The role of proper venting should not be discounted in the total
recipe for quality castings. Here how to avoid costly gas problems.
Vent (vent) a means of escape or passage from a restricted area; an
opening which allows the escape of vapor, heat, gas or liquid.
(Webster's Dictionary)
To the foundry engineer, a vent is a small channel in molds for
letting steam, air or mold gas escape as metal fills the mold. When done
properly, venting will reduce gas-related defects, improve surface
finishing, allow shorter pouring times and result in fewer misruns.
The need to vent cores and molds has been recognized for many years
as a means to avoid the adverse effects of entrapped or evolved gases.
Sources of these gases include heated air in the mold, steam generation
from water in the binders, and products of combustion from binders and
coatings.
However, all too often, venting is forgotten until problems develop.
Ideally, it should be considered as the third leg of the casting quality
"stool" - along with proper feeding and gating practices. If
the gating system is the "plumbing system for molten metal,"
then the venting system is the "plumbing system that allows
entrapped air- and mold-generated gases to escape." In fact, the
venting process is as important to making good castings as the proper
design of the gating/risering system.
Air Expansion and Gas Evolution
Air in the mold cavity can expand to many times its volume as the
molten metal enters the mold. Likewise, the pressures generated from
expanding air can be surprising. Assuming an ideal gas, 1 cubic cm (1cc)
of air at 77F (25C) expands to 62cc at the same pressure when heated to
2822F (1550C). For the same volume, the increased pressure generated
when air at the standard pressure of 10 N/[cm.sup.2] (14.7 psi) is
heated from 77F (25C) to 2822F (1550C) is 628 N/[cm.sup.2] (911 psi).
Moisture in green sand can also be a problem if venting is inadequate.
For every 1 lb of green sand at 3% moisture, there are 13.6 grams of
water. Heated to 2822F (1550C), that 13.6 grams (about 0.5 oz) of
moisture provide 0.5 cu ft of steam.
Table 1. Gas Evolution at 1% Binder for Various Coremaking Methods
Production Gas
Method Evolution
Hotbox 5-7 cc/g
Coldbox 5-10 cc/g
Silicate 2-3 cc/g
Shell 3-5 cc/g
Source: BCIRA Broadsheet 16-3
Gas evolution from binders must also be taken into consideration.
Table 1 lists an approximate guide for gas evolution for each 1% binder in various types of cores.
By knowing the volume of bonded sand from the mold and core that will
burn out and the gas evolution per unit volume, the amount of mold gases
generated can be estimated. This can be another guide as to the amount
of venting necessary. Without adequate venting, these gases can become
entrapped and result in casting defects such as blowholes or scabbing
[ILLUSTRATION FOR FIGURE 1 OMITTED]. There may also be a reaction
between the mold gases and molten metal forming undesirable products in
the casting. Gas pressure can become high enough locally that it will
not allow the molten metal to completely fill the mold cavity, causing
misruns or cold shuts. Furthermore, excessive gas pressure can roughen as-cast surfaces, loosen sand grains, cause mold and core coatings to
buckle, and increase pouring times.
Permeability
Sand permeability also affects the amount of venting needed to ensure
a good casting is produced. Natural openings in the molding sand, as
well as through man-made openings (vents), allow air and gases to
escape. The measure of how fast gases will diffuse through molding sand
is called permeability.
Mold permeability has been defined as: the volume of air in cu cm at
1 cm water gauge pressure that will pass through the test plumbers near me free estimates piece in 1 min
when the test piece is 1 cm long and 1 sq cm in cross sectional area. In
an equation form, it is:
(Volume of Air)*(Height of Specimen)/(Area)*(Time)*(Pressure)
Permeability can be measured with commercially available equipment
and is usually specified as a permeability number. The larger that
number is, the higher its permeability. Permeability can be influenced
by the size of the voids between the sand grains.
Regardless of whether the molding sand is classified as
"coarse" or "fine," the amount of inter-granular
voids is the same. But as sand coarseness increases, voids are fewer and
larger than when compared to finer sand with many smaller voids. Higher
permeabilities are usually associated with coarser sand. The distance
the gases must travel also can influence mold permeability.
Mold and core coatings will greatly reduce permeability through sand.
Care must be taken to keep coatings from blocking vents. Coatings can
also be helpful in directing the way for gases to move toward vents in
core prints.
Casting shape also affects mold permeability. Castings with deep
pockets or sharp concave contours will result in a mold that has a diy plumbing advice more
difficult time in evacuating gases [ILLUSTRATION FOR FIGURE 2 OMITTED].
Additionally, mold compaction comes into play. A high-density mold will
reduce permeability. Areas of the mold cavity closest to the squeeze
head of a high pressure molding machine will see lower permeabilities
than other areas because the sand is more closely compacted in that area
as compared to other parts of the mold. While there is no single optimum
value of permeability, following two guidelines (BCIRA, June 1973):
* There is plumbers near me reviews risk when the permeability is less than 20 in green sand
molds because the margin for error is high if the water content varies.
* If permeabilities over 120 are used in synthetic sands, the
surface finish of the castings may not be acceptable.
Venting Practice
There are many different types of vents. Small diameter rods or stems
[ILLUSTRATION FOR FIGURE 3 OMITTED] can be added to the pattern in
strategic locations to produce a vent as the mold is made. Parting line
vents [ILLUSTRATION FOR FIGURE 4 OMITTED] can be either made with strips
on the pattern or scratched in the mold before the mold is closed.
Remember that parting line vents cease to allow air to escape as the
molten metal rises above the parting line [ILLUSTRATION FOR FIGURE 5
OMITTED]. Additionally, parting line vents, if too large, can cause
run-outs.
Cores can be hollowed out in areas, not only to affect breakdown but
also to help channel the gases toward vents. Commercially available
textile, wax or rope vents [ILLUSTRATION FOR FIGURE 6 OMITTED] are also
available to create channels in molds or cores. Sometimes when an
extremely large vent is needed, pouring tiles or pipes have been used.
Proper gating design should also include a vent at the end of flowoffs
to allow air to escape from' the gating system [ILLUSTRATION FOR
FIGURE 7 OMITTED].
Weep holes in the sides of flasks also serve as vents. If it is
impossible to vent through the sidewalls of flasks, adding small hollow
strips to the flask sidewall can create a channel in the sand for
venting. Bottom boards should also be vented to allow gases to escape
from the bottom of the drag. If bottom boards can't be used, the
floor under the molds should be grooved or the molds should be placed on
a bed of dry sand. Both techniques will allow for venting.
Venting is most successful, however, when included on the pattern
equipment. The proper locations are already laid out and the foundry can
be assured that venting is occurring, rather than relying on people to
perform the operation on the molding line. Mold cycle time is not taken
up with drilling vents. Also, adding vents by hand can disturb the mold
and increase the possibility of loose sand grains falling into the mold
cavity. On the down side, care must be taken to prevent small thin vent
rods from damage during molding and pattern handling. Also, vents can
cause more work in the cleaning room, since they must be removed if
filled with metal.
Venting Blind Risers
When using blind feeders, venting becomes more important as the depth
of sand increases over the riser or as the pouring rate increases.
Failure to properly vent feeders can result in unfilled risers and loss
of atmospheric puncture during solidification.
If possible, it is better to vent through the sand (by placing a
small post on top of the riser) rather than venting through the sleeve.
This is because problems can occur if the vent is too large and the top
of the cope becomes disturbed [ILLUSTRATION FOR FIGURE 8 OMITTED]. This
small rod of metal will solidify before the feeder and therefore
won't allow the feeder to follow the casting during solidification.
The feeder can then become "upset" - especially in skin
forming alloys, causing a late stage shrink at the feeder contact.
General Rules
There is no set answer as to how much venting is necessary, since
every mold can be different. However, there are rules of thumb to follow
for proper venting:
* the total vent area should be at least equal to the choke area;
* venting should be added until there is no change in pouring time;
* watch the flames coming from vents. If they are "pressure
jets," add more venting until they become "lazy" gas
flames;
* if in doubt, vent some more.
http://www.thefreelibrary.com/Moldventing:areturntothebasics.-a020218295
recipe for quality castings. Here how to avoid costly gas problems.
Vent (vent) a means of escape or passage from a restricted area; an
opening which allows the escape of vapor, heat, gas or liquid.
(Webster's Dictionary)
To the foundry engineer, a vent is a small channel in molds for
letting steam, air or mold gas escape as metal fills the mold. When done
properly, venting will reduce gas-related defects, improve surface
finishing, allow shorter pouring times and result in fewer misruns.
The need to vent cores and molds has been recognized for many years
as a means to avoid the adverse effects of entrapped or evolved gases.
Sources of these gases include heated air in the mold, steam generation
from water in the binders, and products of combustion from binders and
coatings.
However, all too often, venting is forgotten until problems develop.
Ideally, it should be considered as the third leg of the casting quality
"stool" - along with proper feeding and gating practices. If
the gating system is the "plumbing system for molten metal,"
then the venting system is the "plumbing system that allows
entrapped air- and mold-generated gases to escape." In fact, the
venting process is as important to making good castings as the proper
design of the gating/risering system.
Air Expansion and Gas Evolution
Air in the mold cavity can expand to many times its volume as the
molten metal enters the mold. Likewise, the pressures generated from
expanding air can be surprising. Assuming an ideal gas, 1 cubic cm (1cc)
of air at 77F (25C) expands to 62cc at the same pressure when heated to
2822F (1550C). For the same volume, the increased pressure generated
when air at the standard pressure of 10 N/[cm.sup.2] (14.7 psi) is
heated from 77F (25C) to 2822F (1550C) is 628 N/[cm.sup.2] (911 psi).
Moisture in green sand can also be a problem if venting is inadequate.
For every 1 lb of green sand at 3% moisture, there are 13.6 grams of
water. Heated to 2822F (1550C), that 13.6 grams (about 0.5 oz) of
moisture provide 0.5 cu ft of steam.
Table 1. Gas Evolution at 1% Binder for Various Coremaking Methods
Production Gas
Method Evolution
Hotbox 5-7 cc/g
Coldbox 5-10 cc/g
Silicate 2-3 cc/g
Shell 3-5 cc/g
Source: BCIRA Broadsheet 16-3
Gas evolution from binders must also be taken into consideration.
Table 1 lists an approximate guide for gas evolution for each 1% binder in various types of cores.
By knowing the volume of bonded sand from the mold and core that will
burn out and the gas evolution per unit volume, the amount of mold gases
generated can be estimated. This can be another guide as to the amount
of venting necessary. Without adequate venting, these gases can become
entrapped and result in casting defects such as blowholes or scabbing
[ILLUSTRATION FOR FIGURE 1 OMITTED]. There may also be a reaction
between the mold gases and molten metal forming undesirable products in
the casting. Gas pressure can become high enough locally that it will
not allow the molten metal to completely fill the mold cavity, causing
misruns or cold shuts. Furthermore, excessive gas pressure can roughen as-cast surfaces, loosen sand grains, cause mold and core coatings to
buckle, and increase pouring times.
Permeability
Sand permeability also affects the amount of venting needed to ensure
a good casting is produced. Natural openings in the molding sand, as
well as through man-made openings (vents), allow air and gases to
escape. The measure of how fast gases will diffuse through molding sand
is called permeability.
Mold permeability has been defined as: the volume of air in cu cm at
1 cm water gauge pressure that will pass through the test plumbers near me free estimates piece in 1 min
when the test piece is 1 cm long and 1 sq cm in cross sectional area. In
an equation form, it is:
(Volume of Air)*(Height of Specimen)/(Area)*(Time)*(Pressure)
Permeability can be measured with commercially available equipment
and is usually specified as a permeability number. The larger that
number is, the higher its permeability. Permeability can be influenced
by the size of the voids between the sand grains.
Regardless of whether the molding sand is classified as
"coarse" or "fine," the amount of inter-granular
voids is the same. But as sand coarseness increases, voids are fewer and
larger than when compared to finer sand with many smaller voids. Higher
permeabilities are usually associated with coarser sand. The distance
the gases must travel also can influence mold permeability.
Mold and core coatings will greatly reduce permeability through sand.
Care must be taken to keep coatings from blocking vents. Coatings can
also be helpful in directing the way for gases to move toward vents in
core prints.
Casting shape also affects mold permeability. Castings with deep
pockets or sharp concave contours will result in a mold that has a diy plumbing advice more
difficult time in evacuating gases [ILLUSTRATION FOR FIGURE 2 OMITTED].
Additionally, mold compaction comes into play. A high-density mold will
reduce permeability. Areas of the mold cavity closest to the squeeze
head of a high pressure molding machine will see lower permeabilities
than other areas because the sand is more closely compacted in that area
as compared to other parts of the mold. While there is no single optimum
value of permeability, following two guidelines (BCIRA, June 1973):
* There is plumbers near me reviews risk when the permeability is less than 20 in green sand
molds because the margin for error is high if the water content varies.
* If permeabilities over 120 are used in synthetic sands, the
surface finish of the castings may not be acceptable.
Venting Practice
There are many different types of vents. Small diameter rods or stems
[ILLUSTRATION FOR FIGURE 3 OMITTED] can be added to the pattern in
strategic locations to produce a vent as the mold is made. Parting line
vents [ILLUSTRATION FOR FIGURE 4 OMITTED] can be either made with strips
on the pattern or scratched in the mold before the mold is closed.
Remember that parting line vents cease to allow air to escape as the
molten metal rises above the parting line [ILLUSTRATION FOR FIGURE 5
OMITTED]. Additionally, parting line vents, if too large, can cause
run-outs.
Cores can be hollowed out in areas, not only to affect breakdown but
also to help channel the gases toward vents. Commercially available
textile, wax or rope vents [ILLUSTRATION FOR FIGURE 6 OMITTED] are also
available to create channels in molds or cores. Sometimes when an
extremely large vent is needed, pouring tiles or pipes have been used.
Proper gating design should also include a vent at the end of flowoffs
to allow air to escape from' the gating system [ILLUSTRATION FOR
FIGURE 7 OMITTED].
Weep holes in the sides of flasks also serve as vents. If it is
impossible to vent through the sidewalls of flasks, adding small hollow
strips to the flask sidewall can create a channel in the sand for
venting. Bottom boards should also be vented to allow gases to escape
from the bottom of the drag. If bottom boards can't be used, the
floor under the molds should be grooved or the molds should be placed on
a bed of dry sand. Both techniques will allow for venting.
Venting is most successful, however, when included on the pattern
equipment. The proper locations are already laid out and the foundry can
be assured that venting is occurring, rather than relying on people to
perform the operation on the molding line. Mold cycle time is not taken
up with drilling vents. Also, adding vents by hand can disturb the mold
and increase the possibility of loose sand grains falling into the mold
cavity. On the down side, care must be taken to prevent small thin vent
rods from damage during molding and pattern handling. Also, vents can
cause more work in the cleaning room, since they must be removed if
filled with metal.
Venting Blind Risers
When using blind feeders, venting becomes more important as the depth
of sand increases over the riser or as the pouring rate increases.
Failure to properly vent feeders can result in unfilled risers and loss
of atmospheric puncture during solidification.
If possible, it is better to vent through the sand (by placing a
small post on top of the riser) rather than venting through the sleeve.
This is because problems can occur if the vent is too large and the top
of the cope becomes disturbed [ILLUSTRATION FOR FIGURE 8 OMITTED]. This
small rod of metal will solidify before the feeder and therefore
won't allow the feeder to follow the casting during solidification.
The feeder can then become "upset" - especially in skin
forming alloys, causing a late stage shrink at the feeder contact.
General Rules
There is no set answer as to how much venting is necessary, since
every mold can be different. However, there are rules of thumb to follow
for proper venting:
* the total vent area should be at least equal to the choke area;
* venting should be added until there is no change in pouring time;
* watch the flames coming from vents. If they are "pressure
jets," add more venting until they become "lazy" gas
flames;
* if in doubt, vent some more.
http://www.thefreelibrary.com/Moldventing:areturntothebasics.-a020218295