Pinholes caused by excessive moisture in molding sand combined with low permeability and poor grain distribution (6 screen sand).
Description
“Blows” or gas holes in castings are cavities, either spherical, flattened or elongated. They are related to localized gas (including entrapped air) pressure that exceeds metal pressure in any locality during
solidification of the metal. One of the greatest problems with core blows is the ease with which they may be confused with mold blows. Since gas travels upwards in the mold it is quite possible for a gas bubble to form from a core and then travel toward the cope surface.
Pinholes, blisters, and body scars as well as certain types of porosity are variations of gas holes. In gas defect problems, there are several possible sources, among which are cores, molding sand and metal. The volume of gas may be kept unchanged but offset by providing easier escape or by providing more metal pressure, forcing gas through existing vents.
Causes
I. Casting and Pattern Design
1) Insufficient print or outlet provided for venting 2) Design that prevents adequate metal pressure 3) lnsufficient vents at the parting line
II. Pattern Equipment
None
Pinholes from excessive permeability and moisture remedied by lowering permeability from 140 to 90-110 and reducing moisture from 4.2% to 2.8-3.3%.
Gas blows caused by clay balls (excess fines).
III. Flask Equipment and Rigging
1) Bars too close to the mold surface 2) Flask wall too close to mold surface 3) Cope too shallow
4) Sprues and risers too close to bars or flask walls 5) Insufficient venting in flasks
IV. Gating and Risering
1) Insufficient cope height 2) Gating causing turbulence
3) Gating design that causes interrupted flow of metal 4) Inadequate riser height or size
5) Improper sprue or gate design allowing air or mold gas to enter metal stream 6) Insufficient spinning speed in centrifugal casting of pipe
7) Insufficient vents and flow-offs
V. Molding Sand
1) High moisture content
2) Inadequate permeability for the volatile materials involved 3) Foreign material in sand-shot, nails, coke, cinders, etc.
4) Clay balls
5) Poorly mixed sand
6) Coarse particles of gas-producing materials
7) Coatings containing excessive gas-producing materials
8) Excessive permeability in skin-dried molds, allowing sweat-back 9) Excessive or insufficient reducing volatile materials
Blow due to excessive moisture in molding sand.
VI. Core Practice
1) Underbaked or undercured cores 2) Excessive binder
3) Inadequate permeability for conditions involved 4) Inadequate venting
5) Excessive or wet core paste
6) Core wash too heavy, skin broken or not dried
7) Excessive gas-producing material either in the core sand or wash 8) Cores that absorb moisture either in storage or in the mold 9) Exposed hooks, wires or hangers
10) Chill or sealing materials not dried
11) Hooks or hanger holes filled and not dried
12) Foreign materials, such as roots, leaves, coke, coal, etc.
13) Excess nitrogen content in resin binders 14) Poorly mixed core sand
Improperly conditioned core sand (lumps of cereal binder and excessive moisture are potential sources of gas and can cause blows).
Broken surface of core wash permitted gas to back up into heavy metal sections.
Vents plugged with iron during pouring caused a major defect on the cope surface.
Vents plugged with iron during pouring caused a major defect on the cope surface.
Blow in manifold at a hot spot, caused by a hard core.
Close-up of the blow shown above.
VII. Molding Practice
1) Combinations of hot and cold materials 2) Improper venting in green or dry sand molds 3) Insufficiently dried molds
4) Hard spots caused by improper ramming for conditions involved 5) Gagger, soldiers, arbors, or bars too close to pattern
6) Clay wash too heavy on gaggers
7) Excessive slicking and patching of green sand 8) Dough roll too wet or too close to mold cavity 9) Wet pouring basins and gating systems 10) Insufficient sand in flask
11) Excessive liquid parting (especially in pockets or depressions) 12) Mold wash or coating too heavy or not dried
13) Failure to lead core vents out of molds 14) Vents too close to surface causing sweatback
Gas blows-cold iron, high moisture.
Blow in the cope side of a pump impeller caused by wet sand and hard ramming.
Result of aluminum in iron.
VIII. Metal Composition
1) Gas dissolved in molten metal
2) Metallic impurities e.g. aluminum, bismuth, lead, tin, antimony and boron 3) Greasy or rusty scrap
4) Enameled scrap containing boron
5) Incorrect balance of aluminum and silicon in steel
Blister on casting caused by hard ramming and wet sand.
Upper section shows a chaplet blow caused by improper coating material or a damp chaplet. Lower section shows a good fusion.
Blows due to cold melted metal.
IX. Melting Practice
1) Steel
a. Too short a boil
c. Tramp elements (e.g. aluminum, lead, tin, antimony, boron, bismuth, enameled iron d. Stripping too early
e. Excessive variation of acid demand of sand
3) Non-ferrous
a. Improper fluxing or degassing procedures b. Excessive melting temperatures
c. Wet refractories
d. Wet or cold melt addition e. Wet or cold skimmer
f. Metallic impurities or tramp elements g. Oxidized metal
X. Pouring Practice
1) Cold, damp, or green crucibles or ladles 2) Pouring cold metal
3) Interrupted pouring
4) Ladle or crucible too high above mold 5) Slow or fast pouring
6) Improperly cleaned ladles 7) Heel of metal left in ladle
8) Insufficient spinning speed in centrifugal casting
XI. Miscellaneous
1) Chills, chaplets, wires which are rusty, improperly coated, oxidized or damp