ppt5.2

Question 1

Most _______ sand molds consist of silica sands bonded
with a _________ mixture

Answer 1

green; bentonite‐water

Question 2

what doe the term “green” means?

Answer 2

It means that the mold, which is tempered with water, is not dried or baked.

Question 3

The ______________________ of the sand are important to the success of the mold making operation.

Answer 3

composition, size, size distribution, purity, and shape

Question 4

Sands are sometimes referred to as ___________

Answer 4

natural or synthetic

Question 5

contain enough naturally occurring clays that they can be mixed with water and used for sand molding.

Answer 5

Natural sands

Question 6

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

Answer 6

synthetic sands

Question 7

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 ____________.

Answer 7

synthetic sands

Question 8

[composition of silica sands] Foundry sands are composed almost entirely of silica (SiO2) in the form of _______.

Answer 8

quartz

Question 9

[composition of silica sands] Some impurities may be present, such as _______, __________, or ________, which is composed of magnesium and ferrous orthosilicate [(Mg,Fe) SiO4]. S

Answer 9

ilmenite (FeO‐TiO2); magnetite (Fe3O4); olivine

Question 10

[composition of silica sands] Some impurities may be
present, such as ilmenite (FeO‐TiO2), magnetite
(Fe3O4), or olivine, which is composed of _________
and __________.

Answer 10

magnesium; ferrous orthosilicate [(Mg,Fe) SiO4]

Question 11

[composition of silica sands] It is used primarily because it is readily available and
inexpensive

Answer 11

silica sand

Question 12

[composition of silica sands] Quartz undergoes a series of _________________ as it is heated.

Answer 12

crystallographic transitions

Question 13

[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
__________.

Answer 13

expansion; tridymite; cristobalite

Question 14

[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.

Answer 14

decrease; increase; increase

Question 15

[shape and distribution of sand grains] The ________________of the sand grains are important in controlling the quality of the mold.

Answer 15

size, size distribution, and shape

Question 16

[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.

Answer 16

sand surface area ; permeability

Question 17

[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.

Answer 17

increase; surface area; amount of bond

Question 18

[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.

Answer 18

low surface‐area‐to‐volume ratio; cores; disadvantage; less binder

Question 19

[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.

Answer 19

greatest surface area

Question 20

[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.

Answer 20

porosity; highest porosity

Question 21

[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.

Answer 21

broadens; fill

Question 22

[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.

Answer 22

burn- in defect; balance

Question 23

[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.

Answer 23

American Foundrymens’ Society (AFS); AFS; surface area per unit weight