9: METAL CASTING

MMET 181 MCQ Practice

What is open mold casting?

process that involves pouring a liquid material into an open-top mold and allowing it to solidify

what is closed mold casting?

process that uses a mold to create parts by injecting resin into a closed cavity and curing it

Why do we have a mix of oil and sand in closed mold casting?

the oil helps the sand hold its shape

Cast cavity always has to be _______ than the shape you want

larger

What happens in shrinkage casting?

Liquid to solid causes less space to be occupied causing the material to “shrink” and have gaps on the sides

What is a shrinkage cavity?

a void or depression that forms within a metal casting during the solidification process

• occurs at the last solidified droplet location

• unavoidable

• irregular shape

• weakens a cast

• can be moved to different site by controlling cooling site

• better if the cavity is not on important areas of the product

How to avoid shrinkage cavities?

• create a riser

  • serves as reservoir

  • pushes air out

What happens to molds in expendable vs permanent mold casting?

• Expendable: can’t re-use mold after job

• Permanent: can re-use mold after job

What are the types of expendable mold casting?

• Sand casting

• Shell-mold casting

• Investment casting

Expendable-mold process: what is sand casting?

creating a mold out of sand, pouring molten metal into the mold, and then removing the metal once it has cooled and solidified

• sand mold produces rough cast surface and other defects due to sand erosion or moisture in the sand.

In these processes metal behaves as a ____

liquid

Expendable-mold process: what is shell-mold casting?

uses heated metal molds to produce hard shells for casting. thin, reusable "shell" mold is created by adhering a resin-coated sand mixture to a pattern

Expendable-mold process: what is investment (lost wax) casting?

a metal casting technique where a detailed wax model of the desired object is created, then coated with a ceramic material, which forms a mold after the wax is melted away, allowing molten metal to be poured in to create the final product with intricate details and precise dimensions

• suitable for products that are precise, have intricate details, have unusual geometry.

• suitable for large parts or mass production with high volume quantity

What are the two types of investment casting?

• shell cast

• flask cast

  • small parts, small quantity production

What is the shell investment casting procedure?

1. form wax pattern (hot wax injected into rubber/Al mold)

2. Make wax tree (pattern arranged and welded onto a central wax trunk)

3. coat wax tree (dipped in slurry, low viscosity mix )

4. air dry

5. melt wax (heated in oven, wax flows out of slurry shell to form a mold)

6. cast metal (molten metal poured into mold)

7. break mold (tree is dipped in water, breaks up covering shell after solidification of cast metal, cast metal tree is washed off and parts are machined off metal trunk)

What is the flask investment casting procedure?

1. form wax pattern (same as shell)

2. make wax tree (same as shell)

3. coat wax tree (tree is put inside a metallic flask, investment - low viscous mixture of different powders and water, is poured into the flask

4. air dry, the flask with investment and wax tree is air dried

5. melt wax (same as shell)

6. cast metal (same as shell

7. break mold (same as shell)

What are the types of permanent mold casting?

• gravity die casting

• pressured die casting

• centrifugal casting

Permanent mold process: what is gravity die casting?

liquid metal is poured and flown by gravity

• limitation: the mold has to have a high melting temp

Permanent mold process: what is pressured die casting?

liquid metal is pressurized and fed into a steel mold

What are important permanent mold features?

• limited to simple shape, small part and low production volume

• if steel molds are used then the cast material needs a lower melting temp

What is the difference between a cold chamber and a hot chamber (permanent mold casting)?

• cold: injection molding for metals

• hot: pressure is created INSIDE the structure

What is centrifugal casting?

a metal casting process that uses centrifugal force to produce high-quality, defect-free metal components

• produces a SINGLE cast that is positioned at center of a mold

• done vertically

What is centrifuge casting?

centrifugal force is used to force molten metal through small runners and compact the liquid metal in a cavity

• produces MULTIPLE casts that are arranged symmetrically about the center of rotation

• done vertically

What is horizontal centrifugal casting?

produces a tube casting from a rotating horizontal mold, aka “true centrifugal casting”

What type of parts is centrifugal casting used for?

beneficial for parts the rotate continuously, ensures the part is balanced

Casting defect: what is a misrun?

metal solidifies/ cools before filling the mold

Casting defect: what is cold shut?

lack of fusion of two metal flows due to early solidification

• only happens if the mold has a “core” (hollow part) metals do not merge

Casting defect: what is cold shot?

caused by splattering during pouring, globules entrapped metal and sand in cast

Casting defect: what is shrinkage cavity?

cavity gives a defect to the part, depression at top due to shrinkage

Casting defect: what is porosity?

AIR POCKETS

bubbles due to entrapped steam or gas in cast

Casting defect: what is hot tear?

• crack due to uneven thermal shrinkage of a large cast

• internal stressors

• mold design made some parts cool down faster than others

Casting defect: what is flash?

two halves did not connect properly or maybe the top of the mold floats (happens when its not clamped together)

Casting defect: what is mold shift?

two halves move/slide while metal is still molten

Casting inspection: what is visual inspection?

visually evaluate:

• dimension inspection: dimension, form, surface

• functional inspection: leak testing, electrical resistance

Casting inspection: what is mechanical inspection?

• destructive: tensile, impact, bending, pressure tests

• nondestructive: x-ray, ultrasonic, x-ray computed tomography, magnetic particle

  aka non-visual

Casting inspection: what is metallurgical inspection?

• grain size and grain growth direction

• inclusion and contamination