)
Reusability - how well is the sand recycled once used in a mould.
Explain why Hot Air and Gasses must be allowed to escape in a Cast
Prevent Porosity - so air bubbles dont form within the solidified metal
Describe the 3 Main types of Defects with Casting
Explain Shell Cast Moulds and Investment Cast Moulds
What Design Considerations must be Made building a Permanent Mould
When should a Permanent Mould be used
What Typical Products are cast using permanent Moulds
- Two Metal Sections that open and close
- Higher Melting Point then Cast Material
- Refractory material (tungsten) to be used if Steel is cast
- High Volume Production, due to high capital cost
- Easy to be Automated
PU pistions, aerospace parts
What is Permanent Mould Casting Also Known as?
Describe the Process
Die Casting
Molten Metal is injected into a cavity (die) at high pressure
Pressure Maintained during solidification
Part is Removed
What are the Advantages and Disadvantages of Die Casting
Compare Shell Moulding with Sand Moulding
Shell Moulding has a Better Surface Finish, from a smoother cavity surface (easier flow)
Shell Mouldimg has a Better Dimensional Accuracy ( No machining required)
Shell Mouldimg has Better Mould Collapsibility
A sand Mould is Cheaper than a Shell Mould
Describe Thermoplastics & Thermoset Plastics
Thermo Plastics
Molecular Structure Remains unchanged during heating and shaping
Thermoset
Molecular Structure permanently changes after heating and shaping, from a curing process
Comment on the Recyclability of Thermo & Thermosetting Plastics
Thermoplastics are suitable for reusing and recycling waste material
Waste Thermoset plastics cannot be remade into other products with sufficient strength and integrity, due to the curing process in production.
PE, PP PS & PVC
What are the names of those Plastics and what products are they typically used in
What are the 3 States of a Thermoplastic
What considerations must be made when heating a thermoplastic prior to mould injection
Glass State , Rubbery State & Viscous Flow
- Softens to Liquid Consitity
- At Polymer Melt Temperature, which is below the Melting point and above the glass transition temperature
What are the Important Properties of the molten plastic prior to mould injection
Viscosity - Resistance to Flow
Viscoelasticity - Tendency to Act as both a Solid and a Fluid
Describe a Problem faced when Moulding Thermoplastics
Polymer Melt Fluid has a High Viscosity
High Flow rate during injection
Causes High Shear Rates and Shear Stresses
Causes Warping, Shrinking & Cracking
Describe what Shear Rate is
Rate of change in velocity of adjacent polymer layers passing over each other
High Shear rate affect its Mechanical Properties Negatively
What is Viscoelastic Deformation
Reversible Over an Extended period compared to Elastic Deformation
Give Examples of Good and Bad Mechanical Properties of polymers
Good
- Corrosion Resistant
-Low Electrical and Thermal Conductivity
- Low Density
- Easy to mould
-Appearance (Transparency, glossy etc.)
- Less energy intensive production
- Impact Resistance
Bad
- Low Stiffness and Strength
- Cannot Tolerate High Temps
- UV degradation
- Cold Flow (Slowly deform over time also know as creeping)
- Thermal Expansion
What is the Following machine?
Describe 2 Principle Components
What is the Following machine?
Describe the Process Sequence
Explain how the Product is Cooled
What is the Following machine?
Describe the Process
What is the Frost Line
Derive the Formula for the Dimension of a mould cavity should be to account for shrinkage
Explain why Sharp corners are avoided in moulds
Sharp Corners cause Uneven Flow During Processing causes a concentration of stress in that area , which can cause defects in the final product.
Explain the Powder Metallurgy (PM) Production Sequence
Blending - Mixing of Homogenised Powders
Pressing - Compression of the metal powder using a punch-and-die into the desired shape, (called a green compact)
Sintering - Green Compacts are heated to a specific temperature below the melting point , bonding the particles into a hard, rigid mass
What are the benefits of Powder Metallurgy (PM)
- Mass Production automation
- Can produce Net and Near Net Shape
- 97% of the material used (Little waste)
- Can be made to varying levels of Porosity (up to 50%)
- Compatible with Metals which other processes are not
- Produces Holes and parts with compicated cross-sectional shapes
What are the benefits to making a product with a porous structure
- Higher Strength-to-Mass ratio
- Harder for micro-organisms to Stick-on and Grow
What does the Phrase Green Compacts refer to?
Compacted into Desired shape but Non-Fully Processed /Un-Sintered
What are Engineering Powders
Name 4 Notable Geometric Features
Metals, or Ceramics solids finely divided
Particle Size & Shape
Distribution
Internal Structure
Surface Area
What is the Main factor that is considered when an Engineering Powder is Produced?
Describe a Common test that can compare this factor between 2 different Powders
Friction - Affects flow and how tightly particles can pack
Angle of Repose
- measure the angle made by the powder when poured through a narrow funnel
- larger the angle , the greater amount of inter-particle friction.
Describe a Powder with the lowest amount of Interparticle Friction
Explain why using Large Sized Particles is a Negative
-Perfectly Sphericle Particles
-Small Particle Size
(Larger contact area from higher surface area & larger Van Der Walls force with Higher Adhesion)
Larger Particle size, means for a smaller number of grain boundaries
Less force is required to cause the particles to move
Reducing the Strength and toughness of the Product
What does the Following Machine do?
Atomisation (Production of Engineering Powder)
Molten Metal is Poured through pressurised coolant
Molten Metal is separated into small droplets and frozen rapidly
What is Bulk Density
How much of the Powder can be accommodated within a given volume
What is True Density
Mass per unit volume of material, if it were melted into a solid mass
What is Packing Factor
Ratio of the volume of the Solid particles to the Bulk volume of the powder
What is the Porosity of Powders
Ratio of the volume of the pores to the Bulk volume of the powder
What are the 4 Functions of Sintering
Bond individual Grains into a Solid Mass
Increase Density
Reduce Porosity
Increase Strength and Hardness
Describe what is Happening with these 4 Steps of Sintering
Explain Isostatic Pressing and Powder Injection Moulding (PIM)
Isostatic - Hydrolyic Pressure applied from all direction during pressing
PIM - Powders are injected into a Mould, used for small and compiled geometries produced in large volumes
What is the Powder Metallurgy Part Classification System
All PM part designs can be classified into 1 of 4 categories
Difficulty of Manufacture Increases from 1-to-4
Number of levels of thickness increases from 1-to-4
What Part features should be avoided in Powder Metallurgy
Side holes & Undercuts
Explain why Chamfers and Radaii are difficult in Powder Metallurgy
What 5 precautions are recommended
While pressing, the punch is under high pressure, if acute angles are used the geometry and stability of the punch may vary, which can cause defects in the part.
Avoid Acute Angles
Larger Angles Between the Top Surface and the Punch
Inside Radius is Desirable over Outside Radius
Avoid Full Outside Corner Radius
Combinations of Radius and Chamfer
Explain the Negatives of Powder Metallurgy (PM)
- Large upfront Cost of Machinery (minimum 10,000 parts )
- Struggles to Produce Parts with non-vertical sides
- Side Holes and Undercuts cannot be produced
- Cant Produce Threading or tapping on parts ( Has to be machined)
- Part Produced must be able to be ejected quickly
Example Q
Explain the 4 steps of powder metallurgy
