Thứ Tư, 21 tháng 7, 2010

Defects and their causes in sand casting

A knowledge of casting defects is essential. If you cannot pinpoint the cause of a defect there is no way of correcting the problem. Some defects are quite obvious, along with the cause. Some types of defects can often resemble each other in appearance and separating them is often difficult A drawing or photograph of a defect is one thing and looking at the actual defect is another. You learn about defects by analyzing your own and those of others. Here is a list of defects and their causes that should include anything you might ever encounter:

1. Poured short: Casting incomplete due to not filling the mold. This is a stupid trick which we all do-cause insufficient metal in the ladle. Going back and touching up the mold will not do it.

2. Bobble: Here the casting will resemble a cold shut or that it was poured short. The problem was a slacked or interrupted pour. When pouring a casting the metal must be poured at a constant choked velocity. If you slack off or reduce the velocity, you can cause this defect. A completely interrupted pour, start-stop-start, if only for a second. A great percentage of lost castings is caused by pouring improperly.

3. Slag inclusion: Slag on the face of the casting and usually down the sides of the sprue. The cause is not skimming the ladle properly, not choking the sprue (keeping it brimming full from start to finish), sprue too big (cannot be kept choked) and gating system improperly choked.

4. Steam gas porosity: This defect usually shows up as round holes like swiss cheese, just under the cope surface of the casting and comes to light during machining. The cause is a wet ladle, where the ladle lining was not properly and thoroughly dried. In extreme cases the metal will kick and boil in the ladle. The practice of pigging the metal in a wet ladle and refilling it with the hopes that the pigged metal finished drying the lade1 is sheer folly.

5. Kish: (cast iron) If the carbon equivalent of the iron is too high for the section poured and its cooling rate is slow, free graphite will form on the cope surface in black shiny flakes free from the casting, causing rough holey defects usually widespread. Carbon equivalent is the relationship of the total carbon, to the silicon and phosphorous content of the iron, which is controlled by the make up charge and silicon added at the spout. It is called carbon equivalent because the addition of silicon or phosphorus is only one third as effective as carbon, therefore the carbon equivalent of the three additives is equal to the total percentage of pure carbon plus one third the percentage of the silicon and phosphorus combined. The carbon equivalent is varied by the foundryman depending on what type of iron he is producing and what he is pouring.

6. Inverse Chill: This defect is found in gray iron castings and is hard or chilled iron in the center sandwiched between soft iron. The cause again could be incorrect carbon equivalent for the job or the presence of non-ferrous metals in the charge, lead, antimony, tellurium which are detrimental impurities.

7. Broken Casting: This could be caused by improper design. Improper filleting along with improper handling any where along the line. Copper base castings, red brass, yellow brass etc., are what are known as hot short, will break easily when hot. Thus, if the casting is shaken out of the mold before it has cooled sufficiently, it can get broken very easily. A mold or core which has too high a hot strength will not give or collapse to give the casting room to move as it shrinks and will break the casting.

8. Bleeder: This defect is caused by shaking the casting out too soon when a portion of it is still liquid. The section runs out leaving a defect. In extreme cases the entire center section will run out on the shake out man's feet.

9. Lead Sweat: A covering of lead tears on the outside of a high leaded bronze or brass casting with underlying holes or porosity (red metals containing a large Bercentage of lead such as leaded bearing bronze). Since the lead has the lowest melting point of the constituents of high leaded bronze (copper 70 percent-tin 5 percent-lead 25 percent) and is not in solution with the copper and tin, it remains liquid until the casting has cooled below 620 degrees Fahrenheit. If the casting is shaken out before it is below this temperature the lead will sweat out &om between the copper and tin crystals.

10. Run Out: This is caused by the metal in the mold running out between the joint of the flask, which drains the liquid metal partially or completely from any portion of the casting above the parting line. A run out can come through or between the drag and the bottom board from a cracked drag mold. Also, it can come from between a loose improperly fitting core and the core print. It is caused by insufficient room between the flask and the cavity, insufficient weight on the cope (cope raises during pouring) or improperly clamped molds. Excessive hydrostatic pressure (sprue too tall for the job) no dough roll used between cope and drag (large jobs) or a combination of all of the above. An attempt to save a run out by placing your foot on top of the cope and applying pressure above the point. where the liquid metal is running out is foolhardy and dangerous. Also trying to stop off the run out flow with sand or clay is folly.

11. Omission of a core: Results and cause obvious.

12. Ram Off: Is a defect resulting from a section of the mold being forced away from the pattern by ramming sand after it has conformed to the pattern contour. This is caused by careless ramming where the mold is rammed vertically and then on an angle, causing the vertically rammed sand to slide sideways leaving a gap between the pattern and the sand, resulting in a deformed casting. Another cause is using a sand with poor or low flowabiity. (Too much clay or sand too he.)

13. Core Rise: This defect is caused by a core rising from its intended position toward the cope surface, causing a variation in wall thickness or if touching the cope no metal at that point. The core has shifted from its position. A green sand core rise is when a green sand core in the drag is cracked at its base (caused when drawing the pattern) and floats toward the cope. Dry sand cores will float if the unsupported span of a thin insufficiently rodded core is too great, it will bend upward by the buoyancy of the metal. Insuficient core prints in number and design, insufficient chaplets, slipped chaplet, chaplets left out by molder, poor design of the core. This defect is easy to spot and remedy.

14. Shifts: These come in two classilications, mold shift and core shift. A mold shift is when the parting lines are not matchedwhen the mold is closed, resulting in a casting offset or mismatched at the parting. The causes are: Excessive rapping of a loose pattern, reversing the cope on the drag, too loose a fit of the pattern pins and dowels, faulty mismatched flasks, too much play between pins and guides, faulty clamping, improper fitting (racked) jackets, and improper placing of jackets. A core shift is caused by not aligning the halves of glued cores true and proper when assembling them.

15. Swell: In this defect you have a casting which is deformed due to the pressure of the metal moving or displacing the sand. It is usually caused by a soft spot or too soft a mold.

16. Sag: A decrease in metal section due to a core or the cope sagging. The cause is insufficient cope bars, too small a flask for the job, insufficient cope depth. This defect will also cause misruns.

17. Fin: This defect is a fin of metal on the casting caused by a crack in the cope or drag, and is caused by wracked flasks, bad jackets, (and setting) uneven warped bottom boards, uneven strike off, insufficient cope or drag depth, bottom board not properly rubbed on drag mold sitting uneven (rocking).

18. Fushion: This defect is a rough glassy surface of fused melted sand on the casting surface either on the outside or on a cored surface. The cause is too low a sintering or melting point of the sand or core. This is quite common when a small diameter core runs through an exceptionally heavy section (of great heat) which actually melts the core. This can also be caused by pouring much too hot (hotter than necessary) for the sand or cores. A mold or core wash can prevent this in some cases but if the sintering point of your sand is too low for the class of work, you need a more refractory sand, zircon etc.

19. Metal Penetration: This defect should not be confused with an expansion scab which is attached to the casting by a thin vein of metal. The defect is a rough unsightly mixture of sand and metal caused by the metal penetrating into the mold wall or the surfaces of a core (not fused). The defect is basically caused by too soft and uneven ramming of the mold or core, making the sand too (open) porous, also too high a pouring temperature, too sharp a comer (insufficient filleting) making it impossible to ram the sand tight enough, localized overheating of the sand due to poor gating practice, or molding with a sand too open for the job. Penetration in brass castings is sometimes traced to excessive phosphorus used in de-oxidizing the metal making it excessively fluid.

20. Rough Surface: This defect can run from mild penetration to spotty rough spots or a completely rough casting. Many factors or combinations are at fault-sand too coarse for the weight and pouring temperature of the casting, improperly applied or insufficient mold coating or core coating, faulty finishing, excessive use of parting compound (dust) hand cut gates not firm or cleaned out, dirty pattern, sand not riddled when necessary, excessive or too coarse of sea coal in the sand, permeability too high for class of work, core or mold wash faulty (poor composition).

21. Blows: Round to elongated holes caused by the generation or accumulation of entrapped gas or air. The usual cause of blow holes is sand rammed too hard (decreasing the permeability), permeability too low for the job, sand and core too wet (excessive moisture), insufficient or closed off core vent, green core not properly dried, incompletely dried core or mold wash, insufficient mold venting, insufficient hydrostatic pressure (cope too short), cope bars too close to mold cavity, wet gagger or soldier too close to mold cavity, poor grain distribution. Any combination of hot and cold materials which would lead to condensation as a hot core set in a cold mold or visa versa. A cold chill or hot chill will cause blows, also wet or rusty chaplets.

22. Blister: A shallow blow covered over with a thin film of metal.

23. Pin Holes: Surface pitted with pin holes which may also be an indicator of subsurface blow holes.

24. Shrink Cavity & Shrink Depression: These defects are caused by lack of feed metal causing a depression on the surface of the casting, a concave surface. The shrink cavity, a cavity below the surface but connected to the surface with a dendrite crystal structure.

25. Cold Shot: Where two streams of metal in a mold coming together fail to weld together. This defect is usually caused by too cold a metal poured too slowly or gating system improperly designed so that the mold cannot be filled fast enough.

26. Misrun: A portion of the casting fails to run due to cold metal, slow pouring, insufficient hydrostatic pressure, sluggish metal (non fluid due to badly gassed or oxidized metal).

27. Scab: Rough thin scabs of metal attached to the casting by a thin vein separated from the casting by a thin layer of sand. Usually found on flat surfaces, caused by hard ramming, low permeability and insufficient hot strength. Sand does not have enough cushion material, wood flour etc., to allow it to expand when heated. Unable to expand it will buckle causing scabs along with, but not always rattails, grooves under the scabs also called pull downs. It is the pull downs that bring about the scab.

28. Blacking or Mold Wash Scab: This is a case when the blacking or wash on the mold or core, when heated, breaks away and lifts off of the surface like a leaf and is retained in or on the metal. The cause is a poor binder in the wash, improperly dried wash or poor wash formula or all of the above

29. Sticker: This is a lump or rat (bump) on the surface of the casting caused by a portion of the mold face sticking to the pattern and being removed with the pattern. This problem is caused by poorly cleaned, shelacked, polished pattern, rough pattern, cheap shellac, tacky shellac, sticky liquid parting, cold pattern against hot sand, insufficient draft.

30. Crush:This defect is caused by the actual crushing of the mold causing indentations in the casting surface. This is caused by flask equipment, such as bottom boards or cores that are too tall or too large for the prints or jackets. Also rough handling.

31. Hot Tears: This defect is actuany a tear or separation fracture due to the physical restriction of the mold and or the core upon the shrinking casting. The biggest cause is too high a hot strength of the core or molding sand. These defects can be external or internal, a core that is overly reinforced with rods or an arbor will not collapse. If you restrict the movement of the casting during its shrinking from solidification to room temperature, it will literally tear itself apart.

32. Gas porosity: This defect is widely dispersed bright round holes which appear on fractured and machined surfaces. This defect is caused by gasses being absorbed in the metal during melting. This gas is released during solidification of the casting. Cause is poor melting practice (oxidizing conditions) and poor de-oxidizing practice.

33. Zinc Tracks: This defect is found on the cope surface of high zinc alloy castings. The defects are caused by the zinc distilling out of the metal during pouring. This zinc oxide floats up to the cope and forms worm track lines on the casting when the metal sets against the cope. The problem is caused by pouring too hot (metal flaring) in ladle or crucible. Pouring the mold too slow, insufficient gates. The mold must be Med quickly before the damage can be done.

34. Drops: This is where a portion of the cope sand drops into the mold cavity before or during pouring. The causes are bumping with weights, rough clamping, weak molding sand (low green strength) rough closing, jackets placed on roughly etc.

35. Washing & Erosion: The sand is eroded and washed around in the mold, some of which finds its way to the cope surface of the casting as dirt sand inclusions. It can come from the gating system or in the mold cavity. The causes are too low a hot strength, too dry a molding sand, poor gating design, a deep drop into the mold, washing at the point of impact, metal washing over a sharp edge at gate, metal hitting against a core or vertical wall during the pouring.

36. Inclusions: Dirt, slag etc. This defect is caused by failure to maintain a choke when pouring, dirty molding, failure to blow out mold properly prior to closing, sloppy core setting causing edges of the print in the mold to break away and fall into the mold. The drag should be blown out, the cores set and blown out again. Dirt falling down the sprue prior to the mold being poured or knocked in during the weighting and jacketing. For the most part, it's just dirty molding.