casting

Disadvantages of sand casting

1- Fins or flush

Castings occur at the surface of the separation of the mold and the evidence of the massage because there is clearance between the two halves of Rizk.

The reason for the clearance of the error during the assembly process of the template or as a result of mismatch of the two halves of the wooden model for this must be taken into account the accuracy of measurement. It can also happen because the mold is not well tied and sometimes due to the pressure of the impulsive metal opens the mold causing these excesses.

The problem of metal appendages is: waste of molten metal, and the difficulty of removing appendages, which increases the cost of production and time, and in some cases also change the dimensions required for the piece.

2 - lack of metal or incomplete (Incomplete Casting):

Lack of metal or incomplete piece due to lack of metal cast in the mold and the main reasons for this defect is the mistake in the design of the casting or feeding system or that the spilled metal is relatively cold and freezes before the completion of the casting process. It is also observed when the metal is not sufficiently liquid, and when gases prevent the filling of the mold with the metal, and also occurs when the metal leaks through the gap formed at the surface of the separation as a result of poor binding of the two halves.

3- Cracks:

Clear cracks can be seen with the naked eye or by using a longitudinal or zigzag magnifier, generally occurring on the sharp edges of the cut and corners. The causes of cracks are numerous and the most important is the poor design of the piece or the wooden model. It is preferable to move away from the right angles and sharp edges and replace them with a round curvature as much as possible.Other important reasons are two different parts of the thickness in the same piece, which causes internal stresses due to different temperatures and in this case it is advisable to use refrigerants or change the design.

 

4. Warp Castings:

It occurs as a result of the uneven thickness of its walls, so the design must be improved and the use of coolers for the thick parts of the castings.

 

5-Docking (Cold Shut)

It is in the form of a groove drop due to the lack of full cohesion between the metal streams entering from different sides.

It occurs because the metal is not sufficiently liquid or the metal is not directed

It is especially true for the metal outage when the mold is filled. Meet the cool part with Hot metal in castings.

6 - gas bubbles (Gas Holes):

They are bubbles of air or gases that form the mold and remain casted in the form of small voids distributed in different parts.

The reasons for the emergence of gas spots in the castings are the lack of permeability of the mold to the gases sufficiently (the intensity of the ram with poor ventilation), and the poor types of sand castings and masses and poor ventilation core, and moisture core when placed in the mold, and pour a metal that has not yet been well disposed of gases. Large horizontal levels of castings should be avoided and dispensed as this can be done using slanted levels.

 

7. Freezing gaps (shrinkage gaps)

They are voids formed as a result of the insufficient metal feeding the foundry in the places where the metal is assembled. The freezing gaps are often seen in castings in the form of a large number of small bubbles causing the crushing of the castings and their porosity.

Reasons for freezing and crushing gaps:

a- Incorrect casting design and estuary and nutrient conditions (technology).

b- Filling the mold with a metal that is hotter than necessary and error in the composition of the metal resulting in excessive shrinkage.

c- The transition should be from the thick parts to the thin parts of the castings

Gradually until this compensates for the temperature difference in the process of metal freezing.

d-The problem of antifreeze gaps becomes very important for castings made of high-strength alloys such as steel.

 

8-Slag lobes (Slag Inclusions):

 

These containments of cast slag in the form of shiny objects or pits in the casted body are located in the mold of the casting powder, and the cause of this defect is poor cleaning of the metal from slag and also error in casting and poor design of the casting system.

This defect can be avoided by adding ceramic filters in the casting system or by changing the casting system from the crucible (from the bottom of the crucible).

 

9 - sand lobes (Sand Inclusions):

Lobes:

It is the metal points that are not fully fused with the casting, are the points that poured into the mold first, and these lobes freeze in castings cast in the form of white hard cast makes it difficult to operate the casting machining.

 

 

10. More casting defects

Green sand mold is a kind of soft mold, so it is not hard enough as the resin sand molds and shell molding. So, there are more casting defects such as sand residuals, sand holes, air holes and shrinkages.

11. Rough surface quality

The casting surfaces by green sand casting process are very rough and coarse. If the iron foundries use the very fine green sand, the rough surfaces will be better. However, there are few iron foundries who use very fine green sands in China. Refer to iron-foundry.com.

 

12. Bad casting dimensions

Since the green sand will have larger shrinkage, the casting dimensions will have larger changes during molding and cooing periods. Therefore, the green sand casting process will cause larger dimensional tolerances. Normally, its could reach casting dimensional tolerances of CT10 to CT12.

13. Unstable casting quality

The temperature has some influence to the casting quality made by green sand casting process. Therefore, the iron foundries should keep the good temperature in cold winter. Moreover, as the manual floor molding, the worker's skill will have key affects to the casting quality.

 Form book:-

 

A-  Effects resulting from incomplete feeding

Many of the more prevalent defects of castings result directly from solidification shrinkage shows typical defects to shrinkage:

         1. External imperfections: The most prevalent type is localized cavities appearing at unted hot spots in the 'casting viz., depressed regions: on cope and upper surfaces. Whenever feeding is grossly inadequate, internal unsoundness is usually indicated by some external imperfection

     i. Wall punctures.

     ii. Deforms by dishing at the weakest point.

     iii. Elongated wormholes appearing at the riser base, at internal angles, on cope surfaces.

    iv. Defects resembling collections of dross where cope surfaces are wrinkled and drawn inward

    v. Small voids appearing to be pinholes.

2-Internal imperfections: Gross shrinkage, center-line shrinkage, micro porosity are internal manifestations of solidification shrinkage:

I-Gross shrinkage is a localized cavity most likely to occur in isolated sections called "Hot Spots" i.e. the last regions of the cavity so solidify.

II. Centre line shrinkage is a narrow. More or less continuous void sometimes found along the center line of castings with extensive plate like sections. It may actually be in a continuous line or may appear as a line of fine shrinkage chevrons (zig-zig) with the tips pointing away from the main direction of feeding. This defect is found only in alloys like steel-which freeze over a relatively narrow temperature range.

iii- Interdendritic shrinkage (Micro porosity) Alloys that freeze over a wider temperature interval tend to exhibit this defect when improperly fed.

 Causes

1- Improper feeding

2- Dissolved gases.

 

Remedies

i.                 Adequate degassing.

ii.                ii. Directional solidification to produce sound castings.

 

3-Internal hot tearing: Internal hot tears 'are radially disposed discontinuitics inside castines. more commonly steel castings, which can be disclosed nondestructively only by radiography. The discontinuities resemble external hot tears, except that they are radial rather than roughly parallel The tears emanate from a low-density area, giving the radio-graph an octopus-like appearance.

Causes

i-                 Contraction stresses

ii-                 Inndequate measures to give mould relief.

Remodies

 i. Cure for internal hot tears is improved feeding.

 4. Gas porosity: Micro porosity, center-line shrinkage und gross porosity (resembling gross shrinkage) may be caused by

I-  Gases dissolved in the metal during melting and pouring.

ii-                Solidification shrinkage.

 Voids in a well-fed section may be caused by gas. Tounded voids with smooth walis arç generally due to gas, called 'Gas Porosity. Angular voids, perhaps with dendrite arms protruding into the voids, are usually caused by imperfect feeding. Fine micro porosity observed occasionally in nonferrous castings may be due to both. i) Gas content, iji) Metal shrinkage.

 

Remedies:

i. Cure for internal hot tears is improved feeding.

5- External bot tears, cold cracks, and warpage

a- External hot tears; are jagged, roughly- parallel tears generally visible to the un sided eye but usually found by x-ray, magnetic powder, or zyglo testing. Tears are often deep and are lormed by surface -rupture near the temperature at which the casting is completely solid appearance of an external hot tear on radiographic film.

  Causes

i-External hot tears, cold cracks, or warpage may occur if castings do not contract relatively freely in the mould as they cool from solidification temperature to room temperature

ii.Stresses developed in the casting by resistance of the mould.

iii-Resistance of thinner parts of the casting to normal contraction upon them, of heavier sections cooling more slowly.

b-Cold cracks; They are somewhat similar to external hot tears. Usually a ” cold crack” is continuous

   Causes

i-it is formed in the cold casting due to stresses imposed during cooling or heat treatment.

c- Warpage: is misalignment. It does not involve any discontinuities like hot tears.

 Causes:

i -Stresses set up by conditions of (Cooling -improper-mould resistance).

B-   Unfused chills and chaplets

Causes: Poor fusion of chaplets  and chills may be due to:

i.                 Low Pouring temperatures.

ii.                 Use of chaplets or chills so large that they cool the metal before surface fusion Can occur,

 Blows from chaplets and chills may be due to:

 i- Moisture.

ii-Other contamination e.g. Oil,, dust.

iii.               Oxide accumulated on the chill or Chaplet before or after it is placed in the mould.

 

iv.               External chills sometimes cause surface blows by similarly contamination.

 

 Remedies

i.                 By choosing and using internal and external chills and chaplets wisely.

ii.                 External and internal chills and Chaplets should have clean, roughened surface such as can be imparted by sand blazing this reduces blows and causes metal to lie quietly against external chills and to fuse with internal chill and chaplets.

c- Moulding delects

1-Sand spots: are caused by

    i- Inadequate control of moulding sand properties.

     2. Oversize castings: are caused by:

   i. Improper moulding

   ii. Rapping a pattern too hard when removing it from the mould

    iii- Excessive allowance in the pattern for metal shrink

     iv. Sand rammed too soft

3-Mismatch, lifts and shift: A mismatch is a slight displacement of the cope half of the casting with respect to the drag half. Mismatch is caused by loose pins on the flask. Lifts and shifts are caused due to cope or core displacements .

       i- Fins are a thin metal flash at the parting line of the mould or core halves. They are readily    removed in the cleaning operation

 

 

D- Other defects in castings .

1-Blow holes: are caused by

i-                 The sand being too wet.

ii-                . Low temperature of the mould

iii-              . Hard ramming

iv-              . Improper venting.

  2. Honeycombing or sponginess: is an external defect consisting of a number of small cavities in close proximity caused by

 i. Dirt or 'scurl' held mechanicallly in suspension in the molten metal.

 ii. Imperfect skimming in the ladle.

iIi. Poor metal,

 

 

3-Sand holes found on external surface or inside the casting: are caused by:

 Loose sand washing into the mould cavity and fusing into the interior of the casting

 ii Rapid pouring of the molten metal.

 4- Swelling: is caused by:

 i. Defective ramming of moulds.

5. Poured short: is caused by:

I.Insufficient metal at one pouring.

 6. Internal air pockets: are caused by:.

 i. Pouring boiling metal.

I i. Rapid pouring of the molten in the mould

E- Defects caused by moulding, core. making and gating

1-Hot tears: are caused by:

 i .Shrinkage cracks, or pulls are formed by the Casting pulling itself apart while cooling in the mould. The cracks have jagged edges and have discolored, oxidized appearance.

Ii. Moulder may have provided insufficient fillets or brackets at the Junctions of sections or have used insufficient feeding iii. Slow running on account of small gates or metal lacking in fluidity.

 

 2. Shifts: These are mismatching of the different parts of a mould at joint lines caused by:

i. The use of undersize moulding-box pins. i. Movement of the face of the moulding sand during rolling over.

3. Fins and fash along the joint lines of castings: are caused:

 i. When the moulder raps the pattern excessively before withdrawal.

ii. If the moulder does not place sufficient weight on the top part of the mould.

 iii. If the moulder fails to see that the parts of the mould are tightly clamped.

4. Cold laps or Misrun castings: are caused:

 i. If the gates are too small. ii. When there are too many restrictions in the gating system.

iii. When metal lacking in fluidity is used.

iii.               Faulty venting of the moulds.

5-Crush: is displacement of sand when the moald is being closed caused by:

i-Carelessness on the part of the moulder

iii-              Excessive weighting of the mould.

iv-              Cores too large

v-                . Core prints too small.

 6. Swell: is a local displacement of the face of the sand by the pressure of the fluid metal caused

 i- These local bulges are often caused by soft reaming.

 7. Sand wash: usually occurs near the ingates as rough lumps on the surface of a casting it appears on the upper surfaces of the casting as rough holes or depression, it is caused by:

 i- Soft ramming.

 ii. Weak sand.

iii-Poor patterns.

iv. Insufficient draft on patterns.

v. Sand wash from cores overbaked or insufficiently bonded.

8. Scabs: are cough slightly-raised areas on the surfaces of castings, often with some sand embedded in the scab caused by:

 i. Uneven ramming.

ii. Slow or intermittent running.

 

9- Sand blow: is an excessively smooth depression the outer surface of a casting, eaused by:

i-Moulding sand with moisture content and low perrmeability.

 ii. Insufficient venting Tbe defect is called 'air lock

iii. Faulty gating system if delect occurs near the ingate where the incoming mectal impinges ou small area of sand.

 10-Core blow: Is is an exoessively saoth depression on the inner surface of a cored cavity or a gas pocket imaiediately above a cored cavity, caused when:

i-                 Moulder forgets to carry tie vent from the core through the mould.

ii-                 Moalder uses cons insufficiently bakod or having picked up moisture after baking.

11- Slag holes are smooth depressions on the upper Surface of Castings. This usually occur oear the ingates, Remedy

i. By inserting slag traps in the gating system.