ppt5.2 Flashcards
Most _______ sand molds consist of silica sands bonded
with a _________ mixture
green; bentonite‐water
what doe the term “green” means?
It means that the mold, which is tempered with water, is not dried or baked.
The ______________________ of the sand are important to the success of the mold making operation.
composition, size, size distribution, purity, and shape
Sands are sometimes referred to as ___________
natural or synthetic
contain enough naturally occurring clays that they can be mixed with water and used for sand molding.
Natural sands
have been washed to remove clay and other
impurities, carefully screened and classified to give a desired size distribution, and then reblended with clays and other materials to produce an optimized sand for the casting being produced
synthetic sands
Because of the demands of modern high‐pressure molding machines and the necessity to exercise close control over every aspect of casting production, most foundries use only ____________.
synthetic sands
[composition of silica sands] Foundry sands are composed almost entirely of silica (SiO2) in the form of _______.
quartz
[composition of silica sands] Some impurities may be present, such as _______, __________, or ________, which is composed of magnesium and ferrous orthosilicate [(Mg,Fe) SiO4]. S
ilmenite (FeO‐TiO2); magnetite (Fe3O4); olivine
[composition of silica sands] Some impurities may be
present, such as ilmenite (FeO‐TiO2), magnetite
(Fe3O4), or olivine, which is composed of _________
and __________.
magnesium; ferrous orthosilicate [(Mg,Fe) SiO4]
[composition of silica sands] It is used primarily because it is readily available and
inexpensive
silica sand
[composition of silica sands] Quartz undergoes a series of _________________ as it is heated.
crystallographic transitions
[composition of silica sands] The first, at 573°C, is accompanied by ______, which can cause mold spalling. Above 870°C, quartz transforms to _______ , and the sand may actually contract upon heating. At still higher
temperatures (> 1470 °C), tridymite transforms to
__________.
expansion; tridymite; cristobalite
[composition of silica sands- mechanism of expansion] As aplha form changes to beta form when heated density _______, resulting in _______ volume and leading to ______ in the linear expansion.
decrease; increase; increase
[shape and distribution of sand grains] The ________________of the sand grains are important in controlling the quality of the mold.
size, size distribution, and shape
[shape and distribution of sand grains] In determining the size, shape, and distribution of the sand grains, it is important to realize that the grain shape contributes to the amount of ________ and that the grain size distribution controls the _______ of the
mold.
sand surface area ; permeability
[shape and distribution of sand grains] As the sand surface area increases, the amount of bonding material (normally clay and water) must _______ if the sand is to be properly bonded. Thus, a change in _________, perhaps due to a change in sand shape or the percentage of core sand being reclaimed, will result in
a corresponding change in the ________ required.
increase; surface area; amount of bond
[shape and distribution of sand grains] Rounded grains have a ____________ and are therefore preferred for making _____ because they require the least
amount of binder. However, when they are recycled into the molding sand system, their shape can be a _________ if the molding system normally uses a high percentage of clay and water to facilitate rapid, automatic molding. This is because rounded grains require ________ than the rest of the system sand.
low surface‐area‐to‐volume ratio; cores; disadvantage; less binder
[shape and distribution of sand grains] Angular sands have the ____________ (except for sands that fracture easily and produce a large percentage of small grains and fines) and therefore require more mulling, bond, and moisture. The angularity of a sand increases with use because the sand is broken down by thermal and mechanical shock.
greatest surface area
[shape and distribution of sand grains]The _______ of the mold controls its permeability, which is the ability of the mold to allow gases generated during pouring to escape through the mold. The ________ will result from grains that are all approximately the same size.
porosity; highest porosity
[shape and distribution of sand grains]As the size distribution _______, there are more grains that are small enough to ______ the spaces between large grains. As grains break down through repeated recycling, there are more and more of the smaller grains, and the porosity of the mold decreases.
broadens; fill
[shape and distribution of sand grains] However, if the porosity of the mold is too great, metal
may penetrate the sand grains and cause a ______. Therefore, it is necessary to _______ the base sand distribution and continue to screen the sand and use dust collectors during recycling to remove fines and to determine the proper bond addition.
burn- in defect; balance
[shape and distribution of sand grains] Most foundries in the United States use the ____________ grain fineness number as a general indication of sand fineness. The ____ grain fineness number of sand is approximately the number of openings per inch of a given sieve that would just pass the sample if its grains were of uniform size, that is, the
weighted average of the sizes of grains in the sample. It is approximately proportional to the ___________ of sand exclusive of clay.
American Foundrymens’ Society (AFS); AFS; surface area per unit weight
